Odd-Nuclearity in Homologous Series of Ni-Tiara Clusters Unlocking Near-Unity NH3 Selectivity in Electrochemical Nitrate Reduction

Abstract Electrochemical nitrate reduction to ammonia offers a sustainable alternative to the Haber-Bosch process while mitigating nitrate pollution. However, the development of nitrate reduction reaction catalysts is hindered by ambiguous active-site structures, poorly understood structure–activity relationships, and limited mechanistic understanding. In this regard, herein we report a homologous series of atomically precise Ni-tiara clusters, [Ni4(L)8], [Ni5(L)10], and [Ni6(L)12] (L = cyclopentylmethanethiolate), which provide a structurally definable molecular platform for systematically decoding nuclearity effects in NO3RR. [Ni5(L)10] exhibits outstanding activity, delivering NH3 Faradaic efficiencies of 95.1% in homogeneous systems and 87.8% in heterogeneous systems at –1.5 V vs Fc+/Fc. Structural analysis by SC-XRD confirmed that the odd nuclearity of Ni5 introduces geometric strain through a nonalternating axial/equatorial thiolate arrangement, generating electronically differentiated Ni centres that promote mixed-valent Ni+/Ni0 intermediates and balanced reduction kinetics. Consequently, Ni5 exhibited balanced reactivity across the multistep reduction pathway with no single rate-limiting step dominating under the optimal operating potential. In contrast, NO2– → NH2OH and NO2– → *NO transformation for Ni4 and Ni6, respectively, were identified as the rate-limiting segment. These results establish a nuclearity-dependent structure–activity relationship and provide molecular-level design principles for efficient multinuclear Ni electrocatalysts for selective ammonia conversion.