Electrocatalytic Oxidation of Hydrogen as an Anode Reaction for the Li-Mediated N₂ Reduction to Ammonia

High-performance Li-mediated nitrogen reduction reaction (NRR) requires an effective anode process, such as the hydrogen oxidation reaction (HOR), to complete the electrosynthesis of ammonia. However, understanding of the mechanism, kinetics, and suitable catalysts for the HOR in organic media relevant to the NRR is limited. To address this, the present work investigates H2 electrooxidation in lithium bis(trifluoromethanesulfonyl)imide tetrahydrofuran solutions using Pt/C and other highly dispersed carbon-supported metal catalysts (Ni/C, Ru/C, Pd/C, Ir/C, Au/C, and PtRu/C). Dc and Fourier transformed ac voltammetric analyses suggest that the HOR under such conditions occurs via a combination of two mechanisms. One is the conventional mechanism involving adsorbed H species, which defines the initial transient activity on reduced metal surfaces. At more positive potentials, H2 can be oxidized through a redox reaction with electrochemically generated oxides on the metal catalyst surface. Extended voltammetric measurements reveal rapid deterioration of the HOR activity of the platinum-group catalysts, which is intensified by the presence of water (≥4 mM), increased electrolyte concentration (2.0 vs 0.1 M), and convection. However, the presence of ethanol - a conventional proton carrier for the NRR - has no pronounced pernicious effect on the activity of Pt/C. Moreover, chronoamperometric assessment of the initial activity demonstrates improved performance of Pt/C after oxidative pre-treatment when C2H5OH is added. However, this effect is transient and the steady-state rates of the HOR on Pt/C after 6 h chronoamperometric tests are not higher than ca. 100 μA cmgeom.-2 at -0.2 V vs ferrocene0/+, irrespective of pre-treatment. More significant improvement is provided by the bimetallic PtRu/C catalytic system, which demonstrates much slower than Pt/C deterioration in activity eventuating in an HOR rate of 350 ± 70 μA cm-2geom. after 6 h of operation at -0.2 V vs ferrocene0/+. We ascribe this to the increased oxyphilic properties of the catalyst surface provided by the presence of Ru. Overall, the results of the present work suggest that a high-performance catalyst for the H2 oxidation as an anode process for the Li-mediated NRR is likely to emerge through the design of multicomponent systems, facilitating the kinetics of both HOR pathways and suppressing poisoning of the catalytic surface.