Defining Nafion Ionomer Roles for Enhancing Alkaline Oxygen Evolution Electrocatalysis

Abstract

The electrocatalytic oxygen evolution reaction (OER) is not strictly a surface reaction, because it takes place in a thin hydrous electrical double layer (EDL). In this work, we perform an in-depth study of Nafion ionomer functions in both catalyst inks and catalyst layers (CL) for high-efficiency OER in alkaline media. Based on cyclic voltammetry results, an analysis method is developed to characterize pseudocapacitance and EDL charging capacitance independently. This enables accurate quantification of interfacial charge transport behavior and active sites for OER. Zeta potential measurements confirm that Nafion ionomer serves as a stabilizing and binding agent in catalyst−solvent inks. The data obtained from the half-cell tests in 1 M KOH reveal that increasing ionomer content in CL reduces OER performance due to higher mass transport resistance and less active sites. The sample with I/C = 1/ 24 (weight ratio of Nafion ionomer to IrO x catalyst) exhibits an approximately 1.7-times higher OER activity than that of I/C = 2/1. Furthermore, the ionomer blocking effect is found to be a common phenomenon, which was observed in a wide range of catalyst loadings and three different catalyst materials. Nevertheless, as demonstrated by Nafion-free samples, the addition of Nafion is indispensable for efficient catalyst utilization. Our study shows that the optimized ionomer content in the CL is 10−30 wt % of catalyst loading. Within this range, Nafion, catalyst particles, and electrolyte solution form efficient interaction, resulting in good connectivity of the charge conduction paths without inhibiting the gas diffusion.

Electrolyte-Dependent Oxygen Evolution Reactions in Alkaline Media: Electrical Double Layer and Interfacial Interactions

“…The weight ratio of IrO x to ionomer was set to be 1/2 for achieving optimal activity and stability based on our previous work. 32 The dispersed ink was then drop-cast on a carbon fiber paper with a microporous layer (MPL) to form a gas diffusion electrode (GDE). The fabricated GDE was dried under an infrared lamp and subsequently cut into 1.5 × 1.5 cm 2 active area.…”

Nafion-Induced Reduction of Manganese and Its Impact on the Electrocatalytic Properties of a Highly Active MnFeNi Oxide for Bifunctional Oxygen Conversion

“…The obtained EOER values were 1.547±0.006 and 1.538±0.007 V vs. RHE for MnFeNiOx/GC and MnFeNiOx-Nafion/GC, respectively, indicating only a slight increase in the apparent OER activity of the electrodes upon addition of Nafion, which could be explained by an improved contact between the conductive MnFeNiOx powder and the GC substrate. [10] Similarly in the case of the ORR, MnFeNiOx-Nafion/GC displayed an EORR value of 0.788±0.003 V vs. RHE, while for MnFeNiOx/GC EORR was 0.776±0.009 V vs. RHE, with a comparatively higher reproducibility in the case of the former as shown in Figure S2. Yet, a more substantial difference in the recorded ORR currents was observed in the kinetic-diffusion mixed control region, which could be related to differences in ORR selectivity.…”

“…[6,7] Depending on the binder and its concentration, additional effects on various properties of electrode films have been reported, including ionic conductivity, hydrophobicity, mass transport, and accessibility to the reaction sites, often leading to an improvement of the catalytic performance. [8][9][10] A popular compound used as a binder is Nafion, an ionconducting ionomer comprising hydrophilic, sulfonic-terminated side chains, and built upon copolymerization of tetrafluoroethylene and perfluorinated vinyl ether monomers. [11] This binder has proved successful in improving the mechanical stability of electrode films, and thus it has been recommended for benchmark electrode preparation protocols.…”

“…[11] This binder has proved successful in improving the mechanical stability of electrode films, and thus it has been recommended for benchmark electrode preparation protocols. [12,13] It is reported that Nafion may induce decreases in overpotential attributed to an increased electrical conductivity and facilitated mass transport, [6] though depending largely on the electrode composition, [8,12,14] and not impacting otherwise the catalytic activity of, for instance, IrO2, [10,12] Pt/C, [6,15] or Pt-Sn/C. [15] However, the nature of additional effects observed with diverse materials remains unclear.…”

The Influence of the Electrodeposition Parameters on the Properties of Mn-Co-Based Nanofilms as Anode Materials for Alkaline Electrolysers

“…However, all of the above-mentioned synthesis methods typically lead to the formation of pure catalyst powder, which requires further processing to produce an ink containing other additives such as a binder and conductive carbon powder [ 15 , 23 ]. This, in turn, may result in very poor stability of the OER electrode [ 24 , 25 ]. One of the promising, alternative synthesis techniques is electrodeposition.…”