PGM-Free Oer Catalyst for Water Splitting – an Interrogation on the Structural Dynamics of Catalyst during Reaction
Authors: Liu Di-Jia 
Published: December 2024

Green hydrogen production through proton exchange membrane water electrolyzer (PEMWE) offers the advantages of high current density and high H2 purity with low footprint. At present, the platinum group metal (PGM) such as Ir are the catalysts of choice for the oxygen evolution reaction (OER) at PEMWE anode. The high cost and limited reserve of Ir add a significant cost to broad implementation of PEM electrolyzer.
Replacing Ir with earth-abundant transition metals could help to reduce the electrolyzer system cost. For PEMWE application, a PGM-free anodic catalyst must be highly active to match the performance of Ir. Furthermore, it must be stable against oxidative acid corrosion. At Argonne, we recently developed a nanofibrous cobalt spinel OER catalyst prepared from cobalt zeolitic methyl-imidazolate framework (Co-ZIF) doped with manganese and lanthanum. [1] A PEMWE containing this catalyst with the anodic loading of ~2.0 mg/cm2 demonstrated a current density of 2A/cm2 at 2.47 volts or 4A/cm2 at 3.00 volts. The catalyst also showed a promising stability under accelerated-stress-test (AST) through voltage cycling as well as galvanostatic tests at different current densities.
To better understand the structure-function relationship, we conducted extensive structural characterizations and computational modeling. For example, our high-resolution electron microscopy study showed a catalyst morphology mimicking that of ZIF precursor but composed of cobalt spinel crystallites with an average size of 3.5 nm. The oxygen atoms on the top layer of the spinel surface are in the relaxed mode. X-ray absorption spectroscopy showed strong oxygen deficiencies around cobalt compared to standard spinel reference. Such deficiency became more predominant at higher cell voltage. Contrary to general perception, the cobalt oxidation state actually decreased instead of increasing at the escalating OER potential and fast turnover rate. Accompanying the change of oxidation state, reduction of oxygen coordination number surrounding Co and growth of Debby-Weller factor of Co-O shell were observed, indicating rapid exchange of the lattice oxygen in spinel during OER. On the other hand, no such changes were observed for the atomically dispersed Mn ion in the spinel as the dopant.
In this presentation, we will discuss in our detailed findings and offer our perspective on the future research direction for PGM-free OER catalyst for PEM water electrolyzer application.

Acknowledgement: This work is supported by the U.S. Department of Energy, Office of Energy Efficiency and Renewal Energy, Hydrogen and Fuel Cell Technologies Office, and by Laboratory Directed Research and Development funding of Argonne National Laboratory, provided by the Director, Office of Science, of the U.S. Department of Energy under Contract No. DEAC02-06CH11357. Work performed at the Center for Nanoscale Materials and Advanced Photon Source, both U.S. Department of Energy Office of Science User Facilities, was supported by the U.S. DOE, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357.
[1] “La- and Mn-doped cobalt spinel oxygen evolution catalyst for proton exchange membrane electrolysis” Lina Chong, Guoping Gao, Jianguo Wen, Haixia Li, Haiping Xu, Zach Green, Joshua D. Sugar, A. Jeremy Kropf, Wenqian Xu, Xiao-Min Lin, Hui Xu, Lin-Wang Wang, Di-Jia Liu, Science 380, 609–616 (2023)