Extracting electrons from water through the oxygen evolution reaction (OER) is a crucial element of carbon neutral fuel generation and other energy storage technologies.^1–3 Large overpotential losses in the OER arise from kinetic barriers associated with multiple proton and electron transfer steps.³ Lowering this barrier is key to improving performance, however a general understanding of the fundamental factors controlling OER kinetics has not yet been reached.

Broadly, two general classes of OER mechanism link applied potential to the rate of catalysis. Electrochemical mechanisms, where the rate determining step (RDS) involves an electron leaving the catalytic intermediate and surface site to be conducted away by the electrode, and chemical mechanisms where no net electron transfer occurs during the RDS.⁴ In the electrochemical mechanism, the current (rate) scales exponentially with the applied potential, as electrode potential is directly correlated to activation energy; whereas in a purely chemical mechanism, voltage only drives the accumulation of catalytic intermediates.⁴ In the simplest case, the rate of a chemical RDS would scale with the coverage of the rate limiting intermediate to the power of its stoichiometric coefficient.⁴ However, more complex behaviour may also arise through interactions between adsorbates changing with coverage, leading to a dependence of activation energy on coverage.⁵

 Beyond fundamental considerations, mechanistic understanding of the OER is challenging due to the complexity of many OER catalysts. For example, despite decades of study, the phase transitions, active oxidation states, active sites and mechanism of cobalt hydroxide/oxyhydroxide – one of the most intensely studied electrocatalysts - remain controversial.^6-9 The position, degree of oxidation and chemical nature of the active sites in CoOOH remains a topic of intense debate - obscuring considerations of the fundamental mechanism of operation in this material.

Such conflicting results reflect a crucial unmet technical challenge in electrocatalysis: there does not exist a widely accessible method of simultaneously measuring populations of catalytic intermediates and proving that the measured intermediate drive current through direct measurement of OER kinetics. Herein, we demonstrate the power of spectroelectrochemical analysis built around employing a multi-channel detector to  measure continuous spectral changes during the polarisation curve at all points of the polarisation curve to overcome this challenge. We  use the example of cobalt oxyhydroxide (hereafter CoOOH) and cobalt-iron Prussian Blue (hereafter CoFe-PB) – another cobalt based catalyst with contrasting behaviour to CoOOH,  This capacity is combined with time resolved measurement of the kinetics of implicated catalytic intermediates as they decay under open circuit conditions to verify turnover frequencies. The richness of the resulting dataset enables us to extract quantitative, operando coverages and turnover frequencies of intermediates, reconstruct the polarisation curve, and prove that the intermediates measured turn over quickly enough to support the measured current. Using this approach, we quantify the extent of energetic destabilisation of the rate limiting intermediate of both catalysts as a function of coverage. Supported by molecular level insight gained from density functional theory calculations we show that the interacting chemical mechanism contributes significantly to the current in both catalysts, despite disperate kinetics and electroadsorpative properties. In light of our conclusions, we examine the contribution this destabilisation makes to OER current as well as examine the validity of simple mechanistic interpretations of Tafel slopes.