PGM-Free Oxygen Evolution Reaction Catalyst for PEM Water Electrolysis – a Mechanistic Study on Activity and Durability for Practical Applications

Abstract

Low temperature proton exchange membrane water electrolyzer (PEMWE) represents a critical technology for green hydrogen production. It offers the 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 platinum group metal (PGM) materials such as Ir black or Ir oxide are catalysts of choice. Their high cost and limited reserve add a significant cost barrier 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.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 National Laboratory, we recently developed a new PGM-free OER of a nanofibrous cobalt spinel catalyst co-doped with lanthanum and manganese (LMCF). [1] The catalyst was synthesized from the precursor of cobalt zeolitic methyl-imidazolate framework (Co-ZIF) doped with manganese and lanthanum. The Co-ZIF was mixed with a polymer solution and electrospun into nanofiber before being converted to fibrous cobalt spinel under low temperature oxidation to remove all the organics and carbon. The new PGM-free catalyst demonstrated a low overpotential of 353 mV (RHE) at 10 mA/cm2 and a low OER degradation over 360 hours in acidic electrolyte. A PEMWE containing this catalyst at its anode at ~2.0 mg/cm2 demonstrated a current density of 2000 mA/cm2 at 2.47 volts (Nafion® 115 membrane) or 4000 mA/cm2 at 3.00 volt (Nafion® 212 membrane). The membrane electrode was also subjected to accelerated-stress-test (AST) in both voltage cycling and galvanostatic run at different current densities. Low degradation rates under both ASTs were observed.In addition to catalyst design and synthesis, we also conducted extensive structural characterizations, stability measurement, as well as computational modeling. Our in situ study found that the OER process involves significant lattice oxygen movement, leading to lowering instead of raising the cobalt oxidation state during the electrolysis. The measurement of the stability number, combined with computational Pourbaix diagram, elucidated the importance of the operating potential in reducing catalyst dissolution. These findings offer in-depth understanding and direction of future development for PGM-free OER catalyst in PEM water electrolyzer. 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)