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Graphene nanoribbons hybridized carbon nanofibers: remarkably enhanced graphitization and conductivity, and excellent performance as support material for fuel cell catalysts† Chaonan Wang,a Hongrong Gao,a Hong Li,a Yiren Zhang,a Bowen Huang,a Junhong Zhao,b Yan Zhu,a Wang Zhang Yuan*a and Yongming Zhang*a High electronic conductivity of the support material and uniform distribution of the catalyst nanoparticles (NPs) are extremely desirable for electrocatalysts. In this paper, we present our recent progress on electrocatalysts for fuel cells with simultaneously improved conductivity of the supporting carbon nanofibers (CNFs) and distribution of platinum (Pt) NPs through facile incorporation of graphene nanoribbons (GNRs). Briefly, GNRs were obtained by the cutting and unzipping of multiwalled carbon nanotubes (MWCNTs) and subsequent thermal reduction and were first used as novel nanofillers in CNFs towards high performance support material for electrocatalysis. Through electrospinning and carbonization processes, GNR embedded carbon nanofibers (G–CNFs) with greatly enhanced graphitization and electronic conductivity were synthesized. Chemical deposition of Pt NPs onto
Received 2nd September 2013 Accepted 28th October 2013
G–CNFs generated a new Pt–G–CNF hybrid catalyst, with homogeneously distributed Pt NPs of 3 nm. Compared to Pt–CNF (Pt on pristine CNFs) and Pt–M–CNF (Pt on MWCNT embedded CNFs), Pt–G–CNF hybrids exhibit significantly improved electrochemically active surface area (ECSA), better CO tolerance for
DOI: 10.1039/c3nr04663j
electro-oxidation of methanol and higher electrochemical stability, testifying G–CNFs are promising
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support materials for high performance electrocatalysts for fuel cells.
1. Introduction Fuel cells (FCs) have played an indispensable role as transportation vehicles for fuel portability, high energy density, zero emissions, relatively low operating temperature, and minimal corrosion problems.1–5 Despite enormous progress in FCs, development of excellent catalysts with minimal loading of platinum (Pt) while maximizing catalytic activity, carbon monoxide (CO) tolerance along with wonderful durability is still one of the core challenges.6–10 Exploration of suitable support materials has proven to be an effective strategy to achieve high performance catalysts. Support materials are of crucial importance in regulating such properties of catalyst nanoparticles (NPs) as shape, size, and dispersion, thus greatly inuencing the fuel cell performances.4,11,12 Generally, proper catalyst support materials must have large specic surface areas to support catalyst NPs, high electrical conductivity to promote a
School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, No. 800 Dongchuan Rd, Minhang District, Shanghai, China. E-mail: [email protected]. cn; [email protected]; Fax: +86-21-54742567; Tel: +86-21-34202613
b
School of Chemistry and Chemical Engineering, Anyang Normal University, No. 65 South Ring Rd, Anyang 455002, China † Electronic supplementary information (ESI) available: Experimental Section; IR; Raman spectra. See DOI: 10.1039/c3nr04663j
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fast electron transfer, strong affinity towards the NPs to immobilize them, and excellent stability under the operating conditions to afford stable catalyst structures.12 Currently, carbon black (CB) is the most widely used catalyst support in FCs.12 However, such negative problems as the electrochemical instability, thermochemical oxidation, the presence of sulfur impurities, and the formation of microporous structures preventing easy access of the reactants, bring long-term degradation of the catalyst performance.13 Accordingly, various support nanomaterials, such as carbon nanotubes (CNTs) and graphene sheets, have attracted tremendous interest due to their higher catalyst loading efficiency, better durability, higher electrical conductivity and lower impurities.14–20 However, these materials also show the disadvantages of undesirable bundling or enfoldment, incomplete functionalization, unsatisfactory CO tolerance, etc. In the past decades, electrospun carbon nanobers (CNFs) have attracted extensive attention due to their remarkable porous structure, close contact between each other and high specic surface area.21–25 The catalyst particles deposited on CNFs can fulll the requirements of proton transfer, reactant access and electronic continuity for fuel cells.21 Specically, compared with traditional chemical vapor deposition (CVD) and vapor growth methods, electrospinning is simple,
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inexpensive and can be used to fabricate various controllable structures at relatively high speeds.26,27 However, electrospun CNFs are mostly produced by carbonization of their polymer precursors, when treated at relatively low temperatures (e.g. 900 C), they normally exhibit quite limited graphitic structures and thus poor electronic conductivity. High temperature carbonization (e.g. 3000 C) is effective to enhance the graphitic structure of CNFs.28,29 However, it requires specialized and expensive equipment and high vacuum operation conditions. Particularly, with increasing temperature, nitrogen components would be dramatically lost, consequently resulting in decreased CO tolerance and durability of Pt catalysts.30–33 Therefore, it is of signicant importance to improve the graphitic structures of CNFs at lower carbonization temperatures in terms of application performance as well as economic factors. To achieve higher conductivity of the CNF support material and better catalytic performance of Pt NPs, in this study, for the rst time, graphene nanoribbons (GNRs) were incorporated into CNFs to prepare G–CNF composites. Graphene oxide nanoribbons (GONRs), which were prepared according to Tour's method,34,35 were initially mixed with polyacrylonitrile (PAN). Then G–CNFs were fabricated through electrospinning and carbonization. Herein, utilization of GONRs is based on the following considerations: (i) they have better dispersibility than GNRs due to the presence of oxygen-containing groups and defects;36,37 (ii) aer carbonization, the residual oxygenated groups are helpful to improve the dispersion as well as CO tolerance of Pt NPs.38 For comparison, Pt NPs on pristine CNFs and MWCNTdoped CNFs (Pt–CNF and Pt–M–CNF) were also prepared. The resultant Pt–G–CNF hybrid catalysts show signicantly increased electrochemical active surface area (ECSA), remarkably enhanced current density, electrochemical stability and moreover CO tolerance when compared to Pt–CNF and Pt–M– CNF hybrids due to their improved graphitization, better electronic conductivity, as well as more uniform distribution of Pt NPs. Moreover, the fraction of expensive Pt NPs could be lowered to 28 wt%. These results clearly indicate that G–CNFs can be used as efficient support materials for high performance electrocatalysts.
2.
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Scheme 1
Preparation route to Pt–G–CNF hybrids.
of the H2PtCl6–G–CNFs mixture utilizing sodium borohydride (NaBH4). Successful preparation of GONRs was conrmed by transmission electron microscopy (TEM) observation. Whereas original MWCNTs show distinct inner and outer wall surfaces (Fig. 1A), upon oxidation, thin GONRs with an average width of 100 nm were obtained, providing signicantly increased accessible area (Fig. 1B and C). The resulting GONRs exhibit excellent dispersibility in polar solvents, such as water, DMF, ethylene glycol (EG), tetrahydrofuran (THF) and N-methyl2-pyrrolidone (NMP). As depicted in Fig. 1D, while MWCNTs are insoluble and precipitated from DMF, GONRs are highly dispersible in DMF, forming a homogenous solution, which can stand for months without precipitation. Such high solvating power of GONRs is attributed to the introduction of functional
Results and discussion
Pt–G–CNF hybrids were prepared according to the route shown in Scheme 1. Briey, GONRs were obtained by the cutting or unzipping of MWCNTs with potassium permanganate (KMnO4) in concentrated sulfuric acid (H2SO4) according to Tour's method.34,35 The resultant GONRs were highly soluble in polar solvents due to the introduction of a number of functional hydroxyl (–OH) and carboxyl (–COOH) groups. Thanks to the excellent dispersibility of GONRs, homogenous GONR–PAN in dimethylformamide (DMF) was obtained. Further electrospinning generated continuous long GONR–PAN (G–PAN) nanobers, which could be converted into G–CNFs via facile carbonization at 900 C. Finally, the resulting Pt–G–CNF hybrids were successfully obtained through chemical reduction
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Fig. 1 TEM images of (A) MWCNTs, (B and C) GONRs and (D) the photographs of MWCNTs and GONRs in DMF.
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oxygen-containing groups, which can be identied from the IR spectrum with stretching vibrations at 1176/1058 (epoxy group, C–O–C), 1742 (–COOH) and 3400/1225 cm1 (–OH) (ESI, Fig. S1†). GONRs can be homogenously mixed with PAN in DMF owing to their good dispersibility. Further electrospinning and subsequent carbonization yielded GONR–PAN and G–CNF composite nanober membranes, respectively. As shown in Fig. 2, with increasing GONR content, colour of the GONR–PAN membrane gradually changes from white to grey-white and then to grey. Notably, the membrane containing 2 wt% MWCNTs (M2–PAN) is even darker than G5–PAN (containing 5 wt% GONRs). It is reasonable that the oxygen-containing functional groups of GONRs have changed the band-gap and optical properties of the MWCNT precursor. ATIR and Raman spectra of the electrospun nanobers verify the successful preparation of such hybrids (ESI, Fig. S1 and S2†). To obtain conductive materials, the membranes must be carbonized. First, preoxidative stabilization at 280 C was carried out to keep the morphology of the nanobers, which yielded brown oxidized membranes. Then carbonization at 900 C under argon generated the resultant black composite CNFs (Fig. 2). Fig. 3 gives typical scanning electron microscopy (SEM) images of electrospun PAN, M2–PAN, G2–PAN (containing 2 wt % GONRs) and their corresponding carbonized counterparts. Clearly, no distinct difference is observed between the morphology of pure PAN and G2–PAN expect the increased diameter of the G2–PAN nanobers owing to the increased solution viscosity by incorporation of GONRs. However, dissimilarly, nanobers with defects and uneven diameters were formed in M2–PAN, which should be ascribed to the poor dispersion of MWCNTs in the PAN–DMF solution. Upon carbonization, smooth and continuous CNFs were obtained from PAN with decreased diameter. Similarly, ultrathin, smooth and uniform G2–CNFs were formed with few deformations and clusters (marked with oval circles). For M2–CNFs, however, large amounts of agglomerations, clusters, fractures and bending distortions are present. These results suggest that GONR–PAN– DMF possesses comparable electrospinning ability to that of PAN–DMF, but far better than that of MWCNT–PAN–DMF, owing to the excellent dispersibility of GONRs. Further structural information and the crystal phase of the nanobers were characterized by X-ray diffraction (XRD). While
Fig. 2 Photographs of varying electrospun nanofiber membranes and their preoxidized and carbonized counterparts. G(M)x–PAN represents a membrane with x wt% GONRs (MWCNTs).
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Fig. 3 SEM images of electrospun nanofibers of (A) PAN, (B) G2–PAN and (C) M2–PAN, and their carbonized counterparts of (D) CNFs, (E) G2–CNFs and (F) M2–CNFs. (G–I) are enlarged images in (D–F). The scale bars in (A–C), (D–F) and (G–I) are 5, 10 and 2 mm, respectively.
MWCNTs show a typical peak at 2q ¼ 26.0 , corresponding to the interlayer space of 0.34 nm, oxidation brings lots of oxygen functionalities and defects to the c-plane of the ribbon layers, resulting in an increased interlayer spacing of 0.89 nm (2q ¼ 9.9 , Fig. 4). G2–PAN hybrids show a similar XRD pattern to that of PAN: no peak at 2q ¼ 9.9 is observed, suggesting GONRs are well dispersed in PAN; meanwhile, the crystalline peak at 2q ¼ 16.7 becomes blunt and a broad amorphous halo at approximately 2q ¼ 25.9 gets intensied, indicating the introduction of GONRs does not signicantly change the structure of PAN nanobers. During carbonization, most oxygen functionalities
Fig. 4 XRD patterns of MWCNTs, GONRs and the electrospun
nanofibers of pristine PAN, M2–PAN, G2–PAN and their corresponding carbonized counterparts of CNFs, M2–CNFs and G2–CNFs.
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and defects were removed and GONRs were thermally reduced into GNRs. The obtained G2–CNFs show a strong and a relatively weak peak at 2q ¼ 26.0 and 43.2 , which correspond to the crystalline (002) and (100) diffractions of graphite, respectively, implying the graphitization of CNFs. Moreover, when MWCNTs or GONRs are incorporated, the graphite peak at 26.0 is remarkably intensied, indicating a signicantly improved graphitization of the CNFs. Further careful inspection reveals that graphitization of G2–CNF is much larger compared to that of M2–CNF as indicated by its even stronger peak. Raman analysis shows that the ratio of the integrated area of the D band to G band (ID/IG) is gradually decreased from 3.7 to 3.1 and then to 2.4 for CNF, M2–CNF and G2–CNF (ESI, Fig. S3†), thus testifying the enhanced graphitic structure of CNFs through incorporation of MWCNTs and GNRs. Though the detailed mechanism of the enhancement of graphitization is not yet clear, there are several considerations: (i) the intrinsic high graphitization of GNRs and MWCNTs; (ii) the ordering or graphitization of GONRs (from GONR to GNR) may have an inducing effect on the ordering of CNFs. However, exploration of the exact mechanism is still in progress. The greatly improved graphitization of CNFs may increase their electronic conductivity, which is of signicant importance to the support materials for the electrocatalysts. The electronic conductivity of various CNFs was measured and the results are given in Fig. 5. As expected, while the conductivity of pristine CNFs is 655 S m1, incorporation of both GNRs and MWCNTs boost it, giving values of 870, 795, 1306, 1635, 1757 and 1878 S m1 for M2–CNFs and Gx–CNFs with 0.5, 1, 2, 3 and 5 wt% GNRs, respectively. Notably, the conductivity enhancement of G2–CNFs is far higher than that of M2–CNFs, indicating GNRs perform better as nanollers than MWCNTs towards highly conductive CNFs. It may be ascribed to the limited enhancement capacity of MWCNTs on graphitization. Meanwhile, MWCNTs cause more defects, fractures and serious distortion, which are also unfavorable to the conductivity. Excellent conductivity of the hybrid G2–CNFs prompted us to fabricate electrocatalysts utilizing them as support materials.
Fig. 5 Electronic conductivity of the CNFs with different contents of GNRs or MWCNTs.
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Typically, Pt NPs were deposited on CNFs through in situ reduction, with 28 or 40 wt%. Morphologies of the hybrids were observed by TEM. Whereas Pt NPs are loosely bound on pristine CNFs with a number of aggregates (Fig. 6A), those on G2–CNFs are homogenously and more densely distributed with a mean diameter of 3 nm (Fig. 6B). It is reported that metal NPs are prone to be anchored on the defects of carbon materials.39,40 The much denser distribution of Pt NPs with fewer aggregates on G2–CNFs than on pristine CNFs might be ascribed to the residual defects on GNRs and moreover the defects caused by the introduction of homogenously dispersed GNRs. Similarly, MWCNTs also bring defects in CNFs, thus making CNFs populated by Pt NPs (Fig. 6C). However, short and broken M2–CNF hybrids are formed with deformations due to the presence of long and entangled MWCNTs (Fig. 6C and D). Meanwhile, MWCNTs with seriously aggregated Pt NPs are found at the breaks of CNFs (Fig. 6E). XRD patterns of the resultant hybrids show strong diffraction peaks at 2q ¼ 40.2 , 46.3 , 67.8 and 81.2 , which are assignable to typical face-centered-cubic (111), (200), (220) and (311) crystalline planes of Pt (Fig. 6F), respectively. The (220) peaks were used to calculate the size of Pt NPs according to Scherrer's equation,41,42 giving the average diameters of 5.6, 4.1 and 4.5 nm for those on CNFs, G2–CNFs and M2–CNFs, respectively. Good dispersion, few aggregates and proper size of Pt NPs on G2–CNFs would bestow them with outstanding catalytic performance. Normally, high conductivity of the support material, small size and homogenous distribution of the catalyst NPs are in favor of good catalytic performance.12,43 Therefore, Pt–G2–CNF and Pt–M2–CNF are expected to display better electrocatalytic performance than Pt–CNF. ECSA is an important parameter to evaluate the electrochemically active sites of the electrocatalysts,44,45 which can be used to compare the performance of varying support materials. ECSA values of various catalysts were measured by cyclic voltammetry (CV) tests. As indicated in Fig. 7A, with the same Pt content of 28 wt%, Pt–G2–CNF exhibits much larger hydrogen adsorption and desorption current peaks than Pt–CNF and Pt–M2–CNF, thus giving a higher ESCA value of 123.1 m2 g1 compared to 56.4 and 74.8 m2 g1 for Pt–CNF and Pt–M2–CNF. With increased Pt loading of 40 wt%, Pt–G2– CNF (40) gives even larger hydrogen adsorption and desorption peaks and area, but with a lowered ECSA value of 110.7 m2 g1, indicating the growing proportion of active surface area is less than that of the catalyst weight. To further test the catalytic activity of varying systems, CV measurements for methanol electrooxidation were carried out. The CV curves show a methanol oxidation current peak in the forward scan and the other oxidation peak in the backward scan. The latter refers to the removal of residual carbonaceous species formed in the forward scan. The peak current density (Im-peak) of Pt–G2–CNF is 560 mA mg1, which is 1.8 and 1.4-fold compared with those of Pt–CNF (311 mA mg1) and Pt–M2–CNF (412 mA mg1) (Fig. 7B). The result clearly suggests that incorporation of MWCNTs and GNRs is helpful to improve the electrocatalytic activity of Pt NPs. Moreover, GNRs perform even better than MWCNTs. The good performance of Pt–G2–
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Fig. 6 TEM images of (A) Pt–CNF, (B) Pt–G2–CNF, (C and E) Pt–M2–CNF and (D) M2–CNF hybrids. (F) XRD patterns of Pt–CNF, Pt–G2–CNF and Pt–M2–CNF. Pt content in the hybrids is 28 wt%.
CNF should be ascribed to the dense and homogenous distribution of relatively small Pt NPs, as well as the high conductivity of G2–CNFs. Strikingly, with a much higher Pt content, the Impeak of Pt–G2–CNF (40) is slightly dropped to 534.2 mA mg1, indicating the remarkably high catalytic activity of Pt–G2–CNF no matter the catalyst loading. Besides high electrocatalytic activity, excellent tolerance to CO toxicity is also indispensible for outstanding
electrocatalysts. Generally, the ratio of the forward to backward anodic peak current (If/Ib) is used to evaluate the CO tolerance. Higher If/Ib values mean better tolerance to intermediate carbon species, and methanol can be oxidized into carbon dioxide more efficiently.46,47 The If/Ib value of Pt–G2–CNF is calculated as 0.99, which is much higher than those of Pt–M2–CNF (0.76) and Pt–CNF (0.86), suggesting Pt–G2–CNF has less intermediate carbon species
CV curves of Pt–CNF, Pt–G2–CNF and Pt–M2–CNF (28 wt% Pt) and Pt–G2–CNF (40) (40 wt% Pt) catalysts measured in (A) 0.5 M H2SO4 and (B) H2SO4–CH3OH (0.5–1 M) aqueous solutions at a scan rate of 50 mV s1 (10 ml of catalyst ink). Fig. 7
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accumulation, better CO tolerance and better catalytic stability. It seems that the If/Ib value and thus the CO tolerance are not signicantly affected by the Pt content, as Pt–G 2–CNF (40) gives the same value of 0.99 as that of Pt–G2–CNF. To get more insights into the catalytic activity of methanol electrooxidation, linear sweep voltammograms (LSV) were obtained. Pt–G2–CNF shows superior catalytic activity as compared to Pt–M2–CNF and Pt–CNF, as indicated by its highest current density (Fig. 8A), which agrees well with the above CV results. It is reasonable that G2–CNFs have much higher electronic conductivity and larger specic area, more homogenous and denser distribution of Pt NPs with proper size and fewer aggregates, which synergistically provide better catalytic conditions for Pt NPs. The current density of Pt–G2–CNF (40) is even greater than that of Pt–G2–CNF, suggesting its remarkably outstanding activity. Further chronoamperometric (CA) tests were conducted to evaluate the long-term performance of Pt–G2 –CNF catalysts towards the methanol oxidation reaction (MOR). For comparison, those of Pt–M2–CNF, Pt–CNF and commercial Pt–XC72R (40 wt% Pt) are also given out (Fig. 8B). Originally, the potentiostatic current is rapidly decreased for all hybrids, potentially owing to the formation of intermediate species during the MOR processes, such as CH 3OHads, CHOads and COads .48 However, the methanol oxidation current of Pt–G2–CNF remains at a much higher level than those of Pt–M2–CNF and Pt–CNF during the current–time test, suggesting its higher mass activity. Meanwhile, it reveals that the effect of intermediate species on current is relatively stable during the whole decay process. It is also noticeable that Pt–G 2–CNF, Pt–M2–CNF and Pt–CNF show superior catalyst stability to that of the commercial Pt– XC72R catalyst (40 wt% Pt), even though they possess much lower Pt contents of 28 wt%. Meanwhile, initially, Pt–G 2–CNF (40) shows slightly lower methanol oxidation currents than
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Pt–G2–CNF, but much higher currents aer 100 s, thus implying its stronger catalyst stability. Apparently, Pt NPs on G2–CNF with different loadings of 28 and 40 wt% both exhibit excellent catalyst stability, thus testifying G 2– CNF hybrids are appropriate support materials for electrocatalysts. All the CV, LSV and CA experiments consistently indicate that both Pt–G2–CNF and Pt–G2–CNF (40) have excellent catalyst activity and stability. Such high electrochemical activity and stability can be explained in terms of highly dispersed Pt NPs (3 nm) and the greatly enhanced graphitization and thus electronic conductivity of the CNFs. Additionally, the residual oxygenated groups on GNRs have promoted the CO tolerance of the catalysts. Considering the excellent performance of the catalysts based on Pt NPs and G–CNFs, G–CNFs are highly promising support materials for next generation electrocatalysts.
3.
Conclusions
GNRs are rst used as novel nanollers for CNFs to support Pt NPs in FCs. The electronic conduction and graphitization degree of support materials show a huge improvement. Pt NPs with proper particle sizes of 3 nm are highly-dispersed on graphene nanoribbons embedded carbon nanobers. Compared to the Pt–M2–CNF and Pt–CNF catalysts, Pt–G2–CNF hybrids show much higher ECSA and better tolerance towards CO toxicity, indicating excellent catalytic activity and long term stability. The ECSA of G2–CNF (40) was slightly smaller than G2–CNF, while shows signicantly improved mass current density and the same better tolerance towards CO toxicity and long term stability. Considering the facile preparation, reasonable cost, and moreover excellent performance, GNRs are quite promising graphitization and electronic conductivity additives for CNFs to improve catalyst activity and reduced the precious metal loading, which can effectively reduce the cost of catalysts and FCs.
Fig. 8 A comparison of methanol oxidation activity of Pt–CNF, Pt–G2–CNF, Pt–M2–CNF (28 wt% Pt) and Pt–G2–CNF (40) (40 wt% Pt) catalysts based on (A) linear sweep voltammograms and (B) current–time curves. In (B), the data of commercial Pt–XC72R (40 wt% Pt) catalyst are also given for comparison (10 ml of catalyst ink).
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Acknowledgements
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This work was nancially supported by the National Natural Science Foundation of China (21104044), the State Hi-tech Research Development Plan of China (863 Project, 2012AA1106015), the National Key Technology R&D Program (2011BAE08B02), and the Shanghai Leading Academic Discipline Project (B202). W. Z. Y. thanks the Start-up Foundation and SMC-Chenxing Young Scholar Program of Shanghai Jiao Tong University.
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Graphene nanoribbons hybridized carbon nanofibers: remarkably enhanced graphitization and conductivity, and excellent performance as support material for fuel cell catalysts† Chaonan Wang,a Hongrong Gao,a Hong Li,a Yiren Zhang,a Bowen Huang,a Junhong Zhao,b Yan Zhu,a Wang Zhang Yuan*a and Yongming Zhang*a High electronic conductivity of the support material and uniform distribution of the catalyst nanoparticles (NPs) are extremely desirable for electrocatalysts. In this paper, we present our recent progress on electrocatalysts for fuel cells with simultaneously improved conductivity of the supporting carbon nanofibers (CNFs) and distribution of platinum (Pt) NPs through facile incorporation of graphene nanoribbons (GNRs). Briefly, GNRs were obtained by the cutting and unzipping of multiwalled carbon nanotubes (MWCNTs) and subsequent thermal reduction and were first used as novel nanofillers in CNFs towards high performance support material for electrocatalysis. Through electrospinning and carbonization processes, GNR embedded carbon nanofibers (G–CNFs) with greatly enhanced graphitization and electronic conductivity were synthesized. Chemical deposition of Pt NPs onto
Received 2nd September 2013 Accepted 28th October 2013
G–CNFs generated a new Pt–G–CNF hybrid catalyst, with homogeneously distributed Pt NPs of 3 nm. Compared to Pt–CNF (Pt on pristine CNFs) and Pt–M–CNF (Pt on MWCNT embedded CNFs), Pt–G–CNF hybrids exhibit significantly improved electrochemically active surface area (ECSA), better CO tolerance for
DOI: 10.1039/c3nr04663j
electro-oxidation of methanol and higher electrochemical stability, testifying G–CNFs are promising
www.rsc.org/nanoscale
support materials for high performance electrocatalysts for fuel cells.
1. Introduction Fuel cells (FCs) have played an indispensable role as transportation vehicles for fuel portability, high energy density, zero emissions, relatively low operating temperature, and minimal corrosion problems.1–5 Despite enormous progress in FCs, development of excellent catalysts with minimal loading of platinum (Pt) while maximizing catalytic activity, carbon monoxide (CO) tolerance along with wonderful durability is still one of the core challenges.6–10 Exploration of suitable support materials has proven to be an effective strategy to achieve high performance catalysts. Support materials are of crucial importance in regulating such properties of catalyst nanoparticles (NPs) as shape, size, and dispersion, thus greatly inuencing the fuel cell performances.4,11,12 Generally, proper catalyst support materials must have large specic surface areas to support catalyst NPs, high electrical conductivity to promote a
School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, No. 800 Dongchuan Rd, Minhang District, Shanghai, China. E-mail: [email protected]. cn; [email protected]; Fax: +86-21-54742567; Tel: +86-21-34202613
b
School of Chemistry and Chemical Engineering, Anyang Normal University, No. 65 South Ring Rd, Anyang 455002, China † Electronic supplementary information (ESI) available: Experimental Section; IR; Raman spectra. See DOI: 10.1039/c3nr04663j
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fast electron transfer, strong affinity towards the NPs to immobilize them, and excellent stability under the operating conditions to afford stable catalyst structures.12 Currently, carbon black (CB) is the most widely used catalyst support in FCs.12 However, such negative problems as the electrochemical instability, thermochemical oxidation, the presence of sulfur impurities, and the formation of microporous structures preventing easy access of the reactants, bring long-term degradation of the catalyst performance.13 Accordingly, various support nanomaterials, such as carbon nanotubes (CNTs) and graphene sheets, have attracted tremendous interest due to their higher catalyst loading efficiency, better durability, higher electrical conductivity and lower impurities.14–20 However, these materials also show the disadvantages of undesirable bundling or enfoldment, incomplete functionalization, unsatisfactory CO tolerance, etc. In the past decades, electrospun carbon nanobers (CNFs) have attracted extensive attention due to their remarkable porous structure, close contact between each other and high specic surface area.21–25 The catalyst particles deposited on CNFs can fulll the requirements of proton transfer, reactant access and electronic continuity for fuel cells.21 Specically, compared with traditional chemical vapor deposition (CVD) and vapor growth methods, electrospinning is simple,
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inexpensive and can be used to fabricate various controllable structures at relatively high speeds.26,27 However, electrospun CNFs are mostly produced by carbonization of their polymer precursors, when treated at relatively low temperatures (e.g. 900 C), they normally exhibit quite limited graphitic structures and thus poor electronic conductivity. High temperature carbonization (e.g. 3000 C) is effective to enhance the graphitic structure of CNFs.28,29 However, it requires specialized and expensive equipment and high vacuum operation conditions. Particularly, with increasing temperature, nitrogen components would be dramatically lost, consequently resulting in decreased CO tolerance and durability of Pt catalysts.30–33 Therefore, it is of signicant importance to improve the graphitic structures of CNFs at lower carbonization temperatures in terms of application performance as well as economic factors. To achieve higher conductivity of the CNF support material and better catalytic performance of Pt NPs, in this study, for the rst time, graphene nanoribbons (GNRs) were incorporated into CNFs to prepare G–CNF composites. Graphene oxide nanoribbons (GONRs), which were prepared according to Tour's method,34,35 were initially mixed with polyacrylonitrile (PAN). Then G–CNFs were fabricated through electrospinning and carbonization. Herein, utilization of GONRs is based on the following considerations: (i) they have better dispersibility than GNRs due to the presence of oxygen-containing groups and defects;36,37 (ii) aer carbonization, the residual oxygenated groups are helpful to improve the dispersion as well as CO tolerance of Pt NPs.38 For comparison, Pt NPs on pristine CNFs and MWCNTdoped CNFs (Pt–CNF and Pt–M–CNF) were also prepared. The resultant Pt–G–CNF hybrid catalysts show signicantly increased electrochemical active surface area (ECSA), remarkably enhanced current density, electrochemical stability and moreover CO tolerance when compared to Pt–CNF and Pt–M– CNF hybrids due to their improved graphitization, better electronic conductivity, as well as more uniform distribution of Pt NPs. Moreover, the fraction of expensive Pt NPs could be lowered to 28 wt%. These results clearly indicate that G–CNFs can be used as efficient support materials for high performance electrocatalysts.
2.
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Scheme 1
Preparation route to Pt–G–CNF hybrids.
of the H2PtCl6–G–CNFs mixture utilizing sodium borohydride (NaBH4). Successful preparation of GONRs was conrmed by transmission electron microscopy (TEM) observation. Whereas original MWCNTs show distinct inner and outer wall surfaces (Fig. 1A), upon oxidation, thin GONRs with an average width of 100 nm were obtained, providing signicantly increased accessible area (Fig. 1B and C). The resulting GONRs exhibit excellent dispersibility in polar solvents, such as water, DMF, ethylene glycol (EG), tetrahydrofuran (THF) and N-methyl2-pyrrolidone (NMP). As depicted in Fig. 1D, while MWCNTs are insoluble and precipitated from DMF, GONRs are highly dispersible in DMF, forming a homogenous solution, which can stand for months without precipitation. Such high solvating power of GONRs is attributed to the introduction of functional
Results and discussion
Pt–G–CNF hybrids were prepared according to the route shown in Scheme 1. Briey, GONRs were obtained by the cutting or unzipping of MWCNTs with potassium permanganate (KMnO4) in concentrated sulfuric acid (H2SO4) according to Tour's method.34,35 The resultant GONRs were highly soluble in polar solvents due to the introduction of a number of functional hydroxyl (–OH) and carboxyl (–COOH) groups. Thanks to the excellent dispersibility of GONRs, homogenous GONR–PAN in dimethylformamide (DMF) was obtained. Further electrospinning generated continuous long GONR–PAN (G–PAN) nanobers, which could be converted into G–CNFs via facile carbonization at 900 C. Finally, the resulting Pt–G–CNF hybrids were successfully obtained through chemical reduction
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Fig. 1 TEM images of (A) MWCNTs, (B and C) GONRs and (D) the photographs of MWCNTs and GONRs in DMF.
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oxygen-containing groups, which can be identied from the IR spectrum with stretching vibrations at 1176/1058 (epoxy group, C–O–C), 1742 (–COOH) and 3400/1225 cm1 (–OH) (ESI, Fig. S1†). GONRs can be homogenously mixed with PAN in DMF owing to their good dispersibility. Further electrospinning and subsequent carbonization yielded GONR–PAN and G–CNF composite nanober membranes, respectively. As shown in Fig. 2, with increasing GONR content, colour of the GONR–PAN membrane gradually changes from white to grey-white and then to grey. Notably, the membrane containing 2 wt% MWCNTs (M2–PAN) is even darker than G5–PAN (containing 5 wt% GONRs). It is reasonable that the oxygen-containing functional groups of GONRs have changed the band-gap and optical properties of the MWCNT precursor. ATIR and Raman spectra of the electrospun nanobers verify the successful preparation of such hybrids (ESI, Fig. S1 and S2†). To obtain conductive materials, the membranes must be carbonized. First, preoxidative stabilization at 280 C was carried out to keep the morphology of the nanobers, which yielded brown oxidized membranes. Then carbonization at 900 C under argon generated the resultant black composite CNFs (Fig. 2). Fig. 3 gives typical scanning electron microscopy (SEM) images of electrospun PAN, M2–PAN, G2–PAN (containing 2 wt % GONRs) and their corresponding carbonized counterparts. Clearly, no distinct difference is observed between the morphology of pure PAN and G2–PAN expect the increased diameter of the G2–PAN nanobers owing to the increased solution viscosity by incorporation of GONRs. However, dissimilarly, nanobers with defects and uneven diameters were formed in M2–PAN, which should be ascribed to the poor dispersion of MWCNTs in the PAN–DMF solution. Upon carbonization, smooth and continuous CNFs were obtained from PAN with decreased diameter. Similarly, ultrathin, smooth and uniform G2–CNFs were formed with few deformations and clusters (marked with oval circles). For M2–CNFs, however, large amounts of agglomerations, clusters, fractures and bending distortions are present. These results suggest that GONR–PAN– DMF possesses comparable electrospinning ability to that of PAN–DMF, but far better than that of MWCNT–PAN–DMF, owing to the excellent dispersibility of GONRs. Further structural information and the crystal phase of the nanobers were characterized by X-ray diffraction (XRD). While
Fig. 2 Photographs of varying electrospun nanofiber membranes and their preoxidized and carbonized counterparts. G(M)x–PAN represents a membrane with x wt% GONRs (MWCNTs).
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Fig. 3 SEM images of electrospun nanofibers of (A) PAN, (B) G2–PAN and (C) M2–PAN, and their carbonized counterparts of (D) CNFs, (E) G2–CNFs and (F) M2–CNFs. (G–I) are enlarged images in (D–F). The scale bars in (A–C), (D–F) and (G–I) are 5, 10 and 2 mm, respectively.
MWCNTs show a typical peak at 2q ¼ 26.0 , corresponding to the interlayer space of 0.34 nm, oxidation brings lots of oxygen functionalities and defects to the c-plane of the ribbon layers, resulting in an increased interlayer spacing of 0.89 nm (2q ¼ 9.9 , Fig. 4). G2–PAN hybrids show a similar XRD pattern to that of PAN: no peak at 2q ¼ 9.9 is observed, suggesting GONRs are well dispersed in PAN; meanwhile, the crystalline peak at 2q ¼ 16.7 becomes blunt and a broad amorphous halo at approximately 2q ¼ 25.9 gets intensied, indicating the introduction of GONRs does not signicantly change the structure of PAN nanobers. During carbonization, most oxygen functionalities
Fig. 4 XRD patterns of MWCNTs, GONRs and the electrospun
nanofibers of pristine PAN, M2–PAN, G2–PAN and their corresponding carbonized counterparts of CNFs, M2–CNFs and G2–CNFs.
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and defects were removed and GONRs were thermally reduced into GNRs. The obtained G2–CNFs show a strong and a relatively weak peak at 2q ¼ 26.0 and 43.2 , which correspond to the crystalline (002) and (100) diffractions of graphite, respectively, implying the graphitization of CNFs. Moreover, when MWCNTs or GONRs are incorporated, the graphite peak at 26.0 is remarkably intensied, indicating a signicantly improved graphitization of the CNFs. Further careful inspection reveals that graphitization of G2–CNF is much larger compared to that of M2–CNF as indicated by its even stronger peak. Raman analysis shows that the ratio of the integrated area of the D band to G band (ID/IG) is gradually decreased from 3.7 to 3.1 and then to 2.4 for CNF, M2–CNF and G2–CNF (ESI, Fig. S3†), thus testifying the enhanced graphitic structure of CNFs through incorporation of MWCNTs and GNRs. Though the detailed mechanism of the enhancement of graphitization is not yet clear, there are several considerations: (i) the intrinsic high graphitization of GNRs and MWCNTs; (ii) the ordering or graphitization of GONRs (from GONR to GNR) may have an inducing effect on the ordering of CNFs. However, exploration of the exact mechanism is still in progress. The greatly improved graphitization of CNFs may increase their electronic conductivity, which is of signicant importance to the support materials for the electrocatalysts. The electronic conductivity of various CNFs was measured and the results are given in Fig. 5. As expected, while the conductivity of pristine CNFs is 655 S m1, incorporation of both GNRs and MWCNTs boost it, giving values of 870, 795, 1306, 1635, 1757 and 1878 S m1 for M2–CNFs and Gx–CNFs with 0.5, 1, 2, 3 and 5 wt% GNRs, respectively. Notably, the conductivity enhancement of G2–CNFs is far higher than that of M2–CNFs, indicating GNRs perform better as nanollers than MWCNTs towards highly conductive CNFs. It may be ascribed to the limited enhancement capacity of MWCNTs on graphitization. Meanwhile, MWCNTs cause more defects, fractures and serious distortion, which are also unfavorable to the conductivity. Excellent conductivity of the hybrid G2–CNFs prompted us to fabricate electrocatalysts utilizing them as support materials.
Fig. 5 Electronic conductivity of the CNFs with different contents of GNRs or MWCNTs.
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Typically, Pt NPs were deposited on CNFs through in situ reduction, with 28 or 40 wt%. Morphologies of the hybrids were observed by TEM. Whereas Pt NPs are loosely bound on pristine CNFs with a number of aggregates (Fig. 6A), those on G2–CNFs are homogenously and more densely distributed with a mean diameter of 3 nm (Fig. 6B). It is reported that metal NPs are prone to be anchored on the defects of carbon materials.39,40 The much denser distribution of Pt NPs with fewer aggregates on G2–CNFs than on pristine CNFs might be ascribed to the residual defects on GNRs and moreover the defects caused by the introduction of homogenously dispersed GNRs. Similarly, MWCNTs also bring defects in CNFs, thus making CNFs populated by Pt NPs (Fig. 6C). However, short and broken M2–CNF hybrids are formed with deformations due to the presence of long and entangled MWCNTs (Fig. 6C and D). Meanwhile, MWCNTs with seriously aggregated Pt NPs are found at the breaks of CNFs (Fig. 6E). XRD patterns of the resultant hybrids show strong diffraction peaks at 2q ¼ 40.2 , 46.3 , 67.8 and 81.2 , which are assignable to typical face-centered-cubic (111), (200), (220) and (311) crystalline planes of Pt (Fig. 6F), respectively. The (220) peaks were used to calculate the size of Pt NPs according to Scherrer's equation,41,42 giving the average diameters of 5.6, 4.1 and 4.5 nm for those on CNFs, G2–CNFs and M2–CNFs, respectively. Good dispersion, few aggregates and proper size of Pt NPs on G2–CNFs would bestow them with outstanding catalytic performance. Normally, high conductivity of the support material, small size and homogenous distribution of the catalyst NPs are in favor of good catalytic performance.12,43 Therefore, Pt–G2–CNF and Pt–M2–CNF are expected to display better electrocatalytic performance than Pt–CNF. ECSA is an important parameter to evaluate the electrochemically active sites of the electrocatalysts,44,45 which can be used to compare the performance of varying support materials. ECSA values of various catalysts were measured by cyclic voltammetry (CV) tests. As indicated in Fig. 7A, with the same Pt content of 28 wt%, Pt–G2–CNF exhibits much larger hydrogen adsorption and desorption current peaks than Pt–CNF and Pt–M2–CNF, thus giving a higher ESCA value of 123.1 m2 g1 compared to 56.4 and 74.8 m2 g1 for Pt–CNF and Pt–M2–CNF. With increased Pt loading of 40 wt%, Pt–G2– CNF (40) gives even larger hydrogen adsorption and desorption peaks and area, but with a lowered ECSA value of 110.7 m2 g1, indicating the growing proportion of active surface area is less than that of the catalyst weight. To further test the catalytic activity of varying systems, CV measurements for methanol electrooxidation were carried out. The CV curves show a methanol oxidation current peak in the forward scan and the other oxidation peak in the backward scan. The latter refers to the removal of residual carbonaceous species formed in the forward scan. The peak current density (Im-peak) of Pt–G2–CNF is 560 mA mg1, which is 1.8 and 1.4-fold compared with those of Pt–CNF (311 mA mg1) and Pt–M2–CNF (412 mA mg1) (Fig. 7B). The result clearly suggests that incorporation of MWCNTs and GNRs is helpful to improve the electrocatalytic activity of Pt NPs. Moreover, GNRs perform even better than MWCNTs. The good performance of Pt–G2–
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Fig. 6 TEM images of (A) Pt–CNF, (B) Pt–G2–CNF, (C and E) Pt–M2–CNF and (D) M2–CNF hybrids. (F) XRD patterns of Pt–CNF, Pt–G2–CNF and Pt–M2–CNF. Pt content in the hybrids is 28 wt%.
CNF should be ascribed to the dense and homogenous distribution of relatively small Pt NPs, as well as the high conductivity of G2–CNFs. Strikingly, with a much higher Pt content, the Impeak of Pt–G2–CNF (40) is slightly dropped to 534.2 mA mg1, indicating the remarkably high catalytic activity of Pt–G2–CNF no matter the catalyst loading. Besides high electrocatalytic activity, excellent tolerance to CO toxicity is also indispensible for outstanding
electrocatalysts. Generally, the ratio of the forward to backward anodic peak current (If/Ib) is used to evaluate the CO tolerance. Higher If/Ib values mean better tolerance to intermediate carbon species, and methanol can be oxidized into carbon dioxide more efficiently.46,47 The If/Ib value of Pt–G2–CNF is calculated as 0.99, which is much higher than those of Pt–M2–CNF (0.76) and Pt–CNF (0.86), suggesting Pt–G2–CNF has less intermediate carbon species
CV curves of Pt–CNF, Pt–G2–CNF and Pt–M2–CNF (28 wt% Pt) and Pt–G2–CNF (40) (40 wt% Pt) catalysts measured in (A) 0.5 M H2SO4 and (B) H2SO4–CH3OH (0.5–1 M) aqueous solutions at a scan rate of 50 mV s1 (10 ml of catalyst ink). Fig. 7
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accumulation, better CO tolerance and better catalytic stability. It seems that the If/Ib value and thus the CO tolerance are not signicantly affected by the Pt content, as Pt–G 2–CNF (40) gives the same value of 0.99 as that of Pt–G2–CNF. To get more insights into the catalytic activity of methanol electrooxidation, linear sweep voltammograms (LSV) were obtained. Pt–G2–CNF shows superior catalytic activity as compared to Pt–M2–CNF and Pt–CNF, as indicated by its highest current density (Fig. 8A), which agrees well with the above CV results. It is reasonable that G2–CNFs have much higher electronic conductivity and larger specic area, more homogenous and denser distribution of Pt NPs with proper size and fewer aggregates, which synergistically provide better catalytic conditions for Pt NPs. The current density of Pt–G2–CNF (40) is even greater than that of Pt–G2–CNF, suggesting its remarkably outstanding activity. Further chronoamperometric (CA) tests were conducted to evaluate the long-term performance of Pt–G2 –CNF catalysts towards the methanol oxidation reaction (MOR). For comparison, those of Pt–M2–CNF, Pt–CNF and commercial Pt–XC72R (40 wt% Pt) are also given out (Fig. 8B). Originally, the potentiostatic current is rapidly decreased for all hybrids, potentially owing to the formation of intermediate species during the MOR processes, such as CH 3OHads, CHOads and COads .48 However, the methanol oxidation current of Pt–G2–CNF remains at a much higher level than those of Pt–M2–CNF and Pt–CNF during the current–time test, suggesting its higher mass activity. Meanwhile, it reveals that the effect of intermediate species on current is relatively stable during the whole decay process. It is also noticeable that Pt–G 2–CNF, Pt–M2–CNF and Pt–CNF show superior catalyst stability to that of the commercial Pt– XC72R catalyst (40 wt% Pt), even though they possess much lower Pt contents of 28 wt%. Meanwhile, initially, Pt–G 2–CNF (40) shows slightly lower methanol oxidation currents than
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Pt–G2–CNF, but much higher currents aer 100 s, thus implying its stronger catalyst stability. Apparently, Pt NPs on G2–CNF with different loadings of 28 and 40 wt% both exhibit excellent catalyst stability, thus testifying G 2– CNF hybrids are appropriate support materials for electrocatalysts. All the CV, LSV and CA experiments consistently indicate that both Pt–G2–CNF and Pt–G2–CNF (40) have excellent catalyst activity and stability. Such high electrochemical activity and stability can be explained in terms of highly dispersed Pt NPs (3 nm) and the greatly enhanced graphitization and thus electronic conductivity of the CNFs. Additionally, the residual oxygenated groups on GNRs have promoted the CO tolerance of the catalysts. Considering the excellent performance of the catalysts based on Pt NPs and G–CNFs, G–CNFs are highly promising support materials for next generation electrocatalysts.
3.
Conclusions
GNRs are rst used as novel nanollers for CNFs to support Pt NPs in FCs. The electronic conduction and graphitization degree of support materials show a huge improvement. Pt NPs with proper particle sizes of 3 nm are highly-dispersed on graphene nanoribbons embedded carbon nanobers. Compared to the Pt–M2–CNF and Pt–CNF catalysts, Pt–G2–CNF hybrids show much higher ECSA and better tolerance towards CO toxicity, indicating excellent catalytic activity and long term stability. The ECSA of G2–CNF (40) was slightly smaller than G2–CNF, while shows signicantly improved mass current density and the same better tolerance towards CO toxicity and long term stability. Considering the facile preparation, reasonable cost, and moreover excellent performance, GNRs are quite promising graphitization and electronic conductivity additives for CNFs to improve catalyst activity and reduced the precious metal loading, which can effectively reduce the cost of catalysts and FCs.
Fig. 8 A comparison of methanol oxidation activity of Pt–CNF, Pt–G2–CNF, Pt–M2–CNF (28 wt% Pt) and Pt–G2–CNF (40) (40 wt% Pt) catalysts based on (A) linear sweep voltammograms and (B) current–time curves. In (B), the data of commercial Pt–XC72R (40 wt% Pt) catalyst are also given for comparison (10 ml of catalyst ink).
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Acknowledgements
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This work was nancially supported by the National Natural Science Foundation of China (21104044), the State Hi-tech Research Development Plan of China (863 Project, 2012AA1106015), the National Key Technology R&D Program (2011BAE08B02), and the Shanghai Leading Academic Discipline Project (B202). W. Z. Y. thanks the Start-up Foundation and SMC-Chenxing Young Scholar Program of Shanghai Jiao Tong University.
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