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

光电化学(PEC)固定氮气是目前探索的利用太阳能生产碳中性氨的可能途径。在这一探索中，我们提出了一个紧密结合的实验和计算工作，以研究PEC氮还原反应(NRR)在一类合理选择的三元铜基氧化物光电阴极上的活性和机理，即CuFeO2和CuBi2O4。这些材料为捕捉阳光提供了合适的带隙，为固定氮气提供了合适的带边位置，更重要的是，提供了p型导电性。关系密切的二元氧化物CuO和Cu2O已经显示出良好的NRR活性。然而，对于这个反应，三元铜氧化物还有待探索。与二元铜氧化物相比，这些三元铜氧化物具有在PEC水溶液操作条件下更稳定的明显优势。此外，它们提供了更多种类的表面位置，具有潜在的双功能。因此，三元铜基氧化物不仅是实现PEC NRR的理想候选者，而且与二元氧化物相比，它们也是研究表面异质中心如何影响铜基氧化物光电阴极的活性、选择性和稳定性的理想模型体系。我们的项目旨在解决三个目标：1)建立组成-结构-活性关系，2)探测工作界面的化学变化，3)阐明CuFeO2和CuBi2O4上的PEC NRR机理。这些目标涉及硝基转化优先计划(SPP 2370)的三个核心领域中的两个，即“催化剂合成及其物理化学表征”和“反应机理的实验和理论研究”。继纯相铜基氧化物光电阴极的合成和PEC表征之后，我们将把原子分辨显微镜和光谱与理论研究相结合来分析PEC NRR条件下的固体-电解液界面。这种组合使我们能够直接比较宏观的PEC特性和原子模拟。我们的实验技术包括用于评估光电阴极稳定性的OPANDO溶解测量、用于探测固体-电解液界面中间体的OPANDO傅立叶变换红外光谱(FTIR)，以及用于在线分析反应产物的差示电化学质谱(DEM)。在我们的理论方法中，我们将实现一种有效的方法来显式处理溶剂化环境。这一实施将使我们能够在密切模拟实验环境的条件下计算热化学NRR路径。我们在整个工作中将实验与互补理论紧密结合，将有助于我们确定PEC NRR的催化相关表面结构，并为相关反应途径提供新的见解。

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.