Activity Enhancement of a Carbon Electrode Material for Vanadium Redox Flow Battery by Electron-Beam Irradiation

The effective addition of surface oxygen groups, which are active sites for redox reactions, on carbon clothes as electrodes by electron beam irradiation in normal air which contains environmental humidity, dry air, or nitrogen atmosphere was carried out. The irradiation introduced 20 at% oxygen at the carbon surface as determined by X-ray photoelectron spectroscopy and the phenol-type hydroxyl group, the carboxylic group, etc., were detected by temperature-programmed desorption. Single-cell measurements indicated the current density at 1.3 V-IR-corrected of the irradiated electrode in normal air was 28% higher than that of the as-received electrode. Since double-layer capacitance between the as-received carbon cloth and irradiated carbon cloth in normal air was similar, the improvement of current density is attributed to the increase of surface oxygen groups. In addition, the radiation in both normal air and dry air improved electrochemical activity similarly. This result suggests the radiation-chemical reaction in this study is dominated by the oxidation reaction with ozone or nitrogen oxides (NOx), while in the meantime, the contribution of the hydroxyl radical from water is considered to be negligible

Highly Active Electrode With Efficiently Added Surface Oxygen Groups for a Vanadium Redox Flow Battery

バナジウムレドックスフロー電池は、太陽光や風力などの時間変動の大きい再生可能エネルギーを利用するための電力貯蔵システムとして注目されている。しかし、電池内部抵抗（反応抵抗）が比較的大きく、反応抵抗低減による電流密度の向上が求められている。本研究では、電極材のカーボンクロスに対して、反応活性点となる酸素官能基を付与する目的に大気中熱処理を行い、電池内部抵抗の低減を目指した。実験では、熱処理温度（500℃－550℃）及び処理時間（1-6 h）をパラメータにカーボンクロスの熱処理を行った。さらに、Nafionを隔膜とする単セルに組み込み、電流-電圧測定及びオーム抵抗の測定により電池性能を評価した。その結果、550℃, 3hで熱処理した試料で最も高い発電性能が得られた。この結果から熱処理による反応面積増加及び反応活性点となる親水性の酸素官能基の付与が発電性能向上に有効であることが示された