Determining the Influence of Catalyst Layer Architecture and Reactant Flow in an MEA for the Electrochemical Nitrogen Reduction Reaction Under Ambient Conditions
Authors: Bi Wei , Gyenge Elod L. , Wilkinson David P. , Shaigan Nima , Malek Ali , Fatih Khalid 
Published: July 2022

Ammonia (NH3) has been increasingly recognized as an advantageous energy/hydrogen carrier beyond agriculture use to assist in the achievement of net-zero and decarbonization goals. For over a century, NH3 production has been dominated by the thermochemical Haber-Bosch process which consumes significant fossil fuel-related energy and feedstocks with a heavy carbon footprint. By contrast, the electrochemical nitrogen reduction reaction (ENRR) presents a sustainable and modular approach to fit remote and small-scale NH3 needs, which has attracted growing interest in recent years. Rational design of electrocatalysts (e.g., introducing defects, size reduction, and heteroatom doping) has been the mainstream approach to address the bottlenecks in the ENRR, especially the low specific activity and poor faradaic efficiency. However, attempts to vary the ENRR electrolytic cell layout/design or the operating conditions have been scarce, and therefore present an opportunity to improve the NH3 electrosynthesis performance by well-thought-out reactor designs coupled with operating conditions.
Our study aims to understand the influence of catalyst layer/electrode architectures and reactant mass transfer on the ENRR performance of model catalysts in a membrane electrode assembly (MEA) cell under different flow configurations. On the anode side, a platinum-coated membrane is used to oxidize the humidified H2, which provides sufficient protons for the cathode reactions. On the cathode side, electrocatalysts with commercial availability and reported ENRR activities are applied as the catalyst (e.g., noble metal, transition metal nitrides or dichalcogenides). The catalyst layer is deposited on the gas-diffusion layer and/or the proton-exchange membrane to construct a gas-diffusion electrode (GDE) and/or a catalyst-coated membrane (CCM). Subsequently, the NH3 electrosynthesis performances are evaluated under different cathode flow conditions for the varied catalyst layer architectures. Initially, the N2-saturated aqueous electrolyte (Na2SO4) is recirculated on the cathode side to flood the cathode surface constantly, which resembles the operating condition in the liquid-filled H-cell present in previous ENRR studies. In these cases, the catalytic sites are predominantly covered by proton donors (e.g., hydronium ions or water molecules) without accessing N2 molecules that only dissolve marginally (0.06 mmol/L) in aqueous media. Hence, in contrast with the “liquid-only” flow configuration, humidified N2 gas is supplied alone (“gas-only”) or in combination with the recirculated electrolyte solution (“gas-liquid”) to increase the presence of N2 near catalytic sites. When gaseous N2 is present, the gas outlet pressure (cell back pressure), as well as membrane drying, were observed to affect the current density and operational stability. Additionally, false positives are diligently recognized which arise from residual N-containing species in the MEA and environmental NH3 contamination. Linear scan voltammetry, the routine electrochemical characterization to distinguish potentially ENRR-active electrocatalysts, was found to be particularly susceptible and contribute significantly to erroneous ENRR performance. In short, This work demonstrates the challenge of getting authentic ammonia production in aqueous systems with improved electrolytic cell designs and operating conditions.
References:

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Wei, M. Pu, Y. Jin and M. Wessling, ACS Appl. Mater. Interfaces, 2021, 13, 21411–21425.

Figure captions: the top two figures illustrate the liquid- and gas-phase flow configurations in this study; and the bottom two figures demonstrate the negligible production of NH3 from the gas-phase reaction condition where significant amounts of erroneous NH3 production was observed from linear scan voltammetry (LSV).

Figure 1