Enhanced Nitrate Reduction Activity from Cu-Alloy Electrodes in an Alkaline Electrolyte

The electrochemical nitrate reduction reaction (NO₃^-RR) offers two-fold advantages─restoring balance to the global nitrogen cycle and a less energy intensive pathway to the production of ammonia. We report the results of voltammetric and spectroscopic measurements examining NO₃^-RR on Cu and Cu-alloyed electrodes (CuAg, CuSn, and CuPt) in an alkaline medium. Electrochemical results demonstrate that the overpotential for the NO₃^-RR is ∼120 mV less on the CuAg catalyst as compared to the Cu-only catalyst. In situ surface enhanced Raman spectroscopy (SERS) obtained from these two Cu samples shows that the presence of dilute Ag maintains the Cu surface in a more reduced state (Cu(I)) during the course of NO₃^-RR, while the neat Cu surface is heavily oxidized during NO₃^-RR in an alkaline medium. Consistent with this behavior, the CuSn alloy also stabilizes Cu(I) on the electrode surface and results in increased NO₃^-RR rates. Alternatively, the CuPt alloy does not yield a stabilized Cu(I) component and consequently results in NO₃^-RR rates lower than those for neat Cu. These results indicate that alloying Cu with different metals can tune the nitrate reduction activity by making the Cu atoms more resistant to oxidation to Cu(II) and stabilizing the Cu atoms in lower oxidation states.