An Ambient Photocatalytic Approach to In‐Situ Prepare Cobalt/Cobalt Phosphide and Cobalt/Cobalt Phosphide@Graphene Heterojunction Nanomaterials With Optimized Charge Separation Effects for Highly Efficient Relayed Electrocatalytic NO _X ⁻ Reduction to NH ₃

ABSTRACT This paper reports a novel visible‐light‐driven photocatalytic method for in‐situ preparation of cobalt/cobalt phosphide (Co/CoP‐ x , where x = Co/CoP molar ratio) heterojunction nanocatalysts for electrocatalytic NO _x ⁻ (NO ₂ ⁻ /NO ₃ ⁻ ) reduction to ammonia (eNO _x RR). A series of Co/CoP‐ x catalysts with different Co:CoP ratios was successfully synthesized at room temperature via the photocatalytic method that utilized tetrabromine fluorescein as a photosensitizer and triethylamine as a sacrificial electron donor in pure aqueous solution. The intimate contact of lattice fringes of Co (0) and CoP confirms the presence of Co/CoP heterostructures. Amongst the catalysts, Co/CoP‐4 was found to deliver the most outstanding performance, including a Faraday efficiency (FE) of 99.38% and an ammonia yield of 797.56 μmol h ⁻¹ cm ⁻² at −0.16 V versus reversible hydrogen electrode (RHE) for eNO ₂ RR, along with a FE of 95.34% and ammonia yield of 552.84 μmol h ⁻¹ cm ⁻² at −0.21 V versus RHE for eNO ₃ RR. Furthermore, when graphene (rGO) was introduced as support material during photosynthesis, the formed Co/CoP@rGO (Co:CoP = 4) catalyst demonstrated enhanced electron transport activity and synergistic electronic effects that enabled promoted ammonia yields up to 3.55 mmol h ⁻¹ cm ⁻² (eNO ₂ RR) and 2.30 mmol h ⁻¹ cm ⁻² (eNO ₃ RR). X‐ray photoelectron spectroscopy revealed an effectively enlarged charge separation effect between Co ( δ ⁺ ) and P ( δ ⁻ ) sites in the Co/CoP‐4 and Co/CoP@rGO, underpinning the excellent catalytic performance. In situ characterization techniques revealed that the excellent charge separation effect significantly enhanced the supply capacity of active hydrogen (*H) and optimized proton transfer kinetics. Meanwhile, the low‐valent state transition characteristics of cobalt active sites accelerated *NO consumption, clarifying the ammonia synthesis relay mechanism where CoP phase was responsible for rapid activation of *NO ₃ to *NO ₂ , which was then transferred to Co phase for subsequent reactions, while the P active sites acted as efficient *H acceptors to promote nitrate hydrogenation. Density functional theory calculations demonstrated that Co/CoP‐4 achieved optimized charge separation for Co ( δ ⁺ )‐P ( δ ⁻ ) through d ‐band center modulation, enhancing NO _x ⁻ adsorption on Co sites and *H generation on P sites, suppressing the hydrogen evolution reaction, and significantly reducing the energy barrier of the rate‐determining step of *NO ₃ → *NO ₂ reaction. This study reveals the significant role of charge separation effect in eNO _x RR for ammonia synthesis, providing insight for the development of low‐cost and highly‐effective electrocatalysts for eNO _x RR to ammonia.