(Invited) PGM-Free Oer Catalysts for PEM Electrolyzer Application

Abstract

Low temperature water electrolysis represents one of the critical technologies in distributed hydrogen production. It produces clean hydrogen with fast response time and works well when coupled with renewable but intermittent power sources such as wind and solar. 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 (x5 improvement) 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, limit the broad implementation of PEM electrolyzer in the renewable energy landscape. Low-cost transition metal based catalysts are known to be active toward OER in alkaline electrolyte but not in acid. Furthermore, traditional catalyst support such as porous carbon cannot sustain the oxidative potential before being oxidized to CO2. Argonne National Laboratory has recently designed and synthesized a new class of PGM-free OER catalyst for PEM electrolyzer. The new catalysts are consisted of highly porous yet stable transition metal composite derived from the metal-organic-frameworks (MOFs). The new catalysts are also integrated into a porous nano-network electrode architecture to improve the conductivity, mass transport and durability against oxidative corrosion. Two catalyst series, ANL-Cat-A and ANL-Cat-B, were developed and investigated. The OER catalyst activity and durability were first measured by the catalytic layer coated over rotating disk electrode (RDE) method or carbon paper in half-cell containing strongly acidic media. Very promising OER activities were achieved. For example, the half-cell OER current density as the function of the polarization potential of a representative ANL-Cat-A (catalyst loading of 2 mg/cm2) was compared with that of Ir-black (catalyst loading of 0.2 mg/cm2). ANL-Cat-A achieved an OER potential of 1.584 V vs. RHE at the current density of 10 mA/cm2, which is only 29 mV higher than that of Ir black benchmark. The catalyst durability was measured through the multiple potential cycling from the voltage of 1.2 V to 2.0 V (vs. RHE) in the acidic electrolyte. The percentage of current density retention against the initial value was measured as the gauge for stability. Both ANL catalysts demonstrated excellent activity and durability over most of PGM-free catalysts in acidic medium. For example, one ANL-Cat-A catalyst retained 90% and 80% current densities at 1.8 V and 2.0 V after 2,000 voltage cycles, respectively. In contrast, the percentage of the current density retention for Ir black was dramatically decreased after only 1000 voltage cycles. Several catalysts from ANL-Cat-A and –B series were also integrated into membrane electrode assemblies and tested in PEM electrolyzer at Giner Inc. under operating condition (60 °C and ambient pressure). Important processing parameters, such as anode catalyst loading, ionomer-to-catalyst ratio, pretreatment and application methods, have been systematically studied. Several MEAs demonstrated OER current density > 200 mA/cm2 at 1.8 V. This work collaborates with DOE HydroGen Consortium in computational modeling, surface property characterization and advanced electron microscopic imaging. Acknowledgement: This work is supported by U. S. Department of Energy, Fuel Cell Technologies Office through Office of Energy Efficiency and Renewable Energy. The works performed at Argonne National Laboratory’s Center for Nanoscale Materials, an U.S. Department of Energy Office of Science User Facility, is supported by Office of Science, U.S. Department of Energy under Contract DE-AC02-06CH11357.