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Pressurized operation is advantageous for many electrolysis and electrosynthesis technologies. The effects of pressure have been studied extensively in conventional oxygen-ion conducting solid-oxide electrochemical cells. In constrast, very few studies have examined pressurized operation in proton-conducting electroceramics. Protonic ceramics offer high proton conductivity at intermediate temperatures (∼400–600°C) that are well-matched to many important thermochemical synthesis processes. Pressurized operation can bring significant additional benefits and/or provide access to synthetic pathways otherwise unavailable or thermodynamically disfavorable under ambient conditions and in higher- or lower-temperature electrochemical devices. Here we examine pressurized steam electrolysis in protonic-ceramic unit-cell stacks based on a BaCe0.4Zr0.4Y0.1Yb0.1O3−δ (BCZYYb4411) electrolyte, a Ni–BZCYYb4411 composite negatrode (fuel electrode) and a BaCo0.4Fe0.4Zr0.1Y0.1O3−δ (BCFZY) positrode (air-steam electrode). The cells are packaged within unit-cell stacks, including metallic interconnects, current collectors, sealing glasses and gaskets sealed by mechanical compression. The assembly is packaged within a stainless steel vessel for performance characterization at elevated pressure. Protonic-ceramic electrolyzer performance is analyzed at 550°C and pressures up to 12 bara. Increasing the operating pressure from 2.1 to 12.6 bara enables a 40% overall decrease in the over-potential required to drive electrolysis at 500 mA cm−2, with a 33% decrease in the cell ohmic resistance and a 60% decrease in the cell polarization resistance. Faradaic efficiency is also found to increase with operating pressure. These performance improvements are attributed to faster electrode kinetics, improved gas transport, and beneficial changes to the defect equilibria in the protonic-ceramic electrolyte, which more than compensate for the slight increase in Nernst potential brought by pressurized operation. Electrochemical impedance spectroscopy (EIS) coupled with distribution of relaxation time (DRT) analysis provides greater insight into the fundamental processes altered by pressurized operation.

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In this work, we present our progress on high-temperature water splitting (HTWS) and hydrogen production at elevated pressure using proton-conducting ceramics. The electrochemical performance of the proton-conducting electrolyzer unit-cell stack is analyzed at 550 °C and pressures up to 12 bar.Proton-conducting ceramics are a promising new class of electrochemical cells due to their high proton conductivity at intermediate temperatures in comparison to more-conventional solid-oxide or molten-carbonate counterparts. Materials exploration continues for fabricating proton conducting cells to achieve better electrochemical performance during operation. Recently, lower ohmic resistance and degradation rates have been observed using the highly proton-conductive and chemically stable perovskite BaCe0.4 Zr0.4 Y0.1 Yb0.1 O3−δ (BCZYYb4411). In this study we work with a composite of Ni–BZCYYb4411 as the fuel electrode (cathode), BCZYYb4411 as the electrolyte and BCFZY as the steam electrode (anode). The cathode and electrolyte layers of the membrane-electrode assemblies used in this study are synthetized using the solid-state reactive sintering (SSRS) method. SSRS is an attractive MEA-fabrication method, as it greatly reduces the number of costly and time-consuming high-temperature sintering processes. During SSRS, single-phase protonic-ceramic perovskite is formed from parent oxides during high-temperature co-sintering of the anode-electrolyte layers. This is in contrast to more-traditional processing, in which the desired phase is first formed in powder form through calcination of parent oxides, while MEA formation is executed in follow-on high-temperature sintering steps.The cathode support is formed by dry-pressing to produce a 57-mm-dia x 1.5-mm-thick disc. The electrolyte (~ 10 mm) layer is deposited using an ultrasonic spray atomizer. The process control and narrow particle size distribution delivered by the ultrasonic atomizer consistently produces a high-density electrolyte, while minimizing thickness. We are now extending the use of ultrasonic spray deposition to other critical components such as the air-steam electrode and interfacial layers.As shown in Figure 1a, the protonic-ceramic membrane-electrode assembly (MEA) is bonded to a composite ceramic frame and assembled into a sealed cell stack with metallic interconnects, current collectors, sealing gaskets and end plates. The stack is placed in a preloaded spring-based mechanical compression system that axially compresses the electrolyzer stack while avoiding any direct compression of the MEA. The assembly is placed in a sealed vessel (Figure 1b); anode and cathode gas pressures are balanced across the MEA, and with the surrounding inert vessel gas (N2). Downstream back-pressure regulators maintain electrolyzer and vessel pressures to minimize the risk of cell fracture.Despite the challenges associated with high pressure operation, this technology is key to address some of the main issues associated with proton conducting ceramics. Higher pressure operation ...