Abstract
The development of active hydrogen oxidation reaction (HOR) and oxygen reduction reaction (ORR) catalysts for use in anion exchange membrane fuel cells (AEMFCs), which are free from platinum group metals (PGMs), is expected to bring this technology one step closer to commercial applications. This paper reports our recent progress developing HOR Pt-free and PGM-free catalysts (Pd/CeO2 and NiCo/C, respectively), and ORR PGM-free Co3O4 for AEMFCs. The catalysts were prepared by different synthesis techniques and characterized by both physical-chemical and electrochemical methods. A hydrothermally synthesized Co3O4 + C composite ORR catalyst used in combination with Pt/C as HOR catalyst shows good H2/O2 AEMFC performance (peak power density of ~388 mW cm−2), while the same catalyst coupled with our flame spray pyrolysis synthesised Pd/CeO2 anode catalysts reaches peak power densities of ~309 mW cm−2. Changing the anode to nanostructured NiCo/C catalyst, the performance is significantly reduced. This study confirms previous conclusions, that is indeed possible to develop high performing AEMFCs free from Pt; however, the challenge to achieve completely PGM-free AEMFCs still remains.
Highlights
Fuel cells are excellent energy convertors due to their advantages of high efficiency and high energy density, and have the potential to provide continuous zero-emission power
Synthesized flame spray pyrolysis (FSP) Pd/CeO2 shows no evidence of metallic Pd, and only a weak signal which can FSP-synthesized FSP Pd/CeO2 shows no evidence of metallic Pd, and only a weak signal which can be attributed to a PdO-type Pd(II) signal (Figure 4b)
Comparing our anion exchange membrane fuel cells (AEMFCs) result using the NiCo-based anode with the rotating disc electrode (RDE) results, it is suggested that the performance of the NiCo AEMFC anode could be limited by mass transfer: While the RDE measurements show the NiCo catalysts having mass activities for the hydrogen oxidation reaction (HOR) in the same order of magnitude as the platinum group metals (PGMs)-based Pd/CeO2 catalysts, e.g., 0.02 and 0.06 mA mg−1 metal at 0.05 V vs reversible hydrogen electrode (RHE), respectively, the mass activity is at least two orders of magnitude lower when the catalyst is used as an AEMFC anode
Summary
Fuel cells are excellent energy convertors due to their advantages of high efficiency and high energy density, and have the potential to provide continuous zero-emission power. Liang et al [14] reported that the catalytic behavior of Co3 O4 grown on reduced graphene oxide (RGO) exhibits good activity towards ORR in alkaline electrolyte, close to that of Pt/C These ex-operando results suggest that Co3 O4 is a promising platinum group metal (PGM)-free electrocatalyst for ORR in AEMFCs. limited work has been reported on the performance of Co3 O4 -based ORR catalysts as real AEMFC cathodes [9,15,16]. A comprehensive review summarizing the development efforts over the past decades on HOR catalysts in alkaline medium was recently reported [17] In this regard, Pd-based electrocatalysts have emerged as one of the few options for Pt-free AEMFCs [18,19]. The catalysts are evaluated for their beginning-oflife performance in H2/O2 AEMFCs single cells
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