Researchers at S. Korea’s Ulsan National Institute of Science and Technology (UNIST) have developed a novel process for the production of hydrogen using various types of biomass, including lignin, as an efficient alternative to water oxidation as an electron source.

In a paper published in the journal ACS Catalysis, the researchers report that this approach allows efficient (photo)electrochemical production of hydrogen with a Faradaic efficiency close to unity at acidic pHs and brings additional economic benefits from by-products.

Schematic diagram of byproduct production and hydrogen evolution through lignin decomposition. Credit: UNIST

In principle, it is possible to produce valuable chemicals such as hydrogen, syngas, hydrocarbons, and alcohols through electrochemical processes using electricity from renewable energy resources. Regardless of final products, it is necessary to extract electrons via oxidation of sacrificial chemicals for the reductive synthesis of the aforementioned chemicals. Conventionally, water is considered a cheap and clean source of electrons; 2H₂O → O₂+4H⁺ +4e^-, E^o = 1.23V vs. reversible hydrogen electrode (RHE).

However, efficient oxidation of water or oxygen evolution reaction (OER) is a still challenging task due to slow kinetics originating from its four-electron transfer process. Despite numerous reports on the development of OER catalysts, there remain limitations such as the use of expensive elements (e.g., Pt, Ru, and Ir), high overpotential (e.g., higher than 1.5 to 1.6 V vs. RHE at 10 mA cm^-2), and low stability of electrodes and catalysts, especially at acid pHs.

In this regard, researchers have also explored alternative electron sources. … In this study, we report that electrons can be readily extracted from various types of biomasses through oxidative depolymerization with phosphomolybdic acids (PMA) at a bias much lower than 1.23V. PMA can act as an electron mediator and a catalyst for biomass depolymerization. Briefly, PMA can be readily reduced at room temperature upon oxidation of abundant biomass wastes, such as lignin and chopped particles of giant miscanthus, acacia, rice straw and larch. We have particularly focused on lignin as an electron source because it has a low utility despite its abundance and high application potential due to difficulties in selective depolymerization.

—Oh et al.

The UNIST system decomposes lignin with a molybdenum (Mo) catalyst to produce high value-added compounds, and the electrons extracted in the process effectively produce hydrogen. The new device has been designed to move electrons from lignin, along the wire to the electrode where the hydrogen evolution reaction (HER) occurs.

With this new system, we can produce hydrogen with less energy (overvoltage) than conventional water electrolysis, as there is no need for oxygen reactions, requiring high energy and precious metal catalysts. Conventional methods require more than 1.5 volts, but the new system was capable of producing hydrogen at a much lower potential (0.95 volts).

—Hyeonmyeong Oh, first author

In addition, vanillin or carbon monoxide (CO), which are produced via lignin breakdown are very useful substances for various industrial processes.

The new biofuel system is a technology that produces hydrogen and valuable chemicals using cheap catalysts and low voltages instead of expensive catalysts such as platinum (Pt). Our work is significant, as it presents a new way to replace oxygen-producing reactions in the electrolysis of water.

—Professor Jungki Ryu, corresponding author

Resources

• Hyeonmyeong Oh, Yuri Choi, Changhwan Shin, et al. (2020) “Phosphomolybdic Acid as a Catalyst for Oxidative Valorization of Biomass and Its Application as an Alternative Electron Source,” ACS Catalysis doi: 10.1021/acscatal.9b04099