Thermodynamic, Kinetic, and Transport Contributions to Hydrogen Evolution Activity and Electrolyte-Stability Windows for Water-in-Salt Electrolytes

Abstract

Concentrated water-in-salt electrolytes (WiSEs) are used in aqueous batteries and to control electrochemical reactions for fuel production. The hydrogen evolution reaction is a parasitic reaction at the negative electrode that limits cell voltage in WiSE batteries and leads to self-discharge, and affects selectivity for electrosynthesis. Mitigating and modulating these processes is hampered by a limited fundamental understanding of HER kinetics in WiSEs. Here, we quantitatively assess how thermodynamics, kinetics, and interface layers control the apparent HER activities in 20 m LiTFSI. When the LiTFSI concentration is increased from 1 to 20 m, an increase in proton activity causes a positive shift in the HER equilibrium potential of 71 mV. The exchange current density, i o , derived from the HER branch for 20 m LiTFSI in 98% purity (0.56 ± 0.05 μA/cm Pt 2 ), however, is 8 times lower than for 20 m LiTFSI in 99.95% (4.7 ± 0.2 μA/cm Pt 2 ) and 32 times lower than for 1 m LiTFSI in 98% purity (18 ± 1 μA/cm Pt 2 ), demonstrating that the WiSE's impurities and concentration are both central in significantly suppressing HER kinetics. The ability and applicability of the reported methods are extended by examining additional WiSEs formulations made of acetates and nitrates.

Correlating Substrate Reactivity at Electrified Interfaces with the Electrolyte Structure in Synthetically Relevant Organic Solvent/Water Mixtures

“…When referencing against Fc +/0 and Me 10 Fc +/0 , the HOR and HER curves in the mixtures with acetone and ACN are, once again, shifted in an exact opposite way (Figure a,b). To verify if the observed shifts arise from a shift of the H 2 O/H 2 thermodynamic potential, we experimentally measured the equilibrium potential of a platinum rotating disk electrode rotated at 2500 rpm immersed in H 2 -saturated solvent/5.0 M water/0.1 M LiClO 4 mixtures, constituting a reversible hydrogen electrode (RHE) (see the Experimental Section and Figures S38–S40 for full details). When the reaction is referenced against the RHE, no shift is observed for the HER and HOR curves between the acetone- and ACN-based mixtures (Figure c).…”

“…A platinized platinum wire was first used to accurately measure the equilibrium potential . However, the measured potential was found to be unstable (Figure S38) and a fade of the black color of the wire was noticeable after the measurements.…”

“…However, the measured potential was found to be unstable (Figure S38) and a fade of the black color of the wire was noticeable after the measurements. A platinum RDE rotated at 2500 rpm was therefore used, giving stable potentials (Figure S39). Great care was taken when cleaning the RDE before measurements, as the measured potentials are sensitive to the cleanliness of the electrode (Figure S40).…”

Cooperative Effects Associated with High Electrolyte Concentrations in Driving the Conversion of CO2 to C2H4 on Copper

“…51,52 It was first introduced for achieving the goal of an aqueous battery owing to its wide electrochemical stability window and suitable conductivity. 53,54 The significantly increased salt solubility and decreased concentration of free water of WiSE compared to conventional low concentration electrolyte provides a pathway to tune the electrolyte for CO2R. As a few studies have shown, applying WiSE for CO2R can mitigate the unwanted HER reaction and control the product selectivity.…”