The solid oxide CO2 electrolyzer has the potential to provide storage solutions for intermittent renewable energy sources as well as to reduce greenhouse gas emissions. One of the key challenges remains the poor adsorption and activity toward CO2 reduction on the electrolyzer cathode at typical operating conditions. Here, we show a novel approach in tailoring a perovskite titanate (La, Sr)TiO3+δ cathode surface, by the in situ growing of SrO nanoislands from the host material through the control of perovskite nonstoichiometry. These nanoislands provide very enhanced CO2 adsorption and activation, with stability up to 800 °C, which is shown to be in an intermediate form between carbonate ions and molecular CO2. The activation of adsorbed CO2 molecules results from the interaction of exsolved SrO nanoislands and the defected titanate surface as revealed by DFT calculations. These cathode surface modifications result in an exceptionally high direct CO2 electrolysis performance with current efficiencies near 100%.Keywords: alkaline earth oxide; carbon dioxide; electrolysis; perovskite; solid oxide electrolyzer;

•Redox reversible ferrite cathode is demonstrated for solid oxide electrolyser.•Promising electrical conductivity is obtained with Pr doping in hydrogen.•High performance of steam electrolysis is achieved with ferrite cathode.In this work, perovskite Sr1−xPrxFeO3-δ (SPF) (x = 0.02, 0.04, 0.06, 0.08 and 0.10) are investigated and employed as solid oxide steam electrolyser cathode at 800 °C. X-ray diffraction (XRD), scanning electron microscope (SEM), and transmission electron microscopy (TEM) analysis together indicate that the Sr1−xPrxFeO3-δ is redox reversible with a phase transition from cubic to orthorhombic structure in redox cycles. The doping of Pr in A site has remarkably enhanced the electronic conduction to 1.0–1.2 S cm−1 at intermediate temperatures in reducing atmosphere. Electrochemical measurements demonstrate that the polarization resistance with Sr0.96Pr0.04FeO3-δ electrode shows the lowest values of 0.25 Ω cm2 in symmetric cells in reducing atmosphere at 800 °C. Direct steam electrolysis with Sr0.96Pr0.04FeO3-δ cathode shows a current density of 1.64 A cm−2 at 2.0 V when fed with 5%H2O/Ar. The hydrogen production rate reaches 4.73, 6.68, 8.35 and 10.23 mL min−1 cm−2 at 1.4, 1.6, 1.8, 2.0 V, respectively, while the highest Faraday efficiency is as high as 97.16% at 1.8 V.

•Metal nanocatalysts are impregnated in LSCM to enhance cathode performance.•The synergetic effect of nanocatalyst and LSCM may improve electrode activity.•The NiCu-LSCM shows optimum cathode performance without degradation.•Superior long-term and cycling ability of cells are obtained.Solid oxide electrolysis cells with La0·75Sr0·25Cr0·5Mn0·5O3-δ (LSCM) cathode can electrolyze CO2 to generate chemical fuels. Nevertheless, the cathode performance is limited by its electrocatalytic activity. In this work, metal nanoparticles including Ni, Cu and NiCu metals are successfully impregnated in LSCM electrode to improve its activity. XRD, XPS, SEM and TEM together confirm the metal nanocatalysts are homogeneously distributed on LSCM backbone and therefore create active electrochemical interface for CO2 splitting. Electrical properties of LSCM with impregnated metal nanoparticles are investigated and correlated to electrode performances. Electrochemical measurements show that the NiCu-LSCM demonstrates the optimum performance without degradation after operation for ∼100 h and ∼10 redox cycles. It is believed that the enhanced performance of CO2 electrolysis may be attributed to the synergetic effect of metal nanocatalyst and LSCM ceramic electrode.

•Sc-doped titanate cathode shows high performance for CO2 electrolysis.•Improved ionic conductivity is achieved with Sc doping in titanate.•Promising electrode polarizations and Faradic efficiencies are obtained.Perovskite oxide (La,Sr)TiO3+δ (LSTO) cathode has demonstrated promising performance for direct CO2 electrolysis due to its unique redox-stable properties. However, insufficient electro-catalytic activity of titanate remains a major drawback that limits electrode performance. In this paper, catalytically active scandium is doped to LSTO to enhance cathode performances. The structure, electronic conductivity and ionic conductivity of La0.2Sr0.8Ti1−xScxO3+δ (LSTSxO) (x = 0, 0.05 and 0.1) are investigated and further correlated with electrode performances. XRD, TEM, TGA and XPS indicate the successful partial replacement of Ti by Sc in the B site of titanate. The improved electrode performances are strongly dependent on scandium doping contents. Promising direct CO2 electrolysis performance is demonstrated with near 100% current efficiency based on La0.2Sr0.8Ti0.9Sc0.1O3+δ at 1.7 V and 800 °C.

This study reported a hierarchically ordered porous Ni-based cathode of a solid oxide electrolysis cell to realise stable CO2 electrolysis without the need of safe gas. The Ni/(Y2O3)0.08(ZrO2)0.92 (YSZ) cathode support has a microchannel structure, which enabled efficient catalyst delivery to the reaction zone between the cathode and the electrolyte and resulted in facilitated gas diffusion through straight channels instead of the tortuous pores intrinsic to conventional porous electrodes. A catalyst network covering a Ni/YSZ scaffold reduced electrode polarisation resistance, prevented carbon formation and suppressed Ni oxidation. The facilitated gas diffusion diminished or eliminated concentration polarisation and carbon formation due to CO accumulation in the reaction zone. The novel hierarchical structure enables stable CO2 electrolysis over conventional Ni-based cathodes with low capital and operational costs.

Perovskite La0.75Sr0.25Cr0.5Mn0.5O3-δ (LSCM) has been used as a typical cathode for CO2 electrolysis in oxide-ion conducting solid oxide electrolyzers; however, the limited electrocatalytic activity still restricts electrode kinetic process. In this work, catalytic-active V2O5 nanocatalyst is impregnated in LSCM cathode to enhance electrocatalytic performance. Electrochemical measurements show that the loading of 2 wt% V2O5 significantly improves electrode activity and accordingly reduces electrode polarization resistance from 2.6 Ω•cm2 for LSCM to 1.2 ΩCm2 for V2O5-LSCM in pure hydrogen. The current densities and Faradic efficiencies with V2O5-LSCM electrode are remarkably enhanced by 30% and 40% in contrast to bare LSCM electrode in the voltage region of 1.2-2.0 V at 800 °C for direct carbon dioxide electrolysis, respectively.

Ionic conduction in perovskite oxide is commonly tailored by element doping in lattices to create charge carriers, while few studies have been focused on ionic conduction enhancement through tailoring microstructures. In this work, remarkable enhancement of ionic conduction in titanate has been achieved via in situ growing active nickel nanoparticles on an oxide surface by controlling the oxide material nonstoichiometry. The combined use of XRD, SEM, XPS and EDS indicates that the exsolution/dissolution of the nickel nanoparticles is completely reversible in redox cycles. With the synergetic effect of enhanced ionic conduction of titanate and the presence of catalytic active Ni nanocatalysts, significant improvement of electrocatalytic performances of the titanate cathode is demonstrated. A current density of 0.3 A cm−2 with a Faradic efficiency of 90% has been achieved for direct carbon dioxide electrolysis in a 2 mm-thick YSZ-supported solid oxide electrolyzer with the modified titanate cathode at 2 V and 1073 K.

Perovskite La0.8Sr0.2MnO3−δ is widely used as an anode for proton-conducting solid oxide steam electrolyzers; however, the insufficient electro-catalytic activity still restricts the electrochemical steam oxidation activity. In this work, catalytically-active scandium is doped into the B-site of the manganate La0.8Sr0.2Mn1−xScxO3−δ (x = 0–0.1) to enhance the electrocatalytic performance. Combined characterizations of XRD, TEM, XPS, SEM and EDS confirm the successful partial replacement of Mn by Sc in the B-site. The doping of Sc remarkably improves ionic conductivity while accordingly decreases electronic conductivity. The electrocatalytic activity has been greatly improved and the composition of La0.8Sr0.2Mn1−xScxO3−δ with x = 0.05 has demonstrated the best electrode polarization performance. The faradic efficiency is significantly enhanced to as high as 80% for La0.8Sr0.2Mn1−xScxO3−δ (x = 0.05) in a proton conducting solid oxide electrolyzer in contrast to a cell with traditional LSM anode for high temperature steam electrolysis.

•Direct conversion of CO2/H2O into syngas has been demonstrated.•Electrochemical process of CO2 reduction with H2O electrolysis is studied.•Loading of Ru catalyst in LSCM electrode promotes cell performances.•Promising electrode polarizations and electrolysis efficiencies are obtained.We have previously reported the electrochemical reduction of CO2 to CO with simultaneous steam electrolysis in a proton conducting solid oxide electrolyzer (PCSOE); however, the conventional Ni-cermet electrode is rapidly oxidized by H2O/CO2 in cathode that causes cell performance degradation. In this work, we report a novel symmetrical PCSOE with redox-stable LSCM ((La0.75Sr0.25)0.95Cr0.5Mn0.5O3−δ) electrode for the electrochemical conversion of CO2/H2O into syngas (CO/H2). The Ru catalyst is impregnated to LSCM to improve electrode activity. Electrochemical measurements demonstrate that the CO2 is electrochemically reduced to syngas (CO/H2) with simultaneous steam electrolysis. The loading of Ru catalyst promotes the electrochemical process with higher Faradic efficiency while induces a more competitive process of hydrogen evolution at 700 °C.

In this study, a potential ilmenite cathode material Ni0.9TiO3 is designed for efficient CO2 electrolysis in an oxide-ion-conducting solid-oxide electrolyzer. Spatially confined catalysis has been successfully achieved to substantially improve cathode activity by in situ growth of catalytically active nickel nanoparticles on a ceramic skeleton. The combined analysis of XRD, SEM, EDS, XPS, TGA and Raman results together confirm that the growth of nickel catalyst is completely reversible in redox cycles. The n-type electrical properties of cathodes are systematically investigated and correlated to electrochemical performance. Efficient CO2 electrolysis with a Faraday efficiency above 90% has been demonstrated with Ni0.9TiO3 in contrast to 60% for a TiO2 cathode at 800 °C.

This work investigated the use of nickel/yttria stabilized zirconia (Ni/YSZ) coated in situ with chromium oxide (Cr2O3) for electrochemical methane (CH4) reforming in solid oxide electrolysers. Combined analysis using X-ray diffraction spectroscopy, transmission electron microscopy, scanning electron microscopy and X-ray photoelectron spectroscopy confirmed the formation of Cr2O3 on the Ni surface with a heterojunction interface formed by reducing nickel chromite (NiCr2O4) to a core–shell structure. The electrical properties of the Cr2O3 coated Ni/YSZ were investigated and correlated to the electrochemical performance. Significant improvements of electrode activity were achieved with Cr2O3 coated Ni/YSZ in contrast to traditional Ni/YSZ in a CH4 atmosphere. Strong carbon deposition resistance was also observed in a methane–carbon dioxide (CH4–CO2; 1:1) atmosphere at 800 °C. Significant enhancement in electrochemical CH4–CO2 reforming was successfully achieved in oxide ion conducting electrolysers with Cr2O3 coated Ni/YSZ cathodes.

•LSM electrode with Fe2O3 nanocatalyst shows promising electrode polarization.•The dependence of electrode performance on Fe2O3 contents is investigated.•The loading of nanocatalyst significantly improves Faraday efficiency.•The oxygen electrode shows stable performance for direct steam electrolysis.Composite electrode based on La0.8Sr0.2MnO3-δ (LSM) can be utilized in a proton-conducting solid oxide electrolyzer for steam electrolysis; however, the insufficient electro-catalytic activity of LSM still restricts the electrode performance and Faraday current efficiency. In this work, catalytic-active iron oxide nanoparticles are loaded on the surface of LSM composite oxygen electrode to improve electro-catalytic performance as well as extend the three-phase boundaries. SEM and EDS results together confirm the loading of Fe2O3 nanoparticles with the size of approximately 20–40 nm on the surface of LSM composite oxygen electrode. The effects on electrode performance due to different contents of Fe2O3 are loaded into LSM composite electrodes are systemically studied using symmetric cells. The electrical property of LSM is investigated and correlated to the electrochemical performance of the composite oxygen electrode in electrolysis cells. The maximum Faraday current efficiency is approximately 65% with the Fe2O3-loaded LSM composite electrode for steam electrolysis in a proton-conducting solid oxide electrolyzer at 800 °C.

Ionic conduction in perovskite oxide is commonly tailored by element doping in lattices to create charge carriers, while few studies have been focused on ionic conduction enhancement through tailoring microstructures. In this work, remarkable enhancement of ionic conduction in titanate has been achieved via in situ growing active nickel nanoparticles on an oxide surface by controlling the oxide material nonstoichiometry. The combined use of XRD, SEM, XPS and EDS indicates that the exsolution/dissolution of the nickel nanoparticles is completely reversible in redox cycles. With the synergetic effect of enhanced ionic conduction of titanate and the presence of catalytic active Ni nanocatalysts, significant improvement of electrocatalytic performances of the titanate cathode is demonstrated. A current density of 0.3 A cm−2 with a Faradic efficiency of 90% has been achieved for direct carbon dioxide electrolysis in a 2 mm-thick YSZ-supported solid oxide electrolyzer with the modified titanate cathode at 2 V and 1073 K.

In this study, a potential ilmenite cathode material Ni0.9TiO3 is designed for efficient CO2 electrolysis in an oxide-ion-conducting solid-oxide electrolyzer. Spatially confined catalysis has been successfully achieved to substantially improve cathode activity by in situ growth of catalytically active nickel nanoparticles on a ceramic skeleton. The combined analysis of XRD, SEM, EDS, XPS, TGA and Raman results together confirm that the growth of nickel catalyst is completely reversible in redox cycles. The n-type electrical properties of cathodes are systematically investigated and correlated to electrochemical performance. Efficient CO2 electrolysis with a Faraday efficiency above 90% has been demonstrated with Ni0.9TiO3 in contrast to 60% for a TiO2 cathode at 800 °C.