SnO2 modified CsH2PO4 (CDP) protonic electrolyte for an electrochemical hydrogen pump†

Abstract

CsH₂PO₄ (CDP) is a well-known super-protonic conductor. However, it must operate under high humidity conditions to prevent dehydration and fast conductivity decay. Herein, we report that adding hydrophilic SnO₂ into CDP can suppress the rate of dehydration of CDP, thus stabilizing protonic conductivity over a broader range of water partial pressures (p_H2O). A total of seven compositions of (1 − x)CDP/(x)SnO₂ were prepared, where 5 ≤ x ≤ 40 (wt%), and examined for their phasal, microstructural, and vibrational properties using X-ray diffraction, field emission scanning electron microscopy, and Raman spectroscopy. The signature of H₂O retained in SnO₂-added CPD was confirmed by Fourier transform infrared (FTIR) spectroscopy. Among these samples, 18 wt% SnO₂ in CDP stood out, showing a stable protonic conductivity of 0.6 × 10⁻² S cm⁻¹ at 250 °C, even at 10% H₂O. We also provide data from pre- and post-test characterization to facilitate the understanding of the observed stability improvement and degradation mechanisms. Finally, we show stable H₂ pumping performance of electrochemical cells with pure CDP and 18 wt% SnO₂–CDP electrolyte and Pt/C electrode. Overall, 18 wt% SnO₂–CDP is the best composition, showing stable conductivity under reduced H₂O conditions and 18 wt% SnO₂–CDP electrolyte with Pt/C electrode is the best membrane electrode assembly (MEA) for electrochemical H₂ pumping for lower water partial pressure applications.