Mechanisms for enhanced performance of platinum-based electrocatalysts in proton exchange membrane fuel cells.

CHEMSUSCHEM MINIREVIEWS DOI: 10.1002/cssc.201300823

Mechanisms for Enhanced Performance of Platinum-Based Electrocatalysts in Proton Exchange Membrane Fuel Cells Liang Su,[a] Wenzhao Jia,[b] Chang-Ming Li,*[c, d] and Yu Lei*[a] As a new generation of power sources, fuel cells have shown great promise for application in transportation. However, the expensive catalyst materials, especially the cathode catalysts for oxygen reduction reaction (ORR), severely limit the widespread commercialization of fuel cells. Therefore, this review article focuses on platinum (Pt)-based electrocatalysts for ORR with better catalytic performance and lower cost. Major breakthroughs in the improvement of activity and durability of electrocatalysts are discussed. Specifically, on one hand, the enhanced activity of Pt has been achieved through crystallographic control, ligand effect, or geometric effect; on the other hand, improved durability of Pt-based cathode catalysts has

been realized by means of the incorporation of another noble metal or the morphological control of nanostructures. Furthermore, based on these improvement mechanisms, rationally designed Pt-based nanoparticles are summarized in terms of different synthetic strategies such as wet-chemical synthesis, Ptskin catalysts, electrochemically dealloyed nanomaterials, and Pt-monolayer deposition. These nanoparticulate electrocatalysts show greatly enhanced catalytic performance towards ORR, aiming not only to outperform the commercial Pt/C, but also to exceed the US Department of Energy 2015 technical target ($30/kW and 5000 h).

1. Introduction Fuel cells are envisaged to be a new generation of power sources, which convert chemical energy into electrical energy with, theoretically, both economic and environmental benefits.[1, 2] Combined with the emerging hydrogen economy, fuel cells have shown great promise in replacing the conventional combustion engine for transportation applications.[3–8] Fuel cells can be categorized by the electrolyte that facilitates the mass transfer of the power generation unit. Electrolytes include aqueous alkaline solution (alkaline fuel cells, or direct borohydride fuel cells), molten substances (molten carbonate fuel cells or phosphoric acid fuel cells), conductive ceramic oxide (solid oxide fuel cells or protonic ceramic fuel cells), and polymer electrolyte (proton exchange membrane fuel cells or alkali anion exchange membrane fuel cells). Figure 1 depicts a schematic configuration of a typical proton exchange membrane [a] Dr. L. Su, Prof. Y. Lei Department of Chemical & Biomolecular Engineering University of Connecticut 191 Auditorium Road, Storrs, CT 06269-3222 (USA) E-mail: [email protected]

fuel cell (PEMFC) containing a membrane electrode assembly (MEA, the combination of cathode, anode, and proton exchange membrane) and two bipolar plates (the backing layers and current collectors). A thin layer of electrocatalyst is coated on each electrode to promote the corresponding half reaction. Hydrogen (H2) is oxidized (losing electrons) on the anode; oxygen (O2) is reduced (gaining electrons) on the cathode. Protons (H + ) are delivered from the anode to the cathode within MEA under the influence of the proton exchange membrane to balance the electric charge in the system; the unidirectional mass transfer of H + constitutes one of the most important characteristics of PEMFCs. As the most important and, incidentally, the most expensive component in fuel cells, electrocatalysts, especially noble metal-based electrocatalysts, have attracted considerable academic and industrial attention. Platinum (Pt) is the most widely used metal catalyst in fuel cells, especially on the cathode.

[b] Dr. W. Jia Department of Nanoengineering University of California-San Diego 9500 Gliman Drive, La Jolla, CA 92093-0448 (USA) [c] Prof. C.-M. Li School of Chemical and Biomedical Engineering Nanyang Technological University 70 Nanyang Drive 637457 (Singapore) E-mail: [email protected] [d] Prof. C.-M. Li Institute for Clean Energy and Advanced Materials Southwest University Chongqing 400715 (PR China)

2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Figure 1. Schematic illustration of a proton exchange membrane fuel cells.

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CHEMSUSCHEM MINIREVIEWS However, the notoriously volatile price of Pt severely limits the popularization of fuel cells. For example, the cost of Pt in a hydrogen PEMFC for a 100 kW (134 hp) vehicle is higher than that of an entire 100 kW gasoline engine.[9] In addition, Pt is vulnerable to poisoning species such as carbon monoxide (CO) Liang Su obtained his Ph.D. degree from the Department of Chemical and Biomolecular Engineering at the University of Connecticut in 2013. His Ph.D. research focused on the development of noble metal-based nanocatalysts for alcohol oxidation reaction and oxygen reduction reaction in the application of fuel cells. He is currently a postdoctoral associate in the Department of Chemical Engineering at Massachusetts Institute of Technology. Wenzhao Jia, Ph.D. is a postdoctoral scholar in the Department of NanoEngineering at the University of California San Diego. She received her Ph.D. in Chemical and Biomolecular Engineering from the University of Connecticut in 2011. Her graduate research focused on the design and synthesis of nanostructured functional materials and their applications in sensing and energy related fields. Chang-Ming Li is a Professor of Institute for Clean Energy & Advanced Materials at Southwest University, China. He received his Ph.D. in 1986 at Wuhan University. His current research focuses on bionanotechnologies, green energies and biosensing/lab-ona chip systems. He has published 385 peer-reviewed journal papers, 8 book chapters and holds 104 patents.

Yu Lei is a Castleman associate professor of Chemical and Biomolecular Engineering at the University of Connecticut, USA. He obtained his Ph.D. degree in 2004 at the University of CaliforniaRiverside in Chemical and Environmental Engineering. His current research combines biotechnology, nanotechnology, and sensing technology, especially as applied to the development of alcohol electro-oxidation, oxygen reduction, gas sensing, electrochemical sensing, and biosensing.


www.chemsuschem.org and to unfavorable operational conditions such as high voltage, causing the inevitable decrease of the lifetime of fuel cells. To make it more competitive in automobile applications, a fuel cell system needs to achieve the price of $30 per kW and the lifespan of 5000 h.[5] Therefore, the cost and durability of electrocatalysts constitute the major challenges in current fuel cell technology. Commercially available Pt catalysts (ETEK, Johnson-Matthey, TKK, and so on) include fine powder of Pt (Pt black) and high surface area carbon-supported Pt nanoparticles (Pt/C). The use of Pt black increases the surface area by decreasing the particle size of the catalyst down to 20 nm, thus providing improved utilization efficiency compared to the bulk Pt. For Pt/C, the incorporation of an activated-carbon support further increases the surface area by providing anchoring sites for the catalyst, which effectively retards the aggregation of Pt nanoparticles with an size of up to 5 nm.[10] However, neither Pt black nor Pt/C can meet the requirement ($30 per kW and 5000 h) set by the U.S. Department of Energy (DOE). Scientists and engineers still face to achieve a further reduction of Pt loading on the cathode by a factor of 20 without compromising activity and durability.[5] In pursuit of this ambitious goal, Pt black and Pt/C often serve as the benchmark to evaluate the performance of newly developed electrocatalysts;[11] kinetic parameters of the electrocatalysis of the oxygen reduction reaction (ORR) are determined by standardized experimental procedure using a thin-film rotating (ring) disk electrode [R(R)DE].[12] One of the current research activities on fuel cells aims to develop nanomaterials with better catalytic performance and lower cost. Proceeding towards this goal, the Review will be focusing on the discussion of platinum-based cathode catalysts for ORR.

2. Fundamentals of ORR Before moving to the cathode catalysts in fuel cells, we will first discuss the reaction they catalyze. Oxygen or air is predominantly used as the cathode fuel because of its abundance. Therefore, ORR has been intensively studied, which is of benefit to the research of almost all types of fuel cells.[13]

2.1. Reaction pathway From a classic point of view, ORR proceeds through either a “direct” four-electron pathway or a “series” two-electron process.[14, 15] In acidic medium: O 2 þ 4 H þ þ 4 e ! 2 H 2 O

E 0 ¼ 1:229 V

ð1aÞ

or O 2 þ 2 H þ þ 2 e ! H 2 O 2

ð1bÞ

H2 O2 þ 2 Hþ þ 2 e ! 2H2 O

ð1cÞ ChemSusChem 2014, 7, 361 – 378

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In alkaline medium: O2 þ 2 H2 O þ 4 e ! 4 OH

ik ¼ n F A k f ðEÞ C O E 0 ¼ 0:40 V

ð2aÞ

or O2 þ H2 O þ 2e ! HO2 þ OH

ð2bÞ

HO2 þ H2 O þ 2e ! 3OH

ð2cÞ

Although the four-electron reaction has been unanimously recognized as the favorable pathway, this traditional ORR mechanism on Pt in a “direct” manner [Eq. (1 a)] has been challenged. For example, based on their mathematical model and experimental observations using Pt-particle electrodes, Chen and Kucernak pointed out that the “indirect” two-electron reactions [Eqs. (1 b) and (1 c)] could be the dominant pathway of ORR on Pt particles in sulfuric acid.[16] Nowadays, the most accepted ORR process on the molecular level is known as the associative mechanism [Eq. (3)]:[17] O2 þ Hþ þ e ! OOHads

ð3aÞ

OOHads þ Hþ þ e ! Oads þ H2 O

ð3bÞ

Oads þ Hþ þ e ! OHads

ð3cÞ

OHads þ Hþ þ e ! H2 O

ð3dÞ

2.2. Average electron transfer number Regardless of arguments over the reaction pathway, the RRDE experiment is still the primary approach to electrochemically determine the average number of electron transfer (n’) for ORR on the as-developed materials [Eq. (4 a)]. n0 ¼ 4 ID =ðID þ IR =NÞ

ð5Þ

where n is the overall electron transfer number; F is the Faraday constant (96 485 C·mol1); A is the area of electrode; kf(E) is the rate constant; CO is the concentration of the dissolved O2. ik is directly related to kf(E), and thus reflects the catalytic ability of the electrocatalyst in a particular system. ik can be determined by rotating disk voltammetry according to the Koutecky–Levich equation: 1=i ¼ 1=ik þ 1=ilev

ð6Þ

where i is the disk current determined directly on the polarization curve (i vs. E) from RDE experiment; ilev is the Levich current (also known as the diffusion limiting current, il), which can be expressed by the Levich equation: ilev ¼ 0:201 n F A DO 2=3 n1=6 C O w1=2

ð7Þ

where DO is the diffusion coefficient of O2 ; n is the kinematic viscosity of the electrolyte; w is the rotation speed of RDE (in the unit of rpm). In a particular system, all parameters except w in Equation (7) are constant. Therefore, the value of ik can be obtained from the intercept of the plot of 1/i versus 1/w1/2 by combining Equations (6) and (7). A simpler way to calculate ik is the direct application of Equation (6) at only one rotation speed with i and ilev obtained from the corresponding polarization curve. ik is usually expressed in terms of mass activity (kinetic current normalized by mass loading) and specific activity [kinetic current normalized by electrochemical surface area (ECSA)], which reflect the utilization efficiency and the intrinsic activity of Pt, respectively. To obtain accurate kinetic parameters from RDE experiments, researchers should examine the influence of the ohmic drop (iR drop) during data acquisition/ processing.[19]

ð4aÞ 2.4. Electrochemical surface area

where ID is the disk current due to electrons being transferred to oxygen atoms on the disk electrode [Eqs. (1 a,b) and (2 a,b)]: ID ¼ I4e þ I2e

ð4bÞ

IR is the ring current resulting from the further reduction of H2O2 or HO2 on the ring electrode [Eqs. (1 c) and (2 c)]. N is the collection efficiency that is only related to the dimension of the electrode. However, as discussed in Section 2.1, it is worth mentioning that the n’ value determined by RRDE is the apparent electron transfer number under the mass transport regime limited by the rotating equipment; the n’ value cannot reflect the detailed ORR mechanism on the catalyst.

2.3. Kinetic current Another parameter of interest in the study of ORR is the kinetic current (ik) defined by Equation (5):[18] 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

To evaluate the ECSA, cyclic voltammetry (CV) of the target catalyst should be carried out in an O2-free electrolyte. The ECSA of the catalyst can then be derived by measuring the charge (Q) associated with the adsorption and desorption of hydrogen after double-layer correction in the hydrogen adsorption/desorption region. The ECSA (cm2 mg1) can be calculated by Equation (8): ECSA ¼

0:5 Q 0:5 AH ¼ m qH m qH s

ð8Þ

where m is the loading of Pt (mg cm2); qH is the adsorption charge of a monolayer hydrogen on the Pt surface, which has been estimated to be 0.21 mC cm2 ; AH is the area of the hydrogen adsorption/desorption region after double-layer correction on the cyclic voltammogram (mAV cm2); s is the scan rate of the CV (V s1). A smaller particle size normally gives rise to a larger ECSA. ChemSusChem 2014, 7, 361 – 378

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3. Increasing the Activity of Pt The cost issue associated with Pt-based cathode catalysts could be circumvented by improving the intrinsic catalytic ability of a certain amount of Pt, which is of primary concern in the development of fuel cell catalysts. 3.1. Crystallographic control ORR relies heavily on the crystallographic orientation of the Pt surface. For example, in a nonadsorbing electrolyte such as perchloric acid (HClO4), the order of the catalytic ability of lowindex, single-crystal Pt(hkl) for ORR at room temperature follows the order:[20] Ptð110Þ > Ptð111Þ > Ptð100Þ,

ð9aÞ

which is consistent with the strength of the interaction between O2 and Pt(hkl). Whereas in adsorbing electrolyte such as sulfuric acid (H2SO4), the order follows:[21] Ptð110Þ > Ptð100Þ > Ptð111Þ,

ð9bÞ

which originates from the sensitivity of Pt(hkl) to the adsorbing anion (SO42 or HSO4) and the corresponding inhibiting effect of the adsorbing anion(s) on O2 adsorption. Similarly, the order of Pt(hkl) in KOH solution follows:[22] Ptð111Þ > Ptð110Þ > Ptð100Þ,

ð9cÞ

which is in accordance with the structure sensitivity to the adsorption of hydroxide ion (OH) and the consequent site blocking effect. The activity rule of crystallographically oriented surfaces can also be applied in the design of Pt nanoparticles towards the development of electrocatalysts for ORR. For example, Sun et al. have demonstrated that, in H2SO4, the specific activity for ORR of Pt nanocubes enclosed by (100) facet was four times higher than that of truncated Pt nanocubes bounded by (100) and (111) facets that resembled the crystallographic structure of Pt on Pt/C.[23] However, it is worth pointing out that the ORR catalytic activity of the low-index Pt(hkl) in H2SO4 is significantly lower than that of the same facet in HClO4 due to the inhibition of the adsorbing species. Therefore, non-adsorbing HClO4 is commonly used as the electrolyte in ORR. High-index facets are believed to have higher reactivity because of the larger number of atomic steps, edges, and kinks that can break down chemical bonds.[24] Xia et al. reported the substantially enhanced specific activity for ORR of Pt concave nanocubes enclosed by (510), (720), and (830) facets in HClO4 compared to that of Pt nanocubes, Pt cuboctahedra, and the commercial Pt/ C.[25] Pt black and Pt/C possess a large ECSA, achieving a high mass activity of Pt. However, for a 5 nm nanocube, surface Pt atoms account for only 32.5 % of all Pt (the diameter of a single Pt atom is 0.27 nm). Therefore, monometallic electrocatalyst has a very limited potential to dramatically increase its 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Figure 2. a) HRTEM image of a single Pd–Pt nanodendrite. b) Mass activity at 0.9 V versus RHE for these three catalysts. For the Pd–Pt nanodendrites or Pt/C catalyst, the metal loading on a RDE was 15.3 mg cm2, whereas the metal loading was 40.8 mg cm2 for the Pt black.[26]

mass activity for ORR due to the low utilization efficiency of Pt. This problem can be effectively alleviated by incorporating/ adopting another metal in/as the core in the catalyst. For instance, Xia et al. ingeniously designed Pd–Pt bimetallic, nanodendritic nanomaterials for ORR, characterized by low Pt loading and high catalytic activity.[26] As shown in Figure 2, truncated Pd octahedra guided the epitaxial growth of Pt. The Pdsupported Pt branches with an average diameter of 3 nm were bounded mostly by (111) with the coexistence of (110) and high-index (311) facets. Compared to Pt/C, the as-developed Pd–Pt nanodendrites showed a 2.5-fold enhanced mass activity for ORR. In this case, Pd performed an inactive but indispensable role as the core of the electrocatalyst. 3.2. Ligand effect This much the second metal does and more. The electrocatalytic activity of Pt for ORR can be greatly improved by incorporating a second transition metal in the catalysts.[27] We will begin by discussing the ligand effect of a second metal from Pt–Ni bimetallic electrocatalysts. The exemplary work by Stamenkovic et al. revealed that a helping hand from the second component can radically enhance the catalytic ability of Pt for ORR by modifying the surface electronic structure and atomic arrangement.[28] Well-defined Pt3Ni(100), Pt3Ni(110), and Pt3Ni(111) alloy electrode surfaces were prepared by sputtering/annealing cycles in ultrahigh vacuum (UHV). The characterization of these single-crystal surfaces were conducted by Auger electron spectroscopy (AES), low-energy ion scattering (LEIS), ultraviolet photoemission spectroscopy (UPS), and lowenergy electron diffraction (LEED) under a UHV environment followed by surface X-ray scattering (SXS), CV, and RRDE under an electrochemical environment. The surface compositions for all three Pt3Ni(hkl) electrodes were similar to each other, with 100 % Pt atoms on the first layer (Pt skin) and 48 % on the second in contrast to the nominal 75 % for the bulk alloy. The direct consequence of this segregation-driven, near-surface compositional change resulted in distinctive electronic properties of the Pt-skin.[29, 30] Positions of the d-band center are structure sensitive and were determined to be 3.14 eV for Pt3Ni(100), 2.70 eV for Pt3Ni(110), and 3.10 eV for Pt3Ni(111), ChemSusChem 2014, 7, 361 – 378

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CHEMSUSCHEM MINIREVIEWS all of which were remarkably smaller than the corresponding value for Pt(hkl). The negative shift of the d-band centers consequentially resulted in the delayed onset of the adsorption of Hupd (underpotentially deposited hydrogen) and OHad (adsorbed hydroxyl species) accompanied by the reduced coverage of Hupd and OHad compared to the corresponding Pt(hkl). This observation, namely the ligand effect of the sub-surface Ni on the Pt-OH bonding, was also supported by theoretical calculation using density functional theory (DFT). The authors also deduced the proportionality of the rate of ORR to be (1qad)x (where qad is the coverage of adsorbed species and x is a constant) on the basis of the multi-electron mechanism of ORR. This proposed expression predicted the synergy between surface geometry and surface electronic structure for ORR, which was verified experimentally as shown in Figure 3. The effective electrocatalyst for ORR was still Pt; the alloyed Ni modified the electronic structure and accordingly, enhanced the catalytic activity of Pt. Taking into account the extremely high activity per surface Pt atom, one can see that the proposed Pt3Ni (111) outperformed Pt/C by an unprecedented factor of 90!

Figure 3. Influence of the surface morphology and electronic surface properties on the kinetics of ORR. RRDE measurements for ORR in HClO4 (0.1 m) at 333 K with 1600 revolutions per minute on Pt3Ni(hkl) surfaces as compared to the corresponding Pt(hkl) surfaces (a horizontal dashed gray line marks specific activity of polycrystalline Pt) are shown. Specific activity is given as a kinetic current density ik, measured at 0.9 V versus RHE. Values of d-band center position obtained from UPS spectra are listed for each surface morphology and compared with corresponding Pt3Ni(hkl) and Pt(hkl) surfaces.[28]

The impressive catalytic activity of Pt3Ni inspired scientists to identify other Pt3M alloy electrocatalysts for ORR using singlecrystal or bulk alloy model surfaces.[31] Stamenkovic et al. creatively generalized the effect of surface electronic structures modified by 3d transition metals in Pt3M (M = Ti, V, Fe, Co, and Ni) on the corresponding ORR activities, which exhibited a “volcano-type” behavior, as shown in Figure 4.[32] The d-band center positions were directly measured by high-resolution valence-band photoemission spectra. As a rule of thumb, on one hand, the adsorption energy of Pt skin is lower (higher coverage) when the d-band center is closer to the Fermi level (less negative value). In this case, the rate of ORR was limited by 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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Figure 4. Relationships between experimentally measured specific activity for the ORR on Pt3M surfaces in 0.1 m HClO4 at 333 K versus the d-band center position for the Pt-skin surfaces.[32]

the availability of vacant Pt sites. On the other hand, higher adsorption energy due to a more negative value of d-band center leads to attenuated adsorption of O2 and consequently insufficient electron transfer to the adsorbed O2. Therefore, the most promising candidate could stand out only by counterbalancing the two opposite effects. Nørskov et al. further extended the trend of the electrocatalysis of Pt3M to early transition metals (M = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Nb, and Mo) according to the experimental data and the computational data from DFT, which were in good agreement with each other.[33] Recently, Kim et al. enriched the series of M by investigating the promoting effect of La in Pt–La alloy electrode prepared by a radio-frequency magnetron co-sputtering system, and experimentally proved the validity of the “volcano behavior” of Pt–M electrocatalysts for ORR.[34] X-ray absorption nearedge structure (XANES) was applied to Pt–La alloys with different compositions (atomic La from 0 % to 33 %) to determine the d-band vacancy of Pt where higher d-band filling (lower vacancy) indicated the downshift of d-band center. The greatly increased ORR activity of Pt3La was attributed to the ligand effect induced by the incorporation of La atoms with depleted electrons into the lattice of Pt with high electron affinity, resulting in the decrease of d-band center position of Pt despite the tensile strain of the surface Pt (which will be discussed in the next section). 3.3. Geometric effect It has been discussed that the electron transfer from M to Pt due to the difference between their electronegativity results in the decrease of the d-band center of Pt, which in turn affects the ORR activity of Pt. However, the statement above is restricted to the “Pt skin” described by the slab model; in other words, the ligand effect fades away over more than a few atomic layers. For dealloyed core–shell catalysts with thicker Pt surfaces, the catalytic activity of Pt is primarily determined by the compressed or expanded arrangement of surface atoms, which is known as the geometric effect.[35] Therefore, the relationship between d-band center and surface strain enables the ChemSusChem 2014, 7, 361 – 378

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continuous tuning of the catalytic activity of the shell by controlling components and/or composition of the core.[36] The paradigmatic work by Strasser et al. specified the relationship between the lattice strain and d-band center of Pt, and clarified the mechanistic origin of the improved catalytic activity of electrochemically dealloyed bimetallic core–shell Pt–Cu nanoparticles.[37] The lattice strain s(Pt) was defined by:

sðPtÞ ¼

ashell aPt 100 aPt

ð10Þ

where aPt is the standard lattice constant of bulk Pt and ashell is the lattice parameter of Pt shell, both of which can be directly measured by anomalous X-ray diffraction (AXRD). For dealloyed Ptshell–PtCucore nanoparticles with the Cu content from 25 % to 75 %, s(Pt) < 0 (compressive strain), which originated from the lattice mismatch between the Ptshell and the PtCucore ; the alloyed core with more Cu resulted in a smaller acore, and, thus, more compressive strain was induced on the shell, which produced the higher ORR activity of the Ptshell. Moreover, this trend held its validity for single crystal surfaces as well. Decreased d-band center with increased compressive strain was observed by X-ray photoelectron spectroscopy (XPS): from 2.87 eV for Pt(111) (no compressive strain) to 3.26 eV for five monolayers of Pt on Cu(111) (2.5 0.3 % compressive strain). Decreased intensity of the anti-bonding resonance of oxygen on Pt with additional compressive strain was also identified by X-ray emission spectra (XES) and X-ray absorption spectra (XAS) for Pt on Cu(111) compared to those for Pt(111), which resulted in the weakened adsorption bond between O and Pt and, consequentially, the improved activity for ORR. In addition, a volcano-type relationship between the ORR activity and the strain on Pt(111) surface model was predicted by DFT calculation, which was in good agreement with the volcano curve for ORR activity and d-band center of Pt. In terms of catalyst design, both ligand and geometric effects should be taken into account; actually, the convolution of these two effects cumulatively influences the catalytic property of Pt-based electrocatalysts.[38] In general, the electronic structure of Pt could be modified by the supporting/alloyed metal with a lesser electronegativity, resulting in the decrease of the d-band center of Pt; most metals could theoretically be applied here since Pt is among the metals with highest electron affinity. XPS can be utilized to examine the change of the electronic structure of Pt in a particular case. On the other hand, the surface strain of Pt could be tuned by the (mono/bi/multi)metallic core with smaller lattice constant, causing the downshift of d-band center of Pt as well; X-ray diffraction (XRD) or even well-documented lattice parameters could be applied to predict the usefulness of certain metal in this regard. In the case that ligand and geometric effects are opposed to each other (such as the Pt3La example discussed in section 3.2), one factor would dominate the overall ORR activity, depending upon the shape and size of catalysts.


4. Improving the Durability of Catalysts The durability issue of cathode catalysts in fuel cells results from the gradual loss of ECSA of Pt under potentiostatic and/ or cycling condition.[39, 40] Two experimental methods, namely accelerated durability test (half cell) and fuel cell/stack test (full cell), are commonly used in the study of the operational stability of the cathode catalysts. While the former requires simpler experimental facilities, the latter provides more information on the scenario of a real-life fuel cell application that is not just confined to the cathode. In PEMFCs, Pt dissolution, Pt particle growth (aggregation and Ostwald ripening), and carbon corrosion account for the loss of ECSA with the first one playing the most significant role.[41] The degradation mechanism by means of Pt dissolution takes place through either a direct pathway for nanoparticles:[42] Pt ! Pt2þ þ 2 e , E 0 ¼ 1:188 V

ð11Þ

or an indirect pathway involving the formation of platinum oxide (PtO) for bulk Pt:[42] Pt þ H2 O ! PtO þ 2 Hþ þ 2 e , E 0 ¼ 0:980 V

ð12aÞ

PtO þ 2 Hþ ! Pt2þ þ 2 e

ð12bÞ

For detailed thermodynamics of Pt dissolution pathways, please refer to the state-of-the-art review by Borup et al.[39] The dissolved Pt may redeposit at other Pt nanoparticles on the cathode[43] or diffuse out of the cathode and deposit in the membrane through hydrogen crossover,[44] and thus severely affects the quality MEA; Pt oxide forming on the surface of metallic Pt dramatically decreases ECSA and inhibits ORR activity of the catalyst. Therefore, effective strategies to impede the dissolution and/or oxidation of Pt are of primary interest in tackling the durability issue of cathode catalysts in PEMFCs. 4.1. Stabilization of Pt by Au Apparently, increasing the oxidation potential of Pt can effectively ameliorate the durability of Pt-based cathode catalysts. The seminal work by Adzic et al. discovered the stabilizing effect of Au on Pt in the electrocatalysis of ORR.[45] Au clusters were deposited on the surface of Pt nanoparticles with an average coverage of 33 %. The catalytic stability of the as-prepared electrocatalyst was examined by accelerated durability test that caused the surface oxidation–reduction cycles of Pt. As shown in Figure 5, after cycling between 0.6 and 1.1 V (vs. RHE) in HClO4 for 30 000 cycles, no appreciable ECSA loss and only 5 mV negative shift of the half wave potential (E1/2) were recorded from Au/Pt/C, whereas a 45 % decrease of ECSA and a 39 mV degradation of E1/2 were observed from Pt/C. As evidenced by XANES, the greatly improved durability of the catalyst was attributed to the increased oxidation potential of Pt in the presence of Au clusters. However, it is worth mentioning here that the incorporation of Au in Pt catalysts is not theoretically favorable in terms of both ligand and geometric effect for ChemSusChem 2014, 7, 361 – 378

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www.chemsuschem.org A further improvement in the durability of this type of catalyst could be achieved by increasing the oxidation potential of the sacrificial component (which should be still lower than that of Pt). One feasible way is to add Au in the Pd core.[46] The higher dissolution potential of Pd in the resultant Pd0.9Au0.1 was proven by CV and X-ray absorption fine structure (EXAFS). Amazingly, the as-prepared PtML/Pd0.9Au0.1/C conserved 70 % mass activity after an unprecedented 200 000 cycles compared to 63 % after 100 000 cycles for PtML/Pd/C, both of which have far exceeded the DOE target (60 % remaining after 30 000 cycles) under the same durability test condition. 4.3. Supportless 1D nanomaterials

Figure 5. Polarization curves for the O2 reduction reaction on a) Au/Pt/C c) and Pt/C catalysts on a rotating disk electrode, before and after 30 000 potential cycles. Sweep rate, 10 mV s1; rotation rate, 1600 rpm. Voltammetry curves for b) Au/Pt/C and d) Pt/C catalysts before and after 30 000 cycles; sweep rate, 50 and 20 mV s1, respectively. The potential cycles were from 0.6 to 1.1 V in an O2-saturated 0.1 m HClO4 solution at room temperature. For all electrodes, the Pt-loading was 1.95 mg (or 10 nmol) of Pt on a 0.164 cm2 glassy carbon rotating-disk electrode. The shaded area in (d) indicates the lost Pt area.[45]

the ORR activity of Pt since Au has a higher electronegativity and larger lattice constant than Pt; another transition metal is often required to balance/mediate the activity and durability of ORR catalysts in the presence of Au.

4.2. Sacrificial dissolution of the core Beyond Au, Adzic et al. also studied the stabilizing effect of the underlying Pd core on the durability of Pt monolayer (PtML) by means of the accelerated fuel cell test.[46] The as-prepared PtML/ Pd/C retained 77 % and 81 % of the original ECSA and mass activity, respectively, compared to 32 % and 32 % for Pt/C after sweeping between 0.7 and 0.9 V (vs. RHE) for 60 000 cycles. For PtML/Pd/C, the loss of Pd in the nanoparticles and the formation of “Pd band” in the Nafion membrane were determined by energy dispersive X-ray spectroscopy (EDXS) and electron energy loss spectroscopy (EELS) after cycling; no loss of Pt or the formation of “Pt-band” was observed in the concurrent measurement. As a metal that is slightly more active than Pt, Pd could be directly dissolved to form metal cations at lower potential (0.915 V) than that of Pt (1.188 V). The oxidization of Pd minimized the further increase of potential on the cathode, and thus protected Pt from being oxidized. In addition, the dissolution of Pd caused the PtML to undergo a small contraction that further increased the dissolution resistance of Pt. In an extreme case after prolonged potential cycling, a Pd hollow core was produced underneath the PtML, which further increased the stability of PtML as predicted by DFT calculation. 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

A carbon support in the form of high surface area carbon black such as Vulcan XC 72R and Ketjenblack provides anchoring sites and electrical contacts for Pt nanoparticles, preventing them from aggregation driven by the minimization of surface energy and connecting them with the electrode. However, the carbon support could be corroded through the following reaction:[39] C þ 2 H2 O ! CO2 þ 4 Hþ þ 4 e

E 0 ¼ 0:207 V

ð13Þ

This reaction is thermodynamically favorable due to the high operating potential on the cathode and kinetically unfavorable because of the high activation energy and low operating temperature. Nevertheless, this carbon oxidation reaction, leading to the corrosion of carbon support and the concomitant deterioration of the durability of Pt, cannot be neglected. Recently, carbon nanotube (CNT)-based supporting materials have shown enhanced durability in comparison with carbon black for PEMFCs.[10, 47] Nevertheless, the relatively high price of CNTs under current technology makes it less competitive than the conventional carbon black support. Inspiration from the advantages associated with CNTs, namely high surface area and improved mass transport, opens up opportunities to exploit supportless 1D nanomaterials as electrode catalysts in PEMFCs. Towards the development of a new generation of electrocatalysts for fuel cells, Yan et al. reported supportless, hollow Pt nanotubes (NT) with a length of 50 mm, diameter of 50 nm, and a wall thickness of 4–7 nm.[48] The high aspect ratio (l/d = 1000) made it less susceptible to Pt dissolution and particle growth, and the exclusion of support avoided the problem of carbon corrosion. As expected, the as-prepared Pt NTs displayed an 8-fold ameliorated durability compared to zero-dimensional (0D) Pt/C after 1000 cycles running between 0 and 1.3 V (vs. RHE) in H2SO4. Motivated by the good performance of Pt NTs, Lei et al. recently developed novel PtCu NTs with simultaneous compositional and morphological controls that enabled the electrocatalyst to outperform commercial Pt/C in terms of both activity and durability.[49] This work provided a paradigmatic strategy to rationally design ORR catalysts with ChemSusChem 2014, 7, 361 – 378

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combined favorable properties. In practical fuel cell applications, the supportless Pt materials have to eventually be dispersed onto a certain type of carbon backing such as carbon cloth or carbon paper to function as the electrode catalysts. A more radical design could be realized by even eliminating such carbon backing. Yu et al. described a promising method to synthesize free-standing Pt nanowire (NW) membrane composed of 1D Pt NWs with an average diameter of 12 nm.[50] After the durability test between 0 and 1.2 V (vs. RHE) in H2SO4 for 3000 cycles, the free-standing Pt NW membrane lost only 18 % ECSA, whereas Pt black and Pt/C decreased by 61 % and 95 %, respectively. A direct observation accounting for the remarkable durability of Pt NWs was their structural integrity after cycling whereas the Pt particles with decreased amount and increased size were observed by scanning electron microscopy (SEM) in both Pt black and Pt/C. 4.4. 3D nanomaterials Moreover, an aesthetic and practical extension of the 1D NWs was accomplished by Rauber et al.[51] A supportless, highly ordered, 3D Pt network with controllable architecture consisting of interconnected 1D NWs with an average diameter of 15 nm was produced through a template-based electrodeposition process. In contrast to Pt black or Pt/C, the 3D Pt networks possessed a consistently large ECSA even at a very high loading of Pt. This free-standing electrocatalyst also survived the durability test (0–1.3 V vs. RHE, 500 cycles in H2SO4), retaining 93 % original ECSA compared to the 71 % and 57 % for Pt black and Pt/C, respectively. Recently, bicontinuous, inverse double gyroid Pt network thin film was synthesized through the electrodeposition of Pt into mesoporous silica followed by the removal of the template using hydrofluoric acid (Figure 6).[52] The as-prepared meso-structured Pt thin film possessed large surface area and high turnover frequency, and more importantly, displayed superior structural and catalytic stability under harsh conditions. The enhanced performance can be attributed to fewer undercoordinated Pt sites on the surface. Specifically, a 10 000-cycle durability test resulted in a 16 % decrease of ECSA and a 7 mV shift of E1/2 for the supportless 3D electrocatalyst, compared to 53 % and 40 mV for the carbon-supported 0D counterpart (ETEK Pt/C).

Figure 6. Synthesis procedure and structural model for mesoporous double gyroid platinum.[52]

4.5. Activity vs. durability As discussed in Section 2.2, Pt3M is a promising class of catalyst for ORR with superior catalytic abilities. The resultant “volcano plot” (activity vs. d-band center) for various M serves as a powerful tool and experimental guideline for the rational design of new electrocatalysts. Along this direction, Nørskov et al. innovatively correlated the d-band center of Pt (that governs the activity of catalysts) with the heat of formation of Pt3M (that determines the stability of catalysts), and ultimately generated a 3D “volcano plot” (Figure 7) for the screening of Pt–M bimetallic electrocatalysts with both high activity and good stability.[33] Although the plot was on based on DFT calculation, 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Figure 7. A three-dimensional “volcano” Scheme of metal-alloy catalysts. (K. Mayrhofer and M. Arenz, Nat. Chem. 2009, 1, 518–519)

proof-of-concept experiments were also conducted to confirm the validity of the theory. Two traditionally overlooked cathode catalysts, Pt3Y and Pt3Sc, prepared in bulk, polycrystalline form showed dramatically enhanced activity and durability in comparison with pure Pt.

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5. Advanced Electrocatalysts for ORR We have discussed the principle behind the design of Pt-based electrocatalysts for ORR in terms of improved specific activity and durability, both of which belong to the intrinsic properties of Pt. In a real scenario, the extremely high price of noble metals demands a high utilization efficiency, namely high mass activity of Pt and any other precious components, for Pt-based electrocatalysts. However, the situation herein is quite intricate since the high specific activity is not equivalent to the high mass activity; high utilization efficiency of Pt does not guarantee the high stability of the catalysts. Therefore, before the discussion about how to translate these principles into the production of practical electrocatalysts for ORR, it is necessary to first investigate the relationship between these important catalytic parameters. 5.1. Particle size effect The catalytic activity of pristine Pt nanoparticles is primarily determined by the size of the electrocatalysts. According to direct observation by transmission electron microscopy (TEM),[53] the particle can be modeled as a cuboctahedron covered by Pt(111), Pt(100), and steps (edges and corners), the percentage of which can be correlated with the diameter of the particle (Figure 8). Based on this model and DFT calculations, Tritsaris et al. predicted that the specific activity should increase monotonically with the increase of particles from 2 nm to 30 nm and the mass activity should exhibit a maximum in the range of 2–4 nm in the electrolyte of 0.1 m HClO4.[54] These simulated results have been corroborated by a number of experimental studies.[55, 56] Recently, Arenz et al. further determined that specific activities of commonly used Pt catalysts in HClO4, H2SO4, and KOH followed the same order: polycrystalline Pt > unsupported Pt black (~ 30 nm) > high surface-area carbon-supported Pt (1—5 nm), which built up the linkage between the extended surface and Pt nanoparticles.[57] Chorkendorff et al. correlated the higher specific activity with more terrace sites (facets) for larger Pt nanoparticles in the range of 2– 11 nm and polycrystalline Pt.[55] The terrace sites on Pt nanoparticles were quantified by means of vacuum CO temperature-programmed desorption (TPD), and identified as the only actives sites for ORR. Shao et al. drew a similar conclusion that the lower specific activity of smaller Pt nanoparticles could be ascribed to the presence of more edge sites where the binding energy of O is very strong.[56] Furthermore, Mayrhofer et al. discovered that particle size-induced potential of zero charge shifted from 0.285 V (vs. RHE) for polycrystalline Pt to 0.245 V for 1 nm Pt nanoparticles, which concomitantly increased the surface coverage of OHads and/or Oads, blocking the active sites for the adsorption of O2 and inhibited the cleavage of OO bond.[58, 59] In addition, the particle size effect on the specific activity can also be extended to Pt alloy catalysts.[60] Although larger particles are theoretically advantageous to higher specific activity, smaller particles are practically required to achieve higher mass activity that reflects the utilization efficiency of Pt. The mass activity (MA) can be derived by the 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Figure 8. a) Model particle with truncated octahedral shape with dissolved edges and corners. Differently colored atoms correspond to active sites of different activity for the oxygen reduction reaction. b) Fractional population n of the (100), (111), and step surface sites of the particle model versus diameter dPt.[54]

product of the specific activity (SA) and the ECSA: MA ¼ SA ECSA

ð14aÞ

where SA increases monotonically with the increase of particle size; ECSA decreases monotonically with the increase of particle size.[56, 57, 61] Therefore, a peak value of MA can be intuitively expected to reflect a balance between the opposite effects of the intrinsic activity per area and the area per Pt loading. The mass activity can also be rationalized as the change of surface coordination number.[62] In detail, Equation 14 a can be alternatively interpreted as the percentage of surface active sites in the overall volume atoms of a particle (in contrast to the percentage of surface active site in the overall surface atoms of a particle): MA / h1 h2

ð14bÞ

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where h1 and h2 are the percentage of the ORR-active atoms on the surface and surface atoms in overall atoms, respectively. Compared to the higher coordination number of (111) facet (9) and (100) facet (8), the under-coordinated atoms on the edge (7) and corner (6) display negligible activity towards ORR.[55] The further decrease of particle size below 2 nm quickly increases the number of under-coordinated sites, and thus quickly decreases h1. Therefore, the highest mass activity in the size between 2–3 nm reflects the highest percentage of surface active terrace sites in the overall (volume) atoms of the nanoparticle. The intrinsic stability of electrocatalysts for ORR is also closely related to the size of Pt nanoparticles. Ceder et al. combined the experimental results from electrochemical scanning tunneling microscopy with ab initio computation to examine the stability of Pt as a function of particle size, and concluded that smaller Pt nanoparticles were more vulnerable to dissolution due to the decreased cohesive energy compared to that of larger nanoparticles.[42] The influence of particle size on the stability of Pt can also be approximated in terms of Gibbs–Thomson equation:[42, 63, 64] E GT ¼ mðdÞ-mð1Þ ¼ 4gW=d

ð15Þ

where EGT is the Gibbs–Thomson energy, m is the chemical potential; g is the surface energy of the particle, W is the atomic volume; d is the diameter of nanoparticles; 1 indicates the bulk Pt. Since larger EGT results in higher dissolution rate, the increase of particle size was predicted to be able to substantially improve the stability of the electrocatalyst for ORR,[64] and has been validated by both accelerated durability test[65] and fuel cell durability test.[61] The improved stability of larger particles under potential cycling conditions can also be explained by the preferential dissolution of smaller particles/clusters and under-coordinated sites as evidenced by in situ anomalous small-angle X-ray scattering[66] and atomic force spectroscopy,[67] respectively. 5.2. Nanopolyhedra—wet chemical synthesis The synergy between crystallographic orientation and surface electronic structure of Pt3Ni(111) prepared by the UHV method has been illustrated in Section 3.2. However, the challenge then becomes the transformation of this highly active surface model into a practical nanoparticulate electrocatalyst in the application of fuel cells. The shape-controlled synthesis of metal nanocrystals promises well-defined crystallography and composition as well as mass production for Pt-based electrocatalysts. In the wet-chemical synthesis, capping agents are normally involved to control the surface energy and the corresponding growth of a certain facet.[68–71] Fang et al. reported a wet chemistry-based synthetic method for the production of nanooctahedra terminated Pt3Ni exclusively with eight (111) facets.[72] The crystallographic control was realized by combined capping agents: oleic acid and oleylamine. The average molar ratio of Pt to Ni was confirmed as 3:1 by inductively coupled plasma-mass spectrometry (ICP-MS) and EDXS. The outer 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

most layer was determined to be a thin Pt layer; this was also supported by the results of Monte Carlo simulation. In HClO4, the mass activity of carbon supported Pt3Ni nano-octahedra for ORR was 4 times higher than that of Pt/C, albeit the particle size of Pt3Ni is three times larger than that of the commercial Pt catalyst. Based on this work, a further improvement in mass activity could be expected by decreasing the particle size. In another study, Yang et al. described the synthesis of high quality, truncated octahedral Pt3Ni electrocatalysts predominantly bounded by (111) facets using wet chemistry with long alkanechain amines as the capping agents.[73] As shown in Figure 9,

Figure 9. a) TEM and b) HRTEM images of truncated octahedron Pt3Ni nanocrystals. c) Specific (mA cm2Pt) and d) mass (A mg1Pt) ORR activities for the t,o-Pt3Ni and reference Pt catalysts. The ORR polarization curves were collected at 1600 rpm.[73]

given the similar average size to that of the commercial Pt/C (50 wt % Pt), the advantage of Pt3Ni nanomaterials was substantiated by the 4-fold higher activity for ORR. Recently, wellfaceted, cubic and cuboctahedral Pt3Ni nanocrystals and octahedral/truncated octahedral Pt–Ni nanocrystals with an average diameter of 14.7 nm were produced on a large scale using dimethylformamide (DMF) as both solvent and reductant.[74] The morphological control did not resort to any capping agent that is hazardous to the electrocatalysis of ORR.[75] Among different compositions, PtNi was determined to possess the highest specific activity that was almost 15 times higher than that of Pt/C. In addition, besides (111) terminated nanopolyhedra, monodisperse Pt3Ni,[72] Pt3Co,[76] and Pt3Fe[77] nanocubes bounded by (100) facets, which are theoretically favorable for ORR in H2SO4, were also prepared through wet chemistry. The general synthetic strategy for the synthesis of Pt3M nanocubes has been summarized elsewhere.[78] 5.3. Pt-skin core–shell catalysts As an important category of model catalysts, Pt-based polycrystalline surfaces have been widely applied in the modern study of heterogeneous catalysis.[79] The pioneering work by ChemSusChem 2014, 7, 361 – 378

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cles with a multilayered Pt skeleton and Pt-skin surface, and their applications in the electrocatalysis of ORR.[82] Monodisperse PtNi nanoparticles were first synthesized in an organic solution using Pt(acac)2 and Ni(ac)2 as the corresponding metal precursors followed by the dispersion of the nanoparticles on TKK carbon black. More imFigure 10. Schematic illustrations of Pt skin and Pt skeleton before and after the exposure to the electrochemical portantly, the acid treatment environment (acidic solution). The blue and red balls represent platinum and nickel, respectively. (0.1 m HClO4) with and without subsequent heat treatment Stamenkovic et al. explored the effect of surface composition (400 8C) produced Pt skeleton and Pt skin, respectively, on the of Pt3Ni alloy electrodes on the electrocatalysis of ORR in 0.1 m PtNi core. In addition, the bulk composition effect on the ORR HClO4.[80, 81] The polycrystalline bulk Pt3Ni alloy was prepared by activity of Pt skeleton was investigated in another study where the conventional metallurgical method. The surface cleaning PtNi was determined as the best candidate among Pt3Ni, PtNi, was performed under UHV by repeating the sputtering–anPtNi2, and PtNi3, probably because it had the most remaining nealing cycles with Ar + ion and oxygen until an ideally clean Ni (27 % versus 11 % for Pt3Ni and 13 % for PtNi3).[83] High-angle surface (no carbon or oxygen would be detected by AES) was annular dark-field scanning transmission electron microscopy produced.[81] As shown in Figure 10, different surface composi(HAADF-STEM) and EDXS confirmed the thickness of 2–3 tions, namely 100 % Pt and 75 % Pt, were generated by annealatomic layers for both Pt skeleton and Pt skin. Both in situ ing at 950 K and mildly sputtering in 0.5 keV Ar + beam, respecXANES and CV indicated a less oxophilic surface of the Pt skin tively, and were evidenced by LEIS. Subsequently, the electrothan that of the Pt skeleton, which was theoretically advantachemical data showed that the total charge of the Hupd region geous to ORR. As expected, the Pt skin exhibited a 2-fold imof sputtered surface was 25 % smaller than that of the polyprovement to Pt skeleton and a 6-fold improvement to Pt/C crystalline Pt, whereas the annealed surface and polycrystalline for both specific and mass activity at 0.95 V (vs. RHE). FurtherPt behaved identically, which confirmed the LEIS results under more, after a 4000-cycle accelerated durability test from 0.6 to UHV. It is worth pointing out that since Ni could be dissolved 1.1 V at 60 8C, both Pt skin and Pt skeleton had a 10 % loss in on exposure to the acidic environment, the stability of the anECSA compared to 40 % for Pt/C; Pt skin had only a 15 % loss nealed surface was much better than that of the sputtered surin the specific activity compared to 57 % for Pt skeleton and face. Accordingly, the resultant surfaces were designated as “Pt 38 % for Pt/C. As confirmed by XANES after cycling, the draskin” (annealed surface in acid) and “Pt skeleton” (sputtered matically decreased specific activity of the Pt skeleton could be surface in acid).[32] More importantly, the order of the specific explained by the dissolution of the underlying Ni, which diminished the ligand effect from the core, whereas no Ni loss was ORR activity at 35 8C was: Pt skin > Pt skeleton > polycrystalline observed for the Pt skin sample. Therefore, for both improved Pt. The higher activity of Pt3Ni compared to that of pure Pt can activity and ameliorated durability, Pt skin nanoparticles with be attributed to the weaker interaction between Pt and OHads a thickness of 2–3 atomic layers were thin enough to benefit as discussed in section 3.2; the higher activity of the Pt skin from the ligand effect bestowed by Ni and thick enough to compared to that of Pt skeleton can be ascribed to the smaller protect Ni from being leached out. number of under-coordinated atoms on the Pt surface, as disOriginating from the stabilization effect of Au on Pt, core– cussed in Section 5.1. In addition, most kinetic parameters shell Pt skin nanocatalysts based on Au/Pt were developed such as Tafel slope, reaction order, activation energy, and numand achieved high activity and superior durability for the elecbers of electron transfer of the Pt skin and Pt skeleton were trocatalysis of ORR.[84, 85] A third transition metal is indispensathe same as those of pure Pt, so that the higher ORR activity primarily originated from the larger pre-exponential factor in ble in this class of catalysts since Au alone would exert a negathe Arrhenius equation. Besides Pt3Ni, the validity of the distive influence on the activity of Pt in terms of both ligand and geometric effects. A non-noble metal could exist either on the cussion above has also been proved with reference to Pt3Co[80] shell or in the core to mediate the activity problem in the presand Pt3Fe,[32] which highlights the great promise of Pt skin ence of Au. Specifically, according to the thin film extended electrocatalysts in the application of fuel cells. surface model, Stamenkovic and his colleagues designed multiWith the recent advance of wet-chemical synthesis, polycrysmetallic Au/Pt3Fe core–shell nanoparticles with tailored mortalline alloy Pt skin models have been successfully transformed into Pt-skin alloy nanoparticles, which exhibit not only excelphology and composition as a highly durable electrocatalyst lent catalytic performance towards ORR but also adaptability for ORR.[84] 7 nm icosahedral Au nanoparticles were first synto mass production. Based on their experience with extended thesized using wet chemistry. Pt3Fe with a thickness of 1.5 nm Pt skin surfaces, especially for the Pt–Ni system, Stamenkovic was subsequently coated on the Au core using wet chemistry et al. demonstrated the synthesis of Pt–Ni bimetallic nanopartias well. Note that Pt3Fe (without Au core) nanoparticles pre 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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Figure 11. Stability characterization of the Pt/C, FePt3/C, and Au/FePt3/C catalysts by 60 000 potential cycles between 0.6 and 1.1 V vs. RHE in oxygen-saturated 0.1 m HClO4 electrolyte at 20 8C with a sweep rate of 50 mV s1: a–c) Summary of the specific surface area, specific and mass activities and d–f) TEM characterization of the catalysts before and after the potential cycling. All the specific surface areas and mass activities are normalized by the initial loading of Pt metal, while the specific activities are the surface area before and after potential cycling, respectively.[84]

pared by a similar method possessed a cuboctahedral shape that had a lower average surface coordination number than icosahedron. After dispersion on TKK carbon black, the as-prepared sample was annealed in air at 450 K to remove surfactants and to generate the surface of the Pt skin. Although both Au/Pt3Fe/C (3-fold) and Pt3Fe/C (2.8-fold) exhibited a similarly improved specific activity compared to Pt/C, the electrocatalyst containing the Au core displayed dramatically improved catalytic durability and structural stability after scanning between 0.6 to 1.1 V (in 0.1 m HClO4 at 20 8C and 50 mV s1) for 60 000 cycles (Figure 11). In another study, AuCu/ Pt core–shell nanoparticles were synthesized through wet chemistry and applied as a highly active and durable electrocatalyst for ORR.[85] In the alloy core, Au stabilized the Pt shell while Cu was responsible for tuning both the ligand and electronic effect of the Au-based core. Consequently, the mass activity of AuCu/Pt for ORR before and after a 30 000-cycle accelerated durability test was improved by a factor of 5 and 8, respectively. The development of Pt-based multimetallic core–shell electrocatalysts mainly concentrated on the disordered alloy core in the form of solid solution. Recently, AbruÇa et al. discovered 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

a new class of structurally ordered intermetallic core–shell nanoparticles with enhanced activity and durability for ORR in comparison with the disordered counterparts.[86] In an archetypical study, Pt/C and Pt3Co/C were prepared by an impregnation method (see Section 5.4 for more details about this preparation method). Pt3Co/C samples were annealed at 400 8C and 700 8C in a H2 atmosphere to generate Pt-skin electrocatalysts with disordered and ordered cores, respectively. As shown in Figure 12 a, the peak positions of the Pt3Co/C-400 and Pt3Co/C700 shifted to higher angles compared to that of Pt/C, indicating a 1.1 % and 1.9 % lattice contraction, respectively, for the bimetallic samples. The unique super periods feature of the ordered intermetallic structure of Pt3Co/C-700 was directly observed by annular dark-field scanning transmission electron microscopy (ADF-STEM), and further illustrated by the simulated result and the idealized model (Figure 12 b to g). The thickness of Pt skin on the ordered core was determined to be 2–3 atomic layers by ADF-STEM and EELS. According to the RDE results, the Pt3Co/C-400 sample exhibited a E1/2 of 0.918 V, a mass activity (at 0.9 V) of 0.16 mA mg1Pt, and a specific activity (at 0.9 V) of 0.31 mA cm2Pt versus 0.875 V, 0.06 mA mg1Pt, and 0.09 mA cm2Pt for Pt/C. Remarkably, the structurally orChemSusChem 2014, 7, 361 – 378

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www.chemsuschem.org cling in N2-saturated 0.1 m HClO4 between 0.05 and 1 V for 5000 cycles, as evidenced by the electrochemical activity test and the EELS composition test. 5.4. Electrochemically dealloyed core–shell catalysts

Figure 12. a) XRD patterns of Pt/C, Pt3Co/C-400 and Pt3Co/C-700. The inset shows the enlarged region of the Pt(220) diffraction peaks, with the black dotted line corresponding to the peak position of pure Pt. The red vertical lines indicate the peak positions of the intermetallic Pt3Co reflections (PDF card # 04-004-5243). b) Atomic-resolution ADF-STEM image of Pt3Co/C-700 after Richardson–Lucy deconvolution with yellow arrows indicating the Ptrich shell. A smaller particle (lower left) overlaps the larger particle in projection. The inset shows the projected unit cell along the [001] axis. c) Diffractogram of the center particle in (b). d) A crop of the super lattice feature from (b). e) The simulated ADF-STEM image of L12 ordered Pt3Co along [001] by a simple incoherent linear imaging model. f) Multislice simulated ADF-STEM (100 kV, probe forming angle = 27.8 mrad, ADF collection angles = 98– 295 mrad) image of the idealized nanoparticle as shown in (b). g) The idealized atomic structure of the Pt3Co core–shell nanoparticle. The white and blue spheres in (e), (g) represent Pt and Co atoms, respectively.[86]

dered intermetallic Pt3Co/C-700 sample presented a further 27 mV positive shift of E1/2, a 3.3-fold increased mass activity, and a 3.5-fold increased specific activity compared to the disordered counterpart. Furthermore, the ordered sample possessed better durability than the disordered sample after potential cy 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

As discussed above, non-noble metal atoms are vulnerable to the acidic environment, especially during the cycling condition of fuel cells, which would deteriorate the catalytic performance of the alloyed electrocatalysts. However, the selective dissolution of the more electrochemically active metal would open up new opportunities to produce dealloyed core–shell nanoparticles on a large scale and to maintain a lower Pt loading during the initial preparation of the pre-activated catalyst. In their proof-of-concept studies, Strasser et al. reported the synthesis of electrochemically dealloyed Pt–Cu nanoparticles with a Pt-rich shell and a Pt–Cu alloy core, and the significantly enhanced (4–6 fold) activity for the electrocatalysis of ORR.[37, 87–89] In detail, Pt25Cu75 alloy nanoparticles of approximately 4.5 nm were prepared using a general, surfactantless impregnation method with a liquid metal-salt precursor, followed by freezedrying and thermal annealing. First, Pt/C was ultrasonicated in aqueous Cu(NO3)2 solution; the thick, mixed slurry was frozen in liquid nitrogen for 5 min and freeze-dried in vacuum overnight; the resultant powder was subsequently annealed at a typical temperature of 800 8C for 7 h under a flow of 4 % H2 in an argon atmosphere. The annealing parameters such as heating rate, duration, and maximum temperature were correlated with the alloy structure, composition, and particle size by means of in situ high-temperature X-ray diffraction.[90] Second, the active electrocatalyst with a particle size of approximately 3.4 nm was obtained by electrochemically dealloying Cu in the Pt25Cu75 sample by means of CV between 0.06 to 1.2 V. As shown in Figure 13 a, the dissolution of Cu was evidenced in the first cycle and the characteristic Hupd and OHads of Pt were acquired in the last cycle. Figure 13 b depicts the nanoparticle models before and after dealloying. EDXS indicated that the nominal Pt/Cu ratio increased from 1:3 to 4:1; HAADF-STEM and XPS confirmed the conclusion of a Pt-rich shell of approximately 0.6 nm in thickness. The formation of such a core–shell structure after dealloying directly depended on the particle size of the catalyst precursor.[91] Moreover, the investigation of the compositional effect of the preactivated nanoparticles on the final catalytic activity towards ORR showed that Pt25Cu75 outperformed Pt50Cu50 and Pt75Cu25, which was primarily due to the higher lattice mismatch between the Pt shell and the Pt–Cu core (Section 3.3). A similar synthetic strategy has also been practiced in the preparation of electrochemically dealloyed Pt–Ni[92] and Pt–Co[93] binary or even Pt–Cu–Co[94] ternary electrocatalysts with the demonstration of improved ORR activities as well. Recently, AbruÇa et al. investigated the effect of electrochemical and chemical etching on structurally ordered intermetallic nanoparticles.[95] The preactivated nanoparticles were also synthesized by a similar procedure as specified above except using H2PtCl6, CuCl2, and Vulcan XC-72 carbon support for the preparation of precursor slurry at the beginning. The ChemSusChem 2014, 7, 361 – 378

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Figure 14. Structural model of distinctly different compositional core–shell fine structures of dealloyed PtxNi1-x catalysts.[98]

Figure 13. a) CV profiles of PtCu alloy nanoparticle electrocatalysts before (dashed) and after (solid line) the electrochemical dealloying process. The broad hydrogen ad-/desorption regime and redox peak couple of Pt (hydr-) oxide are indicated by red and blue areas. Positions (1) and (2) signify the surface dissolution of pure bulk Cu and underpotentially deposited Cu, respectively. b) Illustration of the formation of core–shell bimetallic nanoparticles by electrochemical dealloying. Red and light-gray spheres denote Cu atoms and Pt atoms, respectively.[91]

as-prepared PtCu3/C sample was further annealed at 1000 8C with the protection of H2 for 10 h to form the ordered intermetallic phase. Subsequently, the electrochemical dealloying was carried out by potentially cycling between 0.05 and 1 V in 0.1 m HClO4 at a scan rate of 50 mV s1; the chemical dealloying was performed by immersing the sample in 1 m HNO3 under stirring at 40 8C for 2 days. Intriguingly, the electrochemically dealloyed sample preserved an integral core–shell nanostructure with a 1 nm Pt shell and ordered intermetallic PtCu3 core. In contrast, the chemical leaching gave rise to core–shell nanoporous morphology, the evolution of which could be explained by either an intrinsic dynamic pattern formation process[96] or the nanoscale Kirkendall effect.[97] The resultant alloyed core lost the ordered intermetallic structure and the Pt shell possessed more defects such as steps and kinks. Consequently, in comparison with those of Pt/C (20 wt %), the electrochemically dealloyed sample (5000 cycles) achieved 7.3-fold increased mass activity and 8.2-fold increased specific activity whereas the chemically dealloyed sample (no potential cycling) presented only 3.9- and 3.3-fold improvement, respectively. However, it is worth mentioning that the exposure to an electrochemical environment also improved the chemically dealloyed sample by almost doubling its ORR activity, which could be ascribed to the rearrangement of the surface Pt atoms. As a general and effective treatment, electrochemical dealloying has also been applied to wet-chemically synthesized nanoparticles to produce more active nanocatalysts for ORR. Based on this strategy, Strasser et al. studied the impact of the core–shell compositional structures on their ORR activities.[98] PtNix nanoparticles with different compositions (x = 1, 3, or 5) were prepared using a low-temperature organic solution ap 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

proach followed by electrochemical dealloying processing by means of potential cycling. The RDE results indicated that dealloyed PtNi3 exhibited the highest ORR activity, which originated from the highest residual Ni percentage and more importantly, its distribution. As determined by high-resolution HAADF-STEM and EELS, the unique compositional core–shell fine structure associated with the dealloyed PtNi3 displayed a Ni-enriched inner atomic layer that resulted in a higher extent of the compressive strain on the outermost Pt shell (Figure 14). In another study, Erlebacher et al. explored the effect of particle size on the morphology and catalytic activity of electrochemically dealloyed Pt–Ni nanoparticles.[99] Pt–Ni alloy nanoparticles with controllable size and composition were synthesized by a solvothermal protocol. The dealloying of Ni was performed by potentially cycling in 0.1 m H2SO4 between 0.05 and 1.2 V. As observed by TEM, nanoporosity evolved for average particle sizes larger than 12 nm, below which Pt would quickly passivate the surface during the initial dissolution of Ni. The high ORR activity of the nanoporous catalysts could be attributed to their high surfacearea-to-volume ratio and the tentative nanoconfinement effect of reactant molecules in the nanoporous network with increased residence time.

5.5. Pt monolayer core–shell catalysts Still in the context of core–shell catalysts for ORR, Adzic et al. devised a cutting-edge technique for the monolayer (ML) deposition of Pt on the substrate of another metal or metal alloy, which promises to push the boundary of Pt loading in fuel cells to a theoretical minimum.[100] Figure 15 illustrates the overall procedure for the synthesis of PtML catalyst on an annealed bimetallic nanoparticle, although a monometallic support can also be employed. The heat treatment of the alloyed core (step 1) segregated the noble metal component from the non-noble metal. An adlayer of copper serving as the sacrificial

Figure 15. Model for the synthesis of Pt monolayer catalysts on non-noble metal-noble metal core–shell nanoparticles.[102]

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CHEMSUSCHEM MINIREVIEWS template was then coated on the surface (noble metal) of the core by underpotential deposition (UPD, step 2). Subsequently, PtML was deposited on the core through galvanic replacement reaction by simply exposing the Cu-coated nanoparticle to the solution containing Pt precursor (e.g., K2PtCl4) (step 3). The PtML catalysts (supported on carbon) displayed exciting performances for ORR compared to the commercial Pt/C, and 8-fold and 23-fold enhanced mass activity of Pt were achieved using PtML/ Pd/C[101] and PtML/AuNi10/C,[102] respectively. Similarly, PtML was also deposited on the porous Pd–Cu alloyed core, and achieved 14 times improved mass activity with respect to TKK 46.4 wt % Pt/C.[103] Recently, a novel experimental approach involving PtML on Pd/C was described to directly observe the hazardous role of under-coordinated sites for ORR.[104] The removal of under-coordinated atoms on Pd(/C) and Pd3Co(/C) surfaces was facilitated by the oxidative adsorption and reductive desorption of a bromine (Br) layer during potential cycling in NaOH solution (pH 10). The resultant PtML/Pd/C and PtML/ Pd3Co/C after Br treatment displayed enhanced activity and improved durability compared to those without Br treatment. In another study, the same group demonstrated the scale-up synthesis (1 gram/batch) of PtML on carbon-supported IrNi nanoparticles.[105] The mass-produced PtML/IrNi/C was determined to be 3 times more active than Pt/C in terms of Pt-loading, which resulted from both ligand and geometric effects of the IrNi substrate. Moreover, the as-prepared catalyst showed only a 23 % loss of ECSA and almost no shift of E1/2 after a 50 000-cycle durability test (0.6–1.0 V in air-saturated HClO4). The outstanding operational stability can be attributed to the superior structural stability of both IrNi core and PtML shell as examined by STEM/EELS after the potential cycling. This methodology provides a powerful platform to investigate the geometric effect in terms of the lattice mismatch between the Pt shell of different thicknesses and the metallic core on their catalytic activities.[106] The deposition of mono- or multi-layered Pt on different cores such as Pd and Pd3Co could be mediated by the stepwise UPD of Cu; the corresponding thickness could be determined by EELS coupled with HAADFSTEM. The DFT calculation on the basis of the nanoparticle model (cuboctahedron or icosahedron) ascribed the enhanced specific activity of PtML/Pd to the lattice mismatch-induced compressive strain on (111) facets. In a similar study, multilayers of Pt were introduced onto carbon-supported Ru nanoparticles.[107] Both specific and mass activity of Pt decreased in the following order: Pt2ML/Ru/C > Pt3ML/Ru/C > PtML/Ru/C, which demonstrated that the ORR activity of the Pt shell can be easily modulated by tuning its thickness using the robust stepwise PtML deposition technique. Besides Pt, PdML can be deposited on a conductive core in a similar manner. Therefore, heterogeneous metallic monolayers can be serially deposited on various (bi)metallic cores, which enables the precise control of the geometric effect on the catalytic activity of Pt at the atomic level.[108] In detail, using 3.3 nm Ir2Re with an Ir-enriched surface as the core, a number of PtML-based electrocatalysts, namely, PtML/Ir2Re/C, PtML/PdML/Ir2Re/C, and PtML/Pd2ML/Ir2Re/C, were produced by the step-wise monolayer deposition technique, and their Pt mass activities (A mg1) were determined to 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.chemsuschem.org be 0.38, 0.60, and 0.59, respectively. It is obvious that the incorporation of a Pd interlayer (with the lattice constant aPd of 389.07 pm) effectively mediates the strong compressive strain from Ir (aIr = 383.9 pm) that exceeds the optimal geometric effect on the ORR activity of Pt (aPt = 392.42 pm); the addition of another PdML overcompensated the lattice compression of the core as the mass activity slightly decreased from 0.60 to 0.59. However, it should be pointed out that platinum group metals (PGM) including ruthenium (Ru), rhodium (Rh), Pd, osmium (Os), iridium (Ir), and Pt should be used with caution since the high price of PGM will also increase the overall cost of the catalysts. PtML deposition has also been achieved on well-defined, single-crystal metals to explore the unique physical and chemical properties of this class of electrocatalyst models for the fundamental study of ORR. The ligand effect of the substrate was investigated by comparing PtML-coated metallic surfaces, namely Ru(0001), Rh(111), Pd(111), Ir(111), and Au (111) with Pt(111) surface, which revealed the possibility of fine-tuning the electrocatalytic activity of Pt with an appropriate transition metal.[109] Similar to the discussion in section 3.2,[32, 33] the substrate-induced d-band center change of Pt plays a decisive role in the catalytic ability of PtML for ORR as corroborated by DFT calculation.[110] The mechanism of ORR on Pt can be simplified to two steps: the dissociation of O2 (OO bond-breaking step) and the association of OHad (OH bond-forming step). An acceptable reaction rate for each step is a prerequisite to achieve a high activity of the electrocatalyst for ORR. However, a catalyst that has a higher activity towards the bond-breaking step tends to have a lower activity towards the bond-forming step, and vice versa. The metallic substrate can rationally and effectively tune the d-band center position of Pt, which, in turn, can strike a balance between the two opposite factors. Among all candidates, PtML/Pd(111) was shown to give the highest kinetic current density for ORR, which was even better than that of Pt(111). Adopting Pd(111) as the substrate, the same groups moved forward to study the influence of late-transition metal (M = Ru, Rh, Pd, Re, Os, Ir, and Au) as a second component in the monolayer on the catalytic activity for ORR.[111] It was revealed that the site-blocking and the adverse electronic effects resulting from the high coverage of OHad unequivocally inhibit ORR on Pt in HClO4.[112] The clever idea in this study was to incorporate a second metal (M) that possessed higher affinity with hydroxyl group. The OHad preferentially adsorbed on M at a lower potential would decrease the coverage of OHad on Pt due to the lateral repulsion from the pre-adsorbed OHad (or Oad) on the neighboring metal atoms (adsorbate–adsorbate repulsion). The repulsion of OHad–OHad (or OHad–Oad for Re and Os) calculated by DFT was in the sequence of Os > Re > Ir > Ru > Rh > Pd, Pt > Au. A linear relationship between the repulsion value and the kinetic current density of (Pt0.8M0.2)ML/ Pd(111) successfully substantiated the function of the second metal with an enhancement of more than four times for Os and Re relative to PtML/Pd(111).

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CHEMSUSCHEM MINIREVIEWS 6. Summary and Outlook As the primary electrocatalyst for ORR, Pt has been subjected to intensive and extensive studies. Enhanced activity of Pt has been achieved based on a number of mechanisms such as crystallographic control, ligand effect, and geometric effect. The durability concern associated with Pt can be ameliorated by incorporating another noble metal into the electrocatalyst that can increase the dissolution potential of Pt or serve as the sacrificial dissolution core, and by adopting 1D or 3D supportfree nanostructures, which radically eliminate the carbon corrosion and effectively alleviate the problem of particle growth. According to these principles, a number of Pt-based core–shell electrocatalysts were developed for ORR and have achieved unprecedented catalytic performance. When it comes to the price of the catalysts, the cost of synthetic process must also be considered. Aiming at the commercialization of fuel cells, on one hand, rationally designed electrocatalysts should be produced with controllable dimension, narrow size distribution, and good dispersion; on the other hand, the preparation of electrocatalysts requires a simple and cost-effective procedure that is suitable for scaleup. Considering all factors, solution-based synthesis and impregnation methods have shown better promise for mass production. However, some potential difficulties such as the uniformity of heat and mass transfer, the removal of capping agents, as well as inconsistency from different batches should be carefully investigated by theoretical simulation and benchscale experiments before mass production. Besides transition metals, the catalytic performance of Pt may also be improved with the help of catalyst support materials, such as carbon-based and non-carbon-based nanostructures, which not only anchor and disperse the nanocatalysts, but also favor the mass and charge transfer associated with the reactions.[113] In terms of carbon-based support, nitrogendoped carbonaceous species,[114] nano- and meso-structured carbon materials,[115] and graphene[116] have been demonstrated as attractive supports with improved catalyst dispersion and gas diffusion efficiency. Albeit feasible, the application of carbon-based catalyst supports still faces some challenges such as material synthesis, metal loading, electrode preparation, and, in particular, the carbon corrosion issue during the long-term operation of fuel cells. In contrast, non-carbonaceous substrates, such as metal oxides,[117] nitrides,[118] carbide,[119] and borides,[120] are relatively inert in strong oxidative conditions, and may be considered as potential alternatives to solve the durability issue associated with the carbonaceous counterparts. In addition, to exploit their synergistic function, hybrid materials combining noble metals, non-carbon, and carbon materials (such as Pt-ITO-graphene) have also been extensively developed.[121] The major issue impeding the commercialization of PEMFCs for transportation purposes is the formidably high price of Pt used as the primary catalysts in fuel cells. To overcome this challenge, less expensive noble metals such as Pd (in acidic electrolyte)[122] and Ag (in alkaline electrolyte)[123] have been explored as substitutes for Pt in electrocatalysts. Although this 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.chemsuschem.org category of catalysts hardly outperforms Pt, especially in the acidic electrolyte, the price advantage still makes them worth pursuing. An alternative way to overcome the price problem is to develop electrocatalysts based on non-noble metals,[124–126] which would fundamentally advance the widespread application of fuel cells. In this regard, metal oxides (such as perovskite oxide[127] and Co3O4[128]), carbonaceous materials (such as nitrogen-doped[129] and boron-doped CNTs,[130] nitrogen-doped ordered mesoporous graphitic arrays,[131] nitrogen-doped graphene,[132] and CNT–graphene complexes[133]), and conducting polymers (such as polypyrrole,[134] polyaniline,[135] and PEDOT[136]) have been examined as the electrocatalysts for ORR, and have displayed encouraging activity and superior stability. Furthermore, exhilarating ORR activity has been demonstrated from metal–organic framework (MOF) that has a typical structure of metal–N4, such as phthalocyanine or porphyrinbased coordination complexes.[137] Recently, microscopic and spectroscopic experiments along with DFT calculations predicted the tunable catalytic activity towards ORR of Ag(110)-supported iron phthalocyanine with long-range supramolecular arrangement and local adsorption geometry.[138] A new strategy for the development of organometallic complexes has been inspired by natural enzymes, such as cytochrome c oxidase and multicopper oxidase.[139] These enzymes usually possess Fe- or Cu-based active centers that function cooperatively with each other for ORR. Accordingly, effective electrocatalysts can be precisely designed to mimic the functional groups of the enzymes associated with multi-active sites such as Fe,[140] Co,[141] and Cu[142] catalytic centers and the appropriate chemical environment of the enzymes such as a strongly conjugated nitrogen ring-system that modifies the electronic properties of the metal center. However, similar to the original enzymes, this class of catalysts suffers from lower current output and stability issue that urgently need to be solved. With the advance of modern computational tools that enable the fundamental comprehension of the relationship between synthesis, structure, and property of nanocatalysts, along with the development of state-of-the-art experimental approaches that put the theory into practice, one can expect better fuel cell catalysts with lower cost and higher catalytic performance in the near future.

Acknowledgements We greatly appreciate the partial financial support from the National Science Foundation. Keywords: catalysis · electrochemistry · oxidation reduction reaction · fuel cells · platinum

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Third-body effects of native surfactants on Pt nanoparticle electrocatalysts in proton exchange fuel cells.

Insight into proton transfer in phosphotungstic acid functionalized mesoporous silica-based proton exchange membrane fuel cells.

Realization of Both High-Performance and Enhanced Durability of Fuel Cells: Pt-Exoskeleton Structure Electrocatalysts.

Tetrazole-based, anhydrous proton exchange membranes for fuel cells.

Synthesizing 2D MoS2 Nanofins on carbon nanospheres as catalyst support for Proton Exchange Membrane Fuel Cells.

Mesostructured platinum-free anode and carbon-free cathode catalysts for durable proton exchange membrane fuel cells.

I₃⁻ couple as a liquid cathode for proton exchange membrane fuel cell.

Highly conductive anion exchange membrane for high power density fuel-cell performance.

Iridium-decorated palladium-platinum core-shell catalysts for oxygen reduction reaction in proton exchange membrane fuel cell.

In-Situ Measurement of High-Temperature Proton Exchange Membrane Fuel Cell Stack Using Flexible Five-in-One Micro-Sensor.

Noncovalent immobilization of electrocatalysts on carbon electrodes for fuel production.

Improved Oxygen Reduction Activity and Durability of Dealloyed PtCo x Catalysts for Proton Exchange Membrane Fuel Cells: Strain, Ligand, and Particle Size Effects.

Characterization of Polyethylene-Graft-Sulfonated Polyarylsulfone Proton Exchange Membranes for Direct Methanol Fuel Cell Applications.

Double-layer ionomer membrane for improving fuel cell performance.

H2O2 Fuel Cells?

Durability enhancement of intermetallics electrocatalysts via N-anchor effect for fuel cells.

An electrochemical quartz crystal microbalance study of a prospective alkaline anion exchange membrane material for fuel cells: anion exchange dynamics and membrane swelling.

Interface-designed Membranes with Shape-controlled Patterns for High-performance Polymer Electrolyte Membrane Fuel Cells.

Infrared-activated proton transfer in aqueous nafion proton-exchange-membrane nanochannels.

Hyaluronidase-bound membrane as a biomaterial for implantable fuel cells.

Enhanced biofilm distribution and cell performance of microfluidic microbial fuel cells with multiple anolyte inlets.

Performance of anaerobic fluidized membrane bioreactors using effluents of microbial fuel cells treating domestic wastewater.

Carbon-supported Pt-based alloy electrocatalysts for the oxygen reduction reaction in polymer electrolyte membrane fuel cells: particle size, shape, and composition manipulation and their impact to activity.

Composite materials for polymer electrolyte membrane microbial fuel cells.