Amphi-Funtional Mediator for Proton-Coupled Electron Transfer in Low-Voltage Electrocatalytic-Hydrogenation
Authors: Lee Jisu 
Published: December 2024

Within renewable energy systems, proton exchange membrane (PEM) electrolyzer relies on multiple proton-coupled electron transfer (PCET) reaction on heterogeneous catalyst surface. In PEM electrolyzer, the catalytic process of hydrogen adsorption through Volmer reaction, involving the combination of a proton and an electron on surface, holds significance. Recent attention has been focused on optimizing the proton and an electron coupling by lowering the activation barrier and driving force of the hydrogen adsorption reaction. This focus is primarily on the cathodic part, involving hydrogen evolution reaction (HER) and organic electro-hydrogenation systems, initiating the formation of a surface-adsorbed hydrogen atom (H*) as first step. Among them, hydrogen spillover has emerged as a new method to reduce the activation barrier in HER and hydrogenation systems. This mechanism contains the diffusion of adsorbed H+ from hydrogen-rich to deficient surface. An optimal catalyst for hydrogen spillover is characterized by strong dissociation abilities and weak binding to dissociated hydrogen atoms. Palladium (Pd) exhibits commendable equilibrium of these features, suggesting the potential for performance improvement through alloying with other metals. However, hydrogen spillover is specific to the cathodic part, where the reactant is the surface-adsorbed hydrogen atom, and do not extend to the anodic part, which involves reactants unrelated to the adsorbed hydrogen atom.

Generally, in the anodic part of the electrolyzer system, water-based reactants (H2O in acid/ OH- in based) or organic-based reactants (such as hydrazine, ethanol, and formic acid) are utilized. Nowadays, organic-based reactants are preferred in low-voltage electrolysis due to the sluggish kinetics of electro-decomposition in water-based reactants. Among them, formic acid emerges as a promising organic molecule for achieving high efficiency. Formic acid electrooxidation reaction (FAOR) theoretically exhibits a negative oxidation potential compared to both hydrogen oxidation reaction (HOR) and oxygen evolution reaction (OER): Eo = -0.17 VSHE for HCOOH/CO2 < +0 VSHE for H2/H+ < +1.23 VSHE for H2O/O2. Platinum (Pt)-based catalysts are utilized in FAOR. Unfortunately, since HCOOH oxidation occurs under more positive potential than H* desorption (0.05 V < E < 0.35 VRHE), the FAOR encounters a limitation of overpotential that exceeds the potential of H* desorption. In other words, acceleration of H* desorption on active site could be necessary for the anodic reaction. To facilitate the movement of H* in both anodic and cathodic reactions, it is essential to consider accelerating both H* adsorption and desorption on the active site to ensure the efficiency. Despite the emphasis on activating the target reaction by controlling PCET, there are limited studies on dual mechanisms of H* adsorption and desorption in catalytic system.

Biological energy conversion involves a diverse array of ion-coupled electron transfer reaction. Many organisms in electron transport chain facilitate the movement of electrons from electron donors to acceptors through redox reactions, linking this electron transfer with H+ ions. Here, we report Amphi-functional mediator (AFM) molecules, as a hydrogen atom modulator. Through the electrodeposition method onto Pt and Pd, we facilitate distinct movements of H*, resulting in low overpotential and enhanced catalytic efficiency. Depending on the migration energy of H*, AFM can function as both an acceptor and donor of H* to Pt and Pd, respectively. In a hydrogen-producing electrolyzer employing FAOR as the anodic process (FAOR||HER), AFC reduced the cell potential by 1.15 V at 10 mA cm-2, improving the faradaic efficiency from 87% to 100%. In an electrochemical hydrogen-carrier-producing reactor to hydrogenate toluene to methylcyclohexane (MCH), AFM successfully generated MCH, which cannot be produced without AFM.

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Figure 1