Estimating Electron Transfer Kinetics for Flow Battery Electrodes Using Dense Carbon Films

Redox flow batteries (RFBs) are promising technologies for the efficient and reliable delivery of electricity, offering opportunities to integrate intermittent renewable resources and to support unreliable and/or aging grid infrastructure. Within the RFB, the porous carbonaceous electrode provides surface area for redox reactions, distributes electrolyte, and conducts electrons. Understanding reaction kinetics of the electrode is crucial towards improving RFB output and lowering costs. However, reaction kinetics are driven by an interplay of factors and the complex geometries of porous electrodes invalidate the assumptions in conventional voltametric techniques used to assess electron transfer kinetics, thus frustrating our understanding of performance descriptors.

Here, we outline a strategy to estimate electron transfer kinetics on electrode materials reminiscent of those used in RFBs. First, we describe a bottom-up synthetic process to produce non-porous and planar carbon films to enable evaluation of electron transfer kinetics using traditional electrochemical techniques. Next, we characterize physicochemical properties of the films using a suite of spectroscopic methods. Last, we assess the performance of the films in a custom-designed cell architecture, extracting intrinsic heterogeneous kinetic rate constants in iron-based aqueous electrolytes using standard electrochemical methods (i.e., cyclic voltammetry, electrochemical impedance spectroscopy). We anticipate that the methods and protocols described in this work are broadly applicable for quantitatively assessing electrocatalysts.