High throughput optimisation of PdCu alloy electrocatalysts for the reduction of nitrate ions

PdCu alloy catalysts are optimal for nitrate reduction in alkali media in a small compositional range with a maximum in activity at 84% Cu. This optimum is a result of a combination of a bi-functional and an electronic effect of Pd addition to Cu. [Display omitted] •A high-throughput methodology is used to synthesise and screen PdCu alloys.•The expected equilibrium phases of the PdCu alloys were identified using XRD.•Pd in Cu results in reversible surface oxidation of Cu and oxide destabilisation.•This is a result of bi-functional and electronic effects of Pd in Cu, respectively.•The most active catalyst is in a narrow compositional region around 84%at. Cu. A high-throughput methodology was used to synthesise compositional gradient thin films of PdCu alloys. The expected equilibrium phases of the PdCu alloys as a function of composition were identified using XRD. The electrochemical behaviour of the alloys in NaOH base electrolyte was measured simultaneously on a micro-fabricated array of 100 electrodes. Small concentration of Pd addition to the Cu resulted in a reversible Cu derived surface redox, and higher concentrations decreased its equilibrium potential (a destabilisation of the surface oxide). The former is due to the mediation of the redox reaction by Pd and the latter due to electronic interaction between Pd and Cu. The most active reduction catalyst (rate at constant overpotential or minimum overpotential for a fixed rate) is in a narrow compositional region around 84%at. Cu. The trend in reactivity can be understood by the promotion of nitrate reduction through hydrogen spillover from Pd at low concentrations in the alloy. The reduction in activity as the palladium concentration increases further is concomitant with the destabilisation of oxygen on copper, and therefore likely to be due to the inability to stabilise the nitrate ion at the surface, or extract the first oxygen atom to produce nitrite.