Hiroshi Hashiba

Photo-induced CO2 Reduction with GaN Electrode in Aqueous System

CO2 reduction with water and light illumination is realized using a gallium nitride (GaN) photoelectrode in which excited electrons induce CO2 conversion at the counter electrode. For the counter electrode, a copper (Cu) plate was chosen. The low affinity and wide gap of the nitride semiconductor enable us to create an electron–hole pair which has a sufficient energy for both CO2 reduction and water oxidation, in spite of the fact that a high energy for CO2 reduction is required. Within this system, the generation of formic acid (HCOOH) with 3% Faradic efficiency was confirmed by light illumination alone.

Enhanced CO2 reduction capability in an AlGaN/GaN photoelectrode

Light illumination of a gallium nitride photoelectrode creates separate electron-hole pairs that drive water oxidation and CO2 reduction reactions. Here, we show enhanced photocurrent in an AlGaN/GaN device that consists of an unintentionally doped (uid-) AlGaN photoabsorption layer and an n+-GaN electrical-conduction layer. The production rate of formic acid by CO2 conversion in the uid-AlGaN/n+-GaN photoelectrode is about double that in the uid-GaN/n+-GaN device. This improvement is most likely due to the effect of internal bias in the uid-AlGaN layer generated by the polarization effect, which improves electron-hole separation.

Enhanced Capability of Photoelectrochemical CO2 Conversion System Using an AlGaN/GaN Photoelectrode

We report significantly improved photosynthesis system based on AlGaN/GaN photochemical process. The resultant energy conversion efficiency is 0.13% which is the same level as that of real plants. The capability of this system is enhanced by high cathode potential due to the reduction of energy loss while utilizing the piezoelectric effect in the AlGaN/GaN heterostructure. The Faradaic efficiency of the CO2 conversion to organic materials is enhanced, accompanied by an increment in photocurrent by modification of the AlGaN/GaN photoelectrode structure and electrolytes. Furthermore, reaction products such as C2H4 and C2H5OH are generated by light illumination alone.

CO2 Conversion with Light and Water by GaN Photoelectrode

Light illumination on a photoelectrode creates separate electron and hole pairs that lead to an oxidation and reduction reaction. Here, we show that CO2 reduction by means of water and light is realized by a gallium nitride (GaN) photoelectrode in which excited electrons drive CO2 conversion at the counterelectrode. A copper (Cu) plate was chosen as the counterelectrode. With this system, the generation of formic acid (HCOOH) with 9% Faradic efficiency was confirmed by light illumination alone with the help of NiO co-catalysts.

Highly efficient photochemical HCOOH production from CO2 and water using an inorganic system

We have constructed a system that uses solar energy to react CO2 with water to generate formic acid (HCOOH) at an energy conversion efficiency of 0.15%. It consists of an AlGaN/GaN anode photoelectrode and indium (In) cathode that are electrically connected outside of the reactor cell. High energy conversion efficiency is realized due to a high quantum efficiency of 28% at 300 nm, attributable to efficient electron-hole separation in the semiconductors heterostructure. The efficiency is close to that of natural photosynthesis in plants, and what is more, the reaction product (HCOOH) can be used as a renewable energy source.

Systematic Analysis of Electrochemical CO2 Reduction with Various Reaction Parameters using Combinatorial Reactors

Applying combinatorial technology to electrochemical CO2 reduction offers a broad range of possibilities for optimizing the reaction conditions. In this work, the CO2 pressure, stirring speed, and reaction temperature were varied to investigate the effect on the rate of CO2 supply to copper electrode and the associated effects on reaction products, including CH4. Experiments were performed in a 0.5 M KCl solution using a combinatorial screening reactor system consisting of eight identical, automatically controlled reactors. Increasing the CO2 pressure and stirring speed, or decreasing the temperature, steadily suppressed H2 production and increased the production of other reaction products including CH4 across a broad range of current densities. Our analysis shows that the CO2 pressure, stirring speed, and reaction temperature independently contributed to the limiting rate of CO2 supply to the electrode (Jlim). At a constant temperature, the limiting current density of CH4 increased proportionally with Jlim, illustrating that the production rate of CH4 was proportional to CO2 supply. Varying the CO2 pressure and stirring speed hardly affected the maximum Faradaic efficiency of CH4 production. However, changes to the reaction temperature showed a significant contribution to CH4 selectivity. This study highlights the importance of quantitative analysis of CO2 supply in clarifying the role of various reaction parameters and understanding more comprehensively the selectivity and reaction rate of electrochemical CO2 reduction.

Selectivity Control of CO2 Reduction in an Inorganic Artificial Photosynthesis System

We demonstrated that the selectivity of photo electrochemical CO2 reduction can be controlled in an inorganic artificial photosynthesis system using an AlGaN/GaN photo electrode. By increasing input light intensity and the use of a gold cathode, the Faradaic efficiency of CO dramatically increases from 30% to over 80% while that of H2 decreases. We observed that the cathode potential resulting from illumination determines the ratio of CO and H2. With this system, it is possible to switch the main reaction product from CO to HCOOH, which is also effective even under intense illumination.

A broad parameter range for selective methane production with bicarbonate solution in electrochemical CO2 reduction

This study demonstrated the previously unrecognised capability of potassium bicarbonate (KHCO3) aqueous solutions to assist in the selective generation of methane (CH4) over a wide range of reaction parameters during electrochemical CO2 reduction over a bulk polycrystalline Cu catalyst. Compared with the results obtained with potassium chloride (KCl) electrolytes, a 0.5 M KHCO3 solution maintained a higher faradaic efficiency (FE) for CH4 production at higher current densities. This trend was further emphasised upon increasing the CO2 pressure from near-ambient to 3 atm and incorporating stirring. The result was a FE above 50% over a range of current densities from 90 to more than 330 mA cm−2. The maximum FE reached 80% at 0 °C, while maintaining a broad peak structure when plotted against current density, even over 300 mA cm−2. The buffering ability of KHCO3 appears to play an important role in increasing both the reaction rate and the selectivity for CH4, especially when combined with an optimised CO2 supply to the electrode and an ideal reaction temperature.

Wireless InGaN–Si/Pt device for photo-electrochemical water splitting

We demonstrate a wireless device comprising a gallium nitride (GaN)–silicon-based photo-electrode, and a platinum cathode. Compared with conventional two-electrode photo-electrochemical systems, this wireless monolithic device showed potential for a wider range of applications, and reduced the resistance losses resulting from the wiring and aqueous solution. The efficiency was improved when the electrolyte was changed from KHCO3 to NaOH because water oxidation capability of the surface of the GaN was enhanced. A wider solar spectrum wavelength range was exploited by adopting InGaN as a photo-absorption layer; the improved efficiency for hydrogen generation was 0.90%.

Electrochemical application of Ga2O3 and related materials: CO2-to-HCOOH conversion

We report on the complex catalytic behavior of Ga2O3 for the electrochemical reduction of CO2 to formic acid (HCOOH). Although the experiments were reproducible, the behavior observed during the reaction was complex. A characteristic feature of the reaction was that Faradaic efficiency was strongly dependent on the electric charge during electrolysis. This result implied that the produced HCOOH affected the CO2 reduction reaction on the surface of the electrode, which was confirmed by experiments with initially added acid. The Faradaic efficiency for HCOOH production (η_HCOOH) increased with electric charge, and was further increased by the presence of initially added acid. We also show electrochemical CO2 reduction over other Ga compounds such as GaN and GaP, for which it can be assumed that p electrons and the Ga–Ga distance on the surface of the catalyst have important roles in selective HCOOH production as in the case of Ga2O3.