Rh single-atom catalysts with optimized metal loading for direct CO-feed high-temperature proton exchange membrane fuel cells

Abstract

The presence of carbon monoxide (CO) in crude hydrogen is a significant factor hindering the commercialization of hydrogen fuel cells. Introducing a direct CO fuel cell upstream can selectively oxidize and remove CO from crude hydrogen, thereby releasing electrical energy. The purified crude hydrogen is then fed into the downstream hydrogen fuel cell, which helps mitigate the poisoning effect on platinum (Pt) catalysts. Rhodium (Rh) and iridium (Ir) based single-atom catalysts (SACs) have demonstrated potential in the electrochemical CO oxidation reaction (COOR). However, the low density of SAC active sites (Rh metal loading <1 wt%) hinders their further development for use in direct CO fueled PEMFCs. To address this challenge, a two-step pyrolysis method was developed, yielding a series of atomically dispersed Rh on N-doped carbon with several different Rh metal loadings from 0.25 wt% to 7.43 wt%. Half-cell test results demonstrated that with the increment of Rh loading, the overpotential for the COOR at 1 mA cm⁻² of CO oxidation gradually decreased, while the limiting current density gradually increased, indicating the high COOR activity on the SAC with a higher metal loading. The optimum Rh metal loading was determined to be 2.88 wt% and the limiting current density was found to be 2.4 mA cm⁻² with a mass activity of up to 4.18 A mg_Rh⁻¹. Furthermore, a peak power density of 208.4 mW cm⁻² was achieved in a high-temperature single cell utilizing direct CO feed, thereby demonstrating a stable performance over a 22-hours period. This finding indicates a potential robust CO removal capability.