Solar water splitting is a promising technology for the sustainable production of hydrogen. Despite numerous efforts over decades, it is still suffering from poor performance and stability issues due to many technological challenges, in particular those related to energy-intensive oxygen evolution reactions (OER) that supply protons and electrons for reductive hydrogen evolution reactions. Although many researchers have attempted the development of cheap and efficient catalysts to address such issues, most state-of-the-art water-splitting systems still heavily rely on conventional Pt- and Ir-based catalysts due to sluggish OER kinetics that accompanies the transfer of four electrons and four protons. Recently, researchers start paying attention to the exploration of alternative electron sources for bypassing the energy-intensive OER. Biomass and biomass-derived chemicals have been considered primary candidates due to their abundance and sustainability. Examples include glucose, methanol, ethanol, glycerol, etc. While these chemicals can be readily oxidized to donate electrons and thus significantly improved the performance of solar hydrogen production, they have many limitations for practical applications, such as global food ethics and economic feasibility, because most of them are derived from edible biomass. In this regard, lignin and lignocellulose can be ideal alternatives to OER. In this talk, we report that lignin and lignocellulose be efficiently utilized for solar hydrogen production using soluble redox-active catalysts. Their high solubility and redox activity allow for efficient electrochemical oxidative depolymerization of even solid biomass and efficient mediated electron transfer between biomass and electrodes. As a result, we could achieve a photocurrent density of 19.8 mA/cm2 for hydrogen production along with the production of value-added chemicals from lignin. This study can provide insights into the utilization of both solar energy and biomass for a sustainable future.

Oxidative depolymerization of lignin for solar hydrogen production

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