It is well known that electrocatalytic testing with isotopically labelled reactants enables the elucidation of rate limiting steps involving those species as their heavier weight slow down the chemical reactions in which they participate. To this end, we used 15 NO3 -or D2O to get information about the NO3 -reduction mechanism. Tafel analysis using either 15NO3-or D2O shows an increase in the slope from 151 mV/dec to 181 and 193 mV/dec, respectively (Fig. ). This indicates the participation of both H and N species in the rate limiting step of the NO3R reaction. We then probed the effects of NO3 concentration on the catalytic current. The current density at -0.6 VRHE as a function of the NO3 concentration exhibits 2 linear regimes: from 0 to 40 mM, the current increases at a rate of 3141 mA/MNO3, while from 40 mM to 80 M NO3 -, there is a slower increase of 857 mA/M (Fig. ). This indicates that the most active sites of Cu3Al2OF10 or the surface adsorption motifs of *NO3 -are occupied at modest concentrations (<40mM) and a second subset of active sites/intermediate configurations begins to contribute at higher NO3 -concentrations. An alternative explanation would be the function of bifunctional active sites in which a neighboring site may act to accept or donate species involved in the reaction (e.g. H + or OH -). A comprehensive picture of this is not yet established and is a subject of further investigation. We next took to operando spectroscopy to probe the reaction mechanism on Cu3Al2OF10 surfaces.