Utilizing the Complexity of Transition Metal Phosphide Surfaces to Tune Nitrate Reduction Selectivity

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Electrocatalytic nitrate reduction (NO3RR) is one exciting method of upcycling nitrate in wastewater by conversion to ammonia, an essential component in fertilizers and a potential liquid fuel. Nitrate reduction to NH3 is impeded by the formation of other products such as NO2 - or H2 via the hydrogen evolution reaction (HER). Transition metal phosphide (TMP) nanocrystals have a non-uniform surface charge distribution due to their binary compositions, which leads to a variety of active sites with different binding affinities. We hypothesize that the diversity of active sites on Ni2P can be leveraged to perform complex electrocatalytic reactions such as the NO3RR. Specifically, a diversity of surface sites can create local active site motifs that promote the adsorption of different intermediates, e.g., *NO3 - and *H, that can readily react for the NO3RR. We prepare 5 nm Ni2P and Ni nanocrystals using colloidal syntheses and deposit them onto a carbon support to compare the simpler catalytic surface of Ni with the more complex Ni2P surface. We compare their NO3RR activities with cyclic voltammetry in neutral, buffered aqueous conditions. We also investigate the selectivity between the NO3RR and HER with bulk electrolysis coupled with a colorimetric assay, where Ni2P on carbon shows higher selectivity toward NH3 over NO2 - and H2. With these experiments, we evaluate our hypothesis that the surface complexity of Ni2P will lead to enhanced NO3RR selectivity relative to Ni. This work sets the stage for developing selective electrocatalysts by utilizing ensembles of active sites to drive electrocatalytic selectivity.