Proton-conducting Solid Oxide Electrolysis Research Articles

(Invited) Hydrogen Consortium: Technical Progress on Renewable Hydrogen Production R&D

HydroGEN Energy Materials Network (EMN) is an U.S. Department of Energy (DOE) EERE Hydrogen and Fuel Cell Technologies Office (HFTO)-funded consortium that aims to accelerate the discovery and development of advanced water splitting materials (AWSM) for clean, low-cost hydrogen production. This is in line with the H2@Scale initiative, with the goal to meet U.S. DOE’s Hydrogen Shot production cost target of $1/kg H2 within 1 decade (1 1 1). Materials innovations are key to enhancing performance, durability, and cost of hydrogen generation technologies. Large scale, low cost hydrogen from diverse domestic resources can enable an economically competitive and environmentally beneficial future energy system across multiple sectors.HydroGEN is focused on low technology readiness level AWS technologies, including low- (alkaline exchanged membrane electrolysis) and high-temperature electrolysis (proton-conducting solid oxide electrolysis), photoelectrochecmical (PEC) and solar thermochemical (STCH) water splitting. This presentation will provide an overview of the HydroGEN EMN and technical highlights of a few lab-led and FOA-awarded R&D projects. HydroGEN continues to grow its community of industry, university, and national laboratories, forming a national innovation ecosystem focused on renewable hydrogen production.

A solar-assisted power-to-hydrogen system based on proton-conducting solid oxide electrolyzer cells

Green hydrogen is anticipated to play a major role in a Net-Zero 2050 scenario since it can be produced using sustainable renewable energy sources, resulting in no greenhouse gas emissions. Furthermore, hydrogen has a high energy density and is readily stored and transferred, making it a versatile and convenient fuel for a broad range of applications. In this regard, an attempt has been made to study a solar-assisted power-to-hydrogen system based on proton-conducting solid oxide electrolyzer cells. The article presents a detailed thermo-economic analysis along with genetic algorithm-based optimization studies of the system. The optimum values of energetic efficiency and the cost of hydrogen are found to be 25.15 % and 1.021 $/kg, respectively. Exergy analysis reveals that the highest exergy destruction occurs in solar photovoltaic thermal (67.83 %) and parabolic trough solar collector (13.31 %), respectively. Furthermore, performance results of the solar-assisted power-to-hydrogen system are compared with other hydrogen production technologies.

Effect of CuO–ZnO catalyst layer on proton-conducting electrochemical cell reactor for CO2 reduction reaction

A catalyst layer is coated on the fuel electrode in a proton-conducting solid oxide electrolysis cell reactor, which can optimize electrochemical performance and expand the application range of the reactor. In this work, CuO–ZnO catalyst is prepared by oxalate co-precipitation method and combined with a single cell to obtain a reactor with the structure of La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) ǀ BaZr0.1Ce0.7Y0.1Yb0.1O3-δ (BZCYYb) ǀ NiO-BZCYYb (active layer) ǀ NiO-BZCYYb (support layer) | CuO–ZnO (catalyst layer). The electrolysis current density of this reactor with humid air as oxidant and 20%CO2–80%H2 as fuel is 2751 mA cm−2 under 1.3 V at 750 °C. It operates steadily for 138 h under 1.3 V electrolysis at 650 °C, which is better than the reactor without a catalyst layer under this condition. Additionally, CO and CH4 are detected in the exhaust gas on the fuel electrode side, and the CO selectivity reaches over 90% at 750-550 °C.

Current Leakage and Faradaic Efficiency Simulation of Proton-Conducting Solid Oxide Electrolysis Cells

Proton conducting solid oxide electrolysis cells (H-SOEC) have advantages over its oxygen-ion-conducting (O-SOEC) counterpart due to lower operating temperature (e.g. 300-700oC), relatively lower activation energy, and easier gas separation [1, 2]. However, it suffers from low Faradaic efficiency led by the internal current leakage. Protonic ceramics like BaZr0.90Y0.10O3 (BZY) are generally mixed proton, oxygen ion and electron hole conducting oxides, in which the transference number of each charge carrier depends on the material composition, operating temperature, atmosphere, and polarization current density. Experimental results have been reported that BZY shows significant p-type electronic leakage [3] across the electrolyte resulting in a flux of hydrogen from the negative electrode to the positive electrode and reducing the Faradaic efficiency. However, the fundamental understanding of the formation of electronic charge carriers during electrolysis operation is still unknown.In this research, we will simulate H-SOEC with BZY electrolyte with defect chemistry and Nernst-Planck charge conservation [4] to couple thermodynamic equilibrium of both electrodes under polarized operating conditions. Three mobile charge carriers will be considered in BZY electrolyte, including proton, oxide ion and electron hole, which is balanced with the dopant concentration . The thermodynamic equilibrium will be used to solve for charge species concentration in different gas atmosphere. The thermodynamic parameters will be validated by experimental data of BZY proton concentration [5] under different operating temperature and different O2 or H2O partial pressures. The mobility of each charged species will be validated by overall conductivity of BZY [6]. Then, the model will be used to project internal electronic current leakage and Faradaic efficiency from open circuit voltage to Nernst potential and beyond (as shown in Fig. 1). The correlation between Faradaic efficiency and operating current density will be established to explain the experimental observations. Ultimately, the model could help establish strategies to minimize electronic leakage by tailoring the composition of the electrolyte, optimizing device architecture and manipulating P-SOEC operating conditions.

Advancement of Proton Conducting Solid Oxide Electrolysis Cells (p-SOEC) for Hydrogen Production at Idaho National Laboratory

Proton Conducting Solid Oxide Electrolysis Cells (p-SOEC) is an emerging and attractive technology for hydrogen production through water electrolysis at intermediate temperatures. Economically competitive p-SOEC systems have distinct advantages over conventional oxygen-ion conducting ceramic electrochemical cells, but further technology development and widespread market acceptance will require continuous innovation of materials and structures in order to improve cell performance, enhance system lifetime and reduce cost. Herein, we report the advancement of p-SOEC with materials R&D, interface engineering, as well as cell fabrication and manufacturing in INL. We highlight how DOE support through HydroGEN accelerates move up the technology readiness level.

Dependence of Faraday Efficiency on Operation Conditions and Cell Properties for Proton Ceramic Electrolysis Cells

Proton-conducting solid oxide electrolysis cells (p-SOECs) have attracted much attention due to their low operating temperature and low degradation rate compared with conventional oxygen-ion conducting solid oxide electrolysis cells (o-SOEC). However, p-SOECs suffer from relatively low Faradaic efficiency due to the electronic leakage of the electrolyte. Using an electrolyte charge carrier transport model, we quantified the dependence of Faraday efficiency on the electrolysis operation conditions. Our model describes the transport of charge carriers in the electrolyte when the polarization resistance can not be neglected during cell operations. By accounting for the overpotentials at the interface of electrode and electrolyte in the model, we found that the Faraday efficiency decreases with the increasing current densities at electrolysis mode for both BZY20 and BCZYYb. Our results provide significant insights into the development of highly efficient p-SOECs.

Oxygen Electrode PrBa0.5Sr0.5Co1.5Fe0.5O5+δ-BaZr0.1Ce0.7Y0.1Yb0.1O3-δ with Different Composite Proportions for Proton-Conducting Solid Oxide Electrolysis Cells.

Proton-conducting solid oxide electrolysis cell (H-SOEC), as a hydrogen production device using proton conductor oxides as an electrolyte, has gained attention due to its various advantages of being more suitable for operating conditions at intermediate and low temperatures. However, its commercialization urgently needs to address the issue of insufficient catalytic activity of the oxygen electrode at lower temperatures. In this work, PrBa0.5Sr0.5Co1.5Fe0.5O5+δ-BaZr0.1Ce0.7Y0.1Yb0.1O3-δ (PBSCF-BZCYYb) series composite materials (denoted as PBSCF-BZCYYb46, PBSCF- BZCYYb55, and PBSCF-BZCYYb64 based on the mass ratios of PBSCF and BZCYYb as 4:6, 5:5, and 6:4, respectively) are prepared and applied as oxygen electrodes for H-SOECs. The H-SOECs with the structure of PBSCF-BZCYYb|BZCYYb|NiO-BZCYYb (active layer)|NiO-BZCYYb (support layer) are prepared and recorded as Cell 1, Cell 2, and Cell 3 with PBSCF-BZCYYb46, PBSCF-BZCYYb55, and PBSCF-BZCYYb64 as oxygen electrodes. The H-SOECs exhibit electrolysis current densities of 669.00, 743.80, and 503.30 mA cm-2 under 1.3 V at 650 °C, respectively. The cells also show considerable stability in the constant voltage electrolysis of 179.5, 152.8, and 83.0 h, respectively. Through the comparison of various electrochemical properties, PBSCF-BZCYYb55 is considered the most promising oxygen electrode material in this work.

Current Leakage and Faradaic Efficiency Simulation of Proton-Conducting Solid Oxide Electrolysis Cells

In this study, we simulated BZY electrolyte-supported proton-conducting solid oxide cell by coupling surface defect chemistry reaction with charged species transport. We validated the model parameters by concentration as a function of temperature, conductivity under dry and wet oxygen as a function of oxygen partial pressure and temperature. The results indicate that the high electron-hole mobility (diffusivity) is mainly responsible for the high leaking current under high temperatures. The Faradaic efficiency stays low or even negative under low operating voltage or high temperature and plateaus as the cell voltage increases. The model developed in this study is a useful tool to understand the leaking current in BZY electrolyte and provide design strategies to avoid/mitigate such significant inefficiency for water electrolysis operation.

PrBa0.5Sr0.5Co1.5Fe0.5O5+δ as air electrode for proton-conducting solid oxide cells

Proton-conducting solid oxide cell (H–SOC) is suitable for operation at lower temperatures. However, its commercialization still faces the problem of insufficient catalytic activity of air electrode. Here a triple ionic-electronic conductor PrBa0.5Sr0.5Co1.5Fe0.5O5+δ (PBSCF) is synthesized by glycine-nitrate method, and the PBSCF slurry was screen printed onto the surface of half-cell which is fabricated by aqueous co-tape casting to obtain the single cell with the structure of PBSCF ǀ BaZr0.1Ce0.7Y0.1Yb0.1O3-δ (BZCYYb) ǀ NiO-BZCYYb (active layer) ǀ NiO-BZCYYb (support layer). The peak power density of the single cell with dry air as oxidant and hydrogen as fuel at 750, 700, and 650 °C are 958.75, 872.78, and 709.22 mW cm−2, respectively. When the atmosphere is 20%H2O+80%air at air electrode side and hydrogen at fuel electrode side, the single cell operating as a proton-conducting solid oxide electrolysis cell shows high electrolysis current densities of 4299.4, 2744.6, 1711.8, and 974.6 mA cm−2 under 1.3 V at 750, 700, 650, and 600 °C respectively. Additionally, during the constant voltage electrolysis, the cell shows a good stability in 78 h at 650 °C. The results show that PBSCF is potential as air electrode of H–SOCs, especially in solid oxide electrolysis cell mode.

Interface Problems in Solid Oxide Electrolysis Cells

In this talk, I introduce topics, that I have been working on, in solid oxide electrolysis cells involving complicated interfacial structures and dynamics of interfaces. Then I will focus on my recent work on nickel (Ni) particle migration in electrodes consisting of Ni, yttria-stabilized zirconia (YSZ), and pores during the operation of oxygen-ion conducting solid oxide electrolysis cells (o-SOECs) and Faraday efficiency in proton-conducting solid oxide electrolysis cells (p-SOECs) under electrolysis operations. SOECs can have a significant impact on climate change over the next decade and beyond, in applications such as balancing renewable grid electricity via electrolytic fuel production. However, long-term performance degradation remains a key issue that may limit further implementation of O-SOECs, and the dependency of operation conditions on Faraday efficiency in P-SOECs has been under debate. In particular, in Ni/YSZ/pore electrode of O-SOEC, a phase-field model is proposed that employs the Ni-YSZ 3D microstructure as the initial condition and large-scale numerical simulation is implemented that predicts the directional Ni migration. The results are thus directly comparable to experimental observations. Quantitative predictions of the evolution of the Ni/YSZ/pore system's microstructures due to Ni particles' migration are studied through theoretical analysis and data analysis. In P-SOECs, an electrochemical model is proposed to study the dependency of Faraday efficiency on operation conditions for P-SOECs with yttrium-doped barium zirconates (BZY) and co-doping barium zirconate-cerate oxides with ytterbium and yttrium (BCZYYb) as electrolytes respectively. Our numerical predictions are verified by experimental results obtained in INL. An optimal structure of electrolyte is proposed to boost the Faraday efficiency in P-SOECs.

The effect of sintering aids on BaCe0·7Zr0·1Y0.1Yb0.1O3-δ as the electrolyte of proton-conducting solid oxide electrolysis cells

Proton-conducting solid oxide electrolysis cells (H-SOECs) are attracting attentions of researchers due to their unique advantages. The proton-conducting material BaCe0·7Zr0·1Y0.1Yb0.1O3-δ (BZCYYb) has both the advantages of barium ceria-based and barium zirconate-based materials. BZCYYb material usually is synthesized by solid-state reaction (SSR) method, hence the densification of this electrolyte material is the key to restrict the application of H-SOECs. In this paper, effects of adding 1 wt% of different sintering aids (NiO, CuO, ZnO) to BZCYYb on the grain size and the conductivity are investigated. After adding 1 wt% of NiO and CuO sintering aids, BZCYYb electrolyte achieves ideal density. The electrical conductivity of four samples (BZCYYb without adding sintering aid, BZCYYb with 1 wt% NiO, CuO, ZnO, respectively) is tested under different steam concentrations of air, nitrogen, hydrogen, and nitrogen-hydrogen mixture. The conductivity increased after the sintering aid is added. With the increase of steam concentration, the conductivity decreased slightly and then increased due to electron holes under oxygen atmosphere with steam at high temperature. In other atmospheres, the conductivity increases with the steam concentration. In the atmosphere of 20 vol% H2O-Air and H2, the conductivity of the BZCYYb samples with 1 wt% CuO is about 1.087 × 10−2 S cm−1 and 9.02 × 10−3 S cm−1 at 650 °C, and the conductivity of the BZCYYb samples with 1 wt% NiO is 1.277 × 10−2 S cm−1 and 8.24 × 10−3 S cm−1, respectively. However, compared with NiO, CuO has advantages in promoting hydration reaction and proton conduction of BZCYYb electrolyte.

Performance improvement of the proton-conducting solid oxide electrolysis cell coupled with dry methane reforming

The proton-conducting solid oxide electrolysis cell (H-SOEC) is a clean technology for syngas production from H2O and CO2 through electrochemical and chemical reactions. However, it provides a low CO2 conversion and produces a syngas product with a high H2O content. To improve the H-SOEC for syngas production, H-SOEC coupled with a dry methane reforming process (H-SOEC/DMR) was proposed in this work. The process flowsheet of the H-SOEC/DMR was developed and further used to evaluate the performance of the H-SOEC/DMR process. From the performance analysis of the H-SOEC/DMR process, it was found that the CO2 and CH4 conversions were higher than 90% and 80%, respectively, when the process was operated in the temperature range of 1073–1273 K. In addition, the result showed that a syngas product with a low H2O content could be obtained. Energy efficiency was then considered and the results indicated that the highest energy efficiency of 72.80% could be achieved when the H-SOEC/DMR process was operated at a temperature of 1123 K, pressure of 1 atm, and current density of 2500 A m−2. Based on a pinch analysis, a heat exchanger network was applied to the H-SOEC/DMR process that improved the energy efficiency to 81.46%. Finally, an exergy analysis was performed and showed that the H-SOEC/DMR unit had the lowest exergy efficiency as a high-temperature exhaust gas was released.

(Invited) Advanced Electrode and Electrolyte Materials for Proton Conducting Solid Oxide Electrolysis Cells

Hydrogen production by using proton-conducting solid oxide electrolysis cells (SOECs) has been acknowledged as a promising approach to effectively utilize the clean renewable energies and nuclear heat and to decarbonize. To deliver fast and durable hydrogen production during intermediate-to-low temperature (400~600○C) operation, some critical components such as oxygen electrode and electrolyte are important for achieving high electrolysis performance under realistic conditions, particularly in high steam pressure. In this presentation, the recent developments on praseodymium cobaltite-based PrNixCo1-xO3-δ electrode and Sc-doped BaZrO3-based electrolyte are respectively introduced. The triple conductive electrode demonstrates high activity and stability towards water splitting reaction, and to down-select the optimal composition, a series of optimization study have been carried out to understand structure-property relationship, chemical/performance stability in high steam condition, and interfacial stability. For electrolyte, the hydration behavior, proton conductivity, transference number, and chemical stability are studied to disclose the effect of dopant on increasing effective proton concentration, and the underlying mechanism is also investigated by experimental characterizations.

Enhanced Performance of Protonic Solid Oxide Steam Electrolysis Cell of Zr-Rich Side BaZr0.6Ce0.2Y0.2O3-δ Electrolyte with an Anode Functional Layer.

Proton-conducting solid oxide electrolysis cells (H-SOEC) containing a 15-μm-thick BaZr0.6Ce0.2Y0.2O3−δ (BZCY622) electrolyte thin film, porous cathode cermet support, and La0.6Sr0.4Co0.2Fe0.8O3−δ anodes were fabricated using a reactive cofiring process at approximately 1400 °C. Steam electrolysis was conducted by supplying wet air to the anode at a water partial pressure of 20 kPa. The performance was evaluated using electrochemical measurements and gas chromatography. At 600 °C, the cells generated an electrolysis current of 0.47 A cm–2 at a 1.3 V bias while the Faradaic efficiency reached 56% using 400 mA cm–2. The electrolysis performance was efficiently improved by introducing a 40-nm-thick La0.5Sr0.5CoO3−δ (LSC) nanolayer as an anode functional layer (AFL). The cells with LSC AFL produced an electrolysis current of 0.87 A cm–2 at a 1.3 V bias at 600 °C, and the Faradaic efficiency reached 65% under 400 mA cm–2. Impedance analysis showed that the introduction of the AFL decreased the ohmic resistances and improved interfacial proton transfer across the anode/electrolyte interface and polarization resistances related to the anode reaction. These results demonstrate opportunities for future research on AFL to improve the performance of H-SOECs with Zr-rich BaZrxCe1–x–yYyO3−δ electrolytes.

Electrochemical Reduction of CO2 with Exsolved Metal–Oxide Interfaces in a Proton-Conducting Solid Oxide Electrolyzer

A large amount of carbon dioxide emissions have caused a greenhouse effect and even disrupted the global ecological balance. Therefore, it is urgent and significant to achieve emission reduction of CO2. The use of electrochemistry to reduce carbon dioxide into valuable chemicals and fuels is a sustainable strategy. Here, we present the electrochemical reduction of CO2 into CO in a proton-conducting solid oxide electrolyzer at a low temperature. We exsolve the active metal–oxide interfaces NixCu1–x–NbTi0.4Mn0.1O4−δ (NTMO) in a composite cathode by a synergistic doping strategy to increase the length and tailor the composition of the active interfaces and, hence, to promote CO2 reduction. By adjustment of the ratio of the nickel–copper alloy at the metal–oxide interface, it is found that the best performance can appear with an applied voltage of 0.8 V under the composition of Ni0.5Cu0.5–NTMO. A remarkable current efficiency of 99.8% is achieved with CO production at 0.71 mL min–1 cm–2 at a temperature as low as 600 °C. This illustrates the application prospects of electrochemical reduction of CO2 in a solid oxide electrolyzer through interface engineering.

SrxTi0.6Fe0.4O3−δ (x = 1.0, 0.9) catalysts for ammonia synthesis via proton-conducting solid oxide electrolysis cells (PCECs)

The high concentration of OVs and active sites of S0.9TF promote its high NRR activity. The proton-conducting solid oxide electrolysis cells (PCECs) with S0.9TF material as a cathode showed a high output (bar) and durability (line).

Structure, synthesis, properties and solid oxide electrolysis cells application of Ba(Ce, Zr)O3 based proton conducting materials

Ba(Ce, Zr)O3 based proton-conducting oxides have experienced a surge of attention in the past because of their promising applications in proton-conducting solid oxide electrolysis cells (H+-SOECs). As proton conductor perovskite oxides, Ba(Ce, Zr)O3 based materials demonstrate potential to offer lower activation energy and higher proton transfer properties in low-temperature than oxygen ion conductor perovskite oxides as a result of the smaller ion radius of proton. Herein, a review of the current research progress on Ba(Ce, Zr)O3 based materials, including structural, materials synthesis approaches, conductivity, stability properties, and applications related to H+-SOECs is presented. This review focuses on modification strategies, including enhancing conductivity and chemical stability via the substitution of the B-site element, to improve the performance of H+-SOECs. Finally, more attention should be given to current leakage, performance and long-term stability of H+-SOECs in the future.

Ruddlesden–Popper-Structured (Pr0.9La0.1)2(Ni0.8Cu0.2)O4+δ: An Effective Oxygen Electrode Material for Proton-Conducting Solid Oxide Electrolysis Cells

Proton-conducting solid oxide electrolysis cells (H-SOECs) have attracted a lot of attention due to their high efficiency, energy-saving ability, and environmental friendliness. The ideal H-SOEC ox...

Numerical Study on Biogas Refining System Combined with Proton-Conducting Solid Oxide Electrolyzer

Biogas is one of the most globally distributed carbon-neutral fuels to be used as a local biomass energy source for local society energy demand. However, since the biogas from organic waste is typically a mixture of 60% methane and 40% carbon dioxide, the biogas has a lower energy density compared with coal, oil, LNG, and etc. In this study, biogas refining system including methanation reactor and proton-conducting electrolysis cell has been proposed and investigated based on the numerical calculation including mass and heat balance analysis. Using this system, hydrogen and CO2 derived from electrolysis cell and biogas, respectively, will be converted into methane via methanation reaction. Carbon deposition is a well-known problem to be resolved when carbon-rich gas is supplied to the cell cathode. Based on the calculation results, the gas humidification was found to be necessary to prevent carbon deposition at the cathode inlet. However, carbon deposition was predicted to still occur at the cathode outlet even with the gas humidification when a conventional oxide-ion conducting electrolyte is applied to the electrolysis cell due to the enhanced carbon activity caused by oxide-ion transport from the cathode to the anode. In the case of the proton-conducting electrolyte, proton transport from the anode to the cathode was found to mainly promote reverse water-gas shift reaction and methanation reaction without any carbon deposition. Even though the calculation results should be verified by comparing with experimental results, the numerical study suggested that the biogas refining system combined with p-SOEC is a promising technology for generating carbon-neutral methane with a strongly suppressed risk of carbon deposition.

Numerical Study on Biogas Refining System Combined with Proton-Conducting Solid Oxide Electrolyzer

Biogas refining system including methanation reactor and proton-conducting electrolysis cell has been proposed and investigated based on the simulation including mass and heat balance analysis. Based on the calculation results, carbon deposition was predicted to still occur at the cathode outlet even with the gas humidification when a conventional oxide-ion conducting electrolyte is applied to the electrolysis cell due to the enhanced carbon activity caused by oxide-ion transport from the cathode to the anode. In the case of the proton-conducting electrolyte applied solid oxide electrolysis cell, proton transport from the anode to the cathode was found to mainly promote reverse water-gas shift reaction and methanation reaction without any carbon deposition. Even though the calculation results should be verified by comparing with experimental results, the numerical study suggested that the biogas refining system combined with p-SOEC is a promising technology for generating carbon-neutral methane with a strongly suppressed risk of carbon deposition.