Photoelectrochemical Nitrogen Reduction: Activity, Selectivity and Stability of Cu-based Ternary Oxide Photocathodes

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Photoelectrochemical (PEC) N2 fixation is currently explored as possible route towards harvesting solar energy for carbon-neutral ammonia production. Within this quest, we propose a tightly integrated experimental and computational effort to investigate the activity and mechanism of the PEC nitrogen reduction reaction (NRR) on a rationally selected class of ternary Cu-based oxide photocathodes, namely CuFeO2 and CuBi2O4. These materials offer suitable band gaps for capturing sunlight, appropriate band edge positions for N2 fixation, and, importantly, p-type conductivity. The closely related binary oxides, CuO and Cu2O, have already exhibited promising NRR activity. The ternary Cu oxides, however, are yet to be explored for this reaction. Compared to the binaries, these ternary Cu oxides have the distinct advantage of being more stable under aqueous PEC operating conditions. In addition, they provide a larger variety of surface sites with potential for bifunctionality. Thus, ternary Cu-based oxides are not only promising candidates to realize the PEC NRR, but when compared to the binaries, they are also ideal model systems to study how heterogeneous surface sites influence the activity, selectivity, and stability of Cu-based oxide photocathodes.Our project is designed to address three objectives aimed at 1) establishing composition-structure-activity relationships, 2) probing chemical changes at working interfaces, and 3) elucidating the PEC NRR mechanism on CuFeO2 and CuBi2O4. These objectives tackle two of the three core areas of the Nitroconversion priority program (SPP 2370), namely “catalyst synthesis and their physicochemical characterization” and “experimental and theoretical investigation of reaction mechanisms.” Following the synthesis and PEC characterization of phase-pure Cu-based oxide photocathodes, we will combine atomically-resolved microscopy and spectroscopy with theoretical studies to analyze the solid-electrolyte interface under PEC NRR conditions. This combination allows us to directly compare macroscopic PEC properties with atomistic simulations. Our experimental techniques include operando dissolution measurements to evaluate the stability of photocathodes, operando Fourier-Transform Infrared (FTIR) spectroscopy to probe intermediates at solid-electrolyte interfaces, and Differential Electrochemical Mass Spectrometry (DEMS) for online analysis of reaction products. In our theoretical approach, we will implement an efficient method for the explicit treatment of solvation environments. This implementation will allow us to compute thermochemical NRR pathways under conditions closely mimicking experimental environments. Our tight integration of experiments with complementary theory throughout this work will help us to identify the catalytically relevant surface structures for PEC NRR and provide new insights into the associated reaction pathways.