Disentangling the impact of cation identity on transport and kinetics in nitrate electroreduction to ammonia

The electroreduction of the nitrate anion is surprisingly catalyzed on surfaces that are perceived as negatively charged, conditions where cations from the electrolyte are—on average—closer to the surface in the electrical double layer (EDL). As such, the charge density of alkali metal cations and the extent of their interactions with nitrate could notably impact both transport of nitrate to the surface and the rates of its reduction. We separate these effects quantitatively for the electrocatalytic nitrate reduction reaction (NO3RR) on cobalt in alkaline media, which has high selectivity to ammonia (~90%). A K+-containing electrolyte has higher NO3RR kinetic currents than a Li+-containing one, though the magnitude of this difference depends on catalyst pretreatment. Surprisingly, an ~2x difference in the apparent NO3RR diffusion limited current is observed between Li+ and K+ electrolytes, inconsistent with the reduction of uncharged reactants. This difference arises in part from the availability of active sites, also involved in (oxy)hydroxide formation due to the highly oxidizing nature of the nitrate reactant and nitrite intermediate. The dynamic role of such species during NO3RR is uniquely influenced by the nature of the cation, pointing to a complex coexistence of (hydr)oxides and cations in stabilizing the anionic reactant under the reducing conditions of NO3RR.