Xinxin Yua,
Fang Luob and
Zehui Yang*a
aSustainable Energy Laboratory, Faculty of Materials Science and Chemistry, China University of Geosciences Wuhan, 388 Lumo RD, Wuhan, 430074, China. E-mail: yeungzehui@gmail.com
bSchool of Materials and Science, Hubei University of Technology, 28 Nanli RD, Wuhan 430074, China
First published on 10th October 2016
Sluggish methanol oxidation reaction (MOR) and CO poisoning of platinum electrocatalysts are critical problems in direct methanol fuel cells (DMFCs). Here, we design a stable CO tolerant platinum electrocatalyst via a bottom-up method, in which the platinum nanoparticles are deposited on carbon black after coating with polybenzimidazole (PBI) and poly(vinyl pyrrolidone) (PVP). By comparison with the PVP post-coated electrocatalyst (CB/PBI/Pt/PVP), the PVP pre-coated electrocatalyst (CB/PBI/PVP/Pt) exhibits comparable durability and CO tolerance due to the similar amount of PVP in the electrocatalyst, suggesting the PVP pre-coating method shows negligible effect on CO tolerance and durability, while the Pt utilization efficiency, methanol oxidation activity and power density of CB/PBI/PVP/Pt are 1.6 times higher than those of CB/PBI/Pt/PVP. Thus, the PVP pre-coated electrocatalyst has better activity due to the non-coated Pt nanoparticles. Meanwhile, CB/PBI/PVP/Pt exhibits highly stable CO tolerance during the durability test, while the CO tolerance of the commercial CB/PtRu seriously deteriorates during the durability test due to the dissolution of Ru nanoparticles. To the best of our knowledge, the maximum power density of CB/PBI/PVP/Pt (104 mW cm−2) is one of the highest values in recent publications.
Aside from the sluggish MOR activity, alloying Pt with ruthenium (Ru) was found to be an efficient way to eliminate the CO poisoning problem in DMFCs,4,5 while, as well known, Ru is dissolvable in the acidic medium and accordingly the CO anti-poisoning of PtRu electrocatalysts is unable to be maintained during the long-term operation of the DMFCs due to the highly acidic environment.6 Changing Ru to other transition metals, such as Sn,7,8 Co,9 Au,10 Mo,11 Fe12, etc. has been systematically studied, while the alloyed electrocatalysts still suffered from low durability and sluggish MOR activity since transition metals exhibit low MOR activity.13 Considering the sluggish MOR and CO tolerance simultaneously, design and fabrication of a CO tolerant Pt electrocatalyst is seriously considered because Pt exhibits the highest MOR activity among all the metals. Doping carbon materials with phosphorus14 or boron15 was carried out and it was found that the phosphorus/boron weakened the binding energy between the Pt nanoparticles and CO species. Also, we have reported a promising method to eliminate the CO poisoning problem, in which the Pt nanoparticles were coated with poly(vinylphosphonic acid) (PVPA)16–18 or poly(vinylpyrrolidone) (PVP).19 Due to the presence of the water soluble polymer in the electrocatalyst, the formation of Pt(CO)ads species was accelerated resulting in high CO tolerance; meanwhile, the sluggish MOR is unable to be solved in this method because the polymer partially covers the Pt active sites and the degradation in MOR activity is unavoidable.
In this work, we try to synthesize a new electrocatalyst via a bottom-up design (Fig. 1) in order to maintain high CO tolerance of the electrocatalyst and simultaneously address the sluggish MOR by polymer coating. The carbon black (CB) is firstly coated with polybenzimidazole (PBI) and PVP before the Pt deposition. The PVP layer can be stabilized by the PBI layer due to hydrogen bonding and the Pt nanoparticles will be anchored by PVP via Pt–N bonding.20 Due to the presence of the PVP layer, the CO tolerance would be enhanced because of the acceleration of water adsorption, which is important for enhancement in CO tolerance of the Pt electrocatalyst.21,22 Meanwhile, the Pt nanoparticles are more active compared to the PVP-coated Pt electrocatalyst. The comparison between the PVP pre-coated and post-coated electrocatalysts is systematically studied.
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Fig. 1 Schematic illumination of post- (upper) and pre-coating (lower) of PVP polymer on the synthesized electrocatalyst. |
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Fig. 2 XPS spectra of narrow scan in N1s (a), Pt4f (b) and Ru3p (c) regions of CB/PtRu (black line), CB/PBI/Pt/PVP (blue line) and CB/PBI/PVP/Pt (red line). |
In order to determine the PVP amount in the two electrocatalysts, TGA measurements were carried out and the results are shown in Fig. 3. The PVP amount in CB/PBI/Pt/PVP was evaluated from the decrease in weight residue between CB/PBI/Pt (40.5 wt%) and CB/PBI/Pt/PVP (38.6 wt%). The PVP amount was 5.4 wt% based on a constant weight ratio between CB/PBI and Pt, which is consistent with the result from the weight loss of CB/PBI/Pt/PVP before 300 °C (blue line). The PVP amount in CB/PBI/PVP/Pt was evaluated from the weight loss of CB/PBI/PVP/Pt before 300 °C (red line) since the decomposition temperature of CB and PBI was higher than 300 °C. The PVP amount in CB/PBI/PVP/Pt was determined to be 5.0 wt%. The XPS and TGA results suggested that PVP existed in both of the two electrocatalysts due to the hydrogen bonding between PBI and PVP.30
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Fig. 3 TGA curves of CB/PBI/Pt (green line), CB/PBI/Pt/PVP (blue line) and CB/PBI/PVP/Pt (red line) measured from room temperature to 900 °C under stable oxygen atmosphere. |
It is of importance to measure the morphologies of the two prepared electrocatalysts before the electrochemical measurements. The commercial CB/PtRu was used as the control sample and the PtRu nanoparticles were homogeneously deposited on CB with a diameter of 3.2 ± 0.2 nm as shown in Fig. 4a (for histogram, see ESI, Fig. S3a†). As shown in Fig. 4b and c, the Pt nanoparticles were also uniformly deposited on the carbon materials due to the Pt–N bonding between Pt and PBI or PVP reported by us.20,24 The Pt sizes were 3.0 ± 0.2 nm and 3.2 ± 0.1 nm for CB/PBI/Pt/PVP and CB/PBI/PVP/Pt, respectively (for histogram, see ESI, Fig. S3b and c†), indicating that the three electrocatalysts showed similar particle size. It should be noted that the PVP layer on CB/PBI/Pt was approximately 1 nm.19 Also, the PVP layer was detected by HR-TEM as shown in Fig. S4.†
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Fig. 4 TEM images of CB/PtRu (a), CB/PBI/Pt/PVP (b) and CB/PBI/PVP/Pt (c) before the durability test. |
Electrochemical surface area (ECSA) is an essential parameter to evaluate the activity of the electrocatalyst, which is calculated from the equation: ECSA = QH/(210 × Pt loading amount), where QH is the charge of the hydrogen electro-adsorption from 0.05 to 0.3 V vs. RHE.31,32 From Fig. 5, the initial ECSAs of the commercial CB/PtRu, CB/PBI/Pt/PVP and CB/PBI/PVP/Pt were calculated to be 71.0 m2 gPt−1, 50.3 m2 gPt−1 and 81.7 m2 gPt−1, respectively. By comparison with CB/PBI/Pt/PVP, CB/PBI/PVP/Pt showed 1.6 times higher ECSA due to the coverage of Pt active sites by PVP in CB/PBI/Pt/PVP. The Pt utilization efficiency of CB/PBI/PVP/Pt reached 94%, which is calculated from the equation: Pt utilization efficiency = 6/(ρd × ECSA), where ρ and d are the Pt density and diameter from TEM, respectively.33 However, the Pt utilization efficiencies of commercial CB/PtRu and CB/PBI/Pt/PVP were only 81% and 55%. The lower Pt utilization efficiency of CB/PBI/Pt/PVP is attributed to the coverage of Pt active sites by the PVP layer. The higher Pt utilization efficiency of CB/PBI/PVP/Pt suggested that the pre-coating of PVP polymer was vital for the enhancement in the Pt activity. Subsequently, the durability test was carried out based on the protocol from Fuel Cell Commercialization of Japan (FCCJ, ESI, Fig. S1†), in which the potential was dynamically scanned from 0.6 V to 1.0 V vs. RHE in order to measure the stability of the Pt nanoparticles. As shown in Fig. 5a, the hydrogen adsorption/desorption peaks of the commercial CB/PtRu showed a sharp decrease, which was due to the dissolution of Ru nanoparticles in the acidic medium and easy aggregation of Pt nanoparticles. After the durability test, the electrocatalyst was collected from the electrode and measured using XPS. As shown in Fig. S5,† the Ru3p peak disappeared in CB/PtRu after the durability test indicating that Ru is not stable in the acidic medium. After 4200 potential cycles, the ECSA of the commercial CB/PtRu lost almost 50%. For the newly synthesized CB/PBI/Pt/PVP and CB/PBI/PVP/Pt, the ECSAs were almost stable during the potential cycling and lost only 17% and 20%, respectively, as shown in Fig. 5d. CB/PBI/Pt/PVP showed the highest stability because the Pt nanoparticles were structurally sandwiched by the PBI and PVP polymers. Interestingly, CB/PBI/PVP/Pt showed almost similar loss in ECSA compared to CB/PBI/Pt/PVP, suggesting that the pre-coating or post-coating of PVP polymer showed almost negligible effect on the Pt stability. After the durability test, the nanoparticle size had grown to 6.1 ± 2.7 nm for the commercial CB/PtRu as shown in Fig. 6a, while the Pt sizes increased to 3.8 ± 0.3 nm and 4.0 ± 0.3 nm for CB/PBI/Pt/PVP and CB/PBI/PVP/Pt as shown in Fig. 6b and c, respectively (for histograms, see ESI, Fig. S6†). After the durability test, the Pt utilization efficiency of CB/PBI/PVP/Pt was still 93%, suggesting that the CB/PBI/PVP/Pt showed stability and the highest Pt utilization efficiency during the durability test.
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Fig. 6 TEM images of the commercial CB/PtRu (a), CB/PBI/Pt/PVP (b) and CB/PBI/PVP/Pt (c) after the durability test. |
The CO tolerance of the electrocatalyst was evaluated before and after the durability test. As shown in Fig. 7a, the CO oxidation peak of the commercial CB/PtRu was located at 679 mV vs. RHE before the durability test, which was much lower compared to those of CB/PBI/Pt/PVP (780 mV) and CB/PBI/PVP/Pt (775 mV) due to the presence of Ru facilitating the removal of CO species on the Pt nanoparticles by Ru(OH)ads species. The comparable location of the CO oxidation peak was attributed to the similar PVP amounts in the electrocatalysts, indicating that the pre-coating of the PVP layer showed a negligible effect on the CO tolerance of the electrocatalyst. Meanwhile, the CO oxidation peaks of these two electrocatalysts were negatively shifted compared to that of CB/PBI/Pt as shown in Fig. S7† due to the presence of PVP facilitating the water adsorption to form Pt(OH)ads species because the surfaces of the electrocatalysts are hydrophilic after the introduction of PVP to the electrocatalysts. And Pt(OH)ads species are also important for the enhancement in the CO tolerance of the electrocatalysts since Pt(OH)ads species react with Pt(CO)ads species to recover the CO-poisoned Pt (Pt(OH)ads + Pt(CO)ads → 2Pt + CO2 + H+ + e−). After the durability test, due to the absence of Ru, the CO oxidation peak of the commercial CB/PtRu largely shifted to 1113 mV vs. RHE; in sharp contrast, the CO oxidation peaks of CB/PBI/Pt/PVP and CB/PBI/PVP/Pt were almost stable after the durability test due to the presence of PVP, since PVP plays a prominent role in the CO tolerance of the electrocatalysts.19 Although the commercial CB/PtRu showed better CO tolerance before the durability test, the CO tolerance had seriously deteriorated after the durability test, which shortens the lifetime of DMFCs and degrades the fuel cell performance. Stable CO tolerance is essential for long-term operation of DMFCs. It should be noted that the ECSAs of the commercial CB/PtRu, CB/PBI/Pt/PVP and CB/PBI/PVP/Pt calculated from CO stripping curves (ECSA = QH/(420 × Pt loading amount), QH is the charge of the CO oxidation peak) were 68.5, 47.3 and 79.1 m2 gPt−1, respectively, which were comparable to the ECSA values evaluated from the cyclic voltammetry curves.
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Fig. 7 CO stripping voltammograms of the commercial CB/PtRu (a), CB/PBI/Pt/PVP (b) and CB/PBI/PVP/Pt (c) before (solid line) and after (dotted line) the durability test. |
The methanol oxidation reaction (MOR) was carried out to evaluate the activity of the electrocatalyst. A shown in Fig. 8, the current density of CB/PBI/PVP/Pt was 1.35 A mgPt−1, which was higher compared to those of CB/PBI/Pt/PVP (0.94 A mgPt−1) and commercial CB/PtRu (0.8 A mgPt−1). Relative to CB/PBI/Pt/PVP, CB/PBI/PVP/Pt showed better MOR activity because the Pt nanoparticles were not covered by any polymer and the methanol can be easily adsorbed and oxidized on the Pt nanoparticles. The PVP coating in CB/PBI/Pt/PVP partially blocked the Pt active sites and showed unavoidable loss in activity. Subsequently, the membrane electrode assemblies (MEAs) were fabricated from the three different electrocatalysts and the fuel cell performance was measured at 60 °C. The power density of MEACB/PBI/PVP/Pt/PVP reached 104 mW cm−2, which was much higher compared to MEACB/PBI/Pt/PVP (73 mW cm−2) and MEACB/PtRu (63 mW cm−2) (Fig. 9). Thus, the pre-coating of PVP polymer (CB/PBI/PVP/Pt) enhanced the fuel cell performance and maintained comparable CO tolerance to the post-coating of PVP polymer (CB/PBI/Pt/PVP). To the best of our knowledge, the power density of MEACB/PBI/PVP/Pt was one of the highest values in recent publications as listed in Table 1.
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Fig. 8 Methanol oxidation reaction (MOR) curves of the commercial CB/PtRu (black line), CB/PBI/Pt/PVP (blue line) and CB/PBI/PVP/Pt (red line) before the durability test. |
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra24025a |
This journal is © The Royal Society of Chemistry 2016 |