In the 21th century, energy and environmental issues represent some of the most significant and difficult problems to be solved. The large-scale utilization of solar energy is one of the most attractive candidate methods of providing sustainable, renewable energy and chemical resources. To achieve this, artificial photosynthesis, energy conversion from solar energy to chemical energy, is a subject that many researchers are studying all over the world.
Success in solar CO2 reduction using water as both an electron donor and a proton source leads to a promising artificial photosynthetic process for the conversion of H2O and CO2 into useful organic materials and oxygen. We have established a concept of a photocatalyst composed of a semiconductor photosensitizer and a metal-complex electrocatalyst, which facilitates selective CO2 reduction to C1 chemicals under visible light irradiation. In consequent, the self-sustained artificial photosynthetic process, the solar photoreduction of CO2 to HCOO- (selectivity > 70%) was achieved in aqueous media, in which the system operated without an external electrical bias. The conversion efficiency of solar energy to chemical energy reached 0.14% using CO2, H2O and sunlight only, which is close to that for switchgrass, a promising crop for biomass fuel. A concept for a wireless device operaing in one-compartment rector is also demonstrated.
We report on an energy conversion system which converts CO2 into organic energy source by light and water. For the photo-electrode, we applied the gallium nitride (GaN) with nickel oxide co-catalysts on its surface. It had been difficult to realize this reaction because the energy of excited electron is lower than that of CO2 conversion in most oxide-based photo-catalysts; however, we firstly found that nitride semiconductor makes it possible to solve this problem and succeeded in realizing the CO2 reduction by light illumination alone. The GaN photo-electrode has a hetero structure with Al-doped GaN (AlGaN) in order to take advantage of a polarization effect in AlGaN layer, which enhances the electron-hole separation with suppressing the recombination of them. By the design of thin film structure in nitride semiconductor and adoption of indium (In) cathode, the energy conversion efficiency from solar light to formic acid (HCOOH) reached 0.15 %, which is almost the same level as that of real plants. We can show a demonstration cell which enables us to show a chemical reaction by color change of test reagent. The outdoor experiment is also shown in which the photo current can be observed under the real sun light.
Microbial Electrosynthesis (MES) is an emerging category of bio-electrochemical systems to convert CO2 into commodity chemicals. In MES, electrical current is used to drive microbial production of organics (such as methane, organic acids and alcohols) from CO2. Thus, MES systems using photovoltaic cell (or other renewable sources) as the power sources can be regarded as “artificial photosynthesis systems”. Based on the mechanism of electron transfer from the electrode (cathode) to catalytic microorganisms, MES can be divided into two types, the “indirect” and “direct” MESs. In the “indirect” systems, electrons are used to biotically or abiotically reduce electron-mediator molecules (electron shuttles or electron donors), which are then used by the CO2-reducing microorganisms. In the “direct” systems, on the other hand, electrons are directly utilized to drive the metabolic process of catalytic microorganisms, which were electrically linked to the cathode (called “biocathode”). In this review, with particular focus on the biocathode-utilizing systems, current status of researches on MES is overviewed and technical considerations for implementation of the systems will be discussed.
Catalytic olefin synthesis from CO2 and solar hydrogen is undergoing at ARPChem project, which is mainly funded by METI. Lower olefins such as ethylene, propylene and butenes are the key raw materials of chemical industry. By the replacement of fossil based olefins to the carbon neutral ones, we can expect the drastic reduction of anthropogenic CO2 emission. As realistic catalytic processes, Fisher Tropsh type reaction and MTO (Methanol to Olefin) type reaction seem to be attractive. Both systems have been established by means of fossil feed stocks but they should be modified or re-designed as a carbon neutral GSC process.