Novel Si photoelectrodes architecture with nanostructured Au catalysts to reduce CO₂ into CO

A novel strategy has been proposed for converting carbon dioxide (CO₂) into carbon monoxide (CO) by using solar energy and water. Nanostructured Au, with a superior catalytic property for reducing CO₂ into CO, is placed on a Si photoelectrode as a co-catalyst. Solar-generated carriers in a Si photoelectrode effectively drive CO₂ reduction at the surface of the nanostructured Au, lowering the reaction’s overpotential.

Article  |  Fall 2017

Carbon dioxide (CO₂) has been considered as a main cause of global climate change as its concentration has reached over 400 ppm in the atmosphere. Recently, carbon capture and utilization (CCU) technology has drawn great interest for reducing CO₂ concentrations by converting it into value-added chemical products (e.g. carbon monoxide, alcohols, and ethylene) from renewable energy sources (e.g. solar and wind).

Direct CO₂ conversion has been considered as a possible technological solution to environmental challenges caused by producing fuels and chemical building blocks (e.g. alcohols and ethylene) from renewable energy sources (e.g. solar and wind).

A photoelectrochemical (PEC) CO₂ reduction cell, which is also known as an artificial photosynthesis cell, produces chemical building blocks by using sunlight, water, and CO₂ in semiconductor/electrolyte interfaces. However, numerous challenges hinder the development of practical applications of PEC CO₂ reduction cells. The most critical obstacles are the high energy required for CO₂ reduction due to the stable nature of CO₂ molecules. In addition, few semiconductor materials can absorb the visible spectrum of sunlight while also possessing appropriate energetics to reduce CO₂.

Professor Jihun Oh and his group developed an efficient and selective Si photocathode for CO₂ reduction. His group discovered that nanoporous Au thin film formed by electrochemically reducing an anodized Au thin film shows exceptional CO₂ reduction activity, along with lowered overpotential and improved selectivity of CO₂ reduction. Generally, Au is known as a catalyst for reducing CO₂ to CO. CO is a key component of syngas (a mixture of H2 and CO), which is widely used to produce various hydrocarbon products such as diesel and methanol in the chemical industry. The developed nanoporous Au shows over 90% selectivity for producing CO at 480 mV overpotential as compared to bare Au thin film (~70% selectivity for CO at 480 mV overpotential). Their nanoporous Au catalysts uses 200nm-thick thin film, which is 50,000 times thinner than previously-reported 0.1mm-thick Au nanostructure catalyst and is exceptionally cost-effective.

With this discovery, Prof. Oh’s group further proposed a new artificial photosynthetic cell design to use nanoporous Au thin film as a co-catalyst material for solar CO₂ reduction. As a solar absorber material, Si semiconductor is used since it is earth-abundant and absorbs most of solar spectrum. The proposed Si photoelectrode with nanoporous Au cocatalysts exhibits an exceptional CO₂ reduction property under 1 sun illumination. It achieves 91% CO₂ conversion selectivity in order to produce CO at more positive potential (–0.03 V versus reversible hydrogen electrode) than CO₂/CO equilibrium potential (–0.11 V versus reversible hydrogen electrode) under 1 sun illumination. Prof. Oh claims that the 90% CO₂ conversion rate at more positive potential than equilibrium potential is firstly reported to the best of the team’s knowledge. Prof. Oh believes that their technology provides a general platform to design an efficient photoelectrochemical CO₂ conversion cell.