Impact of solute concentration on the electrocatalytic conversion of dissolved gases in buffered solutions

Effective energy conversion is crucial in developing a sustainable society. Chemical energy can be converted into electricity in the fuel cells system with high efficiency, where hydrogen is oxidized at the anode (hydrogen oxidation reaction; HOR) and oxygen is reduced at the cathode (oxygen reduction reaction; ORR). This reaction, forming H₂O from H₂ and O₂, is exothermic, where the chemical energy stored in the reactant can be released as electricity. Generally, parameters determining the efficiency of electrochemical cell, including fuel cells and electrolysis, are (1) required potential to reach substantial reaction rate (kinetic overpotential), (2) potential loss induced by charged species migration in the cell (ohmic potential loss) and (3) additionally required potential by a concentration gradient near the electrode surface (concentration overpotential).

In the field of electrocatalysis, the active catalysts for these reactions have been explored to reduce the kinetic overpotential, especially for the ORR that is a bottle-neck of the fuel cell technology. Historically, the HOR and ORR have been intensively investigated at extremely alkaline/acidic pH levels, which are kinetically favored. Under such conditions, however, the harsh operating environment requires highly stable catalyst materials and operation plant.

Recently, more attentions have been given to electrochemistry at near-neutral pH levels using buffered solutions, where comparable kinetics is attained compared with extreme-pH conditions. At near-neutral pH, in the absence of buffered species (e.g., phosphate, carbonate and borate), the water molecule has to be involved in the HOR/ORR to achieve a substantial reaction rate due to lack of hydronium and hydroxide ions. The overall performances under such conditions are much inferior to those at acidic and alkaline pH. On the contrary, the presence of buffered species functions to maintain the local pH near the surface of electrodes, resulting in drastically improved surface kinetics.

In any cases, usage of electrolyte solutions (supporting electrolyte) is effective in the electrochemical reactions to reduce the ohmic potential loss. Typically, 0.1 – 0.5 mol L⁻¹ of salts are added to the electrochemical cells. Under highly acidic/alkaline conditions, simple solutions, e.g., sulfuric acid or potassium hydroxide, serve as a supporting electrolyte due to high mobility and activity of hydronium and hydroxide ions. Under buffered near-neutral pH conditions (pH 4 – 10), however, a substantial quantity of salts is needed to obtain a reasonable electrolyte conductivity. Also from the viewpoint of buffering capacity, highly concentrated solution is favored for buffered near-neutral pH systems.

As demonstrated in the article, with increasing solute concentration at near-neutral pH, the concentration overpotential becomes problematic for the HOR/ORR to achieve substantial current density. The properties of water have been adopted in most of the studies to describe the mass transport phenomena in the electrochemical cells even under near-neutral pH conditions. However, in buffered electrolyte solutions, the water properties have been revealed not to be applicable anymore. The electrolyte nature (the kinematic viscosity of the solution, diffusion coefficient and solubility of gases) are drastically altered by adding salts in the cell under buffered near-neutral conditions.

The significance of electrolyte properties, highlighted in the article, has been overlooked in many studies. The electrolyte properties are crucial not only for the ORR/HOR but also for other electrochemical reactions including the hydrogen evolution reaction and oxygen evolution reaction for water splitting. Indeed, recently some literature describes “unusual catalysis” affected by cation and anions in the solution. In the context of the current study, most likely the electrolyte identity induces apparent alteration of the electrochemical performances. For the study on the H₂/O₂ aqueous electrochemistry, the concentration and identity of electrolyte have to be chosen with great care to attain high performances as well as to address surface kinetics properly.