The Mn-doped titanate can lead to the remarkable chemical adsorption of CO2 uncovering the tremendous potential for direct high temperature CO2 electrolysis. Unfortunately, the application of this material is still limited by insufficient carbon dioxide splitting and electrocatalytic activity. This study reports the improved electrocatalytic activity and Faraday efficiency of composite cathode with exsolution of nickel nanocrystals on the surface of doped titanate for CO2 electrolysis. The electrical conductivity has been enhanced in the presence of metallic Ni in contrast to bare Mn doped titanate and further correlated to the electrochemical performance of the composite cathodes. Promising electrode polarization has been demonstrated based on the titanate with nickel nanoparticles and the Faraday efficiency is improved for the direct CO2 electrolysis at high temperatures.

The redox-stable La0.75Sr0.25Cr0.5Mn0.5O3−δ (LSCM) ceramic can be utilized as a solid oxide electrolyzer cathode for direct carbon dioxide electrolysis; nevertheless, the insufficient electro-catalytic activity of ceramic LSCM restricts the electrode performance and current efficiency. In this paper, catalytically active nickel nanoparticles are anchored on the surface of an LSCM substrate through an in situ growth process to improve the electrode performance. The combination of XRD, TEM, XPS, SEM and EDS analyses demonstrate the reversible in situ growth of the nickel catalyst by transforming A-site deficient and B-site rich (La0.75Sr0.25)0.9(Cr0.5Mn0.5)0.9Ni0.1O3−δ (LSCMN) into LSCM and nickel in redox cycles. The conductivities of LSCM and LSCMN are investigated and correlated with electrode performance in symmetrical cells and electrolysis cells. A significant improvement in electrode polarization resistance is observed for the LSCMN cathode. The current efficiency is considerably improved by 30% for LSCMN in contrast to the bare LSCM cathode for direct carbon dioxide electrolysis at 800 °C.

A composite cathode based on lanthanum chromate has uncovered tremendous potential for direct high temperature electrolysis. Unfortunately, a major setback of insufficient electrocatalytic activity limits the efficient carbon dioxide electrolysis. In this paper, catalytically active Ti is doped to partially replace Cr at the B-site in perovskite La0.75Sr0.25Cr0.5−xFe0.5TixO3−δ (LSCrFT, x = 0, 0.1, 0.2, 0.3, 0.4 and 0.5) to improve the polarization and electrocatalytic property of the cathode materials of the solid oxide electrolyzer. The transition of the dominant mechanism from p-type conduction to n-type conduction is observed for the LSCrFT cathode with the Ti doping level at x = 0.3. The doping of Ti in LSCrFT largely improves the electrode activity and therefore reduces electrode polarization resistance. A high-temperature CO2 electrolysis test demonstrates that the Faraday efficiencies of LSCrFT (x = 0.1) cathodes are enhanced by approximately 30% in contrast to bare LSCrF cathodes at 800 °C.

Chemical adsorption of CO2 in composite cathodes at high temperatures plays a significant role in the electrochemical conversion of CO2 into fuels in efficient solid oxide electrolysers. In this work, the active Mn with multi-oxidation states is introduced into the B-site lattice of the redox-stable (La, Sr)TiO3+δ to create oxygen vacancies both in the bulk and on the surface. The ionic conductivities of the Mn-doped titanate are remarkably enhanced by 1–2 orders of magnitude at intermediate temperatures in reducing or oxidizing atmospheres. The chemical adsorption of CO2 is accordingly enhanced by approximately 1 order of magnitude for the Mn-doped titanate and the onset temperature of strong chemical desorption is consequently extended to as high as approximately 800 °C of the common operation temperature of solid oxide carbon dioxide electrolysers. First principles calculations reveal that oxygen vacancy defect sites created by Mn dopants substantially contribute to the chemical adsorption of CO2 and the strong bonding of the oxide ions in CO2 to the nearest cations on the (La,Sr)O- or (Ti,Mn)O2-terminated facets not only activates CO2 molecules but also considerably increases the desorption temperature. The highest current efficiencies of approximately 100% are obtained with the Mn-doped titanate cathodes for the direct electrolysis of CO2 in oxide ion-conducting solid oxide electrolysers.

•The synergetic effect was achieved in the nanometal-loaded vanadate cathode.•The electrode polarization effectively improves with loaded Ni/Fe nanoparticles.•Composite cathode with Ni/Fe nanocatalyst achieves high current efficiency.•Direct steam electrolysis is efficient and dominates the process at high voltage.The use of composite electrodes based on La0.7Sr0.3VO3 (LSV) for steam electrolysis has uncovered the tremendous potential and capacity inherent in this material. Unfortunately, this material has a major setback of inefficient electrolysis triggered by limited electrocatalytic activity. In this work, an infiltration method is employed to load catalytic-active metal nanoparticles onto the composite electrodes in order to achieve an activity-enhanced electrode performance. The electrical properties of LSV are methodically explored and correlated to electrode performance. At 800 °C in either pure H2 or low hydrogen partial pressure (pH2) of 5%H2/N2, the polarization resistance of symmetrical cells with Ni-loaded LSV (LSV-Ni) cathode is largely enhanced, in contrast to bare LSV cathode. Similar improvement is also achieved for the Fe-loaded LSV (LSV-Fe) cathode in a wide range of hydrogen partial pressures of 5%–100%. The Faraday efficiencies of LSV-Ni and LSV-Fe composite cathodes were remarkably improved for electrolysis in either 3%H2O/4.7H2/Ar or 3%H2O/Ar at 800 °C.

•Ni nanoparticles are anchored on cathode skeleton for direct steam electrolysis.•The exsolution of Ni nanoparticles significantly enhance cathode performances.•The synergy of Ni and ceramic cathode leads to stability of steam electrolysis.Doped lanthanum chromates are now commonly used to prepare composite cathodes for direct steam electrolysis. However, the limitation of electrochemical performances and cell current efficiency by insufficient electro-catalytic activity is a major challenge. This paper reports the use of reversibly exsolved catalytic metallic Ni nano-particles on A-site deficient and B-site excess perovskite (La0.75Sr0.25)0.95(Cr0.8Ni0.2)0.95Ni0.05O3−δ (LSCNNi), in order to achieve an activity-enhanced composite cathode via high-temperature reduction under reducing atmospheres. The electrical properties of the ceramic are investigated methodically and correlated with the performances of the composite electrodes in symmetric and electrolysis cells. XRD, SEM, EDS and XPS results, confirm that the exsolution or dissolution of nano metallic catalyst is reversible in redox treatment cycles. The Faradic efficiency reaches approximately 80% for the LSCNNi cathode with the flow of 5%H2/Ar. Ultimately, the unified effect of metallic catalyst and redox-stable ceramic produce striking redox stability as well as electrochemical performances of the titled composite cathode. The results signify that the composite cathode with exsolved nickel nano-particles, is a good potential fuel electrode for direct steam electrolysis in an oxygen-ion conducting solid oxide electrolyzer.

•Ni-loaded LSTO is utilized for direct steam electrolysis to produce hydrogen.•Ni catalyst significantly improves the Faraday efficiency for steam electrolysis.•The stability of Ni-loaded composite cathode is achieved for direct electrolysis.Composite Ni–SDC (Samaria doped Ceria) cathodes are able to operate in strong reducing atmospheres for steam electrolysis, and composite cathodes based on redox-stable La0.4Sr0.4TiO3 (LSTO) have demonstrated promising performances without the reducing gas flow. However, the electro-catalytic activity of cathodes based on LSTO is insufficient for the efficient electrochemical reduction of steam or carbon oxide. In this work, catalytic-active Ni nanoparticles were loaded on a La0.4Sr0.4TiO3−δ–Ce0.8Sm0.2O2−δ cathode (Ni-loaded LSTO–SDC) via an impregnation method to improve the electrode performances for direct steam electrolysis. The synergetic effect of catalytically-active Ni nanoparticles and the redox-stable LSTO–SDC skeleton contributed to the improved performances and the excellent stability of the cathode for direct steam electrolysis. The current efficiency with a Ni-loaded cathode was enhanced by 3% and 17% compared to the values with a bare LSTO–SDC cathode under 2.0 V of applied voltage at 800 °C with a flow of 3% H2O/5% H2/Ar and 3% H2O/Ar to cathodes, respectively.

The use of perovskite titanate as composite cathode for direct steam electrolysis has demonstrated promising performances due to its unique specific redox-stable properties. However, the insufficient electro-catalytic activity of the perovskite titanate remains a major setback that inhibits the electrode performance. This study addresses the drawbacks and reports an investigation of iron doped titanate (La0.2Sr0.8)0.9Ti0.9Fe0.1O3-δ (LSTFO), which can be reduced to in-situ grown iron nano-catalyst on substrate, as a potential composite cathode for steam electrolysis in a solid oxide electrolyser. Results of XRD, SEM, EDS, XPS and TGA analysis together confirm that the exsolution and dissolution of the iron nanoparticles is completely reversible in redox treatment cycles. The electrical properties of the LSTO and LSTFO are investigated and correlated to the performances of the composite electrodes. The presence of exsolved iron nanocatalyst significantly enhanced the polarizations of the symmetrical and electrolysis cells of the LSTFO composite electrode. The synergetic effect of catalytically-active iron and redox-stable LSTO produced excellent short-term stability of the cathode with attendant efficient electrochemical reduction of steam and remarkable enhancement of Faraday efficiency by about 100%.

Composite cathodes based on Sr0.94Ti0.9Nb0.1O3 (STNO) can be utilized for direct steam electrolysis; however, the insufficient electrocatalytic activity limits electrode performance and current efficiency. In this work, redox-reversible (Sr0.94)0.9(Ti0.9Nb0.1)0.9Ni0.1O3 (STNNO) with A-site deficiency and B-site excess has been designed as a cathode material in an oxide-ion-conducting solid oxide electrolyzer for direct steam electrolysis. The XRD, TEM, SEM, EDS, TGA and XPS results together confirm that the exsolution or dissolution of Ni nanoparticles anchored on the STNO surface is completely reversible in the redox cycles. The electrical properties of STNO and STNNO are investigated and correlated to electrode performances. The current efficiency with an STNNO cathode is enhanced by about 20% compared to the values with a bare STNO cathode in 5% H2O/H2/Ar or 5% H2O/Ar at 800 °C. The synergetic effect of catalytically active nickel nanoparticles and the redox-stable STNO skeleton contributes to the improved performance and excellent stability of the cathode for steam electrolysis.

A composite cathode based on redox-stable La0.2Sr0.8TiO3+δ (LSTO) can perform direct carbon dioxide electrolysis; however, the insufficient electro-catalytic activity limits the electrode performances and current efficiencies. In this work, catalytically active scandium is doped into LSTO to enhance the electro-catalytic activity for CO2 electrolysis. The structures, electronic conductivities and ionic conductivities of La0.2Sr0.8Ti1−xScxO (LSTSxO) (x = 0, 0.05, 0.1, 0.15 and 0.2) are systematically studied and further correlated with electrode performances. The ionic conductivities of single-phase LSTSxO (x = 0, 0.05, 0.1 and 0.15) remarkably improve versus the scandium doping contents though the electrical conductivities gradually change in an adverse trend. Electrochemical measurements demonstrate promising electrode polarisation of LSTSxO electrodes and increasing scandium doping contents accordingly improve electrode performances. The Faradic efficiencies of carbon dioxide electrolysis are enhanced by 20% with LSTS0.15O in contrast to bare LSTO electrodes in a solid oxide electrolyser at 800 °C.

This paper reports the in situ growth of Ni nanocatalysts to anchor onto the CeO2 surface to combine the surface oxygen vacancies and form heterogeneous catalysis sites with the aim of improving electrocatalytic activity through direct exsolution of Ni nanoparticles from the Ni-doped CeO2 lattice in a reducing atmosphere at higher temperatures. The combined use of XRD, TEM, SEM and XPS confirms the in situ exsolution of Ni nanoparticles on the CeO2 surface. The doping of CeO2 with nickel leads to a charge redistribution and an increase of oxygen vacancy concentration. The electrical properties of Ce1−xNixO2 (x = 0, 0.05, 0.10, 0.15 and 0.20) are systematically investigated and correlated to their electrochemical performance in symmetrical and electrolysis cells. The electrical properties and electrochemical performances improve with increasing Ni contents. The Ce0.85Ni0.15O2 cathode with anchored Ni nanocrystal shows the best electrochemical performances for carbon dioxide electrolysis with reasonable short-term stability; however, the electro-catalytic activity of Ce0.8Ni0.2O2 with excess Ni particles on the surface rapidly decays because of adverse agglomeration of Ni particles at high temperatures.

Biogas reforming is a renewable and promising way to produce syngas. In this work, we demonstrate a novel strategy to directly and electrochemically convert CH4–CO2 into H2–CO. Electrochemical reforming of dry CH4–CO2 (1:1) mixture is successfully achieved in a 10 μm-thick titanate cathode with oxygen byproduct generated in anode in an oxide-ion-conducting solid oxide electrolyser under external voltages. In addition, loading iron nanocatalyst in titanate cathode or/and increasing applied voltages has further improved CH4–CO2 conversion. The highest methane conversion of approximately 80% is demonstrated for direct electrochemical reforming in cathode in contrast to the low conversion under open circuit condition in oxide-ion-conducting solid oxide electrolyser cathode.

The coupling of surface oxygen vacancies with nano-sized metal can effectively improve the catalytic activity of heterogeneous catalysts. In this work, a high concentration of oxygen vacancies is created in Mn-doped titanate cathode, then iron nanoparticles are exsolved to anchor on the titanate surface and combine the surface oxygen vacancies to form heterogeneous catalysis clusters. The active Mn in the B site of the redox-stable Sr0.95Ti0.8Nb0.1Mn0.1O3.00 (STNMO) creates 0.15 mol oxygen vacancies in the reduced Sr0.95Ti0.8Nb0.1Mn0.1O2.85. With iron doping in the B site, it is found that the exsolution and dissolution of the iron nanoparticles are completely reversible on the titanate surface in redox cycles. The presence of iron nano crystal remarkably increases the ionic conductivity of the titanate solid solution by 0.5 orders of magnitude at intermediate temperatures. Promising electrode polarizations are obtained based on the titanate cathode decorated with heterogeneous electrocatalytic clusters in symmetric cells. The current efficiencies of direct carbon dioxide electrolysis reach as high as 90% in an oxide-ion conducting solid oxide electrolyzer at high temperatures.

This paper investigates the reversible exsolution of a Ni nanocatalyst anchored on the surface of La0.3Sr0.7TiO3−δ (LSTO) for enhancing the electrocatalytic activity of the composite cathode. The metallic Ni nanoparticles significantly enhance the electrode performance and elevate the current efficiency of the electrode in high-temperature steam electrolysis. The combination of XRD, SEM, EDS and XPS results confirms that the exsolution and dissolution of the Ni nanoparticles are completely reversible in redox cycles. The electrical conductivities of the Ni-loaded samples accordingly improved as well. The synergetic effects of the Ni nanocatalyst and redox-stable titanate contribute to the excellent stability and improved performance for direct steam electrolysis. The current efficiencies with Ni-anchored LSTO can be accordingly enhanced by 20% in contrast to the bare cathode with or without reducing gas flowing over the cathode under the applied voltage of 2.0 V at 800 °C.

•In-situ anchoring of Cu nanocatalyst achieved through controlling non-stoichiometry.•The exsolution of Cu nanocatalyst is completely reversible in redox cycles.•The Cu catalyst significantly improves Faraday efficiency for steam electrolysis.•The composite cathode shows short-term stability for direct steam electrolysis.This paper investigates a potential cathode material (La0.2Sr0.8)0.9Ti0.9Cu0.1O3−δ (LSTCO) with A-site deficiency and B-site excess which was designed as a parent material for anchoring exsolved copper nanocatalyst on the surface of La0.2Sr0.8TiO3+δ (LSTO) through a high-temperature reduction. Physical characterization of the samples by combined use of X-ray diffraction, scanning electron microscope, energy-dispersive spectroscopy, thermogravimetric analyzer and X-ray photoelectron spectroscopy indicated that the exsolution and dissolution of the Cu nanoparticles in the cathode was completely reversible in the redox cycles. Electrical properties of LSTO and LSTCO were systematically investigated which correlated closely with electrochemical performance of the composite electrodes in symmetrical cells and electrolysis cells. Polarization resistance (Rp) of the symmetrical cells was improved from 3 Ω cm2 of the LSTO to 1.5 Ω cm2 of the LSTCO in hydrogen atmosphere at 800 °C. Current efficiencies of the solid oxide electrolyzer with Cu-anchored LSTO cathode were found to be enhanced by approximately 20% compared to the bare cathode with or without reducing gas flowing over them under the applied voltage of 2.0 V at 800 °C, respectively.

•LSCM cathode with loaded iron catalyst shows promising electrode polarization.•LSCM anode with loaded iron oxide also shows promising electrode polarization.•Mixed conductivities of LSCM electrode in pulsed redox atmospheres are reported.•Efficient CO2 electrolysis using LSCM-Fe cathode and LSCM-Fe2O3 anode is verified.Composite cathode based on redox-stable La0.75Sr0.25Cr0.5Mn0.5O3−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.

Fluorite-type heterogeneous catalyst ceria is a mixed conductor and widely used as a hydrocarbon-fueled solid oxide fuel cell anode because of its advantage of anti-carbon deposition, redox stability, and thermal compatibility. However, the electrocatalytic activity of a ceria cathode is limited for the catalysis of electrochemical oxidation or reduction reactions. In this work, catalytic-active iron and nickel catalysts are loaded onto a ceria cathode via an infiltration method to enhance electrode performance. Direct electrolysis of carbon dioxide is performed on ceria cathodes loaded with iron and nickel catalysts in solid oxide electrolyzers, respectively. The polarization resistance of symmetrical cells and electrolysis cells loaded with nickel and iron catalysts is largely improved in comparison with the bare ceria. The current efficiencies for carbon dioxide electrolysis for the iron- and nickel-loaded cathodes are 76 and 80 % at 2.0 V and 800 °C, respectively, approximately 25 % higher than that for the bare ceria cathode.

This paper investigates potential fuel electrode materials NbTi0.5M0.5O4 (M = Ni, Cu) for solid oxide steam electrolysers. Efficient catalytic metallic Ni and Cu nanoparticles are exsolved and anchor onto the surface of the highly electronically conducting material Nb1.33Ti0.67O4, forming an enhanced composite fuel electrode in the in situ reduction. The XRD, SEM, EDS and XPS results together confirm that the exsolution or dissolution of the nanometallic catalyst is completely reversible in the redox cycles. The electrical properties of the reduced NbTi0.5M0.5O4 ceramic fuel electrode is systematically investigated and correlated to the electrochemical performance of the composite fuel electrodes in symmetric cells or electrolysis cells. The synergetic effect of the metallic catalyst and ceramic fuel electrode leads to the excellent stability as well as the superior performance of the direct steam electrolysis without a flow of reducing gas over the composite fuel electrodes. The Faradic efficiencies of the steam electrolysis reach 95% and 97% for the Ni- and Cu-enhanced fuel electrodes on flowing reducing gas over the fuel electrodes, respectively, however, comparable performance is achieved for the direct steam electrolysis with Ni- and Cu-enhanced fuel electrodes without flowing reducing gas over the composite fuel electrodes.

Recently, composite cathodes based on doped lanthanum chromates have been widely employed for direct steam electrolysis. However, this approach limits the electrode performances and Faraday efficiency due to insufficient electrocatalytic activity. This study addresses the drawbacks and reports an improved electrocatalytic activity and Faraday efficiency of composite cathode with a reversibly exsolved iron nanoparticles anchored on the surface of doped lanthanum chromates. A-site deficient and B-site excess (La0.75Sr0.25)0.85(Cr0.5Fe0.5)0.85Fe0.15O3−δ (LSCrFF) was designed as the parent material to anchor the exsolved iron nanoparticles on the surface of perovskite chromate (La0.75Sr0.25)(Cr0.5Fe0.5)O3-δ (LSCrF) via high-temperature reduction. The electrical properties of LSCrF and Fe/LSCrF were systematically investigated and correlated with electrochemical performance of the composite electrodes in symmetrical cells and electrolysis cells. The iron nanoparticles significantly improve the electrical conductivity of LSCrF from 1.80 to 6.35 S cm–1 for Fe/LSCrF at 800 °C and Po2 of 10–15 atm. The polarization resistance, Rp, of the symmetrical cells was accordingly enhanced from 4.26 Ω cm2 with LSCrF to 2.58 Ω cm2 with Fe/LSCrF in hydrogen atmosphere at 800 °C. The Faraday efficiency for the direct steam electrolysis showed a marked increase of 89.3% with LSCrFF cathode at 800 °C and 1.8 V as opposed to 76.7% with the cathodes based on LSCrF. The synergetic effect of catalytic-active iron nanoparticles and redox-stable LSCrF substrate produced improved performances and excellent stability for the direct steam electrolysis without a flow of reducing gas over the composite cathodes.Keywords: conductivity; doped lanthanum chromates; iron nanoparticles; solid oxide electrolyzer; steam electrolysis;

In this paper, we demonstrate the direct conversion of CO2/H2O into fuel in a proton-conducting solid oxide electrolyser with the configuration (La0.75Sr0.25)0.95Mn0.5Cr0.5O3−δ (LSCM, oxygen electrode)/BaCe0.5Zr0.3Y0.16Zn0.04O3−δ (BCZYZ, proton-conducting electrolyte)/Ni (fuel electrode) at 600 °C, where 5% H2O/Ar and 100% CO2 are fed into the oxygen electrode and fuel electrode, respectively. AC impedance spectroscopy and I–V testing demonstrate two main processes in the electrochemical process from 0 to 2 V: (1) the reoxidation of the LSCM electrode (Mn3± → Mn4±) below 1.2 V (iR-corrected voltage) and (2) the oxidation of H2O (H2O − 2e → H± ± 1/2O2) above 1.2 V (iR-corrected voltage). The current density reaches ∼0.1 Acm−2 at 2 V versus open circuit voltage (OCV) with a total polarisation resistance of 7.5 Ωcm2. Steam is steadily electrolysed under a 2 V load at 600 °C, and the generated protons in the fuel electrode are simultaneously and completely utilised to electrochemically reduce CO2 with 100% selectivity and ∼90% current efficiency to CO fuel. However, the carbon deposition, poisoning and oxidation of Ni metal in the fuel electrode degrade the cell performance.Highlights► CO2 was electrochemically reduced into CO in H+-type solid oxide electrolysers. ► LSCM-based oxygen electrode shows excellent performance for H2O electrolysis. ► Reoxidation of LSCM is the main process at low voltage during electrolysis. ► Electrochemical oxidation of H2O is main process at high voltage in electrolysis.

•LSCM fuel electrode shows the feasibility of direct steam electrolysis.•Electrical properties of LSCM are studied and correlated to cell performances.•The loading of Fe to LSCM significantly improves fuel electrode and cell performances.•The synergetic effect of Fe and LSCM leads to direct and efficient steam electrolysis.Composite cathodes based on La0.75Sr0.25Cr0.5Mn0.5O3−δ (LSCM) can perform direct steam electrolysis; however, the insufficient electro-catalytic activity still limits the electrode performances and Faradic efficiency. In this work, Fe catalyst is loaded onto an LSCM-based composite fuel electrode and studied for direct steam electrolysis. The dependence of the conductivity of LSCM on temperature and on oxygen partial pressure is investigated and correlated to the cathode performance. The loading of Fe catalysts significantly improved the electrode performance and the Faradic efficiency without the flow of reducing gas over the cathode. The current efficiency is enhanced by 30% and 40% compared to the values with the bare LSCM-based cathode at 800 °C in 3%H2O/5%H2/Ar and 3%H2O/Ar, respectively. The synergetic effect of catalytic-active Fe and redox-stable LSCM leads to excellent stability and better cathode performance for direct steam electrolysis.

•LSM as the composite anode material of proton-conducting SOE is investigated.•Electrical properties of LSM are studied and correlated to cell performances.•Electrolyzer performance improved significantly with catalyst Co3O4-loaded on anode.A composite oxygen electrode based on Co3O4-loaded La0.8Sr0.2MnO3 (LSM)-BaCe0.5Zr0.3Y0.16Zn0.04O3−δ (BCZYZ) is investigated for steam electrolysis in a proton-conducting solid oxide electrolyzer. The conductivity of LSM is studied with respect to temperature and oxygen partial pressure and correlated to the electrochemical properties of the composite oxygen electrodes in symmetric cells and solid oxide electrolyzers at 800 °C. The optimal Co3O4 loading in the composite oxygen electrode is systematically investigated in symmetric cells; loading catalytically active Co3O4 significantly enhances the electrode performance, unlike the bare LSM-BCZYZ electrode. Steam electrolysis was then performed using LSM-BCZYZ and 6 wt.% Co3O4-loaded LSM-BCZYZ oxygen electrodes, respectively. The Co3O4-loaded catalyst significantly improves the electrode process and enhances the current density below a certain applied voltage. The current efficiencies reach approximately 46% with a 10% H2O/air feed for the oxygen electrode.

•Ni-loaded LSCM electrode with synergetic electro-catalytic activity is designed.•Ni-loaded LSCM is utilized for direct steam electrolysis to produce hydrogen.•Ni catalyst significantly improves the Faraday efficiency for steam electrolysis.•The composite cathode shows short-term stability for direct steam electrolysis.Composite cathode based on La0.75Sr0.25Cr0.5Mn0.5O3−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.

Solid oxide electrolyzers have attracted a great deal of interest in recent years because they can convert electrical energy into chemical energy with high efficiency. Ni/YSZ cathodes are generally utilized for high temperature electrolysis of H2O and CO2 in oxygen-ion conducting solid oxide electrolyzers; however, such electrodes can only operate under reducing conditions. In an atmosphere without a flow of reducing gas, cathodes based on La0.2Sr0.8TiO3+δ (LST) are a promising alternative. Solid Oxide Electrolyzers with LST cathodes without pre-reduction were used at 700 °C for the electrolysis of 3%H2O/97%N2 and 100%CO2, and promising polarization impedance data were obtained in both atmospheres. The electrochemical results indicated that the electrochemical reduction of the La0.2Sr0.8TiO3+δ cathode was the main process at low electrical voltages, while the electrolysis was the main process at high voltages because ion transportation in the electrolyte limited the overall efficiency. The electrolysis of H2O was determined to be more efficient than the electrolysis of CO2 under the same conditions. The Faraday efficiencies of H2O and CO2 were 85.0% and 24.7%, respectively, at 700 °C and a 2 V applied potential.Highlights► La0.2Sr0.8TiO3+δ is a promising cathode of SOE at intermediate temperature. ► The electrolysis of H2O and CO2 cathode is efficient in SOE with a La0.2Sr0.8TiO3+δ. ► The main process is reduction of La0.2Sr0.8TiO3+δ at low voltages.

Composite Ni–YSZ fuel electrodes are able to operate only under strongly reducing conditions for the electrolysis of CO2 in oxygen-ion conducting solid oxide electrolysers. In an atmosphere without a flow of reducing gas (i.e., carbon monoxide), a composite fuel electrode based on redox-reversible La0.2Sr0.8TiO3+δ (LSTO) provides a promising alternative. The Ti3+ was approximately 0.3% in the oxidized LSTO (La0.2Sr0.8TiO3.1), whereas the Ti3+ reached approximately 8.0% in the reduced sample (La0.2Sr0.8TiO3.06). The strong adsorption of atmospheric oxygen in the form of superoxide ions led to the absence of Ti3+ either on the surface of oxidized LSTO or the reduced sample. Reduced LSTO showed typical metallic behaviour from 50 to 700 °C in wet H2; and the electrical conductivity of LSTO reached approximately 30 S cm−1 at 700 °C. The dependence of [Ti3+] concentration in LSTO on PO2 was correlated to the applied potentials when the electrolysis of CO2 was performed with the LSTO composite electrode. The electrochemical reduction of La0.2Sr0.8TiO3+δ was the main process but was still present up to 2 V at 700 °C during the electrolysis of CO2; however, the electrolysis of CO2 at the fuel electrode became dominant at high applied voltages. The current efficiency was approximately 36% for the electrolysis of CO2 at 700 °C and a 2 V applied potential.

Composite cathodes based on Sr0.94Ti0.9Nb0.1O3 (STNO) can be utilized for direct steam electrolysis; however, the insufficient electrocatalytic activity limits electrode performance and current efficiency. In this work, redox-reversible (Sr0.94)0.9(Ti0.9Nb0.1)0.9Ni0.1O3 (STNNO) with A-site deficiency and B-site excess has been designed as a cathode material in an oxide-ion-conducting solid oxide electrolyzer for direct steam electrolysis. The XRD, TEM, SEM, EDS, TGA and XPS results together confirm that the exsolution or dissolution of Ni nanoparticles anchored on the STNO surface is completely reversible in the redox cycles. The electrical properties of STNO and STNNO are investigated and correlated to electrode performances. The current efficiency with an STNNO cathode is enhanced by about 20% compared to the values with a bare STNO cathode in 5% H2O/H2/Ar or 5% H2O/Ar at 800 °C. The synergetic effect of catalytically active nickel nanoparticles and the redox-stable STNO skeleton contributes to the improved performance and excellent stability of the cathode for steam electrolysis.

This paper investigates potential fuel electrode materials NbTi0.5M0.5O4 (M = Ni, Cu) for solid oxide steam electrolysers. Efficient catalytic metallic Ni and Cu nanoparticles are exsolved and anchor onto the surface of the highly electronically conducting material Nb1.33Ti0.67O4, forming an enhanced composite fuel electrode in the in situ reduction. The XRD, SEM, EDS and XPS results together confirm that the exsolution or dissolution of the nanometallic catalyst is completely reversible in the redox cycles. The electrical properties of the reduced NbTi0.5M0.5O4 ceramic fuel electrode is systematically investigated and correlated to the electrochemical performance of the composite fuel electrodes in symmetric cells or electrolysis cells. The synergetic effect of the metallic catalyst and ceramic fuel electrode leads to the excellent stability as well as the superior performance of the direct steam electrolysis without a flow of reducing gas over the composite fuel electrodes. The Faradic efficiencies of the steam electrolysis reach 95% and 97% for the Ni- and Cu-enhanced fuel electrodes on flowing reducing gas over the fuel electrodes, respectively, however, comparable performance is achieved for the direct steam electrolysis with Ni- and Cu-enhanced fuel electrodes without flowing reducing gas over the composite fuel electrodes.

Composite Ni–YSZ fuel electrodes are able to operate only under strongly reducing conditions for the electrolysis of CO2 in oxygen-ion conducting solid oxide electrolysers. In an atmosphere without a flow of reducing gas (i.e., carbon monoxide), a composite fuel electrode based on redox-reversible La0.2Sr0.8TiO3+δ (LSTO) provides a promising alternative. The Ti3+ was approximately 0.3% in the oxidized LSTO (La0.2Sr0.8TiO3.1), whereas the Ti3+ reached approximately 8.0% in the reduced sample (La0.2Sr0.8TiO3.06). The strong adsorption of atmospheric oxygen in the form of superoxide ions led to the absence of Ti3+ either on the surface of oxidized LSTO or the reduced sample. Reduced LSTO showed typical metallic behaviour from 50 to 700 °C in wet H2; and the electrical conductivity of LSTO reached approximately 30 S cm−1 at 700 °C. The dependence of [Ti3+] concentration in LSTO on PO2 was correlated to the applied potentials when the electrolysis of CO2 was performed with the LSTO composite electrode. The electrochemical reduction of La0.2Sr0.8TiO3+δ was the main process but was still present up to 2 V at 700 °C during the electrolysis of CO2; however, the electrolysis of CO2 at the fuel electrode became dominant at high applied voltages. The current efficiency was approximately 36% for the electrolysis of CO2 at 700 °C and a 2 V applied potential.

A composite cathode based on redox-stable La0.2Sr0.8TiO3+δ (LSTO) can perform direct carbon dioxide electrolysis; however, the insufficient electro-catalytic activity limits the electrode performances and current efficiencies. In this work, catalytically active scandium is doped into LSTO to enhance the electro-catalytic activity for CO2 electrolysis. The structures, electronic conductivities and ionic conductivities of La0.2Sr0.8Ti1−xScxO (LSTSxO) (x = 0, 0.05, 0.1, 0.15 and 0.2) are systematically studied and further correlated with electrode performances. The ionic conductivities of single-phase LSTSxO (x = 0, 0.05, 0.1 and 0.15) remarkably improve versus the scandium doping contents though the electrical conductivities gradually change in an adverse trend. Electrochemical measurements demonstrate promising electrode polarisation of LSTSxO electrodes and increasing scandium doping contents accordingly improve electrode performances. The Faradic efficiencies of carbon dioxide electrolysis are enhanced by 20% with LSTS0.15O in contrast to bare LSTO electrodes in a solid oxide electrolyser at 800 °C.

Chemical adsorption of CO2 in composite cathodes at high temperatures plays a significant role in the electrochemical conversion of CO2 into fuels in efficient solid oxide electrolysers. In this work, the active Mn with multi-oxidation states is introduced into the B-site lattice of the redox-stable (La, Sr)TiO3+δ to create oxygen vacancies both in the bulk and on the surface. The ionic conductivities of the Mn-doped titanate are remarkably enhanced by 1–2 orders of magnitude at intermediate temperatures in reducing or oxidizing atmospheres. The chemical adsorption of CO2 is accordingly enhanced by approximately 1 order of magnitude for the Mn-doped titanate and the onset temperature of strong chemical desorption is consequently extended to as high as approximately 800 °C of the common operation temperature of solid oxide carbon dioxide electrolysers. First principles calculations reveal that oxygen vacancy defect sites created by Mn dopants substantially contribute to the chemical adsorption of CO2 and the strong bonding of the oxide ions in CO2 to the nearest cations on the (La,Sr)O- or (Ti,Mn)O2-terminated facets not only activates CO2 molecules but also considerably increases the desorption temperature. The highest current efficiencies of approximately 100% are obtained with the Mn-doped titanate cathodes for the direct electrolysis of CO2 in oxide ion-conducting solid oxide electrolysers.

The redox-stable La0.75Sr0.25Cr0.5Mn0.5O3−δ (LSCM) ceramic can be utilized as a solid oxide electrolyzer cathode for direct carbon dioxide electrolysis; nevertheless, the insufficient electro-catalytic activity of ceramic LSCM restricts the electrode performance and current efficiency. In this paper, catalytically active nickel nanoparticles are anchored on the surface of an LSCM substrate through an in situ growth process to improve the electrode performance. The combination of XRD, TEM, XPS, SEM and EDS analyses demonstrate the reversible in situ growth of the nickel catalyst by transforming A-site deficient and B-site rich (La0.75Sr0.25)0.9(Cr0.5Mn0.5)0.9Ni0.1O3−δ (LSCMN) into LSCM and nickel in redox cycles. The conductivities of LSCM and LSCMN are investigated and correlated with electrode performance in symmetrical cells and electrolysis cells. A significant improvement in electrode polarization resistance is observed for the LSCMN cathode. The current efficiency is considerably improved by 30% for LSCMN in contrast to the bare LSCM cathode for direct carbon dioxide electrolysis at 800 °C.