Biophotoelectrochemical systems for solar-driven nitrogen conversion: Interfacial design and redox coupling across the nitrogen cycle

The nitrogen conversion underpins ecosystem stability, chemical synthesis, and energy production. However, anthropogenic perturbations and energy-intensive industrial catalytic processes underscore the urgent need for sustainable nitrogen conversion. Microbial nitrogen conversion encompasses diverse metabolic pathways but remains challenging to regulate. Biophotoelectrochemical (BPEC) systems, which integrate photosensitizers with biocatalysts, provide a platform for coupling solar energy with biological nitrogen transformations, enabling external modulation of reaction pathways and selectivity. Here we systematically summarize recent advances in BPEC systems for nitrogen conversion, with emphasis on the construction and applications of photoelectrode–enzyme/cell hybrids and nanoparticle–enzyme/cell hybrids. Beyond widely studied processes such as nitrogen fixation, denitrification, and anaerobic ammonium oxidation (anammox), we also highlight the emerging and less-explored pathways, including dissimilatory nitrate reduction to ammonium (DNRA), direct ammonium oxidation (dirammox), complete ammonia oxidation (comammox), partial denitrification, and nitrification, within an integrative framework. Particular attention is given to emerging biological nitrogen oxidation routes and the potential role of photocatalytic oxidation in expanding BPEC functionality. Furthermore, we critically examine core challenges in BPEC systems, including interfacial electron transfer, system stability, sacrificial agent dependence, material selection, and detection reliability. Finally, future research directions are outlined to guide the development of BPEC systems toward efficient and sustainable nitrogen cycle regulation, with implications for environmental remediation and green energy synthesis.