Scientists have been working to harness the energy from sunlight to drive chemical reactions to form fuels such as hydrogen, which provide a way to store solar energy for future use.

MADISON, Wis. - In a study published March 9 in Nature Chemistry, University

of Wisconsin-Madison chemistry Professor Kyoung-Shin Choi presents a new
approach to combine solar energy conversion and biomass conversion, two
important research areas for renewable energy.

For decades, scientists have been working to harness the energy from
sunlight to drive chemical reactions to form fuels such as hydrogen, which
provide a way to store solar energy for future use. Toward this end, many
researchers have been working to develop functional, efficient and
economical methods to split water into hydrogen, a clean fuel, and oxygen
using photoelectrochemical solar cells (PECs). Although splitting water
using an electrochemical cell requires an electrical energy input, a PEC can
harness solar energy to drive the water-splitting reaction. A PEC requires a
significantly reduced electrical energy input or no electrical energy at
all.

In a typical hydrogen-producing PEC, water reduction at the cathode
(producing hydrogen) is accompanied by water oxidation at the anode
(producing oxygen). Although the purpose of the cell is not the production
of oxygen, the anode reaction is necessary to complete the circuit.
Unfortunately, the rate of the water oxidation reaction is very slow, which
limits the rate of the overall reaction and the efficiency of the
solar-to-hydrogen conversion. Therefore, researchers are currently working
to develop more efficient catalysts to facilitate the anode reaction.

Choi, along with postdoctoral researcher Hyun Gil Cha, chose to take a
completely new approach to solve this problem. They developed a novel PEC
setup with a new anode reaction. This anode reaction requires less energy
and is faster than water oxidation while producing an industrially important
chemical product. The anode reaction they employed in their study is 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 - a type of cheap and abundant plant matter. FDCA is an
important molecule for the production of polymers.

Biomass conversion can offer a viable pathway to generate chemicals used in
industrial processes without using petroleum products. Conventional biomass
conversion processes use high-pressure oxygen for the conversion of HMF to
FDCA at high temperatures. Choi and Cha, however, developed an efficient
electrochemical method to oxidize HMF to FDCA at room temperature and
ambient pressure using water as the oxygen source. Then they employed this
oxidation reaction as the anode reaction of the PEC that produces hydrogen
at the cathode. By doing so, they demonstrated the utility of solar energy
for biomass conversion as well as the feasibility of using an oxidative
biomass conversion reaction as an anode reaction in a hydrogen-forming PEC.

"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," says
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," Choi says. "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."

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.