A Durable CO-Tolerant Polymer-Coated Electrocatalyst Supported on a Nanoporous Carbon
Authors: Nakashima Naotoshi , Yang Zehui , Moriguchi Isamu 
Published: February 2020

Electrochemical energy productions are considered as an alternative energy sources because those energy consumptions are recognized to be more environmentally friendly and sustainable. The fuel cells, especially the direct methanol fuel cells (DMFCs), have received a lot of attention due to the higher energy density (5.04 KWh/L), easy storage and transportation compared with the hydrogen fuel cells (0.53 KWh/L). Also the direct conversion of methanol has a voltage similar (2CH3OH+3O2→2CO2+4H2O+6e-, E=1.19 V) to that of hydrogen (E=1.23 V). However, the DMFCs still suffer from three problems on the anode side; namely, i) the carbon monoxide (CO) poisoning of the platinum nanoparticles (Pt-NPs), which is generated from the uncompleted methanol oxidation reaction (MOR), ii) the sluggish methanol oxidation reaction (MOR) compared to the hydrogen oxidation reaction (HOR) and iii) the low durability of the electrocatalyst in terms of Pt stability and carbon corrosion, which degrades the fuel cell performance.

To commercialize the direct methanol fuel cells (DMFCs), the durability of the anodic electrocatalyst needs to be highly considered, especially under high temperature and methanol concentration conditions. The low durability caused by the carbon corrosion as well as the carbon monoxide (CO) poisoning of the platinum nanoparticles (Pt-NP) leading to the decrease of the active Pt-NPs and increase in the inactive Pt-NPs covered by CO species. We previously reported that poly(vinylphosphonic acid) (PVPA) plays an important role in enhanced CO tolerance of the electrocatalyst due to the acceleration of the reaction between Pt and H2O to form the Pt(OH)ads that consumes the CO poisoned Pt, namely, Pt(CO)ads.
1 In this study, we deposited Pt-NPs on poly[2,2’-(2,6-pyridine)-5,5’-bibenzimidazole] (PyPBI)-wrapped nanoporous carbon (NanoPC)2,3) and coated the as-synthesized electrocatalyst with poly(vinylphosphonic acid) (PVPA). The durability of the as-synthesized NanoPC/PyPBI/Pt/PVPA was tested in 0.1M HClO4 electrolyte at 60 oC by cycling the potential from 1.0 to 1.5 V vs. RHE, and the results indicated that the NanoPC/PyPBI/Pt/PVPA showed an ~5 times better durability by comparison with that the commercial CB/Pt. The methanol oxidation reaction (MOR) of the electrocatalyst was tested before and after the potential cycling in the presence of 4M or 8M methanol at 60 oC and found that the CO-tolerance of the electrocatalyst was ~3 times higher than that of the commercial CB/Pt. Such a higher CO tolerance is due to the coating of the PVPA, which was proved by an EDX mapping measurement. The NanoPC/PyPBI/Pt/PVPA showed a high durability and CO tolerance under high temperature and high methanol concentration conditions indicating that the electrocatalyst would be used in real fuel applications.

References

1) N. Nakashima et al., J. Mater. Chem. A
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2, 18875-18880.

2) N. Nakashima et al., ACS Appl. Mater. Interfaces, 2016, in press.

3) N. Nakashima et al., ACS Appl. Mater. Interfaces, 2015, 7, 9800-9806.