Accounting for surface coordination: The exploration of heterogeneous catalysts using first-principles calculations can be daunting because the large number of atoms and possible surface geometries. Calle-Vallejo et al. describe a simpler metric for assessing optimal reactivity: a weighted average of surface coordination that includes second-nearest neighbors (see the Perspective by Stephens et al. ). The calculations identified three approaches for introducing cavity sites into the platinum(111) surface to improve its performance for the oxygen reduction reaction used in fuel cells. Science , this issue p. 185 , see also p. 164

Electrocatalytic recycling of waste nitrate (NO₃⁻) to valuable ammonia (NH₃) at ambient conditions is a green and appealing alternative to the Haber−Bosch process. However, the reaction requires multi-step electron and proton transfer, making it a grand challenge to drive high-rate NH₃ synthesis in an energy-efficient way. Herein, we present a design concept of tandem catalysts, which involves coupling intermediate phases of different transition metals, existing at low applied overpotentials, as cooperative active sites that enable cascade NO₃⁻-to-NH₃ conversion, in turn avoiding the generally encountered scaling relations. We implement the concept by electrochemical transformation of Cu−Co binary sulfides into potential-dependent core−shell Cu/CuO_x and Co/CoO phases. Electrochemical evaluation, kinetic studies, and in−situ Raman spectra reveal that the inner Cu/CuO_x phases preferentially catalyze NO₃⁻ reduction to NO₂⁻, which is rapidly reduced to NH₃ at the nearby Co/CoO shell. This unique tandem catalyst system leads to a NO₃⁻-to-NH₃ Faradaic efficiency of 93.3 ± 2.1% in a wide range of NO₃⁻ concentrations at pH 13, a high NH₃ yield rate of 1.17 mmol cm⁻² h⁻¹ in 0.1 M NO₃⁻ at −0.175 V vs. RHE, and a half-cell energy efficiency of ~36%, surpassing most previous reports.