Tailorable Electrocatalytic 5-Hydroxymethylfurfural Oxidation and H₂Production: Architecture-Performance Relationship in Bifunctional Multilayer Electrodes

Water electrocatalytic splitting is considered as an ideal process for generating H2 without byproducts. However, in the water-splitting reaction, a high overpotential is needed to overcome the high-energy barrier due to the slow kinetics of the oxygen evolution reaction (OER). In this study, we selected the 5-hydroxymethylfurfural (HMF) oxidation reaction, which is thermodynamically favored, to replace the OER in the water-splitting process. We fabricated three-dimensional hybrid electrocatalytic electrodes via layer-by-layer (LbL) assembly for simultaneous HMF conversion and hydrogen evolution reaction (HER) to investigate the effect of the nanoarchitecture of the electrode on the electrocatalytic activity. Nanosized graphene oxide was used as a negatively charged building block for LbL assembly to immobilize the two electroactive components: positively charged Au and Pd nanoparticles (NPs). The internal architecture of the LbL-assembled multilayer electrodes could be precisely controlled and their electrocatalytic performance could be modified by changing the nanoarchitecture of the electrode, including the thickness and position of the metal NPs. Even with a composition of the identical constituent NPs, the electrodes exhibited highly tunable electrocatalytic performance depending on the reaction kinetics as well as a diffusion-controlled process due to the sequential HMF oxidation and the HER. Furthermore, a bifunctional two-electrode electrolyzer for both the anodic HMF oxidation and the cathodic HER, which had an optimized LbL-assembled electrode for each reaction, exhibited the best full-cell electrocatalytic activity.