The cover art is attributed to an article entitled “Measurement methods on electrodes and electrocatalysts for water electrolysis” by Professor Shigenori Mitsushima et al. Editor’s Choice of this issue is an invited review article of comprehensive work on measurement methods for water electrolysis, transcribed from the article published in Denki Kagaku in 2022 (DOI: 10.5796/denkikagaku.22-TE0003) as a guidance for researchers. Development of highly efficient methodology of water electrolysis by renewable energy is highly demanded to realize carbon-neutral society with sustainable growth. However, absence of rigorous evaluation protocol for the performance of water electrolysis may hinder the sound progress of relevant research fields. The authors introduced common measurement techniques of two representative water electrolysis, alkaline water electrolysis (AWE) and proton exchange membrane water electrolysis (PEMWE). This work will help researchers to avoid making unfair comparison and prevent misjudgment of the experimental results obtained in different institutes, that is ultimately beneficial to the community.

Toward decarbonized society with sustainable growth, the use of renewable energy as a primary power source is highly demanded. Water electrolysis is promising to produce hydrogen from renewable energy for energy storage and transportation. In developments of new technologies related to alkaline water electrolysis (AWE) and proton exchange membrane water electrolysis (PEMWE), common measurement techniques are required to compare experimental results fairly among different institutes. In this paper, we introduce common measurement techniques, such as electrochemical half cell and full cell for AWE and PEMWE, evaluation techniques for catalytic activities and durability of catalysts under steady and non-steady operation, advanced synchrotron radiation analysis of catalysts, operando XAFS analysis, and optical observation of gas bubbles in detail, that are recently developed in the project research commissioned by the New Energy and Industrial Technology Development Organization (NEDO).

There is considerable interest in all-solid-state sodium-ion batteries from the perspectives of safety and resource efficiency. However, it is known that the interfacial resistance of oxide-based solid electrolytes such as Na_1+xZr₂Si_xP_3−xO₁₂ (NASICON), which is used in all-solid-state sodium-ion batteries, is high because of poor contact at the solid/solid interface. In this study, to reduce the interfacial resistance, we conceived of introducing a ductile electrolyte, Na(CB₉H₁₀)_0.7(CB₁₁H₁₂)_0.3 (NaCBH), Na_2.25Y_0.25Zr_0.75Cl₆ (NYZC), and a polymer electrolyte (PE), into the NASICON/hard carbon interface. The ionic conductivity of NaCBH was the highest among three ductile electrolytes. In impedance measurements using a symmetric cell, the interfacial resistance was high for NYZC but low for NaCBH and the PE. This suggests that good contact was formed at the NaCBH/NASICON and NaCBH/PE interfaces and that NaCBH is an excellent electrolyte as an intermediate layer. In addition, when NaCBH and hard carbon were combined, charged, and discharged, a capacity comparable to that of organic electrolytes was obtained. Furthermore, the interfacial resistance between the solid electrolyte and hard carbon was reduced by introduction of NaCBH, proving that NaCBH is an effective intermediate layer on the anode side for use in all-solid-state sodium-ion batteries.