Elevated concentrations of atmospheric greenhouse gases (GHGs), particularly carbon dioxide (CO 2 ), have affected the global environment, causing climate deviations and threatening the biodiversity. [1,2] Deep-cutting solutions and new technologies are thus imperative to repay the carbon debt. [3] Hydrogen (H 2 ) is an ideal fuel to enable a successful transition to a global zero emission economy: With a gravimetric energy density more than double that of conventional fuels like diesel, gasoline, and natural gas it is a lightweight energy carrier which can be converted to electricity and water via fuel cells and thus holds great promise to cut emissions of the transport and energy sectors to zero. Furthermore, it is an energy dense chemical which is already used in the chemical industry (i.e., ammonia via the Haber-Bosch process) and steel manufacturing. However, these industries currently use brown (made from coal via gasification and subsequent water gas shift reaction) and gray (made by steam methane reforming) hydrogen which both have significant CO 2 footprints. Thus, synthesizing green (meaning renewable) hydrogen cost-effectively is a solution which could green such industries and the transport sector and simultaneously meet our growing demands for viable energy storage solutions. However, switching from fossilfuels to a hydrogen economy requires efficient technologies that permit clean, sustainable and cost-effective production, storage, and utilization of H 2 . [4][5][6] Water electrolysis is a clean technology for green H 2 production, where water is split into H 2 at the cathode while O 2 is generated at the anode. Thermodynamically, the energy input required for this reaction equals an applied voltage of 1.23 V but in practice >1.8 V is commonly applied due to the sluggish kinetics of the oxygen evolution reaction (OER). The kinetics of the OER are complex. In addition to the thermodynamic potential of 1.23 V, even the best OER catalysts to date show significant overpotentials (0.35-0.4 V) which is due to suboptimal scaling relation between OOH and OH adsorption energies. [7,8] As a consequence, a significant amount of energy is spent on Biomass is recognized as an ideal CO 2 neutral, abundant, and renewable resource substitute to fossil fuels. The rich proton content in most biomass derived materials, such as ethanol, 5-hydroxymethylfurfural (HMF) and glycerol allows it to be an effective hydrogen carrier. The oxidation derivatives, such as 2,5-difurandicarboxylic acid from HMF, glyceric acid from glycerol are valuable products to be used in biodegradable polymers and pharmaceuticals. Therefore, combining biomass-derived compound oxidation at the anode and hydrogen evolution reaction at the cathode in a biomass electrolysis or photo-reforming reactor would present a promising strategy for coproducing high value chemicals and hydrogen with low energy consumption and CO 2 emissions. This review aims to combine fundamental knowledge on photo and electro-assisted catalysis to provide a comprehensive und...
