Pressurized solid phosphate electrolyzer for medium temperature water splitting

A concept for electrolytic water splitting is presented, applying super-proton-conducting cesium-dihydrogen-phosphate (CsH₂PO₄) mixed with polyvinylbutyral shaped as disks. Our two-chamber pilot setup enabled continuous water electrolysis at above ~ 240 °C/26 bar of argon/water. Anodes consisted of ruthenium-dioxide (RuO₂) and cathodes of nickel or platinum-coated steel felts. The conditions during electrolysis (flow rates of H₂O, current density) were investigated. The water vapor pressure in the anode chamber had a decisive influence on the electrolysis efficiency: the CsH₂PO₄ electrolyte needs a stable supply of humidity to retain its super-proton-conductivity. H₂O forms O₂ gas at the anode and H⁺ ions depart to the cathode and react with electrons to form H₂. Without stable water addition, higher potentials are needed to maintain efficiency for stable current densities. Our investigations also comprised measurements of polarization curves at ~ 240 °C for Ru-coated steel anodes and Ni or Pt cathodes, with the latter reaching a current density of 250 mA/cm² at 2 V.