How do you run a car on sunlight? While batteries are a popular option today, their bulk can limit a vehicle’s range. For a compact alternative, researchers are exploring renewable hydrogen gas, generated by electrolysis: a process that splits water molecules with electricity.
Efficient electrolytic technologies such as proton exchange membrane water electrolysers (PEMWE) may be vital for a greener future. “Unlike other systems, PEMWE is compact, can operate at high current densities, and responds rapidly to electrical fluctuations,” explained Mingwu Tan, a Senior Scientist and Group Leader at the A*STAR Institute of Sustainability for Chemicals, Energy and Environment (A*STAR ISCE2). “These features make PEMWE suited to intermittent energy sources such as solar and wind.”
However, a critical component of PEMWE systems—the anode catalyst—relies heavily on iridium: a rare earth element so scarce that only a few tonnes are extracted each year worldwide. What’s more, many PEMWE designs involve anodes that operate in strong acids at high voltages. These conditions can gradually degrade both the catalytic component and its support structures, wasting valuable iridium.
To circumvent these issues, a team of researchers from A*STAR ISCE2 and Zhejiang University, China, recently investigated a new engineering approach for longer-lasting PEMWE catalysts: strengthening their foundations, rather than their catalytic components alone.
“Previous research focused on optimising the iridium active phase while treating the catalyst support as a passive carrier,” said Tan. “Our team therefore asked if the support could be used to actively control the electronic structure, reaction pathways and stability of iridium species in the catalyst.”
The researchers introduced another rare earth element, thulium (Tm), into a PEWME catalyst support comprised of manganese dioxide (MnO2). Through structural and electrochemical studies, they showed that thulium doping created a coordination environment that improved the support’s stability, promoted the formation of ultrasmall iridium oxide (IrOx) clusters, and enhanced interactions between IrOx and the support.
“Thulium changed not only the catalyst’s activity and stability, but the reaction pathway it used for oxygen evolution,” said Tan. “This unexpected change demonstrated that catalyst supports can directly influence the reaction mechanism of active iridium.”
The team’s IrOx/Tm-MnO2 catalyst achieved 1.62V at 1 A cm-2 with an iridium loading of only 0.25 mgIr cm-2, and maintained its operational stability for over 800 hours: an exceptional electrolytic performance compared to other existing PEMWE systems. The thulium-modified support also strongly anchored iridium to the anode, resulting in only 7.4 percent activity loss in the catalyst during accelerated testing.
“These findings suggest that support engineering could help reduce iridium consumption while improving PEMWE system durability,” Tan noted. “However, larger-area cells, longer-term stack testing and techno-economic assessment will still be required before practical deployment.”
The team aims to optimise their design while screening for more economical elements with similar effects as thulium. They will also evaluate their catalyst in larger devices under conditions that more closely represent solar- and wind-powered electrolysis.
The work described was supported by the National Natural Science Foundation of China and the A*STAR Career Development Fund.
The A*STAR-affiliated researchers contributing to this research are from the A*STAR Institute of Sustainability for Chemicals, Energy and Environment (A*STAR ISCE2).
