Electrochemical hydrogenation of non-aromatic carboxylic acid derivatives as a sustainable synthesis process: from catalyst design to device construction

In situ electrochemical regeneration of active 1,4-NADH for enzymatic lactic acid formation via concerted functions on Pt-modified TiO2/Ti

Nicotinamide adenine dinucleotide (NAD+) and its reduced form (NADH) are key cofactors serving as essential hydrogen acceptors and donors to facilitate energy and material conversions under mild conditions. We demonstrate direct electrochemical conversion to achieve highly efficient regeneration of enzymatically active 1,4-NADH using a Pt-modified TiO2 catalyst grown directly on a Ti mesh electrode (Pt-TOT). Spectral analyses revealed that defects formed by the inclusion of Pt species in the lattice of TiO2 play a critical role in the regeneration process. In particular, Pt-TOT containing approximately 3 atom% of Pt exhibited unprecedented efficiency in the electrochemical reduction of NAD+ at the lowest overpotential to date. This exceptional performance led to the production of active 1,4-NADH with a significantly high yield of 86 ± 3% at -0.6 V vs. Ag/AgCl (-0.06 V vs. RHE) and an even higher yield of 99.5 ± 0.4% at a slightly elevated negative potential of -0.8 V vs. Ag/AgCl (-0.2 V vs. RHE). Furthermore, the electrochemically generated NADH was directly applied in the enzymatic conversion of pyruvic acid to lactic acid using lactate dehydrogenase.

Developing electrochemical hydrogenation towards industrial application

Electrochemical hydrogenation reactions gained significant attention as a sustainable and efficient alternative to conventional thermocatalytic hydrogenations. This tutorial review provides a comprehensive overview of the basic principles, the practical application, and recent advances of electrochemical hydrogenation reactions, with a particular emphasis on the translation of these reactions from lab-scale to industrial applications. Giving an overview on the vast amount of conceivable organic substrates and tested catalysts, we highlight the challenges associated with upscaling electrochemical hydrogenations, such as mass transfer limitations and reactor design. Strategies and techniques for addressing these challenges are discussed, including the development of novel catalysts and the implementation of scalable and innovative cell concepts. We furthermore present an outlook on current challenges, future prospects, and research directions for achieving widespread industrial implementation of electrochemical hydrogenation reactions. This work aims to provide beginners as well as experienced electrochemists with a starting point into the potential future transformation of electrochemical hydrogenations from a laboratory curiosity to a viable technology for sustainable chemical synthesis on an industrial scale.

Advancement of Electrochemical Thermoelectric Conversion with Molecular Technology

Thermocells are a thermoelectric conversion technology that utilizes the shift in an electrochemical equilibrium arising from a temperature difference. This technology has a long history; however, its low conversion efficiency impedes its practical usage. Recently, an increasing number of reports have shown drastic improvements in thermoelectric conversion efficiency, and thermocells could arguably represent an alternative to solid thermoelectric devices. In this Minireview, we regard thermocells as molecular systems consisting of successive molecular processes responding to a temperature change to achieve energy generation. Various molecular technologies have been applied to thermocells in recent years, and could stimulate diverse research fields, including supramolecular chemistry, physical chemistry, electrochemistry, and solid-state ionics. These research approaches will also provide novel methods for achieving a sustainable society in the future.

Cu Modified TiO2 Catalyst for Electrochemical Reduction of Carbon Dioxide to Methane

Electrochemical reduction of CO2 (ECO2R) is gaining attention as a promising approach to store excess or intermittent electricity generated from renewable energies in the form of valuable chemicals such as CO, HCOOH, CH4, and so on. Selective ECO2R to CH4 is a challenging target because the rate-determining step of CH4 formation, namely CO* protonation, competes with hydrogen evolution reaction and the C–C coupling toward the production of longer-chain chemicals. Herein, a Cu-TiO2 composite catalyst consisting of CuOx clusters or Cu nanoparticles (CuNPs), which are isolated on the TiO2 grain surface, was synthesized using a one-pot solvothermal method and subsequent thermal treatment. The Cu-TiO2 catalyst exhibited high selectivity for CH4, and the ratio of FE for CH4 to total FE for all products in ECO2R reached 70%.

Heterointerface Created on Au-Cluster-Loaded Unilamellar Hydroxide Electrocatalysts as a Highly Active Site for the Oxygen Evolution Reaction

The oxygen evolution reaction (OER) is a critical element for all sorts of reactions that use water as a hydrogen source, such as hydrogen evolution and electrochemical CO₂ reduction, and novel design principles that provide highly active sites on OER electrocatalysts push the limits of their practical applications. Herein, Au-cluster loading on unilamellar exfoliated layered double hydroxide (ULDH) electrocatalysts for the OER is demonstrated to fabricate a heterointerface between Au clusters and ULDHs as an active site, which is accompanied by the oxidation state modulation of the active site and interfacial direct OO coupling ("interfacial DOOC"). The Au-cluster-loaded ULDHs exhibit excellent activities for the OER with an overpotential of 189 mV at 10 mA cm^-2 . X-ray absorption fine structure measurements reveal that charge transfer from the Au clusters to ULDHs modifies the oxidation states of trivalent metal ions, which can be active sites on the ULDHs. The present study, supported by highly sensitive spectroscopy combining reflection absorption infrared spectroscopy and modulation-excitation spectroscopy and density functional theory calculations, indicates that active sites at the interface between the Au clusters and ULDHs promote a novel OER mechanism through interfacial DOOC, thereby achieving outstanding catalytic performance.

Electrochemical Molecular Conversion of α-Keto Acid to Amino Acid at a Low Overpotential Using a Nanoporous Gold Catalyst

A nanoporous gold (NPG) electrode prepared through a facile anodization technique was employed in the electrochemical reductive amination of biomass-derivable α-keto acids in the presence of a nitrogen source to produce the corresponding amino acids. NPG showed a clear reductive current in the presence of α-keto acid and NH₂OH, and the electrolysis experiments confirmed the production of L-amino acid. A reductive voltammetric signal at the NPG electrode appeared at a more positive potential by 0.18-0.79 V, compared with those at the planar-gold electrode without anodization and other previously reported electrode systems, indicating the high activity of the prepared nanostructure for the electrochemical reaction. Maximum Faradaic efficiencies (FEs) of 74-93% in the reductive molecular conversion to amino acids of Ala, Asp, Glu, Gly, and Leu were obtained under the optimized conditions. The FE values were strongly dependent on the applied potential in the electrolysis, suggesting that the hydrogen evolution reaction at the electrode surface was more significant as the applied potential became more negative. The effect of potential at the NPG was lower than that at the planar-gold electrode. These results indicate that nanostructurization decreases the overpotential for the electrochemical reductive amination, resulting in high FE.

Inversely polarized thermo-electrochemical power generation via the reaction of an organic redox couple on a TiO2/Ti mesh electrode

We demonstrate thermo-electrochemical (TEC) conversion using a biocompatible redox couple of lactic acid and pyruvic acid on earth-abundant TiO₂. The TEC cell exhibited a positive Seebeck coefficient of 1.40 mV K^-1. DFT calculations figured out that the adsorption of intermediate species and protons on TiO₂ controls both the redox reaction and current polarity.

Selectivity enhancement in the electrochemical reduction of oxalic acid over titanium dioxide nanoparticles achieved by shape and energy-state control

The morphology of TiO2 nanoparticles has a substantial impact on the product selectivity in an electrochemical reduction reaction.

Electrosynthesis of amino acids from biomass-derivable acids on titanium dioxide

Seven amino acids were electrochemically synthesized from biomass-derivable α-keto acids and NH2OH with faradaic efficiencies (FEs) of 77-99% using an earth-abundant TiO2 catalyst. Furthermore, we newly constructed a flow-type electrochemical reactor, named a "polymer electrolyte amino acid electrosynthesis cell", and achieved continuous production of alanine with an FE of 77%.

Catalytic enhancement on Ti–Zr complex oxide particles for electrochemical hydrogenation of oxalic acid to produce an alcoholic compound by controlling electronic states and oxide structures

Ti_0.9Zr_0.1O₂ complex oxide particles exhibit superior catalytic performances for the direct power storage into glycolic acid via electroreduction of oxalic acid due to favorable crystallinity.