Electrochemical conversion from fuel to electricity and vice versa, with high efficiency, is a key technology for achieving carbon neutrality. Electrochemical devices that operate at intermediate temperatures of around 300°C are an undeveloped technological area and thus offer promising possibilities for new applications. Particularly, in terms of catalytic reactions, the intermediate temperature range is expected to provide better control over product selectivity and reaction activity compared to low-temperature or high-temperature devices. Currently, only a few electrochemical devices for CO₂ conversion in this temperature range have been reported, utilizing CsH₂PO₄ proton conductors as electrolytes [1,2].

 In this study, instead of CsH₂PO₄, which lacks long-term stability, a new type of electrochemical cell was fabricated using a phosphate glass proton conductor developed by alkali-proton substitution (APS) method [3,4] as the electrolyte in combination with a cermet-type porous substrate.

 Previously, single cells for fuel cells were fabricated by hot pressing glass electrolyte blocks onto Pd plates [5]. However, for practical reasons, such as cell scale-up and cost considerations, it became necessary to develop a cell with an electrolyte fabricated on a conventional support. Therefore, we developed a process to form a glass electrolyte on a porous cermet, similar to those used in SOFCs and SOECs. The glass electrolyte, with a composition of 36HO_1/2 - 4NbO_5/2 - 2BaO - 4LaO_3/2 - 4GeO₂ - 1BO_3/2 - 49PO_5/2 [3], was prepared by APS method and then powdered for cell fabrication. This powder was coated on a porous Ni-Gd_0.1Ce_0.9O_1.95 cermet support (φ18mm), and a dense glass electrolyte layer was formed by hot-pressing at 330^oC. The electrolyte layer's thickness, around 40μm, was confirmed by cross-sectional SEM observations. The proton conductivity of the glass electrolyte remained consistent post hot-pressing, measuring 2x10^-3 Scm^-1 at 300°C under H₂ atmosphere using a Pd sputtered film electrode (φ10mm). This demonstrates that the powdering and hot-pressing procedure did not affect the proton conductivity. We successfully fabricated a cermet-supported type phosphate glass electrolyte electrochemical cell. To confirm its potential in CO₂ reduction reactions, experiments were carried out using cermet-supported type cell employing Ru fine particles as the electrocatalyst. 100% H₂ gas was supplied to the anode and 100% CO₂ gas to the cathode at 300^oC and the exhaust gas was analyzed by gas-chromatography. When a voltage of 5V was applied, corresponding to the direction of proton transport from the anode to the cathode, hydrogen production was observed, and CO and CH₄ gases were also detected. Faraday efficiencies for each gas production were 19%, 12% and 68% for H₂, CO and CH₄, respectively. The high Faraday efficiency for CH₄ production indicates that the supplied H₂ gas is being efficiently utilized in the reaction with CO₂, and this result proves that our cell can be effectively used for CO₂ reduction. The ability to electrochemically control the potential of reactants opens up the possibility of developing a new field of research in reaction chemistry using ionic conductors. Currently, we are carefully examining whether an electrochemical reaction acceleration effect is being observed.