Defect & Nanostructure Engineering of Copper-based Materials for (Photo) Electrochemical H₂ and NH₃ Productions

Abstract

Copper-based oxides (e.g., CuO, Cu₂O, CuBi₂O₄, and CuFe₂O₄) are widely used materials for (photo)electrochemical (PEC) water splitting and nitrate reduction to produce hydrogen and ammonia. Due to its wide bandgap value (1.2 to 3 eV), it provides many promising light absorber candidates for applications of solar energy conversion. It is worth noting that the conduction band edges in most copper-based oxides surpass the hydrogen evolution potential, thereby thermodynamically promoting the water reduction reaction. Additionally, the d-orbital electron of Cu can inject charge into the high-energy π- orbital of nitrate to active N=O, thus promoting the electrochemical nitrate reduction reaction (e-NO₃RR) to synthesize ammonia. However, the stability of the copper-based catalysts needs to be improved. For PEC water splitting, we fabricated nanoporous CuBi₂O₄ (np-CBO) photocathodes using a facile solution method and studied their rapid thermal processing in controlled atmospheres (O₂, N₂, and vacuum) to control the surface point defects. We illuminated that controlling the RTP atmospheres and sequence strongly influenced the formation of point defects (copper/oxygen vacancy and Cu¹⁺), which is evident from the scanning transmission electron microscopy, X-ray photoelectron spectroscopy and electron paramagnetic resonance analyses. Significantly, the O₂-RTP treated CBO photocathode exhibited a greatly enhanced photocurrent density and stability than the pristine CBO. Also, we showed the reversibility of the formation of point defects and photocurrent responses via sequential RTP treatments. Conclusively, surface point defect engineering via RTP treatment in a controlled atmosphere is a rapid and facile strategy to improve charge transport and transfer properties of photoelectrodes for efficient solar water splitting. For the e-NO₃RR study, we synthesized nanoporous defect-rich CuO nanowires (nd-CuO NWs) electrocatalysts using a facile solution-flame (sol-flame) reduction strategy, and successfully controlled both surface defects and morphology to construct a highly active surface for converting NO₃- to NH₃. Obviously, the surface defects of oxygen vacancies were analyzed by X-ray photoelectron spectroscopy and electron paramagnetic resonance spectroscopy. Significantly, the nd-CuO NWs exhibited a greatly enhanced NH₃ yield rate, Faradaic efficiency, and selectivity of 0.48 mmol h-1 cm-2, 97.3%, and 86.2% at a lower reduction potential of -0.2 V vs. RHE in 1 M KOH with 2000 ppm NO₃- electrolyte, which are higher than other controlled samples. Noticeably, the defect-rich, manoporous structure provided a high electrochemically active surface area and fast electron transfer properties, leading to a high e-NO₃RR performance and stability. Hence, this work provided a rational design strategy for the rapid fabrication of defect-rich nanoporous catalysts for efficient electrochemical nitrate-to-ammonia conversion. Our study provides new insights into nanoporous structure design and point defects engineering to develop efficient PEC water splitting and electrochemical e- NO₃RR. Furthermore, it is expected that more efficient (photo)electrodes can be developed through additional research and development in the future. Also, the optimized synthesis method on the defects control applies to various oxide synthesis methods and is expected to be expanded to other energy fields, such as (photo)electrochemical, energy conversion, and storage devices.