Assessing the Impact of BaZr0.7Y0.3-YPryO3- d Composition on Water Splitting Kinetics in Composite Pr2NiO4+Δ-BaZr0.7Y0.3-YPryO3- d Electrodes in Proton-Conducting Electrolysis Cells

Abstract

Protonic-ceramic electrolysis cells (PCECs), with proton-conducting oxide electrolytes, provide high current densities between 500-700°C because electrolytes such as BaCe0.8-xZrxY0.1Yb0.1O3-δ (BCZYYb) have protonic conductivities in this temperature range. The protonic conductivities of BCZYYb are almost an order of magnitude higher than oxide-ion conductivities in conventional solid-oxide electrolytes. To convert such high ionic fluxes in PCECs into efficient hydrogen production, robust positive electrodes (positrodes) must be developed with reduced overpotentials associated with water-splitting kinetics and proton incorporation [1]. The positrodes with Ruddlesden-Popper oxides like Pr2NiO4+δ(PNO) have demonstrated effective water-splitting kinetics as well as mechanical and chemical compatibility with barium zirconate electrolytes [2]. As a mixed conductor, PNO conducts polarons, oxide ions, and protonic defects (OH●) into the crystal structure [3]. However, its relatively low proton and oxide-ion conductivities limit charge transfer reaction near three-phase boundaries with the electrolyte, which increases positrode overpotentials. To improve the surface conductivity in PNO-based positrodes, our team, in collaboration with West Virginia University [4], has explored the impact of very thin overlayers of mixed ion-conducting BaZr0.7Y0.15Pr0.15O3- d (BZYP) on PNO positrode layers. Electrochemical characterization showed that the BZYP overlayers reduce the area-specific resistance of the PNO positrodes by ≈50% at 550-700°C: this suggests the extension of the electrochemically active surface area required for the water-splitting reaction beyond three-phase boundaries. The present study focuses on optimizing composite PNO-BZYP porous positrodes in conventional negatrode-supported PCECs to understand the water-splitting reaction mechanism for efficient electrolysis.Composite PNO-BaZr0.7Y0.3-yPryO3- d (BZYP) positrodes were fabricated for a range of Y/P cation ratios. The 22 mm diameter negatrode-supported button cells included 1 mm thick porous negatrodes of BaCe0.4Zr0.4Y0.1Yb0.1O3-δ(BCZYYb4411)-NiO and a spray-coated 5-10 μm thick BCZYYb4411 electrolytes as shown in figure 1. These negatrode-electrolytes were sintered at 1550°C for 6 h. The PNO and BZYP positrode powders were synthesized by solid-state reaction at 1350°C and 1500°C, respectively. PNO was identified as an orthorhombic crystal structure, and BZYP as a tetragonal crystal structure. Reactivity tests PNO and BZYP (1:1 wt ratio) at 1050°C, 1150°C, and 1250°C for 5 h showed no phase change. After negatrode-electrolyte sintering, both single-phase PNO (reference), and composite PNO-BZYP slurries were painted on the exposed electrolyte layers and sintered at 1150°C for 5 h to provide porous positrodes.Electrochemical impedance spectroscopy (EIS) and current-voltage measurements on BZYP-PNO positrode were compared with PNO positrode in the temperature range of 550-700°C. During these measurements, steam, and oxygen partial pressures were varied by 3-50%. EIS measurements informed the resistances associated with the different processes in the water-splitting reaction mechanism. The impact of both proton/oxygen ion and electron conductivity on the water-splitting kinetics could be identified based on different BZYP compositions by investigating the effects of temperatures, steam, and oxygen partial pressures. Different Y/P cation ratios in BZYP supported the investigation of how different (proton/oxygen ion and electron) conductivities in the BZYP impact the electrochemical water-splitting reaction. Figure 1