Protonic-ceramic materials, exemplified by BaCe_0.8-xZr_xY_0.1Yb_0.1O_3-δ (BCZYYb), can facilitate efficient H₂O electrolysis between 500 and 700 °C when integrated with effective H₂O-splitting electrocatalysis [1]. Several studies have demonstrated that Ruddlesden-Popper oxides like Pr₂NiO_4+δ (PNO) are efficient water-splitting electrocatalysts [2–4]. PNO conducts both electrons and oxygen ions and can also incorporate protonic defects (OH^●) into the crystal structure, which makes it a triple H⁺/O²⁻/e⁻ conductor and effective for H₂O-splitting [4]. In the range of operating temperatures, PNO's electronic conductivity remains significantly higher than its proton and oxygen-ion conductivities, which limit the region for water-splitting charge transfer reactions near PNO/electrolyte interfaces and increase positrode overpotentials. Collaborative research with West Virginia University has introduced the concept of conformal BaZr_0.7Y_0.3-yPr_yO_3-δ(BZYP) overlayers on PNO positrode to enhance performance by increasing surface conductivity and consequently, the active surface area. To explore the impact of BZYP overlayers on PNO electrode performance and surface chemistry, BCZYYb electrolyte-supported cells were developed using thin-film PNO positrodes with and without BZYP overlayers via pulsed laser deposition. The thin-film electrodes enabled in operando environmental X-ray photoelectron spectroscopy (XPS) to observe surface chemistry of water-splitting on PNO positrodes.

Electrochemical characterization of pre-hydrated cells has been conducted from 550 to 700°C at a positrode H₂O partial pressure up to 0.44 bar and negatrode flow 5% H2/Argon. Open circuit potentials (OCP) measured of 0.925 V exhibited 1.2% deviation from thermodynamic value of 0.937 V at 550°C, which increased to 3.6% at 700°C where measued OCP is 0.843 V and thermodynamic is 0.875 V, attributable to increased electronic leakage. V-I polarization curves and electrochemical impedance spectroscopy (EIS) have shown that BZYP overlayers decreased the total area-specific resistance of PNO positrodes by approximately 50% across the range of operating temperatures. EIS data suggested that BZYP overlayers substantially reduced the bulk resistance, which aligns with the hypothesis that BZYP enhances surface conductivity and electrochemically active areas of the electrodes. However, BZYP overlayers did not significantly reduce polarization resistance.

Environmental XPS at elevated temperatures up to 500 °C in 1.5 mbar H2O pressure on thin-film PNO (uncoated and BZYP-coated) electrodes were performed with and without positive bias in a single-chamber in a Scienta-Omicron HiPP Lab system. The Pr⁴⁺/Pr³⁺ ratio for BZYP surface is higher compared to the PNO surface in biased cell condition by 35%. Monitoring surface ratios of Pr⁴⁺/Pr³⁺ and Ni³⁺/ Ni²⁺ enables the evaluation of surface kinetics in response to temperature variations and applied bias. This analysis will offer insights into the water-splitting processes on PNO and BZYP coated electrodes.