www.advmat.de www.MaterialsViews.com
COMMUNICATION
Electrochemically Fabricated Polypyrrole and MoSx Copolymer Films as a Highly Active Hydrogen Evolution Electrocatalyst Tanyuan Wang, Junqiao Zhuo, Kuangzhou Du, Bingbo Chen, Zhiwei Zhu, Yuanhua Shao, and Meixian Li* Hydrogen is an ideal energy carrier because of its clean and renewable properties.[1] Therefore, the efficient generation of hydrogen from water splitting by the hydrogen evolution reaction (HER) has caught great attention.[2] Pt-group metals are the most efficient electrocatalysts for HER. However, they suffer from high cost and low abundance. Thus, the exploration of highly active non-precious metal electrocatalysts has become a main research focus for the realistic use of hydrogen.[3] Molybdenum sulfides (MoSx) have attracted great interest as promising electrocatalysts for HER[4] since Nørskov and coworkers reported the catalytic activity of MoS2 nanoparticles for HER.[5] Subsequent studies have suggested that some other molybdenum sulfide structures, such as cubane-type [Mo3S4]4+,[6] amorphous MoSx,[7] or even molecules that contain an MoS2 edge site[8] could also promote the evolution of hydrogen in addition to MoS2.[9] Moreover, the HER activity of MoSx could be optimized by increasing the conductivity and active sites of the catalysts by, for instance, supporting them on Au[10] or carbon nanomaterials,[9b,11] as well as by doping with some other transition metal elements.[12] In addition, exposing more active edge sites was made possible by engineering the surface structure[4e] or using chemical exfoliation,[9d] as reported. Recent work has also pointed out that the catalytic species are likely related to the bridging S22− or apical S2− for the MoSx-based materials.[7b] Although various efforts have been made to improve the MoSx-based materials for HER, their catalytic activity was still inferior to Pt with large overpotentials and Tafel slopes. In order to produce a more active electrocatalyst, various parameters such as the conductivity, roughness, the attachment of catalysts on the electrodes, and the active edge sites need to be well controlled.[3,13] Electro-deposition, using (NH4)2MoS4 as the precursor, has been proved to be an easy and efficient way to produce an active MoSx film for HER by Hu and coworkers.[7a] On the other hand, polypyrrole (PPy) is a conducting polymer that is widely used in the energy conversion and storage field because of its excellent conductivity, large electrochemical T. Y. Wang, J. Q. Zhuo, K. Z. Du, B. B. Chen, Prof. Z. W. Zhu, Prof. Y. H. Shao, Prof. M. X. Li College of Chemistry and Molecular Engineering Peking University Beijing 100871, P.R.China E-mail: [email protected]
DOI: 10.1002/adma.201400265
Adv. Mater. 2014, DOI: 10.1002/adma.201400265
surface area, as well as good stability.[14] Moreover, It has been demonstrated that [MoS4]2− and some other molybdenum sulfide anions can be doped into PPy during the polymerization process,[15] which makes it an ideal carrier for MoSx. Herein, we demonstrate a simple way to fabricate polypyrrole/MoSx hybrid (PPy/MoSx) films by a one-step electrochemical copolymerization. The PPy/MoSx films exhibit an outstanding HER performance that is comparable to that of commercial Pt/C catalysts. The highly active PPy/MoSx films for HER were prepared in 0.1 M NaClO4 containing 0.5 M pyrrole (Py) and 2 mM (NH4)2MoS4 by electro-polymerization at 0.75 V vs. SCE. The mixed solution was aged overnight before use. Figure 1a displays the catalytic performances of electrodes modified with different films for HER in 0.5 M H2SO4. The PPy/MoSx film exhibits an excellent HER performance. It can clearly be seen that hydrogen evolution occurs at 0 V vs. RHE, which is nearly the same as that of the commercial Pt/C. This is the best result reported to date from MoSx-based materials in electrocatalysis for HER. The electrode modified with an (NH4)2MoS4 film also showed a relatively high HER activity even though the film was not stable in acid, but it is not as good as the commercial Pt/C catalyst. The PPy-modified electrode has no HER activity even at a potential of –0.3 V vs. RHE. The MoSx film that was electrodeposited at 0.75 V vs. SCE with (NH4)2MoS4 as the precursor exhibits an onset potential of –0.24 V vs. RHE for HER, which is inferior to the result obtained by Hu and coworkers[7a] but can be explained by the fact that the deposition potential was much more positive, and its performance was also not as good as that of the (NH4)2MoS4 and PPy/MoSx-modified electrodes. The hydrogen evolution mechanism at the PPy/MoSx films was further investigated by the Tafel plot. Figure 1b is the Tafel plot of the PPy/MoSx modified electrode. A Tafel slope of 29 mV dec−1 is observed, which is much smaller than those of commonly reported MoSx with good HER activity (about 40 mV dec−1),[7,9b] suggesting that the HER mechanism of PPy/ MoSx might be a Volmer–Tafel mechanism with the Tafel reaction as the rate-determining step, which is similar to the HER mechanism on Pt. For practical applications, a small Tafel slope is desirable as it will be helpful for a faster increase of the HER rate with increasing overpotential. Furthermore, the exchange current density of PPy/MoSx was calculated to be 5.6 × 10−4 A cm−2, and the catalytic current density at –0.06 V vs. RHE is 50 mA cm−2, which are both significantly larger than those for the reported MoSx-based materials[4b,9,12a] and NiMo-based
© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
wileyonlinelibrary.com
1
www.advmat.de
COMMUNICATION
www.MaterialsViews.com
Figure 2. SEM images of different electro-polymerized films. a,b) PPy/ MoSx, c) PPy, and d) MoSx.
Figure 1. a) Polarization curves of electrodes modified with: PPy (a), MoSx (b), (NH4)2MoS4 (c), PPy/MoSx (d); and a commercial Pt/C electrode (e). b) Tafel plot and the fitting curve (dash line) of the PPy/MoSx films in 0.5 M H2SO4 at a scan rate of 2 mV s−1.
materials,[16] as well as cobalt-based catalysts.[17] Furthermore, the onset potentials for most of these catalysts are more negative than the value of –0.06 V vs. RHE where the PPy/MoSx films already exhibit a large catalytic current. Atomic emission spectrometry was used to measure the loading of MoSx in the hybrid films by dissolving the films in nitric acid. A small loading of 7.8 µg cm−2 for Mo was obtained, which meant a high HER efficiency at the films.[4b] The scanning electronic microscopy (SEM) images show that the electrodeposited PPy/MoSx films on a glassy carbon electrode (GCE) have a rather rough structure owing to the agglomeration of particles with sizes of hundreds of nanometers or smaller (Figure 2a,b and Figure S1, Supporting Information), which are different from the plain PPy films and MoSx films (Figure 2c and 2d). A possible reason for this is that the polymerization of Py and the deposition of MoSx are competitive reactions during the formation of the PPy/MoSx films.[18] For the PPy films, the deposition current was quite large when pyrrole was polymerized in the absence of (NH4)2MoS4, which meant that too much pyrrole reacted when the potential was applied, thus leading to the formation of accumulated large particles.
2
wileyonlinelibrary.com
PPy/MoSx films that consisted of small nanostructured particles were formed as the polymerization current decreased in the presence of (NH4)2MoS4. For the pure MoSx films, the electrodeposition mechanism of (NH4)2MoS4 was quite different from that of pyrrole, so flat films were formed during deposition. The relative roughness of the films was further evaluated by cyclic voltammetry (Figures S2 and S3, Supporting Information).[4d] Compared to the electrodeposited MoSx-modified electrode, the PPy/MoSx-modified electrode shows a 900-time increase in relative roughness, which illustrates that the electrochemical surface area of the PPy/MoSx-modified electrode increases significantly. Electrochemical impedance spectroscopy (EIS) presents (Figures S4 and S5, Supporting Information) a quite low chargetransfer resistance of only 50 Ω at an overpotential of 30 mV, which is much lower than that obtained for MoS2 nanoparticles grown on graphene[9b] and the electrodeposited MoSx,[19] thus suggesting relatively fast HER kinetics of the PPy/MoSx films, which can be ascribed to the super high conductivity and high activity of the copolymer films as well as its good attachment to the substrate electrode.[3,14] X-ray photoelectron spectroscopy (XPS) was then employed to characterize the elemental composition and bonding configuration of the electrodeposited films. Figure S6 (Supporting Information) shows the survey spectra of the plain PPy films. An intense N peak was detected in addition to a C and O peak. Cl was also observed due to the doping of ClO4− during the electro-polymerization process. For the PPy/MoSx films, however, signals of Mo and S appear in the XPS spectrum (Figure 3a), and the relative intensity of Cl decreases, which implies that fewer ClO4− ions are doped into the film and that the molybdenum sulfide anion also acts as the counter ion during the polymerization of Py. Both Mo (IV) and Mo (VI) could be observed, as well as S with two different binding energies that represent bridging S22− and/or apical S2− and terminal S22− and/or S2− (Figure 3b,c).[7a] The XPS spectra of (NH4)2MoS4 (Figure 3d–f) and MoSx films (Figure 3g–i) were also recorded. Only Mo (IV) is observed for the (NH4)2MoS4 films and the intensities of the two types of S are nearly the same. The MoSx films also exhibit two kinds of Mo and two kinds of S, but their valent compositions seem different from those of the PPy/
© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Mater. 2014, DOI: 10.1002/adma.201400265
www.advmat.de www.MaterialsViews.com
COMMUNICATION Figure 3. XPS survey spectra for a) PPy/MoSx, d) (NH4)2MoS4, and g) MoSx. XPS spectra of the Mo 3d region for b) PPy/MoSx, e) (NH4)2MoS4, and h) MoSx. XPS spectra of the S 2p region for c) PPy/MoSx, f) (NH4)2MoS4, and i) MoSx.
MoSx films. This suggests that the bonding configurations of Mo and S in these films are different. The ratio of S to Mo for the PPy/MoSx films is calculated to be 5.0:1, which is much larger than commonly reported for MoSx (x ≈ 3)[7] and the MoSx film prepared in this experiment (2.7:1), and it is even larger than the theoretical ratio of doped MoS42−. This suggests that an extremely S-rich molybdenum sulfide structure has been formed in these films. Thus we presume that molybdenum polysulfide with more active S edge sites that is highly active for HER is produced during the electro-deposition process, similar to the structures reported before by theoretical calculation.[20] This is further verified by our experimental results, whereby the intensity of the S 2p3/2 peak at 163.4 eV, which represents bridging S22− and/or apical S2−, is significantly higher than that of the S 2p3/2 peak at 162.0 eV, which represents terminal S22− and/or S2−, compared to the ratio found in XPS spectra of (NH4)2MoS4 and MoSx films, which is also distinct from that of reported MoSx-based materials.[7] One possible reason for this phenomenon is that molybdenum polysulfide anions might be produced during the aging process.[21] They would not only serve as counter ions during the co-polymerization process, but also react at the deposition potential, producing more bridging S22− and/or apical S2−, which are considered to be active sites for HER.[7b] In order to verify the formation of molybdenum polysulfide anions, UV–vis spectroscopy was employed to characterize the mixed solution of Py and (NH4)2MoS4. Figure S7 in the Supporting Information presents the UV–vis absorption spectra for the different solutions of Py, (NH4)2MoS4, and their mixture before and after aging. The Py intensity exhibits
Adv. Mater. 2014, DOI: 10.1002/adma.201400265
an increase in the absorption spectrum after aging because of its oxidization during aging. (NH4)2MoS4 shows two intense peaks at 468 nm and 316 nm, and no change is observed after aging, which suggests that it is quite stable without the existence of Py. The freshly prepared mixture solution of Py and (NH4)2MoS4 also displays two peaks at 468 nm and 316 nm, but the intensities of these two peaks decrease and a new peak at 395 nm appears after aging. This new peak might possibly be attributed to the formation of molybdenum polysulfide anions, such as MoS92−,[21] which illustrates that there is an interaction between Py and (NH4)2MoS4. During the electro-deposition process, S-edge enrichment would be achieved because of the existence of these anions, thus forming extremely active PPy/ MoSx films for HER. This is also the reason why the highly efficient electrocatalytic PPy/MoSx films could not be obtained from the freshly prepared solution without aging. Raman characterization also confirmed that more bridging S22− is formed during the process of co-polymerization (Figure S8, Supporting Information). No peak appears for PPy in the range of 100–600 cm−1. (NH4)2MoS4 exhibits two typical bands at 455 and 475 cm−1 corresponding to the symmetric and asymmetric Mo-S vibrations, whereas MoSx shows a broad band at 540 cm−1 corresponding to the S-S vibration of S22− ligands in addition to other broad bands at 235, 350, and 450 cm−1.[22] The PPy/MoSx films also display broad bands at 450 and 550 cm−1, which can be assigned to the Mo-S and S-S vibrations, respectively, even though the signals are quite weak. The S-S vibration band is shifted to higher wavenumbers compared to that of the plain MoSx films, which attests that more bridging
© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
wileyonlinelibrary.com
3
www.advmat.de
COMMUNICATION
www.MaterialsViews.com
Figure 4. Polarization curves of the the PPy/MoSx films deposited a) at various potentials, b) for different durations at 0.75 V vs. SCE in 0.5 M H2SO4 at a scan rate of 2 mV s−1.
S22− is formed during the co-polymerization process as the typical band of bridging S22− (553 cm−1) is higher than that of terminal S22− (522 cm−1),[22] thus leading to the good HER performance of the hybrid films. All of these confirm that the highly active PPy/MoSx films for HER have a different structure compared to the precursor (NH4)2MoS4 and MoSx films reported in the literature.[7,11,13] The hybrid films exhibit a higher S to Mo ratio and more bridging S22− active sites, therefore, displaying outstanding HER activity. In order to further understand the formation of the highly active PPy/MoSx films, different deposition potentials from 0.6 to 0.9 V vs. SCE were tested, and the reaction currents were obtained (Figure S9, Supporting Information). The resulting films display different electrocatalytic activities for HER (Figure 4a), and the hybrid films deposited at 0.75 V vs. SCE show the best performance. When the potential is relatively negative (0.6 V), electrodeposition of MoSx would be the main reaction rather than the polymerization of Py, resulting in a low reaction current and a film with a different morphology (Figure S10, Supporting Information). XPS characterization (Figure S11–S13, Supporting Information) also confirmed that the relative intensity of Mo and S is much stronger than
4
wileyonlinelibrary.com
that shown in Figure 3a. These indicate that the films deposited at 0.6 V vs. SCE are more similar to MoSx films rather than to PPy/MoSx films. On the other hand, if the potential is increased to 0.8 V vs. SCE, the films do show a similar morphology compared to that of the PPy/MoSx films deposited at 0.75 V vs. SCE (Figure S14, Supporting Information), however, overoxidization occurs in the films. A new type of S ion with a higher valence could be observed, as well as an increase in Mo (VI) (Figure S15–S17, Supporting Information), thus leading to an inferior HER performance. In addition, the ratio of S to Mo for the PPy/MoSx films deposited at 0.6 or 0.8 V vs. SCE was 2.2 and 2.3:1, respectively, which is much smaller than that of the hybrid films deposited at 0.75 V vs. SCE, suggesting that S-enrichment is not achieved for these films. For even higher deposition potentials (0.90 V vs. SCE), the films do not exhibit any obvious HER activity. In short, the binding configurations of PPy/MoSx films deposited at other potentials are quite different from that of the PPy/MoSx films deposited at 0.75 V vs. SCE. Our particular PPy/MoSx films show a good conductivity and large electrochemical surface area, as well as a higher amount of active S edge sites, thus generating an extremely high catalytic activity for HER. Similarly, the concentrations of Py and (NH4)2MoS4 in the solution had an influence on the HER activity of the resulting hybrid films. Only the hybrid films that were prepared from a solution with a molar ratio of 250:1 (Py/(NH4)2MoS4) displayed a high HER activity (Figure S18, Supporting Information). Moreover, the electro-deposition time also affects the HER performance of the hybrid films, as shown in Figure 4b. A shorter deposition time led to lower active films as the loading is smaller, whereas a longer time resulted in the formation of unstably thick films with inferior activity. Therefore, 100 s was chosen as the deposition time for preparing the PPy/MoSx films. These phenomena reveal that the aging process that produces molybdenum polysulfide anions is not the only key factor for the construction of the highly active PPy/ MoSx films. Although we managed to fabricate PPy/MoSx films with an unique structure, the formation mechanism of these films is still unclear, because the formation conditions are complicated, as they involve the interaction of Py and (NH4)2MoS4 in solution, the competitive reaction between the polymerization of Py and the deposition of MoSx,[18] as well as the reconstruction of the films during the process of electro-polymerization. The stability of the PPy/MoSx films was also evaluated. Unfortunately, its stability was not very good without the protection of Nafion. The HER activity would be lost after several cycles in 0.5 M H2SO4 as the composition of the films readily changed in this electrolyte. Figure 5a and Figure S19–S20 (Supporting Information) show the XPS spectra of the PPy/ MoSx films after polarization in 0.5 M H2SO4. It can be seen that SO42− is incorporated into the films (Figure 5a) and the ratio of S (lower valence) to Mo decreases to 3.4:1. However, by modifying with Nafion, the stability of the PPy/MoSx films for HER improved greatly. Only a slight decrease in current was observed after 1000 cycles from 0.54 to –0.04 V vs. RHE at a scan rate of 100 mV s−1 (Figure 5b). These results also imply the existence of molybdenum polysulfide anions with a high ratio of S to Mo in the PPy/MoSx films, which results in the high HER activity of the films, as Nafion could hinder the dissolution of anions from the film into the solution. Durability tests
© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Mater. 2014, DOI: 10.1002/adma.201400265
www.advmat.de www.MaterialsViews.com
COMMUNICATION
PPy/MoSx film can be attributed to the following three aspects: i) PPy is a well-known conducting polymer that easily forms porous structures. Therefore, PPy offers the good conductivity and large electrochemical surface area of the PPy/MoSx films; ii) the electro-deposition technique provides the good attachment of the PPy/MoSx films to the substrate electrode; iii) the interaction between Py and (NH4)2MoS4 results in the formation of a film with a high S to Mo ratio (5.0:1) with more active S edge sites forming during the polymerization process and after aging. This research proposes a new scheme to construct highly efficient MoSx-based electrocatalysts for HER. Further work is needed to elucidate the electro-deposition mechanism.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements This work was financially supported by the National Natural Science Foundation of China (Grants 21075003 and 21275010). Received: January 17, 2014 Revised: February 14, 2014 Published online:
Figure 5. a) XPS spectra of the S 2p region for the PPy/MoSx films electrodeposited at 0.75 V vs. SCE after polarization in 0.5 M H2SO4 without the protection of Nafion. b) Stability test of the PPy/MoSx films protected with Nafion using a cyclic potential scanning from 0.54 to –0.04 V vs. RHE in 0.5 M H2SO4 at a scan rate of 100 mV s−1.
over longer periods of time of the PPy/MoSx films for HER, however, revealed that the stability of our films is not so good as compared to reported MoS2-based materials.[13b] Although the HER current density easily reached tens of mA cm−2 even at a potential of –0.05 V vs. RHE because of the high activity of the catalyst, an obvious decrease in current was observed after about 5000 s with chronoamperometry, as shown in Figure S21 (Supporting Information). A possible reason for this is that the Nafion film is damaged by the large amounts of bubbles produced during the hydrogen evolution reaction. This means that more research is needed to improve the stability of the catalyst. In conclusion, we have fabricated an extremely active PPy/ MoSx structure for HER with Py and (NH4)2MoS4 as the reactants by electrochemical copolymerization. Although the mechanism for the formation of the PPy/MoSx films is still unclear, the films demonstrate outstanding HER performance. A small Tafel slope of 29 mV dec−1, a positive onset potential of 0 V vs. RHE, and a high exchange-current density of 5.6 × 10−4 A cm−2 were observed. These are the best results reported to date for MoSx-based materials, and they are comparable to those of commercial Pt/C catalysts. The outstanding HER activity of the
Adv. Mater. 2014, DOI: 10.1002/adma.201400265
[1] a) M. S. Dresselhaus, I. L. Thomas, Nature 2001, 414, 332; b) N. S. Lewis, D. G. Nocera, Proc. Natl. Acad. Sci. USA 2006, 103, 15 729. [2] a) M. G. Walter, E. L. Warren, J. R. McKone, S. W. Boettcher, Q. X. Mi, E. A. Santori, N. S. Lewis, Chem. Rev. 2010, 110, 6446; b) J. A. Turner, Science 2004, 305, 972. [3] A. B. Laursen, S. Kegnæs, S. Dahl, I. Chorkendorff, Energy Environ. Sci. 2012, 5, 5577. [4] a) Q. J. Xiang, J. G. Yu, M. Jaroniec, J. Am. Chem. Soc. 2012, 134, 6575; b) D. Merki, X. L. Hu, Energy Environ. Sci. 2011, 4, 3878; c) M. L. Tang, D. C. Grauer, B. Lassalle-Kaiser, V. K. Yachandra, L. Amirav, J. R. Long, J. Yano, A. P. Alivisatos, Angew. Chem. Int. Ed. 2011, 50, 10 203; d) J. D. Benck, Z. B. Chen, L. Y. Kuritzky, A. J. Forman, T. F. Jaramillo, ACS Catal. 2012, 2, 1916; e) J. Kibsgaard, Z. B. Chen, B. N. Reinecke, T. F. Jaramillo, Nat. Mater. 2012, 11, 963; f) B. Seger, A. B. Laursen, P. C. K. Vesborg, T. Pedersen, O. Hansen, S. Dahl, I. Chorkendorff, Angew. Chem. Int. Ed. 2012, 51, 9128. [5] B. Hinnemann, P. G. Moses, J. Bonde, K. P. Jørgensen, J. H. Nielsen, S. Horch, I. Chorkendorff, J. K. Nørskov, J. Am. Chem. Soc. 2005, 127, 5308. [6] T. F. Jaramillo, J. Bonde, J. D. Zhang, B. L. Ooi, K. Andersson, J. Ulstrup, I. Chorkendorff, J. Phys. Chem. C 2008, 112, 17 492. [7] a) D. Merki, S. Fierro, H. Vrubel, X. L. Hu, Chem. Sci. 2011, 2, 1262; b) Y. H. Chang, C. T. Lin, T. Y. Chen, C. L. Hsu, Y. H. Lee, W. J. Zhang, K. H. Wei, L. J. Li, Adv. Mater. 2013, 25, 756; c) H. Vrubel, X. L. Hu, ACS Catal. 2013, 3, 2002. [8] a) H. I. Karunadasa, E. Montalvo, Y. J. Sun, M. Majda, J. R. Long, C. J. Chang, Science 2012, 335, 698; b) A. M. Appel, D. L. DuBois, M. R. DuBois, J. Am. Chem. Soc. 2005, 127, 12 717. [9] a) F. K. Meng, J. T. Li, S. K. Cushing, M. J. Zhi, N. Q. Wu, J. Am. Chem. Soc. 2013, 135, 10 286; b) Y. G. Li, H. L. Wang, L. M. Xie, Y. Y. Liang, G. S. Hong, H. J. Dai, J. Am. Chem. Soc. 2011, 133,
© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
wileyonlinelibrary.com
5
www.advmat.de
COMMUNICATION
www.MaterialsViews.com
[10]
[11]
[12]
[13]
6
7296; c) I. Hatay, P. Y. Ge, H. Vrubel, X. L. Hu, H. H. Girault, Energy Environ. Sci. 2011, 4, 4246; d) M. A. Lukowski, A. S. Daniel, F. Meng, A. Forticaux, L. S. Li, S. Jin, J. Am. Chem. Soc. 2013, 135, 10 274. a) T. F. Jaramillo, K. P. Jorgensen, J. Bonde, J. H. Nielsen, S. Horch, I. Chorkendorff, Science 2007, 317, 100; b) T. Y. Wang, L. Liu, Z. W. Zhu, P. Papakonstantinou, J. B. Hu, H. Y. Liu, M. X. Li, Energy Environ. Sci. 2013, 6, 625. a) H. Vrubel, D. Merki, X. L. Hu, Energy Environ. Sci. 2012, 5, 6136; b) L. Liao, J. Zhu, X. J. Bian, L. N. Zhu, M. D. Scanlon, H. H. Girault, B. H. Liu, Adv. Funct. Mater. 2013, 23, 5326. a) D. Merki, H. Vrubel, L. Rovelli, S. Fierro, X. L. Hu, Chem. Sci. 2012, 3, 2515; b) P. D. Tran, M. Nguyen, S. S. Pramana, A. Bhattacharjee, S. Y. Chiam, J. Fize, M. J. Field, V. Artero, L. H. Wong, J. Loo, J. Barber, Energy Environ. Sci. 2012, 5, 8912. a) A. B. Laursen, P. C. K. Vesborg, I. Chorkendorff, Chem. Commun. 2013, 49, 4965; b) J. F. Xie, H. Zhang, S. Li, R. X. Wang, X. Sun, M. Zhou, J. F. Zhou, X. W. Lou, Y. Xie, Adv. Mater. 2013, 25, 5807; c) W. F. Chen, J. T. Muckerman, E. Fujita, Chem. Commun. 2013, 49, 8896.
wileyonlinelibrary.com
[14] a) E. Frackowiak, F. Beguin, Carbon 2002, 40, 1775; b) R. Bashyam, P. Zelenay, Nature 2006, 443, 63. [15] a) S. Sadki, P. Schottland, N. Brodie, G. Sabouraud, Chem. Soc. Rev. 2000, 29, 283; b) D. Bélanger, G. Laperrière, F. Girard, D. Guay, G. Tourillon, Chem. Mater. 1993, 5, 861. [16] J. R. Mckone, E. L. Warren, M. J. Bierman, S. W. Boettcher, B. S. Brunschwig, N. S. Lewis, H. B. Gray, Energy Environ. Sci. 2011, 4, 3573. [17] S. Cobo, J. Heidkamp, P. A. Jacques, J. Fize, V. Fourmond, L. Guetaz, B. Jousselme, V. Ivanova, H. Dau, S. Palacin, M. Fontecave, V. Artero, Nat. Mater. 2012, 11, 802. [18] S. Lavallée, G. Laperrière, D. Bélanger, J. Electroanal. Chem. 1997, 431, 219. [19] H. Vrubel, T. Moehl, M. Grätzel, X. L. Hu, Chem. Commun. 2013, 49, 8985. [20] B. Wang, N. Wu, X. B. Zhang, X. Huang, Y. F. Zhang, W. K. Chen, K. N. Ding, J. Phys. Chem. A 2013, 117, 5632. [21] M. Draganjac, E. Simhon, L. T. Chan, M. Kanatzidis, N. C. Baenziger, D. Coucouvanis, Inorg. Chem. 1982, 21, 3321. [22] T. Weber, J. C. Muijsers, J. W. Niemantsverdriet, J. Phys. Chem. 1995, 99, 9194.
© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Mater. 2014, DOI: 10.1002/adma.201400265
COMMUNICATION
Electrochemically Fabricated Polypyrrole and MoSx Copolymer Films as a Highly Active Hydrogen Evolution Electrocatalyst Tanyuan Wang, Junqiao Zhuo, Kuangzhou Du, Bingbo Chen, Zhiwei Zhu, Yuanhua Shao, and Meixian Li* Hydrogen is an ideal energy carrier because of its clean and renewable properties.[1] Therefore, the efficient generation of hydrogen from water splitting by the hydrogen evolution reaction (HER) has caught great attention.[2] Pt-group metals are the most efficient electrocatalysts for HER. However, they suffer from high cost and low abundance. Thus, the exploration of highly active non-precious metal electrocatalysts has become a main research focus for the realistic use of hydrogen.[3] Molybdenum sulfides (MoSx) have attracted great interest as promising electrocatalysts for HER[4] since Nørskov and coworkers reported the catalytic activity of MoS2 nanoparticles for HER.[5] Subsequent studies have suggested that some other molybdenum sulfide structures, such as cubane-type [Mo3S4]4+,[6] amorphous MoSx,[7] or even molecules that contain an MoS2 edge site[8] could also promote the evolution of hydrogen in addition to MoS2.[9] Moreover, the HER activity of MoSx could be optimized by increasing the conductivity and active sites of the catalysts by, for instance, supporting them on Au[10] or carbon nanomaterials,[9b,11] as well as by doping with some other transition metal elements.[12] In addition, exposing more active edge sites was made possible by engineering the surface structure[4e] or using chemical exfoliation,[9d] as reported. Recent work has also pointed out that the catalytic species are likely related to the bridging S22− or apical S2− for the MoSx-based materials.[7b] Although various efforts have been made to improve the MoSx-based materials for HER, their catalytic activity was still inferior to Pt with large overpotentials and Tafel slopes. In order to produce a more active electrocatalyst, various parameters such as the conductivity, roughness, the attachment of catalysts on the electrodes, and the active edge sites need to be well controlled.[3,13] Electro-deposition, using (NH4)2MoS4 as the precursor, has been proved to be an easy and efficient way to produce an active MoSx film for HER by Hu and coworkers.[7a] On the other hand, polypyrrole (PPy) is a conducting polymer that is widely used in the energy conversion and storage field because of its excellent conductivity, large electrochemical T. Y. Wang, J. Q. Zhuo, K. Z. Du, B. B. Chen, Prof. Z. W. Zhu, Prof. Y. H. Shao, Prof. M. X. Li College of Chemistry and Molecular Engineering Peking University Beijing 100871, P.R.China E-mail: [email protected]
DOI: 10.1002/adma.201400265
Adv. Mater. 2014, DOI: 10.1002/adma.201400265
surface area, as well as good stability.[14] Moreover, It has been demonstrated that [MoS4]2− and some other molybdenum sulfide anions can be doped into PPy during the polymerization process,[15] which makes it an ideal carrier for MoSx. Herein, we demonstrate a simple way to fabricate polypyrrole/MoSx hybrid (PPy/MoSx) films by a one-step electrochemical copolymerization. The PPy/MoSx films exhibit an outstanding HER performance that is comparable to that of commercial Pt/C catalysts. The highly active PPy/MoSx films for HER were prepared in 0.1 M NaClO4 containing 0.5 M pyrrole (Py) and 2 mM (NH4)2MoS4 by electro-polymerization at 0.75 V vs. SCE. The mixed solution was aged overnight before use. Figure 1a displays the catalytic performances of electrodes modified with different films for HER in 0.5 M H2SO4. The PPy/MoSx film exhibits an excellent HER performance. It can clearly be seen that hydrogen evolution occurs at 0 V vs. RHE, which is nearly the same as that of the commercial Pt/C. This is the best result reported to date from MoSx-based materials in electrocatalysis for HER. The electrode modified with an (NH4)2MoS4 film also showed a relatively high HER activity even though the film was not stable in acid, but it is not as good as the commercial Pt/C catalyst. The PPy-modified electrode has no HER activity even at a potential of –0.3 V vs. RHE. The MoSx film that was electrodeposited at 0.75 V vs. SCE with (NH4)2MoS4 as the precursor exhibits an onset potential of –0.24 V vs. RHE for HER, which is inferior to the result obtained by Hu and coworkers[7a] but can be explained by the fact that the deposition potential was much more positive, and its performance was also not as good as that of the (NH4)2MoS4 and PPy/MoSx-modified electrodes. The hydrogen evolution mechanism at the PPy/MoSx films was further investigated by the Tafel plot. Figure 1b is the Tafel plot of the PPy/MoSx modified electrode. A Tafel slope of 29 mV dec−1 is observed, which is much smaller than those of commonly reported MoSx with good HER activity (about 40 mV dec−1),[7,9b] suggesting that the HER mechanism of PPy/ MoSx might be a Volmer–Tafel mechanism with the Tafel reaction as the rate-determining step, which is similar to the HER mechanism on Pt. For practical applications, a small Tafel slope is desirable as it will be helpful for a faster increase of the HER rate with increasing overpotential. Furthermore, the exchange current density of PPy/MoSx was calculated to be 5.6 × 10−4 A cm−2, and the catalytic current density at –0.06 V vs. RHE is 50 mA cm−2, which are both significantly larger than those for the reported MoSx-based materials[4b,9,12a] and NiMo-based
© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
wileyonlinelibrary.com
1
www.advmat.de
COMMUNICATION
www.MaterialsViews.com
Figure 2. SEM images of different electro-polymerized films. a,b) PPy/ MoSx, c) PPy, and d) MoSx.
Figure 1. a) Polarization curves of electrodes modified with: PPy (a), MoSx (b), (NH4)2MoS4 (c), PPy/MoSx (d); and a commercial Pt/C electrode (e). b) Tafel plot and the fitting curve (dash line) of the PPy/MoSx films in 0.5 M H2SO4 at a scan rate of 2 mV s−1.
materials,[16] as well as cobalt-based catalysts.[17] Furthermore, the onset potentials for most of these catalysts are more negative than the value of –0.06 V vs. RHE where the PPy/MoSx films already exhibit a large catalytic current. Atomic emission spectrometry was used to measure the loading of MoSx in the hybrid films by dissolving the films in nitric acid. A small loading of 7.8 µg cm−2 for Mo was obtained, which meant a high HER efficiency at the films.[4b] The scanning electronic microscopy (SEM) images show that the electrodeposited PPy/MoSx films on a glassy carbon electrode (GCE) have a rather rough structure owing to the agglomeration of particles with sizes of hundreds of nanometers or smaller (Figure 2a,b and Figure S1, Supporting Information), which are different from the plain PPy films and MoSx films (Figure 2c and 2d). A possible reason for this is that the polymerization of Py and the deposition of MoSx are competitive reactions during the formation of the PPy/MoSx films.[18] For the PPy films, the deposition current was quite large when pyrrole was polymerized in the absence of (NH4)2MoS4, which meant that too much pyrrole reacted when the potential was applied, thus leading to the formation of accumulated large particles.
2
wileyonlinelibrary.com
PPy/MoSx films that consisted of small nanostructured particles were formed as the polymerization current decreased in the presence of (NH4)2MoS4. For the pure MoSx films, the electrodeposition mechanism of (NH4)2MoS4 was quite different from that of pyrrole, so flat films were formed during deposition. The relative roughness of the films was further evaluated by cyclic voltammetry (Figures S2 and S3, Supporting Information).[4d] Compared to the electrodeposited MoSx-modified electrode, the PPy/MoSx-modified electrode shows a 900-time increase in relative roughness, which illustrates that the electrochemical surface area of the PPy/MoSx-modified electrode increases significantly. Electrochemical impedance spectroscopy (EIS) presents (Figures S4 and S5, Supporting Information) a quite low chargetransfer resistance of only 50 Ω at an overpotential of 30 mV, which is much lower than that obtained for MoS2 nanoparticles grown on graphene[9b] and the electrodeposited MoSx,[19] thus suggesting relatively fast HER kinetics of the PPy/MoSx films, which can be ascribed to the super high conductivity and high activity of the copolymer films as well as its good attachment to the substrate electrode.[3,14] X-ray photoelectron spectroscopy (XPS) was then employed to characterize the elemental composition and bonding configuration of the electrodeposited films. Figure S6 (Supporting Information) shows the survey spectra of the plain PPy films. An intense N peak was detected in addition to a C and O peak. Cl was also observed due to the doping of ClO4− during the electro-polymerization process. For the PPy/MoSx films, however, signals of Mo and S appear in the XPS spectrum (Figure 3a), and the relative intensity of Cl decreases, which implies that fewer ClO4− ions are doped into the film and that the molybdenum sulfide anion also acts as the counter ion during the polymerization of Py. Both Mo (IV) and Mo (VI) could be observed, as well as S with two different binding energies that represent bridging S22− and/or apical S2− and terminal S22− and/or S2− (Figure 3b,c).[7a] The XPS spectra of (NH4)2MoS4 (Figure 3d–f) and MoSx films (Figure 3g–i) were also recorded. Only Mo (IV) is observed for the (NH4)2MoS4 films and the intensities of the two types of S are nearly the same. The MoSx films also exhibit two kinds of Mo and two kinds of S, but their valent compositions seem different from those of the PPy/
© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Mater. 2014, DOI: 10.1002/adma.201400265
www.advmat.de www.MaterialsViews.com
COMMUNICATION Figure 3. XPS survey spectra for a) PPy/MoSx, d) (NH4)2MoS4, and g) MoSx. XPS spectra of the Mo 3d region for b) PPy/MoSx, e) (NH4)2MoS4, and h) MoSx. XPS spectra of the S 2p region for c) PPy/MoSx, f) (NH4)2MoS4, and i) MoSx.
MoSx films. This suggests that the bonding configurations of Mo and S in these films are different. The ratio of S to Mo for the PPy/MoSx films is calculated to be 5.0:1, which is much larger than commonly reported for MoSx (x ≈ 3)[7] and the MoSx film prepared in this experiment (2.7:1), and it is even larger than the theoretical ratio of doped MoS42−. This suggests that an extremely S-rich molybdenum sulfide structure has been formed in these films. Thus we presume that molybdenum polysulfide with more active S edge sites that is highly active for HER is produced during the electro-deposition process, similar to the structures reported before by theoretical calculation.[20] This is further verified by our experimental results, whereby the intensity of the S 2p3/2 peak at 163.4 eV, which represents bridging S22− and/or apical S2−, is significantly higher than that of the S 2p3/2 peak at 162.0 eV, which represents terminal S22− and/or S2−, compared to the ratio found in XPS spectra of (NH4)2MoS4 and MoSx films, which is also distinct from that of reported MoSx-based materials.[7] One possible reason for this phenomenon is that molybdenum polysulfide anions might be produced during the aging process.[21] They would not only serve as counter ions during the co-polymerization process, but also react at the deposition potential, producing more bridging S22− and/or apical S2−, which are considered to be active sites for HER.[7b] In order to verify the formation of molybdenum polysulfide anions, UV–vis spectroscopy was employed to characterize the mixed solution of Py and (NH4)2MoS4. Figure S7 in the Supporting Information presents the UV–vis absorption spectra for the different solutions of Py, (NH4)2MoS4, and their mixture before and after aging. The Py intensity exhibits
Adv. Mater. 2014, DOI: 10.1002/adma.201400265
an increase in the absorption spectrum after aging because of its oxidization during aging. (NH4)2MoS4 shows two intense peaks at 468 nm and 316 nm, and no change is observed after aging, which suggests that it is quite stable without the existence of Py. The freshly prepared mixture solution of Py and (NH4)2MoS4 also displays two peaks at 468 nm and 316 nm, but the intensities of these two peaks decrease and a new peak at 395 nm appears after aging. This new peak might possibly be attributed to the formation of molybdenum polysulfide anions, such as MoS92−,[21] which illustrates that there is an interaction between Py and (NH4)2MoS4. During the electro-deposition process, S-edge enrichment would be achieved because of the existence of these anions, thus forming extremely active PPy/ MoSx films for HER. This is also the reason why the highly efficient electrocatalytic PPy/MoSx films could not be obtained from the freshly prepared solution without aging. Raman characterization also confirmed that more bridging S22− is formed during the process of co-polymerization (Figure S8, Supporting Information). No peak appears for PPy in the range of 100–600 cm−1. (NH4)2MoS4 exhibits two typical bands at 455 and 475 cm−1 corresponding to the symmetric and asymmetric Mo-S vibrations, whereas MoSx shows a broad band at 540 cm−1 corresponding to the S-S vibration of S22− ligands in addition to other broad bands at 235, 350, and 450 cm−1.[22] The PPy/MoSx films also display broad bands at 450 and 550 cm−1, which can be assigned to the Mo-S and S-S vibrations, respectively, even though the signals are quite weak. The S-S vibration band is shifted to higher wavenumbers compared to that of the plain MoSx films, which attests that more bridging
© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
wileyonlinelibrary.com
3
www.advmat.de
COMMUNICATION
www.MaterialsViews.com
Figure 4. Polarization curves of the the PPy/MoSx films deposited a) at various potentials, b) for different durations at 0.75 V vs. SCE in 0.5 M H2SO4 at a scan rate of 2 mV s−1.
S22− is formed during the co-polymerization process as the typical band of bridging S22− (553 cm−1) is higher than that of terminal S22− (522 cm−1),[22] thus leading to the good HER performance of the hybrid films. All of these confirm that the highly active PPy/MoSx films for HER have a different structure compared to the precursor (NH4)2MoS4 and MoSx films reported in the literature.[7,11,13] The hybrid films exhibit a higher S to Mo ratio and more bridging S22− active sites, therefore, displaying outstanding HER activity. In order to further understand the formation of the highly active PPy/MoSx films, different deposition potentials from 0.6 to 0.9 V vs. SCE were tested, and the reaction currents were obtained (Figure S9, Supporting Information). The resulting films display different electrocatalytic activities for HER (Figure 4a), and the hybrid films deposited at 0.75 V vs. SCE show the best performance. When the potential is relatively negative (0.6 V), electrodeposition of MoSx would be the main reaction rather than the polymerization of Py, resulting in a low reaction current and a film with a different morphology (Figure S10, Supporting Information). XPS characterization (Figure S11–S13, Supporting Information) also confirmed that the relative intensity of Mo and S is much stronger than
4
wileyonlinelibrary.com
that shown in Figure 3a. These indicate that the films deposited at 0.6 V vs. SCE are more similar to MoSx films rather than to PPy/MoSx films. On the other hand, if the potential is increased to 0.8 V vs. SCE, the films do show a similar morphology compared to that of the PPy/MoSx films deposited at 0.75 V vs. SCE (Figure S14, Supporting Information), however, overoxidization occurs in the films. A new type of S ion with a higher valence could be observed, as well as an increase in Mo (VI) (Figure S15–S17, Supporting Information), thus leading to an inferior HER performance. In addition, the ratio of S to Mo for the PPy/MoSx films deposited at 0.6 or 0.8 V vs. SCE was 2.2 and 2.3:1, respectively, which is much smaller than that of the hybrid films deposited at 0.75 V vs. SCE, suggesting that S-enrichment is not achieved for these films. For even higher deposition potentials (0.90 V vs. SCE), the films do not exhibit any obvious HER activity. In short, the binding configurations of PPy/MoSx films deposited at other potentials are quite different from that of the PPy/MoSx films deposited at 0.75 V vs. SCE. Our particular PPy/MoSx films show a good conductivity and large electrochemical surface area, as well as a higher amount of active S edge sites, thus generating an extremely high catalytic activity for HER. Similarly, the concentrations of Py and (NH4)2MoS4 in the solution had an influence on the HER activity of the resulting hybrid films. Only the hybrid films that were prepared from a solution with a molar ratio of 250:1 (Py/(NH4)2MoS4) displayed a high HER activity (Figure S18, Supporting Information). Moreover, the electro-deposition time also affects the HER performance of the hybrid films, as shown in Figure 4b. A shorter deposition time led to lower active films as the loading is smaller, whereas a longer time resulted in the formation of unstably thick films with inferior activity. Therefore, 100 s was chosen as the deposition time for preparing the PPy/MoSx films. These phenomena reveal that the aging process that produces molybdenum polysulfide anions is not the only key factor for the construction of the highly active PPy/ MoSx films. Although we managed to fabricate PPy/MoSx films with an unique structure, the formation mechanism of these films is still unclear, because the formation conditions are complicated, as they involve the interaction of Py and (NH4)2MoS4 in solution, the competitive reaction between the polymerization of Py and the deposition of MoSx,[18] as well as the reconstruction of the films during the process of electro-polymerization. The stability of the PPy/MoSx films was also evaluated. Unfortunately, its stability was not very good without the protection of Nafion. The HER activity would be lost after several cycles in 0.5 M H2SO4 as the composition of the films readily changed in this electrolyte. Figure 5a and Figure S19–S20 (Supporting Information) show the XPS spectra of the PPy/ MoSx films after polarization in 0.5 M H2SO4. It can be seen that SO42− is incorporated into the films (Figure 5a) and the ratio of S (lower valence) to Mo decreases to 3.4:1. However, by modifying with Nafion, the stability of the PPy/MoSx films for HER improved greatly. Only a slight decrease in current was observed after 1000 cycles from 0.54 to –0.04 V vs. RHE at a scan rate of 100 mV s−1 (Figure 5b). These results also imply the existence of molybdenum polysulfide anions with a high ratio of S to Mo in the PPy/MoSx films, which results in the high HER activity of the films, as Nafion could hinder the dissolution of anions from the film into the solution. Durability tests
© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Mater. 2014, DOI: 10.1002/adma.201400265
www.advmat.de www.MaterialsViews.com
COMMUNICATION
PPy/MoSx film can be attributed to the following three aspects: i) PPy is a well-known conducting polymer that easily forms porous structures. Therefore, PPy offers the good conductivity and large electrochemical surface area of the PPy/MoSx films; ii) the electro-deposition technique provides the good attachment of the PPy/MoSx films to the substrate electrode; iii) the interaction between Py and (NH4)2MoS4 results in the formation of a film with a high S to Mo ratio (5.0:1) with more active S edge sites forming during the polymerization process and after aging. This research proposes a new scheme to construct highly efficient MoSx-based electrocatalysts for HER. Further work is needed to elucidate the electro-deposition mechanism.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements This work was financially supported by the National Natural Science Foundation of China (Grants 21075003 and 21275010). Received: January 17, 2014 Revised: February 14, 2014 Published online:
Figure 5. a) XPS spectra of the S 2p region for the PPy/MoSx films electrodeposited at 0.75 V vs. SCE after polarization in 0.5 M H2SO4 without the protection of Nafion. b) Stability test of the PPy/MoSx films protected with Nafion using a cyclic potential scanning from 0.54 to –0.04 V vs. RHE in 0.5 M H2SO4 at a scan rate of 100 mV s−1.
over longer periods of time of the PPy/MoSx films for HER, however, revealed that the stability of our films is not so good as compared to reported MoS2-based materials.[13b] Although the HER current density easily reached tens of mA cm−2 even at a potential of –0.05 V vs. RHE because of the high activity of the catalyst, an obvious decrease in current was observed after about 5000 s with chronoamperometry, as shown in Figure S21 (Supporting Information). A possible reason for this is that the Nafion film is damaged by the large amounts of bubbles produced during the hydrogen evolution reaction. This means that more research is needed to improve the stability of the catalyst. In conclusion, we have fabricated an extremely active PPy/ MoSx structure for HER with Py and (NH4)2MoS4 as the reactants by electrochemical copolymerization. Although the mechanism for the formation of the PPy/MoSx films is still unclear, the films demonstrate outstanding HER performance. A small Tafel slope of 29 mV dec−1, a positive onset potential of 0 V vs. RHE, and a high exchange-current density of 5.6 × 10−4 A cm−2 were observed. These are the best results reported to date for MoSx-based materials, and they are comparable to those of commercial Pt/C catalysts. The outstanding HER activity of the
Adv. Mater. 2014, DOI: 10.1002/adma.201400265
[1] a) M. S. Dresselhaus, I. L. Thomas, Nature 2001, 414, 332; b) N. S. Lewis, D. G. Nocera, Proc. Natl. Acad. Sci. USA 2006, 103, 15 729. [2] a) M. G. Walter, E. L. Warren, J. R. McKone, S. W. Boettcher, Q. X. Mi, E. A. Santori, N. S. Lewis, Chem. Rev. 2010, 110, 6446; b) J. A. Turner, Science 2004, 305, 972. [3] A. B. Laursen, S. Kegnæs, S. Dahl, I. Chorkendorff, Energy Environ. Sci. 2012, 5, 5577. [4] a) Q. J. Xiang, J. G. Yu, M. Jaroniec, J. Am. Chem. Soc. 2012, 134, 6575; b) D. Merki, X. L. Hu, Energy Environ. Sci. 2011, 4, 3878; c) M. L. Tang, D. C. Grauer, B. Lassalle-Kaiser, V. K. Yachandra, L. Amirav, J. R. Long, J. Yano, A. P. Alivisatos, Angew. Chem. Int. Ed. 2011, 50, 10 203; d) J. D. Benck, Z. B. Chen, L. Y. Kuritzky, A. J. Forman, T. F. Jaramillo, ACS Catal. 2012, 2, 1916; e) J. Kibsgaard, Z. B. Chen, B. N. Reinecke, T. F. Jaramillo, Nat. Mater. 2012, 11, 963; f) B. Seger, A. B. Laursen, P. C. K. Vesborg, T. Pedersen, O. Hansen, S. Dahl, I. Chorkendorff, Angew. Chem. Int. Ed. 2012, 51, 9128. [5] B. Hinnemann, P. G. Moses, J. Bonde, K. P. Jørgensen, J. H. Nielsen, S. Horch, I. Chorkendorff, J. K. Nørskov, J. Am. Chem. Soc. 2005, 127, 5308. [6] T. F. Jaramillo, J. Bonde, J. D. Zhang, B. L. Ooi, K. Andersson, J. Ulstrup, I. Chorkendorff, J. Phys. Chem. C 2008, 112, 17 492. [7] a) D. Merki, S. Fierro, H. Vrubel, X. L. Hu, Chem. Sci. 2011, 2, 1262; b) Y. H. Chang, C. T. Lin, T. Y. Chen, C. L. Hsu, Y. H. Lee, W. J. Zhang, K. H. Wei, L. J. Li, Adv. Mater. 2013, 25, 756; c) H. Vrubel, X. L. Hu, ACS Catal. 2013, 3, 2002. [8] a) H. I. Karunadasa, E. Montalvo, Y. J. Sun, M. Majda, J. R. Long, C. J. Chang, Science 2012, 335, 698; b) A. M. Appel, D. L. DuBois, M. R. DuBois, J. Am. Chem. Soc. 2005, 127, 12 717. [9] a) F. K. Meng, J. T. Li, S. K. Cushing, M. J. Zhi, N. Q. Wu, J. Am. Chem. Soc. 2013, 135, 10 286; b) Y. G. Li, H. L. Wang, L. M. Xie, Y. Y. Liang, G. S. Hong, H. J. Dai, J. Am. Chem. Soc. 2011, 133,
© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
wileyonlinelibrary.com
5
www.advmat.de
COMMUNICATION
www.MaterialsViews.com
[10]
[11]
[12]
[13]
6
7296; c) I. Hatay, P. Y. Ge, H. Vrubel, X. L. Hu, H. H. Girault, Energy Environ. Sci. 2011, 4, 4246; d) M. A. Lukowski, A. S. Daniel, F. Meng, A. Forticaux, L. S. Li, S. Jin, J. Am. Chem. Soc. 2013, 135, 10 274. a) T. F. Jaramillo, K. P. Jorgensen, J. Bonde, J. H. Nielsen, S. Horch, I. Chorkendorff, Science 2007, 317, 100; b) T. Y. Wang, L. Liu, Z. W. Zhu, P. Papakonstantinou, J. B. Hu, H. Y. Liu, M. X. Li, Energy Environ. Sci. 2013, 6, 625. a) H. Vrubel, D. Merki, X. L. Hu, Energy Environ. Sci. 2012, 5, 6136; b) L. Liao, J. Zhu, X. J. Bian, L. N. Zhu, M. D. Scanlon, H. H. Girault, B. H. Liu, Adv. Funct. Mater. 2013, 23, 5326. a) D. Merki, H. Vrubel, L. Rovelli, S. Fierro, X. L. Hu, Chem. Sci. 2012, 3, 2515; b) P. D. Tran, M. Nguyen, S. S. Pramana, A. Bhattacharjee, S. Y. Chiam, J. Fize, M. J. Field, V. Artero, L. H. Wong, J. Loo, J. Barber, Energy Environ. Sci. 2012, 5, 8912. a) A. B. Laursen, P. C. K. Vesborg, I. Chorkendorff, Chem. Commun. 2013, 49, 4965; b) J. F. Xie, H. Zhang, S. Li, R. X. Wang, X. Sun, M. Zhou, J. F. Zhou, X. W. Lou, Y. Xie, Adv. Mater. 2013, 25, 5807; c) W. F. Chen, J. T. Muckerman, E. Fujita, Chem. Commun. 2013, 49, 8896.
wileyonlinelibrary.com
[14] a) E. Frackowiak, F. Beguin, Carbon 2002, 40, 1775; b) R. Bashyam, P. Zelenay, Nature 2006, 443, 63. [15] a) S. Sadki, P. Schottland, N. Brodie, G. Sabouraud, Chem. Soc. Rev. 2000, 29, 283; b) D. Bélanger, G. Laperrière, F. Girard, D. Guay, G. Tourillon, Chem. Mater. 1993, 5, 861. [16] J. R. Mckone, E. L. Warren, M. J. Bierman, S. W. Boettcher, B. S. Brunschwig, N. S. Lewis, H. B. Gray, Energy Environ. Sci. 2011, 4, 3573. [17] S. Cobo, J. Heidkamp, P. A. Jacques, J. Fize, V. Fourmond, L. Guetaz, B. Jousselme, V. Ivanova, H. Dau, S. Palacin, M. Fontecave, V. Artero, Nat. Mater. 2012, 11, 802. [18] S. Lavallée, G. Laperrière, D. Bélanger, J. Electroanal. Chem. 1997, 431, 219. [19] H. Vrubel, T. Moehl, M. Grätzel, X. L. Hu, Chem. Commun. 2013, 49, 8985. [20] B. Wang, N. Wu, X. B. Zhang, X. Huang, Y. F. Zhang, W. K. Chen, K. N. Ding, J. Phys. Chem. A 2013, 117, 5632. [21] M. Draganjac, E. Simhon, L. T. Chan, M. Kanatzidis, N. C. Baenziger, D. Coucouvanis, Inorg. Chem. 1982, 21, 3321. [22] T. Weber, J. C. Muijsers, J. W. Niemantsverdriet, J. Phys. Chem. 1995, 99, 9194.
© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Mater. 2014, DOI: 10.1002/adma.201400265
