Considered from the context of transition-state theory, the shift in onset potential towards less-cathodic values caused by the CeO2-δ catalyst support (Figure ) could be explained fundamentally by two factors: (1) the electron tunneling re-organization energy for the rate-limiting step has decreased and/or (2) the Δ𝐺 𝑎𝑑𝑠 for rate-limiting reaction intermediates (e.g. Δ𝐺 𝑁𝑂 3 -,𝑎𝑑𝑠 ) has decreased (become more favorable). We suggest the latter to be the case in the instance of CeO2-δ versus FTO as support structures for Cu, given the comparable Tafel slopes in 0.1 M NaNO3 (Figure ) and 0.1 M NaNO2 (Figure ).