Engineering low-valent molybdenum sites in CoMoO4 nanosheets to boost electrochemical nitrogen-rich wastewater treatment

Electrochemical coupling nitrate-to-ammonia (NO3–-to-NH3) with urea oxidation reaction (UOR) is attractive for both energy-saving ammonia synthesis and comprehensive nitrogen-rich wastewater treatment. However, developing the efficient electrocatalyst that simultaneously promotes hydrogenation of NO3– and UOR is still challenging. Here, we engineered the porous CoMoO4 nanosheets with rich low-valent Mo sites (Mo(L)-CoMoO4-x) as a difunctional electrocatalyst for both NO3–-to-NH3 and UOR. The Mo(L)-CoMoO4-x displayed high Faradaic efficiency (93.33%) and selectivity (91.16%) for NO3–-to-NH3 and low potential (1.31 V vs. RHE) at 100 mA cm−2 toward UOR. Impressively, a low cell voltage of 1.83 V was needed for coupling UOR with NO3–-to-NH3 in two-electrode system. Mechanism study revealed that the introduction of low-valent Mo sites in CoMoO4 nanosheets accelerated the activation kinetics and multistep hydrogenation of NO3–, and also promoted the reconstruction of active CoOOH species for UOR, resulting the difunctional catalytic property for NO3–-to-NH3 and UOR.