(Invited) On the Structural and Mechanistic Studies of PGM-Free Oer Catalysts for PEM Electrolyzer

Abstract

Low temperature water electrolysis represents a critical technology for green hydrogen production. Low temperature electrolysis can be operated using either proton exchange or alkaline membrane electrolyte. Compared to alkaline electrolyzer, proton exchange membrane (PEM) electrolyzer offers advantages of significantly higher current density and higher H2 purity, rendering it a preferred technology when high energy efficiency and low footprint are essential.Working in the oxidative and acidic environment under high polarization voltage, however, adds substantial demand to the electrode catalyst and the support. This is particularly the case at anode where the oxygen evolution reaction (OER) takes place. At present, the PGM materials such as Ir black or Ir oxide are catalysts of choice. Their high cost and limited reserve, however, adds a significant cost to PEM electrolyzer, which contributes to the overall expense of hydrogen production next only to the cost of electricity. Replacing Ir with earth-abundant transition metal oxides could help to reduce the electrolyzer system cost. These materials are known to be applicable in the alkaline electrolyte but not in acid due to dissolution. Since the traditional porous carbon cannot be used as the support under the oxidative potential during OER, their stand-alone conductivity represents another critical consideration.Argonne National Laboratory has recently designed and prepared a new class of PGM-free OER catalyst for PEM electrolyzer. The new catalysts are consisted of highly porous yet stable transition metal oxide derived from the metal-organic-frameworks (MOFs).Two catalyst series, ANL-Cat-A and ANL-Cat-B, were developed and investigated. The OER catalyst activity and durability were first measured by rotating disk electrode (RDE) method and in half-cell in the acidic media. Very promising OER activities were achieved. The catalyst durability was also measured through the multiple potential cycling from the voltage of 1.2 V to 2.0 V (vs. RHE) in the acidic electrolyte. Both ANL catalysts demonstrated promising activity and durability in the acidic medium. These PGM-free OER catalysts were also integrated into the membrane electrode assemblies and tested in PEM water electrolyzer under operating condition (60 °C to 80 °C and ambient pressure). Several MEAs demonstrated promising OER current density of 2000 mA/cm2 at 2.2 V iR-corrected .Extensive structural characterizations were carried out, both in static state and under the reaction condition using various tools such as high resolution electron microscopy and in situ X-ray absorption spectroscopy. Interesting correlation between the structure and property relationship was found. Computational modeling was also performed to understand the fundamental mechanism behind electron conductivity and the acid tolerance behind this new class of OER catalysts. Acknowledgement: This work is supported by U. S. Department of Energy, Hydrogen and Fuel Cell Technologies Office through Office of Energy Efficiency and Renewable Energy and by Office of Science, U.S. Department of Energy under Contract DE-AC02-06CH11357.