Defect-Driven Optimization of TiO₂-Based Electrodes for High-Efficiency Electrochemical 1,4-NADH Generation

The electrochemical reduction of nicotinamide adenine dinucleotide (NAD) using water as a hydrogen source is a promising strategy for the efficient and environmentally friendly production of the active enzymatic cofactor 1,4-NADH, which is a key for the further application of enzymatic systems in various industrial fields. However, the efficient regeneration of 1,4-NADH (NADH-reg) remains a major challenge. The rate-limiting step in the electrochemical conversion of the oxidized NAD is the second electron transfer to an NAD radical, which is formed by the reduction of NAD+, at a large overpotential, whereas the other side reactions proceed readily. To surmount this obstacle and promote NADH-reg, we used Ni nanoparticle-loaded TiO2 on a Ti electrode (Ni-TOT) containing a sufficient number of defects as active sites, which are formed at 300 °C in the H2 atmosphere. Ni-TOT facilitated the formation of enzymatically active 1,4-NADH with a superior yield and significantly reduced overpotential compared to those on untreated TOT. We found that hydrogen spillover promotes the formation of active sites on Ni-TOT. This study highlights the potential of engineered defect-enriched electrodes as a means to advance NADH-reg.