Proton-conducting Solid Oxide Electrolysis Research Articles

Modeling and optimization of proton-conducting solid oxide electrolysis cell: Conversion of CO2 into value-added products

Abstract Proton-conducting solid oxide electrolysis cells (SOEC-H + ) are a promising technology that can utilize carbon dioxide to produce syngas. In this work, a detailed electrochemical model was developed to predict the behavior of SOEC-H + and to prove the assumption that the syngas is produced through a reversible water gas-shift (RWGS) reaction. The simulation results obtained from the model, which took into account all of the cell voltage losses (i.e., ohmic, activation, and concentration losses), were validated using experimental data to evaluate the unknown parameters. The developed model was employed to examine the structural and operational parameters. It is found that the cathode-supported SOEC-H + is the best configuration because it requires the lowest cell potential. SOEC-H + operated favorably at high temperatures and low pressures. Furthermore, the simulation results revealed that the optimal S/C molar ratio for syngas production, which can be used for methanol synthesis, is approximately 3.9 (at a constant temperature and pressure). The SOEC-H + was optimized using a response surface methodology, which was used to determine the optimal operating conditions to minimize the cell potential and maximize the carbon dioxide flow rate.

Enhancement in Chemical Stability of Proton Conducting Solid Oxide Electrolysis Cell (SOECs)

Developments of fuel cell technology are a great achievement for researcher’s to fulfill the need of clean and renewable energy. Among the various type of fuel cells, high temperature ceramic fuel cells inherit several advantages such as high efficiency, fuel flexibility and no precious-metal catalyst [1], but the solid oxide fuel cell (SOFC) typically uses yttria-stabilized zirconia (YSZ) at operating temperature of 800-1000°C, resulting in high cost, cell degradation and material compatibility issues. Many researchers investigated protonic ceramic conducting fuel cells (PCFCs) and have presented their possibility as intermediate temperature ceramic fuel cells [2, 3]. Proton-conducting oxides due to high conductivity with lower activation energy (0.3-0.6eV) can be operated at lower temperature of 400-600°C, which increases the cell life-time and reduces the fabrication cost. Compared with SOFCs when operating on hydrocarbon fuel, particularly CH4 conversion rate is high due to direct conversion of proton (hydrogen). A few studies try to use solid oxide electrolysis cell (SOEC) to produce hydrogen by electrolyzing steam at the IT range. [4]. In conventional oxygen-ion SOECs, steam is fed into the fuel electrode side, then the oxygen-ions move toward the air electrode, while the hydrogen is produced at fuel electrode side. One of the technical issues in SOECs is the oxidation of Ni based electrode in steam conditions, resulting in cell degradation. In proton-conducting SOECs, steam is fed at air electrode and hydrogen is produced at fuel electrode. In IT-SOFCs, the BSCF cathode is considered to be as an excellent material [5], but it can be degraded and decompose in high humid air condition. Hence, we should develop durable materials against high water vapor pressure. In this study we design the electrode material to develop highly durable cathode materials for SOECs. Moreover, in order to investigate the practical use of these cell, the accelerating durability test is designed and carried out. The durability test is conducted by periodically repeated both in fuel cell and electrolysis modes. To find the degradation analysis of the cell, micro-structural analysis is performed with SEM, TEM and XRD [6, 7], the impedance spectra and over-potential analysis are performed. E. D. Wachsman, and K. T. Lee, Science , 334, 935 (2011).C. Duan, J. Tong, M. Shang, S. Nikodemski, M. Sanders, S. Ricote, A. Almansoori and R. O’Hayre, Science, 349, 6254 (2015).L. Bi, S. Boulfrad and E. Traversa, J. Chem. Soc. Rev., 43, 8255 (2014).M. Ni, M. K. H. Leung and D. Y.C. Leung, Int. Ass. Hydrogen Eng., 33, 4040 (2008).P. K. Lohsoontorn, D. J. L. Brett, N. Laosiripojana, Y. M. Kim, J-M. Bae, Int. J. Hydrogen energy, 35, 3958 (2010).G. Kim, N. Lee, K. -B. Kim, B. -K. Kim, H. Chang, S. J. Song, J. -Y. Park, Int. J. Hydrogen energy, 38, 1571 (2013).H. Xiao, T. L. Reitz, M. A. Rottmayer, J. Power sources, 183, 49 (2008). * Corresponding authors: jyoung@sejong.ac.kr (J.-Y. Park)

Characterization of Proton-Conducting Solid Oxide Electrolysis Cells Using High Valence State Manganate(V) Ba3(MnO4)2 As an Oxygen Evolution Reaction Electrocatalyst

There have been strong concerns in clean and renewable energy production due to finite fossil fuel sources, increasing oil prices and environmental concerns. Hydrogen is regarded as the leading candidate fuel, because it releases H2O alone during combustion and it is compatible to use in high efficiency fuel cell system. Steam reforming of hydrocarbon gas is currently the main way to produce hydrogen but still relies on fossil fuel consumption. On the contrary, water electrolysis using electric power generated by renewable energy is attractive as sustainable hydrogen production method. Especially, steam electrolysis using solid electrolyte cells is promising because high-temperature heat partly offers the energy for water electrolysis, leading favorable kinetics and thermodynamics. Hence, it is motivated to investigate on solid oxide electrolysis cell (SOEC) using proton-conducting ceramics to achieve highly efficient conversion from electrical power into chemical fuel gas directly. However, sufficient performance has not be achieved yet in the current system because large overpotential is needed for oxygen evolution reaction at anode owing to the relatively slow kinetics and the limited active zone in the anode/electrolyte interfaces due to the mismatch of ionic carriers, Accordingly, it is a great challenge to develop high performance oxygen electrode with efficient electrocatalytic ability for 4 electron transfer oxygen evolution reaction. Recently, it is reported that high valence state metal oxide reveal superior electrocatalytic activity for water oxidation because the energy levels between the occupied metal orbital and the O 2p orbital are very close, causing a strong hybridization and facilitating O-O bond formation [1]. Herein, we examined the electrocatalytic performance of high valence state Mn(V) oxide Ba3(MnO4)2as an anode for SOEC. This oxide has been reported to be very stable at elevated temperatures in oxidative conditions. Proton-conducting BaZr0.4Ce0.4Y0.2O3- δ (BZCY) was used as proton conducting electrolyte. Bulk electrolyte cell was constructed with a BZCY disc which was prepared by solid state reactive sintering (SSRS) method. The electrolyte precursor powder was prepared by mixing proper amount of BaCO3, CeO2, ZrO2, and Y2O3 according to the desired stoichiometry with the addition of 2.0 wt.% NiO as a sintering aid. This mixture was ball-milled for 48 h and uniaxially pressed under 20 MPa for 1 min and then cold-isostatic-pressed under 100 MPa for 1 min. Finally, the green pellets were calcined at 1500°C for 8 h so as to obtain dense electrolyte disc (2 mmd, 9 mmΦ). Pt paste was applied at one side of the surface as a cathode. LSCF or LSCF/Ba3(MnO4)2mixed ink were screen-printed at the other side of the surface as anode materials. Samples were evaluated by XRD and XAS. The electrochemical impedance spectroscopy and I-V measurements were carried out to evaluate the SOEC properties. Two kinds of anode materials were examined in this research, namely, the cell-1: Pt | BZCY | LSCF and cell-2: Pt | BZCY | Ba3(MnO4)2/LSCF. The cell-2 showed superior steam electrolytic performance compared to cell-1. The current density of steam electrolysis of cell-2 was 145 mA cm-2 meanwhile cell-1 was 95 mA cm-2 in bias voltage of 1.5 V at 600°C. Impedance spectroscopy was conducted to evaluate the anodic polarization resistance. LSCF anode gives 6 Ω cm2, however, the Ba3(MnO4)2/LSCF composite anode gives reduced resistances, 3 Ω cm2. Furthermore, the spectral features were completely different between both. The spectrum of LSCF anode had three semi-circles: high frequency arc (105-104 Hz), middle frequency arc (104-102 Hz) and low frequency arc (102-10-1 Hz). On the other hand, Ba3(MnO4)2/LSCF involves only two semi-circles: high frequency arc (104-102 Hz) and low frequency arc (102-10-1 Hz). These results indicate Ba3(MnO4)2 changes the reaction pathway of water oxidation electrode/solid electrolyte interface. The oxygen evolution rate was measured by gas chromatography when electrolysis was performed at a constant current density of 100 mA, 200 mA, and 300 mA. The flux of oxygen from anode side was corresponding to the calculated from the current density, indicating that the faradaic efficiency was almost 100%. XRD pattern of the sample after electrolysis showed that there were no secondary phases, indicating Ba3(MnO4)2 is stable in SOEC anodic condition. The above results suggest the Ba3(MnO4)2is promising for oxygen electrode material in proton-conducting solid oxide electrolysis cell. [1] Zhiyi Lu et al. Nature Communications 5, 4345, 2014

Tailoring Electrode Materials for Proton-Conducting Solid Oxide Fuel/Electrolysis Cells with Chemically Stable BaZrO3 Electrolyte

Proton-conducting oxides offer an alternative solution for solid oxide cells with its unique advantages, such as low activation energy, high ionic conductivity at intermediate temperatures (1, 2). In the community, it is generally realized that the use of chemically stable electrolyte material BaZrO3 is the most promising way for the development of proton-conducting solid oxide cells. Although the use of BaZrO3 has been achieved with desirable fuel cell performance (2), the design of its air electrode is still a challenge, especially for the proton-conducting solid oxide electrolysis cells. In this presentation, we report our recent work on the tailor of the air electrode materials for proton-conducting solid oxide cell, including the use of traditional Sr-doped LaMnO3 (LSM) cathode for BaZrO3-based fuel cells with desirable cell performance and the design of mixed proton-electronic mixing conducing air electrode for proton-conducting electrolysis cells. LSM, which is one of the most classical air electrode materials for solid oxide cells, is regarded can be only used above 700 oC due to its poor catalytic activity at intermediate temperatures (around 600 oC). In our study, we successfully employed the inkjet printing technique, enabling LSM work properly as the air electrode for proton-conducting solid oxide fuel cells with BaZrO3 electrolyte. After the optimization, the fuel cell performance reached 200 mW cm-2 at 600 oC, which is encouraging considering the air electrode material is LSM. A further improvement of fuel cell performance was achieved by using mixed electronic-oxygen conductor air electrode material, reaching the highest power output for the BaZrO3-based cells up to now. In addition to the work on fuel cells, research on the air electrode material for solid oxide electrolysis cells was also carried out. Following to our previous work using proton-electronic mixed conductor for proton-conducting fuel cells (3, 4), a similar strategy was proposed for electrolysis cells. Coupling BaZr0.5Pr0.3Y0.2O3 (BZPY30, a mixed protonic-electronic conductor) with La0.6Sr0.4Co0.2Fe0.8O3 (LSCF) as the air electrode, the cell showed a dramatic electrolysis cell improvement compared with the LSCF- BaZr0.7Pr0.1Y0.2O3 (BZPY10, a pure protonic conductor). This result suggests the extension of proton conduction at the air electrode is critical for the transportation of proton at air electrode side and thus facilitates the electrochemical reactions for the proton-conducting electrolysis cells.

In-situ constructing hybrid oxygen electrode of porous Co3O4 nanowire array on La0.8Sr0.2MnO3−δ for steam electrolysis

Porous Co3O4 nanowire array has been in-situ grown on the La0.8Sr0.2MnO3−δ (LSM) electrode by a facile hydrothermal process aiming at enhancing the electrochemical performance of LSM anode for steam electrolysis in intermediate temperature proton-conducting solid oxide electrolyzer. X-ray diffraction (XRD) and scanning electron microscopy (SEM) techniques have been employed to characterize phase structure, morphology and composition of the obtained hybrid. The high activity of the Co3O4 nanoarray improves the oxygen reduction performance of LSM at intermediate temperature. Significant improvement has been obtained in terms of catalytic activity of the hybrid electrode compared with bare LSM electrode in air. The maximum current efficiency of the hybrid electrode reaches 71.64% for steam electrolysis in a proton-conducting solid oxide electrolyzer at 700 °C, which is approximately 1.3 times higher than that for the bare LSM electrode.

Demonstration of direct conversion of CO2/H2O into syngas in a symmetrical proton-conducting solid oxide electrolyzer

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.

Composite manganate oxygen electrode enhanced with iron oxide nanocatalyst for high temperature steam electrolysis in a proton-conducting solid oxide electrolyzer

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.

Scandium-Doped LSM Anode for Steam Electrolysis in a Proton-Conducting Solid Oxide Electrolyzer

Perovskite La0.8Sr0.2MnO3-δ has been widely used as anode for proton conducting solid oxide steam electrolyzer; however, the insufficient catalytic activity still restricts the activity for electrochemical oxidation of steam. In this work, Sc-doped anode material La0.8Sr0.2Mn1-xScxO3-δ (x=0, 0.02, 0.05, 0.08, 0.1) with the aim of enhancing anode performance is studied. Combined characterization using XRD, TEM, XPS, SEM and EDS confirmed the successful partial replacement of Mn by Sc in B site. The doping of Sc remarkably improves ionic conductivity while accordingly decreases the mixed conductivity due to the increase of oxygen vacancies indicated by TGA and XPS. Electrochemical measurements and ionic conductivity measurements demonstrate LSMS0.05 has the most optimized polarization properties. With LSMS0.05 anode, the Faradic efficiency is maximized to 80% and accordingly enhanced by 25% compared with the electrolyzer with LSM anode for high temperature steam electrolysis by exposing anode to 5%H2O/air.

Y-doped BaZrO3as a chemically stable electrolyte for proton-conducting solid oxide electrolysis cells (SOECs)

A proton-conducting SOEC with chemically stable Y-doped BaZrO3is fabricated for the first time for electrolysis applications.

Composite oxygen electrode LSM-BCZYZ impregnated with Co3O4 nanoparticles for steam electrolysis in a proton-conducting solid oxide electrolyzer

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.

Energy transport inside a three-phase electrode and application to a proton-conducting solid oxide electrolysis cell

This work focuses on the modelling of thermal processes inside a planar high temperature steam electrolyzer that use cermets as electrodes. While the continuity equation for mass and charge have been demonstrated in a previous publication, energy balance for thermal transfers inside the electrode assembly is established via a control volume method. A non-dimensional number is built from different criterion used in the literature in order to validate the local thermal equilibrium assumption (LTE) inside the porous electrodes. A parametric analysis is carried out on a proton-conducting solid oxide electrolysis cell in galvanostatic mode. The results show that the heat sources are mainly ohmic and that their locations are not dependent on inlet current and inlet velocity of gases. This observation allows us to build an original thermal resistance network in order to analytically evaluate the temperature inside each component of the cell. This modelling strategy reduces computation time, allows reverse physical analysis and gives a precise estimation on the maximum temperatures attained in the components of the cells.

Composite Oxygen Electrode Based on LSCF and BSCF for Steam Electrolysis in a Proton-Conducting Solid Oxide Electrolyzer

This paper investigates composite oxygen electrodes based on La0.6Sr0.4Co0.2Fe0.8O3−δ (LSCF) and Ba0.5Sr0.5Co0.8Fe0.2O3−δ (BSCF) for proton-conducting solid oxide electrolyzer using BaCe0.5Zr0.3Y0.16Zn0.04O3−δ (BCZYZ) proton conductor. The conductivities of LSCF and BSCF are studied versus 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, respectively. Steam electrolysis is performed in a proton-conducting solid oxide electrolyzer based on composite oxygen electrode LSCF-BCZYZ and BSCF-BCZYZ at 800°C, respectively. I-V and AC impedance spectroscopy tests demonstrate that the oxidation of oxygen electrode is the main process at low voltage; however, the steam electrolysis dominates the whole process at high voltages. The current efficiencies of steam electrolysis is enhanced by ∼20% when the gas fed into oxygen electrode changes from 3%H2O/Air to 3%H2O/Ar.

Composite Oxygen Electrode Based on LSCM for Steam Electrolysis in a Proton Conducting Solid Oxide Electrolyzer

This paper presents a ceramic composite oxygen electrode (La0.75Sr0.25)0.95Mn0.5Cr0.5O3-δ (LSCM) – BaCe0.5Zr0.3Y0.16Zn0.04O3-δ (BCZYZ) for steam electrolysis in a proton-conducting Solid Oxide Electrolyzer (SOE) with proton conducting electrolyte BCZYZ and fuel electrode BCZYZ/Ni at 700°C. AC impedance spectroscopy and I-V tests demonstrate two main processes from 0 to 2 V with 3%H2O/N2 vs 5% H2/Ar: reoxidation of the LSCM electrode below 1.2 V and oxidation of H2O above 1.2 V. The current density reaches 2 Acm−2 at 2 V with electrode polarization of 0.1 Ωcm2 and steam electrolysis shows short term stability with current efficiency of 22%.