Cu), NO 2 -is an intermediate that can dissolve into solution and presumably re-reduce on the surface , yielding an eventual product of NH3/NH4 + (Fig. ) . This has important implications on the condition at which products are measured and compared, as products and FE will depend on the amount of NO 3 -reduced, or converted (tied to the amount of charge passed and convoluted with the frequently used measure of time passed). Explicit measurement of the electrochemical potential at which NO 2 -can be reduced is also informative in understanding the NO3RR mechanism and product distribution . Although isotopic labeling is generally unnecessary to distinguish that NH3/NH4 + originates from NO 3 -given high yields, it can be informative in identifying possible contributions from dissolved intermediates, like NO 2 -20