Electrochemical performance of solid oxide electrolysis cell electrodes under high-temperature coelectrolysis of steam and carbon dioxide

The SOEC electrodes during steam (H₂O) electrolysis, carbon dioxide (CO₂) electrolysis, and the coelectrolysis of H₂O/CO₂ are investigated. The electrochemical performance of nickel-yttria stabilised zirconia (Ni-YSZ), Ni-Gd_0.1Ce_0.9O_1.95 (Ni-GDC), and Ni/Ruthenium-GDC (Ni/Ru-GDC) hydrogen electrodes and La_0.8Sr_0.2MnO_3−δ-YSZ (LSM-YSZ), La_0.6Sr_0.4Co_0.8Fe_0.2O_3−δ (LSCF), and La_0.8Sr_0.2FeO_3−δ (LSF) oxygen electrodes are studied to assess the losses of each electrode relative to a reference electrode. The study is performed over a range of operating conditions, including varying the ratio of H₂O/H₂ and CO₂/CO (50/50 to 90/10), the operating temperature (550-800 °C), and the applied voltage. The activity of Ni-YSZ electrodes during H₂O electrolysis is significantly lower than that for H₂ oxidation. Comparable activity for operating between the SOEC and solid oxide fuel cell (SOFC) modes is observed for the Ni-GDC and Ni/Ru-GDC. The overpotential of H₂ electrodes during CO₂ reduction increases as the CO₂/CO ratio is increased from 50/50 to 90/10 and further increases when the electrode is exposed to a 100% CO₂ (800 °C), corresponding to the increase in the area specific resistance. The electrodes exhibit comparable performance during H₂O electrolysis and coelectrolysis, while the electrode performance is lower in the CO₂-electrolysis mode. The activity of all the O₂ electrodes as an SOFC cathode is higher than that as SOEC anodes. Among these O₂ electrodes, LSM-YSZ exhibits the nearest to symmetrical behaviour.     

Effect of Fe infiltration to Ni/YSZ solid‐oxide‐cell fuel electrode on steam/CO2 co‐electrolysis

This study proposes a novel methodology for controlling syngas production from high‐temperature CO₂/steam co‐electrolysis. The co‐electrolysis of CO₂/steam mixtures is one of the most promising methods to reduce CO₂ emissions and mitigate climate change. CO₂ and steam are reduced to produce synthetic gas (H₂ and CO) through thermo‐electrochemical reactions occurring in a solid‐oxide‐cell fuel electrode. To make this technology viable, it is essential to improve electrochemical cell performance and obtain controllability of gas conversion and product gas selectivity. In this study, Fe infiltration to the Ni/YSZ fuel electrode and subsequent in situ alloying of Ni‐Fe is used to enhance the cell performance and syngas productivity. Impregnation of Fe‐oxide nanoparticles on the fuel electrode support of solid oxide cells and subsequent in situ alloying Ni‐Fe is obtained. Their homogeneous morphology and distribution are obtained by using an advanced infiltration technique. Results show that the Ni‐Fe/YSZ fuel electrode enhances CO selectivity and lowers an overvoltage imposed on the cell. This may result in syngas production with higher carbon contents and a higher co‐electrolysis system efficiency. In addition, its long‐term durability for 500‐hour operation is also evidenced with stable syngas production and negligible cell degradation.     

Synthesis and electrochemical properties of LSM and LSF perovskites as anode materials for high temperature steam electrolysis

La_0.8Sr_0.2MnO₃ (LSM) and La_0.8Sr_0.2FeO₃ (LSF) perovskites used as the anode materials for high temperature steam electrolysis (HTSE) were synthesized by sol–gel self-propagating method. These two powders were mixed with yttria-stabilized zirconia (YSZ) powders, respectively to fabricate composite anodes of solid oxide electrolysis cells (SOECs). The LSM–YSZ and LSF–YSZ composite anodes were tested at 1073 K SOEC working temperature under electrolysis conditions, using cells with a YSZ electrolyte and a Pt counter electrode. Their electrochemical performances were compared and the possibilities of using as SOEC anodes were discussed.     

Efficient carbon dioxide electrolysis in a symmetric solid oxide electrolyzer based on nanocatalyst-loaded chromate electrodes

Composite cathode based on redox-stable La_0.75Sr_0.25Cr_0.5Mn_0.5O_3−δ (LSCM) can be handled to perform for direct CO₂ electrolysis without a flow of reducing gas over the electrode; however, the insufficient electrocatalytic activity of the ceramic composite cathode still limits the electrode performances and current efficiencies. In this case, catalytic-active iron nanocatalyst and iron oxide catalyst were loaded into the LSCM-based composite cathode and anode, respectively, to improve the electrode performances. Then efficient direct CO₂ electrolysis was demonstrated by using the symmetric solid oxide electrolyzer based on LSCM loaded with 2 wt% Fe₂O₃ at 800 °C. The dependences of conductivity of LSCM were studied on temperature and oxygen partial pressure and further correlated to the electrode performance. The loading of nanocatalyst considerably improves the electrode performance and the current efficiency of CO₂ electrolysis was accordingly enhanced by approximately 75% for the impregnated LSCM-based electrode at 800 °C. The synergistic effect of catalyst-active iron nanoparticles and redox-stable LSCM perovskite ceramic leads to the excellent stability and better cathode performance for the direct CO₂ electrolysis at high temperatures.     

Results of recent high temperature coelectrolysis studies at the Idaho National Laboratory

Some results of CO₂/H₂O electrolysis experiments performed to date using button cells and three different 10-cell planar solid oxide stacks are presented and discussed. These results include electrolysis performance at various temperatures, gas mixtures, and electrical settings. Product gas compositions, as measured via an in-line micro gas chromatograph (GC), are compared to predictions obtained from an INL-developed chemical equilibrium coelectrolysis model (CECM). Better understanding of the feasibility of producing syngas using high temperature electrolysis may initiate the systematic investigation of nuclear-powered synfuel production as a bridge to the future hydrogen economy and ultimate independence from foreign energy resources.     

High activity oxide Pr0.3Sr0.7Ti0.3Fe0.7O3−δ as cathode of SOEC for direct high-temperature steam electrolysis

A high activity ferrite Pr_0.3Sr_0.7Ti_0.3Fe_0.7O_3?δ (PSTF) has been synthesized and examined as a cathode of solid oxide electrolysis cell (SOEC) for direct high-temperature steam electrolysis. The SOEC with a configuration of PSTF|YSZ|LSM-YSZ was operated under H₂O concentrations ranging from 20%H₂O/Ar to 60%H₂O/Ar and exhibited excellent electrochemical performances. Polarization resistance of the electrolyzer was as small as 0.43 Ω cm² in 60%H₂O/Ar at 1.85 V at 800 °C. According to AC impendence spectra analyzing, gas diffusion process was the rate-determine-step (RDS) under smaller current density, while under larger current density, transport properties in the electrodes and the interfaces of electrode/electrolyte was RDS. The electrochemical properties of PSTF cathodes were systematically investigated and compared when they were exposed to gas atmosphere with and without safe gas (H₂). The obtained results demonstrated that PSTF electrode could conceivably avoid any hydrogen feeding for steam electrolysis.     

Performance and stability of (La,Sr)MnO3–Y2O3–ZrO2 composite oxygen electrodes under solid oxide electrolysis cell operation conditions

The electrochemical performance and stability of (La,Sr)MnO₃–Y₂O₃–ZrO₂ (LSM-YSZ) composite oxygen electrodes is studied in detail under solid oxide electrolysis cells (SOECs) operation conditions. The introduction of YSZ electrolyte phase to form an LSM-YSZ composite oxygen electrode substantially enhances the electrocatalytic activity for oxygen oxidation reaction. However, the composite electrode degrades significantly under SOEC mode tested at 500 mA cm⁻² and 800 °C. The electrode degradation is characterized by deteriorated surface diffusion and oxygen ion exchange and migration processes. The degradation in electrode performance and stability is most likely associated with the breakup of LSM grains and formation of LSM nanoparticles at the electrode/electrolyte interface, and the formation of nano-patterns on YSZ electrolyte surface under the electrolysis polarization conditions. The results indicate that it is important to minimize the direct contact of LSM particles and YSZ electrolyte at the interface in order to prevent the detrimental effect of the LSM nanoparticle formation on the performance and stability of LSM-based composite oxygen electrodes.     

Preparation of NiO–YSZ composite powder by a combustion method and its application for cathode of SOEC

Nickel oxide and yttria-stabilized zirconia (NiO–YSZ) composite powders were synthesized by a new situ-combustion method in this paper. The adding amount of CO(NH₂)₂ was calculated by the combustion reaction equation. The products were characterized by X-ray diffraction, field emission scanning electronic microscope and electrochemical impedance spectra (EIS). The results showed that the products were well crystallized with NiO coating on YSZ particles. The optimized ratio of CO(NH₂)₂ to Ni(NO₃)₂ was 2:1. A single solid oxide electrolysis cell made from NiO–YSZ composite cathode with the powder prepared at optimized ratio 2:1 exhibited better performance than other samples with the electrolytic voltage of 0.98 V. The electrolytic cell was operated steadily at 900 °C for 2 h with the current of 0.33 A cm⁻² when the stream of 80% H₂O + 20% H₂ was input. EIS analysis indicated that H₂O adsorption and diffusion of the Ni–YSZ electrode were the limited step in the whole electrolysis reaction.     

Synthesis and evaluation of (La0.6Sr0.4)(Co0.2Fe0.8)O3 (LSCF)–Y0.08Zr0.92O1.96 (YSZ)–Gd0.1Ce0.9O2−δ (GDC) dual composite SOFC cathodes for high performance and durability

This paper investigates a (La_0.6Sr_0.4)(Co_0.2Fe_0.8)O₃ (LSCF)–Y_0.16Zr_0.92O_1.96 (YSZ)–Gd_0.1Ce_0.9O_2−δ (GDC) dual composite cathode to achieve better cathodic performance compared to an LSM/GDC–YSZ dual composite cathode developed in previous research. To synthesize the structures of the LSCF/GDC–YSZ and LSCF/YSZ–GDC dual composite cathodes, nano-porous composite cathodes containing LSCF, YSZ, and GDC were prepared by a two-step polymerizable complex (PC) method which prevents the formation of YSZ–GDC solid solution. At 800 °C, the electrode polarization resistance of the LSCF/YSZ–GDC dual composite cathode showed to be significantly lower (0.075 Ω cm²) compared to that of a commercial LSCF–GDC cathode (0.195 Ω cm²), a synthesized LSCF/GDC–YSZ dual composite cathode (0.138 Ω cm²), and an LSM/GDC–YSZ dual composite cathode (0.266 Ω cm²) respectively. Moreover, the Ni–YSZ anode-supported single cell containing the LSCF/YSZ–GDC dual composite cathode achieved a maximum power density of 1.24 W/cm² and showed excellent durability without degradation under a load of 1.0 A/cm² over 570 h of operation at 800 °C.     

A high-performance ceramic composite anode for protonic ceramic fuel cells based on lanthanum strontium vanadate

The composite electrodes for protonic ceramic fuel cells (PCFC) were fabricated by infiltration of (La_0.8Sr_0.2)FeO_3−δ (LSF) cathode and (La_0.7Sr_0.3)V_0.90O_3−δ (LSV) anode into a porous protonic ceramic, Ba(Ce_0.51Zr_0.30Y_0.15Zn_0.04)O_3−δ (BCZY-Zn), respectively. Further, Pd-ceria catalysts were added into the composite anode. In the same method, the oxygen ion conducting fuel cells with the yttria-stabilized zirconia as an electrolyte (YSZ cell) were also fabricated. At 973 K, the non-ohmic area specific resistance (ASR) of PCFC (0.09 Ω cm²) was much smaller than that of the YSZ cell (0.28 Ω cm²) although the protonic conductivity of BCZY-Zn was slightly smaller than the oxygen ion conductivity of YSZ. According to the analysis of the symmetric cells with BCZY-Zn as an electrolyte, the LSV-composite anode showed better performance than the LSF-composite cathode at low temperatures.     

A vanadium-doped BSCF perovskite for CO2 electrolysis in solid oxide electrolysis cells

CO₂ electrolysis through solid oxide electrolysis cells (SOECs) is a promising strategy for converting CO₂ to useful fuels and chemicals powered by renewable energy. Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3-δ (BSCF) is a widely-investigated anode material of SOEC on account of its high and catalytic activity. However, its poor stability under CO₂ atmosphere limits its application as cathode in CO₂ electrolysis. Herein, we develop a vanadium-doped BSCF material Ba_0.5Sr_0.5(Co_0.8Fe_0.2)_1-xV_xO_3-δ (BSCFV_x) for SOEC cathode. The doping of V into B site of BSCF decreases the basicity of perovskite oxide and increases its stability under CO₂ atmosphere, thus improving the CO₂ electrolysis performance. Compared to BSCF, the cell of BSCFV_0.01 cathode shows a better tolerance to CO₂ and achieves 48.3% increase in current density at 1.6 V and 800 °C.     

Long-term tests of a Jülich planar short stack with reversible solid oxide cells in both fuel cell and electrolysis modes

A two-cell planar stack in the Jülich F-design with solid oxide cells has been built and the reversible operation between fuel cell and electrolysis modes has been demonstrated. The cells were anode supported cells (ASC) with yttria-stabilized zirconia (YSZ) electrolytes, Ni/YSZ hydrogen electrodes and perovskite oxygen electrodes with lanthanum strontium cobalt ferrite (LSCF). This paper summarizes and discusses the preliminary experimental results on the long-term aging tests of the reversible solid oxide planar short stack for fuel cell operation (4000 h) at a current density of 0.5 A cm⁻² which shows a degradation of 0.6% per 1000 h and for steam electrolysis operation (3450 h) and co-electrolysis operation of CO₂ and H₂O (640 h) under different current densities from −0.3 to −0.875 A cm⁻² which show different degradation rates depending on current density and on steam or co-electrolysis.     

A high-performance solid oxide fuel cell anode based on lanthanum strontium vanadate

Ceramic composites were prepared by infiltration of La_0.7Sr_0.3VO_3.85 (LSV) into porous scaffolds of yttria-stabilized zirconia (YSZ) and tested for use as solid oxide fuel cell (SOFC) anodes. There was no evidence for solid-state reaction between LSV and YSZ at calcination temperatures up to 1273 K. For calcination at 973 K, LSV formed a continuous film over the YSZ. The LSV phase reduced easily upon heating in H₂ to 973 K, with the reduction forming pores in the LSV and greatly increasing its surface area. The electrodes showed high electronic conductivity after reduction, with a 10-vol% LSV-YSZ composite exhibiting a conductivity of 2 S cm⁻¹ at 973 K. In the absence of an added catalyst, the LSV-YSZ electrodes showed relatively poor performance; however, an electrode impedance of approximately 0.1 Ω cm² was achieved at 973 K in humidified H₂ following addition of 0.5 vol% Pd and 2.8 vol% ceria The LSV-YSZ composites were stable in CH₄ but there was evidence for poisoning of the Pd catalyst by V following high-temperature oxidation.     

Electrochemical characteristics of an La0.6Sr0.4Co0.2Fe0.8O3–La0.8Sr0.2MnO3 multi-layer composite cathode for intermediate-temperature solid oxide fuel cells

An La_0.6Sr_0.4Co_0.2Fe_0.8O₃–La_0.8Sr_0.2MnO₃ (LSCF–LSM) multi-layer composite cathode for solid oxide fuel cells (SOFCs) was prepared on an yttria-stabilized zirconia (YSZ) electrolyte by the screen-printing technique. Its cathodic polarization curves and electrochemical impedance spectra were measured and the results were compared with those for a conventional LSM/LSM–YSZ cathode. While the LSCF–LSM multi-layer composite cathode exhibited a cathodic overpotential lower than 0.13 V at 750 °C at a current density of 0.4 A cm⁻², the overpotential for the conventional LSM–YSZ cathode was about 0.2 V. The electrochemical impedance spectra revealed a better electrochemical performance of the LSCF–LSM multi-layer composite cathode than that of the conventional LSM/LSM–YSZ cathode; e.g., the polarization resistance value of the multi-layer composite cathode was 0.25 Ω cm² at 800 °C, nearly 40% lower than that of LSM/LSM–YSZ at the same temperature. In addition, an encouraging output power from an YSZ-supported cell using an LSCF–LSM multi-layer composite cathode was obtained.     

Electrolysis of carbon dioxide in Solid Oxide Electrolysis Cells

Carbon dioxide electrolysis was studied in Ni/YSZ electrode supported Solid Oxide Electrolysis Cells (SOECs) consisting of a Ni-YSZ support, a Ni-YSZ electrode layer, a YSZ electrolyte, and a LSM-YSZ O₂ electrode (YSZ = Yttria Stabilized Zirconia). The results of this study show that long term CO₂ electrolysis is possible in SOECs with nickel electrodes.The passivation rate of the SOEC was between 0.22 and 0.44 mV h⁻¹ when operated in mixtures of CO₂/CO = 70/30 or CO₂/CO = 98/02 (industrial grade) at 850 °C and current densities between −0.25 and −0.50 A cm⁻².The passivation rate was independent of the current density and irreversible when operated at conditions that would oxidise carbon. This clearly shows that the passivation was not caused by coke formation. On the other hand, the passivation was partly reversible when introducing hydrogen. The passivation may be a consequence of impurities in the gas stream, most likely sulphur, adsorbing on some specific nickel sites in the cathode of the SOEC. Activation can be carried out by hydrogen reacting with adsorbed sulphur to form the volatile compound H₂S. Because adsorption of sulphur is site specific, only a part of the nickel sites were passivated and long-time operation of CO₂ electrolysis in these Ni/YSZ electrode supported Solid Oxide Electrolysis Cells seems therefore feasible.     

Gadolinium doped ceria-impregnated nickel-yttria stabilised zirconia cathode for solid oxide electrolysis cell

Steam electrolysis (H₂O → H₂ + 0.5O₂) was investigated in solid oxide electrolysis cells (SOECs). The electrochemical performance of GDC-impregnated Ni-YSZ and 0.5% wt Rh-GDC-impregnated Ni-YSZ was compared to a composite Ni-YSZ and Ni-GDC electrode using a three-electrode set-up. The electrocatalytic activity in electrolysis mode of the Ni-YSZ electrode was enhanced by GDC impregnation. The Rh-GDC-impregnated Ni-YSZ exhibited significantly improved performance, and the electrode exhibited comparable performance between the SOEC and SOFC modes, close to the performance of the composite Ni-GDC electrode. The performance and durability of a single cell GDC-impregnated Ni-YSZ/YSZ/LSM-YSZ with an H₂ electrode support were investigated. The cell performance increased with increasing temperature (700 °C-800 °C) and exhibited comparable performance with variation of the steam-to-hydrogen ratio (50/50 to 90/10). The durability in the electrolysis mode of the Ni-YSZ/YSZ/LSM-YSZ cell was also significantly improved by the GDC impregnation (200 h, 0.1 A/cm², 800 °C, H₂O/H₂ = 70/30).     

High performance yttria-stabilized zirconia based intermediate temperature solid oxide fuel cells with double nano layer composite cathode

A composite double layer cathode of La_0.6Sr_0.4Co_0.8Fe_0.2O_3?δ/La_0.8Sr_0.2FeO_3?δ (LSCF/LSF) was successfully fabricated by infiltration method to accelerate the sluggish oxygen reduction reaction (ORR) processes. In this composite cathode, both LSF and LSCF layers are uniformly distributed on Yttria-stabilized Zirconia (YSZ) scaffold by optimizing the infiltrating solution components. LSF serves as a protective layer between LSCF and YSZ. The introduction of the LSCF exterior layer has greatly improved cell performance compared with the cell with sole LSF cathode. At 600 °C, the maximum power density of the cell with LSCF/LSF/YSZ composite cathode reaches up to 0.559 W cm^?2. The evolution of the cathode polarization resistance verifies that the ORR activity has been greatly enhanced. Therefore, the results indicate that the high cell performance at intermediate temperatures can be obtained by adopting the LSCF cathode into YSZ-based SOFCs using protective layer and that the infiltration method is a practical way for constructing electrode.     

Composite cathode based on Ni-loaded La0.75Sr0.25Cr0.5Mn0.5O3−δ for direct steam electrolysis in an oxide-ion-conducting solid oxide electrolyzer

Composite cathode based on La_0.75Sr_0.25Cr_0.5Mn_0.5O_3−δ (LSCM) can be utilized for direct steam electrolysis in an oxide-ion conducting solid oxide electrolyzer; however, the insufficient electro-catalytic activity of LSCM still restricts the electrode performance and the Faraday efficiency. In this work, catalytic-active Ni particles are loaded to cathode via impregnation method. The electrical properties of LSCM are investigated and further correlated to the electrochemical performance of LSCM-SDC cathodes. The AC impedance spectroscopy and current–voltage tests demonstrate that the electrochemical reduction of LSCM cathodes is the main process at low voltages; however, the steam electrolysis is the dominant process at high voltages. The Faraday efficiency with Ni-loaded LSCM was enhanced by 20% in 4.96%H₂/Ar/3%H₂O and 11% in 97%Ar/3%H₂O in contrast to the bare LSCM-SDC cathodes, respectively. The synergetic effect of catalytic-active Ni and redox-stable LSCM contributes to improved performance and the stability of the cathode for direct steam electrolysis.     

Nano-structured (La, Sr)(Co, Fe)O3 + YSZ composite cathodes for intermediate temperature solid oxide fuel cells

A nano-structured La_0.8Sr_0.2Co_0.5Fe_0.5O₃ + Y₂O₃ doped ZrO₂ (LSCF + YSZ) composite cathode was prepared by impregnation of a LSCF-containing solution into porous YSZ structure presintered on the YSZ electrolyte. The result shows that the LSCF phase was formed at 700 °C, forming a nano-structured, effective and functional LSCF + YSZ composite cathode that not only produces high triple phase boundaries for the O₂ reduction reaction, but also provides a structurally stable interface between the LSCF and YSZ. The electrode polarization resistance for the O₂ reduction reaction is from 0.539 to 0.047 Ω cm² between 600 and 750 °C, indicating the promising potential the LSCF + YSZ as a high performance cathode for intermediate temperature solid oxide fuel cells.     

Perovskite Sr2Fe1.5Mo0.5O6−δ as electrode materials for symmetrical solid oxide electrolysis cells

Perovskite Sr₂Fe_1.5Mo_0.5O_6−δ (SFM) has been successfully prepared by a microwave-assisted combustion method in air and employed as both anode and cathode in symmetrical solid oxide electrolysis cells (SOECs) for hydrogen production for the first time in this work. Influence of cell operating temperature, absolute humidity (AH) as well as applied direct current (DC) on the impedance of single cells with the configuration of SFM|La_0.9Sr_0.1Ga_0.8Mg_0.2O₃ (LSGM)|SFM has been evaluated. Under open circuit conditions and 60 vol.% AH, the cell polarization resistance, R_P is as low as 0.26 Ω cm² at 900 °C. An electrolysis current of 0.88 A cm⁻² and a hydrogen production rate as high as 380 mL cm⁻² h have been achieved at 900 °C with an electrolysis voltage of 1.3 V and 60 vol.% AH. Further, the cell has demonstrated good stability in the long-term steam electrolysis test. The results showed that the cell electrolysis performance was even better than that of the reported strontium doped lanthanum manganite (LSM) – yttria stabilized zirconia (YSZ)|YSZ|Ni–YSZ cell, indicating that SFM could be a very promising electrode material for the practical application of SOEC technology.