in the supplementary material, with the sum of the transfer coefficients, α a + α c , either being equal to one or two. This is a consequence of the fact that the transfer coefficients must fulfil the general relationship α a + α c = n/ν, where n = 2 is the overall number of electrons transferred in the HOR/HER. The stoichiometry number ν counts how many times the RDS of a given mechanism is visited, i.e., ν = 2 for the Tafel-Volmer mechanism with Volmer RDS, yielding α a + α c = 1, and ν = 1 for all other mechanisms, corresponding to