A Methodology for the Analysis of Water Oxidation Electrocatalysts in the Absence of Limiting Current that Avoids the Pitfalls of Existing Methods

Water oxidation is an important reaction studied as a way to generate electrons from water, to promote water splitting and the formation of green hydrogen. When using electrodes to drive homogeneous water oxidation catalysis, cyclic voltammograms are analyzed to provide catalytic rate constants. There are two main methods, foot-of-the-wave analysis (FOWA) and limiting current analysis. FOWA relies on approximations inherent to analyzing water oxidation catalysis, such as determining the formal potential of the catalytic intermediate, <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics> <mml:mrow> <mml:msub><mml:msup><mml:mi>E</mml:mi> <mml:mn>0</mml:mn></mml:msup> <mml:mrow><mml:mi>c</mml:mi> <mml:mi>a</mml:mi> <mml:mi>t</mml:mi></mml:mrow> </mml:msub> </mml:mrow> <mml:annotation>$$ {{E}^{0}}_{cat} $$</mml:annotation></mml:semantics> </mml:math> . Limiting current methods are the optimal way to analyze catalyst performance but rely on observable limiting current, which is virtually never seen in water oxidation. To avoid those issues, a method is proposed for analyzing nonideal cyclic voltammetry waveshapes in water oxidation: by analyzing rate data across a large range of potentials, an optimal potential, <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics> <mml:mrow> <mml:msub><mml:mrow><mml:mi>E</mml:mi> <mml:mo>'</mml:mo></mml:mrow> <mml:mrow><mml:mi>c</mml:mi> <mml:mi>a</mml:mi> <mml:mi>t</mml:mi></mml:mrow> </mml:msub> </mml:mrow> <mml:annotation>$$ {{E}^{\text{&#x00026;amp;amp;amp;amp;amp;aposx;}}}_{cat}$$</mml:annotation></mml:semantics> </mml:math> , can be obtained, where catalytic current, <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics> <mml:mrow><mml:msub><mml:mi>i</mml:mi> <mml:mrow><mml:mi>c</mml:mi> <mml:mi>a</mml:mi> <mml:mi>t</mml:mi></mml:mrow> </mml:msub> </mml:mrow> <mml:annotation>$i_{c a t}$</mml:annotation></mml:semantics> </mml:math> , is nearly independent of scan rate and has a linear dependency on buffer concentration. The method is applied to four homogeneous water oxidation catalysts with prior extensive electrochemical elucidation, all of which lack an ideal, purely kinetic waveshape in cyclic voltammetry. Application of the method avoids the biases of the other methods cited for the kinetic analyses of water oxidation catalysts.