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UW-Madison team develops novel hydrogen-producing photoelectrochemical cell using solar-driven biomass conversion as anode reaction

Researchers at the University of Wisconsin-Madison have developed an innovative hydrogen-producing photoelectrochemical cell (PEC), using solar-driven biomass conversion as the anode reaction. In a paper in the journal Nature Chemistry, the duo reports obtaining a near-quantitative yield and 100% Faradaic efficiency at ambient conditions without the use of precious-metal catalysts for this reaction, which is also thermodynamically and kinetically more favorable than conventional water oxidation at the anode. They thus demonstrated the utility of solar energy for biomass conversion (rather than catalysts) as well as the feasibility of using an oxidative biomass conversion reaction as an anode reaction in a hydrogen-forming PEC.

Chemistry Professor Kyoung-Shin Choi and postdoc Hyun Gil Cha said that their results suggest that solar-driven biomass conversion can be a viable anode reaction that has the potential to increase both the efficiency and the utility of PECs constructed for solar-fuel production.

Photoelectrochemical cells (PECs) can directly utilize photogenerated electron–hole pairs in semiconductor electrodes for fuel production, as nature does through photosynthesis. In a typical PEC, fuels are formed by reduction reactions at the cathode that consume photo-excited electrons. Examples include the reduction of water to give H2 and the reduction of CO2 to give carbon-based fuels such as methanol and methane. To complete the circuit, oxidation reactions occur at the anode and consume photogenerated holes.

In general, water oxidation to give O2 is used as the anode reaction, which is environmentally benign and does not require additional species in the electrolyte. Another critical role of water oxidation as the anode reaction for a sustainable PEC operation is the generation of H+ (2H2O → O2 + 4H+) to offset the H+ consumption accompanied by the cathode reaction, which reduces water or CO2. However, water oxidation is not a kinetically favoured reaction, and its product, O2, is not of significant value.Therefore, to identify an anode reaction that has more favourable kinetics and can generate value-added chemicals would be beneficial to increase the overall efficiency and utility of PECs.

The production of building-block chemicals as well as of fuels using renewable energy sources is critical for a complete independence of fossil fuels. To achieve this goal, as well as to address the aforementioned issues, oxidatively producing building-block organic molecules using biomass-derived intermediates as alternative anode reactions of PECs is an exciting and desirable strategy.

—Cha and Choi (2015)

For the anode reaction in the current study, Choi and Cha employed the oxidation of 5-hydroxymethylfurfural (HMF) to 2,5-furandicarboxylic acid (FDCA). HMF is a key intermediate in biomass conversion that can be derived from cellulose; FDCA is an important molecule for the production of polymers. Accordingly, the oxidation of HMF into FDCA has already received a great deal of research and development attention.

Most of the earlier work exploring the conversion of HMF into FDCA utilized aerobic oxidation using heterogeneous catalysts. However, an alternative approach is electrochemical oxidation in which the oxidation is driven by the electrochemical potential applied to the electrode—eliminating the use of O2 or other chemical oxidants.

Choi and Cha developed an efficient electrochemical method to oxidize HMF to FDCA at room temperature and ambient pressure using water as the oxygen source and​2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) as a mediator. Then they employed this oxidation reaction as the anode reaction of the PEC that produces hydrogen at the cathode.

Since the photoelectrochemical cell is built for the purpose of hydrogen production and HMF oxidation simply replaces oxygen production at the anode, in essence, no resources are used specifically for HMF oxidation.

—Professor Choi

In other words, FDCA is a bonus byproduct from a PEC that generates hydrogen. The production of FDCA, a valuable chemical, at the anode lowers the production cost for hydrogen. This new approach therefore presents new possibilities for research in both solar conversion and biomass conversion.

When we first started this study, we were not sure whether our approach could be really feasible. However, since we knew that the impact of the study could be high when successful, we decided to invest our time and effort on this new research project at the interface of biomass conversion and solar energy conversion.

—Professor Choi

Developing and optimizing every piece of the full solar cell setup demonstrated in the study took the researchers about two years. Choi expects that the development of more diverse and efficient electrochemical and solar-driven biomass conversion processes will increase the efficiency and utility of solar-fuel-producing PECs.

Resources

  • Hyun Gil Cha & Kyoung-Shin Choi (2015) “Combined biomass valorization and ​hydrogen production in a photoelectrochemical cell” Nature Chemistry doi: 10.1038/nchem.2194

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