Self-sustainable protonic ceramic electrochemical cells using a triple conducting electrode for hydrogen and power production

Abstract

The protonic ceramic electrochemical cell (PCEC) is an emerging and attractive technology that converts energy between power and hydrogen using solid oxide proton conductors at intermediate temperatures. To achieve efficient electrochemical hydrogen and power production with stable operation, highly robust and durable electrodes are urgently desired to facilitate water oxidation and oxygen reduction reactions, which are the critical steps for both electrolysis and fuel cell operation, especially at reduced temperatures. In this study, a triple conducting oxide of PrNi_0.5Co_0.5O_3-δ perovskite is developed as an oxygen electrode, presenting superior electrochemical performance at 400~600 °C. More importantly, the self-sustainable and reversible operation is successfully demonstrated by converting the generated hydrogen in electrolysis mode to electricity without any hydrogen addition. The excellent electrocatalytic activity is attributed to the considerable proton conduction, as confirmed by hydrogen permeation experiment, remarkable hydration behavior and computations.

Fig. 1. Water splitting reaction on oxygen electrode and PNC’s hydration.

a Mixed oxygen-ion O²⁻ and electron conducting electrode. b Triple (H⁺, O²⁻, and electron) conducting electrode to extend reaction to entire electrode surface. c Chemical expansion of PNC perovskite structure observed in high-temperature X-ray diffraction at 600 °C when exposed to wet air.

Fig. 2. Study of hydration behavior, hydrogen permeation and proton migration energy.

a Comparison of hydration ability for PCO, PNC, LSCF, and PBSCF electrodes at 500 °C in wet air condition. b, c Proton migration pathway I along inter-octahedron direction and pathway II along intra-octahedron direction. d, e Minimum energy paths for proton migration along pathway I and II. f Hydrogen permeation fluxes of PNC dense membrane.

Fig. 3. Electrochemical performances of PCEC with PNC oxygen electrode.

a Performance in fuel cell mode (pure H₂ is used as fuel). b Current–voltage curves measured in electrolysis mode at various temperatures and dry 10% H₂ and wet air (~10% H₂O) are used as reactant gas in hydrogen electrode and oxygen electrode, respectively. c Current density response at different constant voltage of 1.2, 1.4 and 1.6 V at 600 °C. d Durability testing of the cell in different steam concentrations (20 and 30% H₂O) at 500 °C. e Stability testing of the cell at constant voltage of 1.4 V and 1.6 V at 500 °C. f Thermal cycle durability of the cell between 400 and 600 °C.

Fig. 4. Nanofiber-structured PNC mesh with high porosity and active nanoparticles.

a–d Scanning electron microscopy images of the electrode mesh with different magnification showing hollow-fiber-like string self-architectured to form mesh structure. e, f Transmission electron microscopy images of a single nanofiber composed of ~50 nm particles.

Fig. 5. Enhanced electrochemical performances with nanofiber-structured electrode.

a Current–voltage curves measured in electrolysis mode at various temperatures and dry 10% H₂ and wet air (~10% H₂O) are used as reactant gas in hydrogen electrode and oxygen electrode, respectively. b Performance in fuel cell mode (pure H₂ is used as fuel). c Comparison of impedance spectra under 1.4 V at 500 °C for cells with regular or 3D PNC electrode. d Durability testing of the cell in electrolysis mode at 500 °C. e Performance comparison of representative SOECs at electrolysis voltage of 1.4 V for oxide-ion conducting SOEC (O-SOEC), first-generation proton conducting SOEC (H-SOEC) operating at high temperatures (600~700 °C), second-generation H-SOEC at intermediate temperatures (<600 °C) and high-performing electrolysis cell developed in this work.

Fig. 6. Reversible operation and analysis in hydrogen and power generation.

The cycling reversible operation of the electrochemical cell between electrolysis mode (hydrogen production) and fuel cell mode (electricity generation) at different current densities to examine the capability of converting hydrogen generated by electrolysis into electricity. a Reversible operation with external hydrogen supply: current density responses as working mode transiently changing between electrolysis and fuel cell at 500 °C. b Schematic of self-sustainable reversible operation without external hydrogen feeding. c Voltage observation under switchable electrolysis and fuel cell current densities at 500 °C. d Average hydrogen production based on results on current density and Faradaic efficiency and hydrogen yield assuming the cell is operated for 24 h. e Average electricity generation rate and electricity yield in 24 h.