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.     

Steam electrolysis performance of intermediate-temperature solid oxide electrolysis cell and efficiency of hydrogen production system at 300 Nm h

The steam electrolysis performance of an intermediate-temperature solid oxide electrolysis cell (SOEC) was measured at 650 °C at various steam concentrations. The cell voltage decreased with increasing steam concentration, which was attributed to a decrease in the steam electrode polarization. The highest performance of the SOEC was 1.32 V at 0.57 A cm⁻². On the basis of the electrolytic characteristics of this cell, the efficiency of a hydrogen production system operating at a capacity of 300 N m³ h⁻¹ was estimated. The system efficiency reached a higher heating value (HHV standard) of 98% due to the effective recovery of thermal energy from exhaust gas.     

Theoretical study on pressurized operation of solid oxide electrolysis cells

In this paper the influence of pressure on the performance of solid oxide electrolysis cells is theoretically analyzed in a pressure range between 0.05 and 2 MPa. A previously validated electrochemical model of a solid oxide fuel cell stack is used to predict electrolysis behavior. The effect of pressure on thermodynamics, kinetics and gas diffusion is discussed. It is shown that thermodynamics are negatively influenced by an increase in pressure whereas kinetics and gas transport are improved. Overall pressure effects are therefore only small. At low current density the electrolysis cell shows better performance at low pressure whereas performance improves with pressure at high current densities.     

Oxygen electrode degradation in solid oxide cells operating in electrolysis and fuel cell modes: LSCF destabilization and interdiffusion at the electrode/electrolyte interface

Three long-term experiments have been performed in SOEC and SOFC modes at different operating temperatures. The durability tests confirm a higher degradation in electrolysis mode with respect to fuel cell operation. In addition, a larger increase of the ohmic resistance is observed for the cell operated at higher temperature in electrolysis mode. The oxygen electrodes of the pristine and tested cells have been characterized by synchrotron X-ray micro-diffraction and micro-fluorescence to assess the relation between the material destabilization and the formation of insulating phases due to interlayer diffusion. The analyses of the pristine cell confirm the presence after the electrode sintering of strontium zirconate and a Gd-rich interdiffusional layer in the electrolyte just below the zirconates. Moreover, evolutions in the LSCF unit cell volume reveal strontium segregation after aging. The associated material destabilization is linked to the accumulation of SrZrO₃ at the barrier layer/interdiffusional layer interface in operation and both phenomena are found to be thermally-activated and promoted in electrolysis mode. Finally, the crystallographic evolution of the interdiffusional layer in electrolysis mode has been investigated by X-ray diffraction. A slight increase of the phase peaks intensity detected at the highest temperature is correlated to the largest formation of SrZrO₃ observed in this condition. Based on these preliminary results, it is proposed that the loss of Zr⁴⁺ from the electrolyte due to the zirconates formation could facilitate the interdiffusion of Gd, reducing the local ionic conductivity and thus significantly contributing to the largest increase in the ohmic resistance observed in this case.     

The effect of electrode infiltration on the performance of tubular solid oxide fuel cells under electrolysis and fuel cell modes

The electrochemical performance of two different anode supported tubular cells (50:50 wt% NiO:YSZ (yttria stabilized zirconia) or 34:66 vol.% Ni:YSZ) as the fuel electrode and YSZ as the electrolyte) under SOFC (solid oxide fuel cell) and SOEC (solid oxide electrolysis cell) modes were studied in this research. LSM (La_0.80Sr_0.20MnO_3−δ) was infiltrated into a thin porous YSZ layer to form the oxygen electrode of both cells and, in addition, SDC (Sm_0.2Ce_0.8O_1.9) was infiltrated into the fuel electrode of one of the cells. The microstructure of the infiltrated fuel cells showed a suitable distribution of fine LSM and SDC particles (50–100 nm) near the interface of electrodes and electrolyte and throughout the bulk of the electrodes. The results show that SDC infiltration not only enhances the electrochemical reaction in SOFC mode but improves the performance even more in SOEC mode. In addition, LSM infiltrated electrodes also boost the SOEC performance in comparison with standard LSM–YSZ composite electrodes, due to the well-dispersed LSM nanoparticles (favouring the electrochemical reactions) within the YSZ porous matrix.     

Hydrogen production through steam electrolysis: Model-based dynamic behaviour of a cathode-supported intermediate temperature solid oxide electrolysis cell

Hydrogen production via steam electrolysis may involve less electrical energy consumption than conventional low temperature water electrolysis, reflecting the favourable thermodynamics and kinetics at elevated temperatures. In the present paper, a one-dimensional model of a cathode-supported planar intermediate temperature solid oxide electrolysis cell (SOEC) stack is employed to study the dynamic behaviour of such an electrolyser. The simulations found that step changes in the average current density cause the stack temperature to alter during both exothermic and endothermic operation. However, the temperature control, by the variation of the air flow through the stack, was predicted to be capable of returning the stack temperature to the initial value. Furthermore, the proposed control strategy is observed to reduce the interim temperature excursions between the initial and final steady states, suggesting that such a control strategy has a good potential to prevent the issues of cell component fracture, and transitions in stack operating mode, which are related to the temperature fluctuations during dynamic operation of an SOEC stack.     

Dynamic modelling and performance analysis of reversible solid oxide fuel cell with syngas

Reversible solid oxide fuel cell (rSOFC) is able to both generate and store energy in a single device under dual-mode operation conditions, i.e., fuel cell and electrolysis, which is favorable for intermittent energy supply and storage in application of renewable energies. The dynamic behaviors such as quick startup, short response time and good stabilization performance are highly required to optimize the performance in the dual-mode conditions, which has not been investigated well in consideration of the co-redox reactions with syngas. In this work, a 2D unsteady planar rSOFC cell model is developed with the experimental validation performed, to evaluate the dynamic behaviors in terms of multi-physics transport processes coupled with the co-redox reactions occurring in the rSOFC. The dynamic characteristics such as the long term performance, quick response and good stabilization are investigated by changing the working conditions including the starting-up and mode-switching procedures. It is concluded that the diffusion of the gases is the major limitation on the dynamic performance. The response and stabilization performance when the syngas is applied is not as good as that of H₂/H₂O fueled. It takes long time for the water-gas shift reaction to response and stabilize when the mode is switched in dual-mode work. The dynamic performance of the electrochemical reaction with responding current density is obviously affected with the fluctuant variation of the operating voltage for the case with syngas under dual-mode work. High temperature condition is beneficial to improve the quick response performance in dual-mode operation.     

Complementary techniques for solid oxide electrolysis cell characterisation at the micro- and nano-scale

High-temperature steam electrolysis by solid oxide electrolysis cells (SOEC) is a method with great potential for transforming clean and renewable energy from non-fossil sources to synthetic fuels such as hydrogen, methane or dimethyl ether, which have been identified as promising alternative energy carriers. With the same technology, fuel gas can be used in a very efficient way to reconvert chemically stored energy into electrical energy, since SOECs also work in the reverse mode, operating as solid oxide fuel cells (SOFC). As solid oxide cells (SOC) perform at high-temperatures (700–900 °C), material degradation and evaporation can occur, e.g., from the cell-sealing material, leading to poisoning effects and aging mechanisms that decrease the cell efficiency and long-term durability. To investigate such cell degradation processes, thorough examination of SOCs often requires a chemical and structural characterisation at a microscopic and nanoscopic level. The combination of different microscopic techniques such as conventional scanning electron microscopy (SEM), electron probe microanalysis (EPMA) and the focused ion beam (FIB) preparation technique for transmission electron microscopy (TEM) allows for post-mortem analysis at a multi-scale level. These complementary techniques can be used to characterise structural and chemical changes over a large and representative sample area (micro-scale) as well as at the nano-scale level for selected sample details. This article presents a methodical approach for the structural and chemical characterisation of changes in aged cathode-supported electrolysis cells produced at Risø DTU, Denmark. Additionally, we present results from the characterisation of impurities at the electrolyte/hydrogen interface caused by evaporation of sealing material.     

Modelling of effect of pressure on co-electrolysis of water and carbon dioxide in solid oxide electrolysis cell

As a promising renewable energy storage device, the solid oxide electrolysis cell (SOEC) attracts wide attention in the world. In this study, the effect of operating pressure on the co-electrolysis of water and carbon dioxide in SOEC is investigated by a three-dimensional model, in which the reversible water gas shift reaction and direct internal reforming reaction are considered. After comparison with experimental data, the influence of operating pressure on the polarization loss, electrolysis reaction rate, chemical reaction rate and thermal-neutral voltage is also studied in detail. The results show that the cell voltage increases below 8 atm and then decreases with the operating pressure. In addition, the effects of thermal insulation boundary condition, gas flow configuration and gas utilization rate under different operating pressures are also discussed. It is found that the reverse direct internal reforming reaction is activated under the operating pressure higher than 3 atm. Moreover, compared to co-flow arrangement, counter-flow arrangement is more helpful to cell performance improvement at high current densities.     

Hydrogen production through steam electrolysis: Model-based steady state performance of a cathode-supported intermediate temperature solid oxide electrolysis cell

Hydrogen production via steam electrolysis may involve less electrical energy consumption than conventional low temperature water electrolysis, reflecting the improved thermodynamics and kinetics at elevated temperatures. The present paper reports on the development of a one-dimensional dynamic model of a cathode-supported planar intermediate temperature solid oxide electrolysis cell (SOEC) stack. The model, which consists of an electrochemical model, a mass balance, and four energy balances, is here employed to study the steady state behaviour of an SOEC stack at different current densities and temperatures. The simulations found that activation overpotentials provide the largest contributions to irreversible losses while concentration overpotentials remained negligible throughout the stack. For an average current density of 7000 A m⁻² and an inlet steam temperature of 1023 K, the predicted electrical energy consumption of the stack is around 3 kW h per normal m³ of hydrogen, significantly smaller than those of low temperature stacks commercially available today. However, the dependence of the stack temperature distribution on the average current density calls for strict temperature control, especially during dynamic operation.     

Effects of sulfur poisoning on degradation phenomena in oxygen electrodes of solid oxide electrolysis cells and solid oxide fuel cells

The durabilities of a single solid oxide electrolysis cell (SOEC) and a solid oxide fuel cell (SOFC) operating at 0.3 A cm^?2 and 973 K under different air supply conditions were investigated. In the SOEC, S penetration was observed mainly at the gadolinium-doped ceria (CGO) electrolyte/lanthanum strontium cobalt oxide (LSC) oxygen electrode interface. In contrast, during SOFC operation, S was distributed widely within the LSC. The reaction governing S penetration into the LSC is an oxidizing one. Thus, it is likely that the high oxygen partial pressure at the CGO electrolyte/LSC oxygen electrode interface accelerated the penetration of S. When air was supplied using an activated carbon filter during SOEC operation, the degradation rate decreased to 0.6% kh^?1 within 3000 h. Finally, the results of accelerated tests performed using air containing 0.2 ppm SO₂ suggested that the effect of S poisoning was greater during SOEC operation than during SOFC operation.     

Process analysis of a novel coal-to-methanol technology for gasification integrated solid oxide electrolysis cell (SOEC)

China's carbon peaking and carbon neutrality goals present a significant challenge for coal chemical technology, which is critical to securing the energy structure. Combining coal chemical industry technology with new energy is an effective approach to transform the development of the coal chemical industry. This paper proposes and studies a novel coal-to-methanol (CTM) technology of gasification integrated solid oxide electrolysis cell (SOEC). SOEC electrolytic hydrogen production technology is an advanced electrolytic water technology with the advantages of large scale and high efficiency, which is very suitable to be combined with industrial technology and can solve the painful problem of H₂ deficiency in the conventional coal to methanol process. In this study, from mechanistic analysis and model simulations, it is observed that by increasing the SOEC capacity, the novel CTM system can create more methanol at the same coal consumption and simultaneously reduce CO₂ emissions. The novel CTM system can produce up to 2.2 times more methanol and reduce CO₂ emissions by 94% by replacing the water-gas-shift (WGS) process with the SOEC unit. The novel CTM increases energy consumption. In addition, the novel CTM technology will effectively improve the economics of coal to methanol, taking into account the carbon tax. At the methanol price of 2900 RMB/t and SOEC capacity of 250 MW, the economic benefits of novel CTM were 2.1 times greater than CTM technology.     

Modelling of anode delamination in solid oxide electrolysis cell and analysis of its effects on electrochemical performance

Degradation of a solid oxide electrolysis cell (SOEC) during long-term operation remains to be the key obstacle to their massive production and commercialization. One of degradation processes within SOEC is anode delamination. The anode of SOEC splits at the interface with solid electrolyte due to elevated pressure of oxygen that is produced by electrochemical reactions. The main assumption that anode delamination starts at the fuel inlet is based on post-mortem analysis of SOEC. This paper addresses numerical modelling of a single, electrolyte supported, SOEC. The anode delamination is modelled by implementing the modifications of SOEC's geometry. A brief overview of the model is also given. Verification of the implemented model relies on the measurement data from literature. The simulation results show that increasing the area of delaminated anode (A_delaminated) increases the operating voltage of the SOEC if a constant electrolysis current is applied. This strongly influences the conversion efficiency (η) of the SOEC. Indeed, if linear growth of A_delaminated over time is assumed, the η of SOEC degrades very fast at the beginning of SOEC's operation. The presented model also helps analyze the hot spots of current density, where high pressure of oxygen appears.     

Micro modelling of solid oxide electrolysis cell: From performance to durability

An in-house micro model has been built to describe the electrochemical mechanisms governing both H₂ and O₂ electrodes operating in SOEC mode. A special attention has been paid to take into account the microstructure properties of the ionic, electronic and gas phases as well as the processes occurring therein.     

Computing approach of cathodic process within solid oxide electrolysis cell: Experiments and continuum model validation

Classical solid oxide fuel cell anode (Ni-cermet) could be employed as solid oxide electrolysis cell cathode. Ni-cermet has been synthesized and tested as solid oxide electrolyzer cathode using three-electrode techniques between 700 °C and 900 °C. yttria stabilized zirconia was used as the electrolyte and Pt as the counter electrode. Polarization curves and impedance spectra have been analyzed for two gas compositions. The presented results demonstrated an influence of Ni-cermet electrode behavior upon gas composition and temperature. The present results highlight a mechanism changing on Ni-cermet electrode upon gas composition. In a second part, a one-dimensional steady state model is developed to predict the cathodic behavior of Ni-cermet. This model takes into account mass and charge conservation, transport of species and reaction kinetics. It considers the porous electrode to be a homogeneous medium characterized. The influence of varying chemical and electrochemical steps kinetic on the shape of polarization curves is discussed. At high overpotential values the model with two rate-limiting steps has been validated using numerical optimization method.     

Electrolytic effect in solid oxide fuel cells running on steam/methane mixture

A two-dimensional model is developed to study the performance of a planar solid oxide fuel cell (SOFC) running on steam/methane mixture. The model considers the heat/mass transfer, electrochemical reactions, direct internal reforming of methane (CH₄), and water gas shift reaction in an SOFC. It is found that at an operating potential of 0.8 V, the upstream and downstream of SOFC work in electrolysis and fuel cell modes, respectively. At the open-circuit voltage, the electricity generated by the downstream part of SOFC is completely consumed by the upstream through electrolysis, which is contrary to our common understanding that electrochemical reactions cease under the open-circuit conditions. In order to inhibit the electrolytic effect, the SOFC can be operated at a lower potential or use partially pre-reformed CH₄ as the fuel. Increasing the inlet gas velocity from 0.5 m s⁻¹ to 5.0 m s⁻¹ does not reduce the electrolytic effect but decreases the SOFC performance.     

Performance evaluation of a 4-stack solid oxide module in electrolysis mode

To support the current trend of testing bigger reversible Solid Oxide Cell (rSOC) modules, CEA has built the 120 kW_DC Multistack platform. It was used to test SOLIDpower recently developed-Large Stack Module (LSM) in electrolysis mode.Results show high thermal performance of the LSM, with homogeneous temperature distribution and losses in the kilowatt range above 700 °C. A performance map was recorded between 712 and 744 °C over 22.4-to-29.6 kg h^?1 steam flowrates using a fast control strategy to avoid endothermic operation. A peak power of 74 kW_DC was converted into more than 50 kg day^?1 of H₂ (35.5 kWh_DC kgH₂^?1). In addition, fuel utilization of more than 90% and steam conversion above 80% were demonstrated at the module level. In the end, the modular design of the LSM seems well suited for system scale up, paving the way for mutualization of auxiliaries and CAPEX reduction.     

Concept,design, and energy analysis of an integrated power-to-methanol process utilizing a tubular proton-conducting solid oxide electrolysis cell

An alternative power-to-methanol process based on an integration of a tubular proton-conducting solid oxide electrolysis cell into a methanol synthesis unit is explained, energetically evaluated and technically presented. Being currently developed in the joint research project DELTA, the novel process has the potential for a significant increase in system efficiency, if the heat from the exothermic synthesis reaction can be utilized and/or kinetic advantages can be achieved.For the experimental proof of the concept and a comprehensive characterization of the process, a test platform is currently under construction. The design of the flexible test facility with the complex technical integration of both processes (electrolysis and synthesis) is described briefly. The chemical reactor, where electrolysis and synthesis are taking place, allows for an operation at temperatures (for electrolysis) up to 700 °C, pressures of 10 MPa and a current (across the electrolysis cell) of up to 100 A. Moreover, a precise pressure balancing system between both gas volumes, an axial temperature measurement and the possibility of regulating both processes inside the pressure vessel are pivotal properties of the test facility.     

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.