High Pressure Operation of Reversible Protonic Ceramic Electrochemical Cells
Authors: Herradon Carolina , Le Long , Meisel Charlie , Kim You-Dong , Cadigan Christopher , O'Hayre Ryan , Sullivan Neal P. 
Published: July 2021

We present our development of proton-conducting ceramics for high-temperature water splitting (HTWS) and hydrogen production at elevated pressure. One of the challenges of intermittent renewable energy production from solar and wind is the mismatch between generation and demand. This decoupled production and consumption of energy has motivated extensive research in electrochemical energy storage. Depending on the application and timescale, a variety of reversible electrochemical cells are needed to address the energy storage needs of intermittent renewable energy sources. For long-term energy storage and electrical-to-chemical energy conversion, high-temperature solid-oxide electrolyzers are attractive. In this work we present some of the advantages and recent developments of electrolyzers based on protonic-ceramic materials. Protonic-ceramic electrolyzers (PCECs) have shown an encouraging performance trajectory in recent years. PCECs can deliver a pure-H2 product stream, simplifying downstream separations. Higher-pressure PCEC operation (~ 10 bar) has been demonstrated, but is at an early stage. In this presentation, we highlight a unique installation that permits the study of these systems in a variety of configurations under high-pressure conditions.

Reversible protonic ceramic electrochemical cells (RePCEC) are attracting attention due to their potential low cost, energy flexibility and high proton conductivity at intermediate temperatures (300 – 700 ºC). Previous studies focus either on protonic ceramic fuel cells (PCFCs) for power generation or protonic ceramic electrolysis cells (PCECs) for dry hydrogen production. Electronic leakage through the protonic-ceramic electrolyte presents a concern; high Faradaic efficiency has been demonstrated, but has also seen wide variance across studies. Low electrode activity can also reduce Faradaic efficiency, especially at higher current densities and driving voltages. Additionally, although Hydrogen is well known for its excellent characteristics as a fuel, its low density represents a challenge since it must be compressed to high pressures for storage and transportation.

In this context, high-pressure operation of RePCECs is presented as key technology to address the challenges described. Higher total pressure operation should lead to higher efficiencies by decreasing the electron hole concentration in the electrolyte, thereby diminishing the electron leakage problem. High pressure can also boost electrode activity, enabling higher currents at lower driving voltages. From a cost perspective, high-pressure electrolysis reduces downstream compression needs. Operating at pressure is potentially even more important for electrofuels, where green hydrogen is transformed into higher-value chemicals; these chemistries often require high temperatures and pressures to achieve meaningful production rates. Ammonia serves as an example; the second-most-produced global chemical that contributes nearly 1.5% of global annual CO2 emissions.

In this work, we present our progress on high-pressure operation of RePCECs. The electrochemical performance of protonic-ceramic electrolyzer stacks is characterized at 550 ºC and pressures up to 12 bar. The figure shows assembly of a unit-cell stack; 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. This assembly is sealed within a stainless-steel vessel; the operating pressure of the reactants is balanced by the surrounding gas within the vessel, minimizing pressure differentials between the stack and its surroundings. The cross-cell pressure differential between the fuel and the air-steam electrodes brings the risk of cell fracture. Downstream electronic back-pressure regulators serve to maintain stable pressures throughout.

The vessel, fuel side, and air side of the installation all have independently regulated flows at their respective inlets and a common regulated pressure at their outlets. Additional systems are in place to introduce steam, execute electrochemical characterizations, and monitor outlet flow rates. Furthermore, an in-line/in-situ mass spec and gas chromatograph provide continuous gas composition measurements.

All of the integrated tools in the high-pressure installation are used to study hydrogen production in electrolysis mode, electricity production in fuel-cell mode, and chemical production using catalyst beds at the electrochemical reactor outlet. This powerful combination is very useful for optimizing the performance of electrodes and electrolytes at high pressure conditions and giving us new insight into the application of protonic-ceramic materials.

Figure 1