4180034

On the fundamental mechanism of oxygen evolution reaction over PGM-free electrocatalysts in operating proton-exchange membrane water electrolyzer

Date
March 27, 2025

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 metals (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 (Science 2023). 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 fundamental mechanism and structure-function relationship at the electrocatalyst surface, we conducted extensive characterizations and computational modeling. For example, high resolution electron microscopy 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 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.

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