Electrolysis Cell Operation Research Articles

Reversible Long-Term Operation of a MK35x Electrolyte-Supported Solid Oxide Cell-Based Stack

High-temperature reversible solid oxide cells (rSOC) combine electrolyzer and fuel cell in one process unit which promises economic advantages with unrivaled high electrical efficiencies. However, large temperature gradients during dynamic rSOC operation can increase the risk of mechanical failure. Here, the degradation behavior of a 10-cell stack of the type “MK35x” with chromium-iron-yttrium (CFY) interconnects and electrolyte-supported cells (ESC) developed at Fraunhofer IKTS was investigated in rSOC operation at DLR. Its degradation was evaluated during nominal rSOC operation for more than 3400 h with 137 switching cycles between solid oxide fuel cell and solid oxide electrolysis cell operation of 24 h reflecting intermittent availability of solar energy. The voltage degradation rates of +0.58%/kh and −1.23%/kh in electrolysis and fuel cell operation, respectively, are among the lowest reported in literature. Comparison to a previously published long-term test in steam electrolysis did not show any indication for an increased degradation. Electrochemical impedance spectroscopy measurements were performed for all repeat units to evaluate the degradation behavior in detail. An overall polarization resistance decrease due to an improvement of the oxygen electrode was observed during electrolysis operation but was absent during fuel cell operation.

Optimal operation of solid-oxide electrolysis cells considering long-term chemical degradation

Optimizing the performance of solid oxide electrolysis cells (SOECs) for long-term hydrogen (H2) production at high temperatures is crucial, as prolonged operation leads to efficiency losses and shorter cell lifespans due to chemical degradation. In this work, we adopt a quasi-steady state approach for dynamic optimization over extended operational periods to address the disparity in timescales between cell operation and degradation. Integrating a 2-D non-isothermal SOEC model with balance-of-plant (BOP) equipment, we explore three optimization objectives: minimizing terminal degradation, maximizing integral efficiency, and minimizing the levelized cost of H2 (LCOH). Our dynamic optimization algorithm reduces LCOH by 9.5% and 16% compared to strategies focusing solely on terminal degradation and integral efficiency, respectively. For electricity prices of 0.03 $/kWh and 0.3 $/kWh, optimal replacement schedules range from 5 to 2 years, depending on the operational mode. Furthermore, a flexible operational mode yields additional improvements in LCOH over traditional galvanostatic and potentiostatic modes.

Dynamic operation of metal-supported solid oxide electrolysis cells

Symmetric-structure metal-supported solid oxide fuel cells and electrolysis cells (MS-SOFCs, MS-SOECs) offer several advantages over conventional solid oxide cells, including the use of inexpensive materials, high mechanical strength, and rapid ramp-up ability. Aggressive operation of MS-SOCs in fuel cell mode is well-established, including extremely fast start-up, redox tolerance, and imbalanced pressure. Here, we extend dynamic operation to MS-SOCs in SOEC mode with high steam content for both small button cells and a large rectangular cell, including: steam cycling, thermal cycling, redox cycling and power cycling. Steam cycling entailed switching between 3:97 and 50:50 steam:hydrogen ratio. For thermal cycling, the temperature was rapidly varied between 150 °C and 700 °C for 50 cycles. Redox cycling involved switching the steam side gas between 50 % humidified H2 and 50 % humidified N2 for 5 cycles. Power cycling was performed by operating the cell under variable current density, resulting in cell voltage between 1.3 V and 2.8 V. Degradation rates for each testing strategy were compared to a baseline cell, and found to be similar. The excellent tolerance to dynamic operation increases confidence that MS-SOECs will be compatible with dynamic or intermittent renewable resources.

Enhancing Efficiency in Alkaline Electrolysis Cells: Optimizing Flow Channels through Multiphase Computational Fluid Dynamics Modeling

The efficient operation of alkaline water electrolysis cells hinges upon understanding and optimizing gas–liquid flow dynamics. Achieving uniform flow patterns is crucial to minimize stagnant regions, prevent gas bubble accumulation, and establish optimal conditions for electrochemical reactions. This study employed a comprehensive, three-dimensional computational fluid dynamics Euler–Euler multiphase model, based on a geometric representation of an alkaline electrolytic cell. The electrochemical model, responsible for producing hydrogen and oxygen at the cathode and anode during water electrolysis, is integrated into the flow model by introducing mass source terms within the user-defined function. The membrane positioned between the flow channels employs a porous medium model to selectively permit specific components to pass through while restricting others. To validate the accuracy of the model, comparisons were made with measured data available in the literature. We obtained an optimization design method for the channel structure; the three-inlet model demonstrated improved speed and temperature uniformity, with a 22% reduction in the hydrogen concentration at the outlet compared to the single-inlet model. This resulted in the optimization of gas emission efficiency. As the radius of the spherical convex structure increased, the influence of the spherical convex structure on the electrolyte intensified, resulting in enhanced flow uniformity within the flow field. This study may help provide recommendations for designing and optimizing flow channels to enhance the efficiency of alkaline water electrolysis cells.

Understanding the phase stability of yttria stabilized zirconia electrolyte under solid oxide electrolysis cell operation conditions

The degradation mechanism of yttria-stabilized zirconia (YSZ) with varying Y2O3 compositions under an applied electric potential.

Enhanced performance of reversible solid oxide fuel cells by densification of Gd0.1Ce0.9O2−δ (GDC) barrier layer/La0.6Sr0.4Co0.2Fe0.8O3−δ (LSCF6428)-GDC oxygen-electrode interface

In reversible solid oxide cells (RSOCs) with yttria-stabilized zirconia (YSZ) electrolyte and LSCF6428-based oxygen electrode, a GDC barrier layer is used to prevent inter-diffusion of constituents. However, the performance of such RSOC is limited to the achievable density of GDC at prevailing temperatures of traditional fabrication methods. In this work, the GDC barrier layer was densified via infiltration with polyvinylpyrrolidone (PVP) chelated metal precursors sol and subsequent sintering at 1200 °C. The NiO-YSZ fuel electrode-supported RSOCs with YSZ electrolyte, porous or infiltrated GDC barrier layer and LSCF6428-GDC composite oxygen electrode were fabricated. The RSOCs were analyzed with respect to operating temperature (650–800 °C), water partial pressure at fuel electrode (pH2OFE=0.03–0.5 atm), oxygen partial pressure at oxygen electrode (pO2AE=0.02–0.21 atm) and current loading. The performance evaluation, using current-voltage curve and electrochemical impedance spectroscopy (EIS), showed that the RSOC with an infiltrated GDC barrier layer exhibited a 32–48% increase in performance in fuel cell (FC) mode in the 650–800 °C range, compared to that of the RSOC with a porous GDC layer. Similarly, in electrolysis cell (EC) mode, comparing current density at 1.3 V, chosen as the reference voltage for ease of comparison, based on a thermoneutral voltage of 1.285 V in electrolysis cell (EC) operation at 750 °C[1], the RSOC with an infiltrated GDC barrier layer exhibited values of over 1.2 A∙cm−2, in contrast to the RSOC with a porous GDC barrier layer, which showed 0.8 A∙cm−2. In the case of the RSOC with infiltrated GDC, a durability test was conducted at 750 °C under an EC current density of 1A∙cm−2for 100 h. Our experimental results demonstrate that the RSOC with an infiltrated GDC barrier layer not only exhibits enhanced performance but also maintains stability. Comparative analysis was conducted using Distribution of Relaxation Time (DRT) analysis of Electrochemical Impedance Spectroscopy (EIS) data, to further understand the differences between the cells.

(Invited) Materials Stability in Anode-Supported Solid Oxide Cells

Professor Anil Virkar has been a pioneer in the development and characterization of high-performance anode-supported solid oxide fuel cells, and in understanding degradation processes in such cells operated in electrolysis mode. This talk reviews methods for improving anode-supported cells, particularly for yielding high performance at reduced operating temperature. The understanding of degradation mechanisms occurring during solid oxide electrolysis cell operation will be discussed.

Performance optimization of solid oxide electrolysis cell for syngas production by high temperature co-electrolysis via differential evolution algorithm with practical constraints

Co-electrolysis of water and carbon dioxide using solid oxide electrolysis cell combined with the catalytic reactor to produce renewable synthesis fuel is considered as a promising strategy for carbon dioxide utilization. However, in order to achieve a target product with a remarkable system-level efficiency, a certain proportion of carbon monoxide and hydrogen from solid oxide electrolysis cell must be sent to the catalytic reactor directly, which imposes practical constraints on the solid oxide electrolysis cell performance. Take this constraint into consideration, we proposed a performance design methodology that enables a rapid and effective optimization of the solid oxide electrolysis cell operating conditions. Firstly, a physical model is developed by integrating chemical equilibrium, electrochemistry, and energy conservation involved in the solid oxide electrolysis cell. Secondly, a physical model is utilized to generate a dataset for the development of neural network model and optimization algorithm. This approach allows for the efficient determination of optimal conditions while considering practical constraints. In the case study, under the constraint that the desired fuel is a long carbon chain fuel with a carbon dioxide conversion rate ranging from 80 % to 90 %, the highest efficiency achieved by the solid oxide electrolysis cell is optimized to 66.75 %. Moreover, this approach is extended to other two target fuels (dimethyl ether and methane) under different carbon dioxide conversion rate, and the optimal performance is designed. According to the results, there is a trade-off between high efficiency and high carbon dioxide conversion rate, as the low carbon dioxide conversion rate often leads to higher efficiency. This method allows for flexible design of operating conditions to meet different fuel requirements and guide practical applications.

Control strategy of solid oxide electrolysis cell operating temperature under real fluctuating renewable power

Power-to-liquid (PtL) technology through solid oxide electrolysis cells (SOECs) can efficiently convert renewable energy sources into synthetic fuels for long-time energy storage. However, due to its fluctuation and intermittence, renewable energy sources only produce time-varying currents, which directly impact the operating temperature of SOEC, generating high thermal stresses for SOEC and reducing its durability. To deal with this issue, a control-oriented SOEC dynamic model containing an electrochemical part, a reversible water gas shift (RWGS) part, and a thermodynamic part, was developed in this paper. Afterward, main control parameters, able to adjust SOEC operating temperature effectively, were determined through the Extended Fourier Amplitude Sensitivity Test (EFAST). The corresponding results show that the air flow rate was the best choice to control SOEC temperature when it works in a low current density case, while the fuel flow rate becomes optimal when SOEC operates in a high current case. Given this knowledge, an adaptive feedback control strategy was proposed to stabilize the operating temperature of SOEC in practice. In the end, the effectiveness of the proposed control was demonstrated through step signals of current in both low- and high-current density cases and an actual fluctuating current from real solar cells.

The Effect of Operating Temperature on Galvanostatic Operation of Solid Oxide Electrolysis Cells

The purpose of this work is to study the degradation of Nickel Yttrium Stabilized Zirconia (Ni-YSZ) supported Solid Oxide Electrolysis Cells at different temperatures during galvanostatic steam electrolysis.Four long-term experiments (>=700 h) electrolysis tests are carried out in galvanostatic mode (i.e. constant current), at a current density of 1 A/cm2 and testing temperatures of 925, 850, 800 and 750 oC. The cells’ performance and degradation are comprehensively analyzed via operando Electrochemical Impedance Spectroscopy (EIS). To quantify the degradation, EIS data from Nyquist, Bode and Distribution of Relaxation Times (DRT) plots are utilized in conjunction with complex-non-linear-least-square (CNLS) fitting on suitable equivalent circuit models (ECM).The durability tests show that increasing the operational temperature has significantly beneficial effect on the long-term performance of the cells. More specifically, the cells operated at 850 oC and 925 oC not only did not reach 1.3 V during operation but they also have considerable low voltage degradation rates.To highlight the beneficial effect of increasing the operational temperature, one more durability test at 925oC and 1.25 A/cm2 is conducted. It is showed that even at higher current density the cell shows superior performance compared to lower temperatures and lower current densities.The results obtained from the CNLS fitting of the EIS data revealed that the observed degradation is attributed mainly to the increase of fuel electrode’s resistance and the corresponding increase of fuel electrode’s overpotential. At higher temperatures, the increase of fuel electrode’s resistance and overpotential is significantly lower during operation. The data from the CNLS fitting are used in order to approximate the time evolution of the different overpotentials of the cell.In addition, the first and last EIS measurements under current of each of the durability tests are used for fitting to an ECM which contains Transmission Line Model (TLM) elements. The physical parameters obtained by the TLM fitting are compared with the results of the microstructure analysis. More specifically, post-mortem analysis of the cells using Scanning Electron Microscopy (SEM) and Energy-dispersive X-ray spectroscopy (EDS) provide extra info about the microstructure changes in the Ni-YSZ and the depleted layer of Ni close to the electrode/electrolyte interface.The findings of this work highlight the significance of preserving a low fuel electrode overpotential during SOEC operation by increasing the operational temperature and therefore, by improving the reaction kinetics. It is suggested that Ni coarsening does not have a detrimental effect on long-term performance and that the overpotential-driven degradation mechanisms, such as Ni migration, mainly determine the life time of a SOEC.

Optimization of Time-of-Flight Secondary Ion Mass Spectrometry for Analysis of Porous Transport Layers

As we move towards commercialization of the proton exchange membrane water electrolyzers (PEMWEs), fundamental understandings of individual components and their integration with focus on material composition and its correlation to performance and lifetime of electrolysis cell operation is of significant importance. Porous Transport Layers (PTLs) increase overpotential of the cell when the titanium passivates, i.e. develops a titanium oxide surface layer of a certain thickness. To prevent this passivation, platinum group metal coatings are typically applied to the PTL, but this adds significant costs.This presentation will cover recent developments in characterization of PTL coatings and PTL-coating interfaces using Time of Flight Secondary Ion Mass Spectrometry (ToF-SIMS) in correlation with Scanning Transmission Electron Microscopy and Energy Dispersive X-ray Spectroscopy (STEM-EDS) analysis. Although this technique is extremely beneficial, ToF-SIMS is typically applied to flat substrates and therefore has been mainly utilized in other scientific fields such as the thin film industry. This talk will discuss method validation and optimization that has been conducted for PTL analysis to allow its characterization by ToF-SIMS. Topics include understanding parameters and limitation of using SIMS on these morphologically challenging samples, as well as looking at elemental and species identification, and applying depth profiling to gauge information on coating and oxide thickness and their consistency. Correlations between real and flat substrates prepared under similar conditions are made, allowing standardization of depth profiling for future analysis of a wider variety of samples.

Dynamic response and safety performance of an anode-supported solid oxide electrolysis cell operating under electrical transients

Solid oxide electrolysis cells have emerged as a promising technology for renewable energy storage. The intermittent operation, however, could jeopardize durability. Understanding the transient response characteristics of the solid oxide electrolysis cell under diverse electrical inputs is crucial to comprehend the dynamic durability of the electrolysis cell. Yet, the electrochemical and temperature dynamic responses are challenging to be in-situ measured. In this study, a three-dimensional electrochemical-mass-heat coupled dynamic model of the solid oxide electrolysis cell is established and validated by the experimental I–V curve and electrochemical impedance spectroscopy data. The current and voltage manipulations on the transient responses are evaluated and the cause of voltage overshoot during current control is analyzed. The effects of amplitude and switching time of current upward manipulation on the electrochemical and thermal responses are also investigated. It is discovered that 40% current upward (60% power upward) slightly exceeds the upper limit of the temperature gradient of 8 K/cm and approaches the FU limit of 90%. On the other hand, the influence of switching time on safety performance is negligible.

The Effect of Operating Temperature on Galvanostatic Operation of Solid Oxide Electrolysis Cells

This study aims to investigate the effect of operating temperature on the galvanostatic operation of SOECs. Durability tests are conducted at four different temperatures but under the same current density. The analysis of the EIS data recorded during operation shows that the polarization and ohmic resistance increase considerably faster at lower temperatures. It also reveals that the main cause of degradation originates from the degradation of the Ni-YSZ electrode. Microstructure analysis reveals changes in the porosity and the %Ni atomic percentage in the active Ni-YSZ electrode layer, indicating that Ni migrates from the electrode/electrolyte interface towards the neighboring support layer. Over time, the resistance and overpotential attributed to Ni-migration exhibit linear increase. However, increase of temperature decreases their degradation rates. Overpotential thresholds for severe degradation of the Ni-YSZ electrode are identified at 750 oC and 800 oC but not at 850 oC and 900 oC due to the lower developed overpotentials.

Improving Electrocatalytic Activity of Cobalt-Free Barium Ferrite-Based Perovskite Oxygen Electrodes for Proton-Conducting Solid Oxide Cells via Introducing A-Site Deficiency

Proton-conducting solid oxide cells are regarded as promising solid-state energy conversion devices to realize the high-efficiency conversion between electrical energy and chemical energy. However, the high cost, easily reducible ion characteristics, and large thermal expansion coefficient related to the valence state of the traditional high-catalytic cobalt-based oxygen electrode are unavoidable problems. Cobalt-free barium ferrite-based oxides with triple-conducting properties are considered the most promising prospective candidates for highly active oxygen electrode materials because of the increased electrochemical reaction active sites, excellent thermal stability, and low thermal expansion coefficient. In this work, by introducing 5% A-site deficiency at the atomic scale, the oxygen vacancy concentration is increased so that the catalytic reaction kinetics and proton-conduction ability of ABO3-type perovskite oxygen electrode material BaFe0.5Sn0.2Bi0.3O3−δ are improved. The effects of non-stoichiometry on the phase composition, microstructure, hydration capacity, thermal expansion characteristics, conductivity, and chemical stability are investigated. In both fuel cell and electrolysis cell operation modes, the proton-conducting solid oxide cells with the A-site-deficient Ba0.95Fe0.5Sn0.2Bi0.3O3−δ oxygen electrode demonstrate excellent electrochemical performance, giving a peak power density of 0.69 W cm–2 (increased by 19.0%) and an electrolysis current density of 1.64 A cm–2 (increased by 30.2%) at 700 °C. Moreover, even at 600 °C with 50% high steam partial pressure, electrolysis for hydrogen production is maintained for about 100 h without significant attenuation, confirming the vital role of A-site defect engineering in the design of advanced oxygen electrode materials.

Undissolved alumina in dynamic chunks piercing through the bath - metal interface in industrial aluminum electrolysis cells

In Hall–Héroult cells the uninvolved alumina precipitates on the top of the cathode to form resistive deposits which could significantly impact the energy consumption. A dynamic energy conservation (DEC) approach has been developed to quantify the flux of undissolved alumina piercing the interface between the electrolyte and the liquid metal inHall–Héroultcells. The proposed DEC model considers both electrochemical and interfacial phenomenon between the electrolyte and the liquid metal. A critical radius at or below which a particle of undissolved alumina is retained at the electrolyte/liquid metal pad interface is defined and quantified considering the heat transfer and electrolysis cell operating parameters: temperature, electrolyte composition, alumina porosity, and current density. The proposed approach appears reliable to quantify the amount of undissolved alumina which pierces the electrolyte/liquid metal pad interface and precipitates to the bottom of the cell to form a deposit on the cathode.

Development of an electrochemical membrane bioreactor for succinic acid production and in situ separation with engineered Yarrowia lipolytica cultivated on municipal biowaste hydrolysate

A novel electrochemical membrane bioreactor (EMB) integrating succinic acid (SA) production and in situ separation in the anode compartment through an anion exchange membrane was employed in fed-batch fermentations with Y. lipolytica PSA02004 using hydrolysates from the organic fraction of municipal solid waste (OFMSW) as feedstock. The initiation of electrolysis cell operation and the reduction of pH from 6 to 5.5 at 30 h in a 6.7 L bioreactor improved the SA production efficiency, resulting in 66.7 gSA/L, 0.51 g/g yield, 0.78 g/(L·h) productivity, high coulombic efficiency (66.2%) and relatively low electricity consumption for SA separation (2.6 kWh/kgSA). The recirculation of the fermentation broth in the cathode compartment and the OH– produced by water reduction reduced NaOH consumption (35.4%) for pH control during fermentation. The fermentation was efficiently replicated in a 30 L bioreactor with a low membrane surface area (100 cm2) electrolysis cell, but it failed with a higher membrane surface area (702 cm2) electrolysis cell indicating that yeast cell viability, cell design and EMB configuration are important aspects for process scale-up. SA crystals were purified, at 99.95% purity and 95% yield, from the anolyte solution via activated carbon treatment, evaporation, crystallization and drying. Cell removal via centrifugation and acidification stages were not required as in conventional SA purification processes. The produced SA crystals were suitable for the production of polyester polyols for polyurethane urea dispersions applications.

Experimental investigation on performance evaluation of PEM electrolysis cell by using a Taguchi method

Proton exchange membrane (PEM) electrolysis cells play a vital role in high-purity and sustainable hydrogen production by utilizing the DC power from renewable energy sources and can help minimize harmful greenhouse gasses. The performance of PEM electrolysis cells can be improved significantly by optimizing the operating and design conditions. In the present study, experiments have been conducted to investigate the influences of the operational parameters (cell voltage, temperature, and water flow rate) on the performance of the PEM electrolysis cell with a total surface area of 9 cm2. Design of experiment and Taguchi method were also applied to the experimental system to optimize PEM electrolysis cell operating parameters. Cell voltage, temperature, and water flow rate are the three main control factors that have been varied between 1.6 and 2.4 V, 40–80 °C, and 16.5–30.6 mL/min, respectively. The results were analyzed using analysis of variance (ANOVA) and the signal-to-noise (S/N) ratios by Minitab software for the optimal operational factors. The results show that the best electrolysis cell performance is obtained at a temperature of 80 °C, cell voltage of 2.4 V, and water flow rate of 16.5 mL/min. The maximum current and hydrogen flow rate for the optimal combinations of the input factors were 17.398 A and 152.019 mL/min, respectively. At the highest cell performance, LHV (low heating value) and HHV (high heating value) energy efficiencies were calculated as 59.6% and 70.4%, respectively. The cell voltage has considerably affected the current and hydrogen flow rate with contributions of 91.91% and 90.81%, respectively. The water flow rate has a minimal effect, with contributions of 1.04% and 1.25%, respectively. Our results indicated that the current and hydrogen flow rate reflecting the cell performance increased with increasing the cell voltage, the temperature and decreasing the water flow rate. Therefore, the ideal combination of operating factors is essential to provide the best cell performance and lower operational costs.

Biohydrogen Production in Microbial Electrolysis Cell Operating on Designed Consortium of Denitrifying Bacteria.

This study provides insight into the use of a designed microbial community to produce biohydrogen in simple, single-chamber microbial electrolysis cells (MECs). The ability of MECs to stably produce biohydrogen relies heavily on the setup and microorganisms working inside the system. Despite having the most straightforward configuration and effectively avoiding costly membranes, single-chamber MECs are prone to competing metabolic pathways. We present in this study one possible way of avoiding this problem using characteristically defined, designed microbial consortium. Here, we compare the performance of MECs inoculated with a designed consortium to MECs operating with a naturally occurring soil consortium. We adapted a cost-effective and simple single-chamber MEC design. The MEC was gastight, 100 mL in volume, and equipped with continuous monitoring for electrical output using a digital multimeter. Microorganisms were sourced from Indonesian environmental samples, either as denitrifying bacterial isolates grouped as a designed consortium or natural soil microbiome used in its entirety. The designed consortium consisted of five species from the Pseudomonas and Acinetobacter genera. The headspace gas profile was monitored periodically with a gas chromatograph. At the end of the culture, the composition of the natural soil consortium was characterized by next generation sequencing and the growth of the bacteria on the surface of the anodes by field emission scanning electron microscopy. We found that MEC using a designed consortium presented a better H2 production profile, with the ability of the system to maintain headspace H2 concentration relatively stable for a long time after reaching stationary growth period. In contrast, MECs inoculated with soil microbiome exhibited a strong decline in headspace H2 profile within the same time frame. This work utilizes a designed, denitrifying bacterial consortium isolated from Indonesian environmental samples that can survive in a nitrate-rich environment. Here we propose using a designed consortium as a biological approach to avoid methanogenesis in MECs, as a simple and environmentally friendly alternative to current chemical/physical methods. Our findings offer an alternative solution to avoid the problem of H2 loss in single-chamber MECs along with optimizing biohydrogen production through bioelectrochemical routes.

Dynamic behavior of high-temperature CO2/H2O co-electrolysis coupled with real fluctuating renewable power

Direct utilization of fluctuational renewable powers leads to rapid changes of working conditions and brings difficulties in the operation of solid oxide electrolysis cells (SOECs). Herein, a multi-physics SOEC model is established to investigate its dynamic characteristics using a real photovoltaic power supply for co-electrolysis of H2O and CO2. Dynamic responses of key performances including the current density, the average SOEC temperature, the H2O/CO2 conversion rate and the output H2/CO ratio are analyzed over a whole day. It is found that a high CO2 mole fraction can help inhibit average temperature fluctuation, where the maximum temperature difference decreases from 110 to 57 K with the inlet CO2 mole fraction increasing from 0.2 to 0.8. Besides, the largest temperature gradient occurs in the middle of the cell in the morning and gradually migrates to the inlet. Generally, a high inlet gas temperature can increase the outlet H2/CO ratio especially at low voltages. The outlet H2/CO ratio is also found to be closely related with the gas utilization rate, where a utilization rate of 0.6 shows 10% higher H2/CO ratio than that of 0.8. This study can provide a guideline for the performance optimization of SOECs with fluctuating power supply.

Techno-economic assessment of a novel power-to-liquid system for synthesis of formic acid and ammonia, based on CO2 electroreduction and alkaline water electrolysis cells

The power-to-liquid concept is a promising strategy to convert the power plants' flue gas to value-added liquid fuels using renewable energy. This technology could potentially reduce global greenhouse gases emissions and mitigate the environmental problems associated with the fossil fuels industry. In this regard, the main objective of the present study is to propose a novel power-to-liquid plant for the synthesis of formic acid and ammonia from power plants' flue gas, emphasizing the role of electrochemical technologies and renewable energy. The system's basis is developed by the integration of CO2 electroreduction cell, alkaline water electrolysis cell, and photovoltaic panel technologies. The plant performance is evaluated by a techno-economic analysis by considering the operating conditions of the electrochemical cells as operating variables. The results show that the Power-to-liquid energy efficiency is continuously reduced with the CO2 electroreduction cell's current density, while it reaches a peak at the alkaline electrolysis cell's current density of 0.2 A/cm2. Also, the study of the effect of alkaline electrolysis cell's operating temperature and pressure on the Power-to-liquid energy efficiency shows that the plant is more efficient at higher temperatures and lower pressures. From an economic aspect, the plant's cost of the product decreases with cells' current densities. Moreover, the plant's cost of the product is reduced by the increase in the alkaline electrolysis cell's operating temperature while it increases with the operating pressure. At design conditions, the alkaline electrolysis and CO2 electroreduction cells' energy efficiencies were calculated as 60.80% and 33.80%, respectively. Also, the Power-to-liquid energy efficiency and plant's cost of the product were obtained 37.60% and 22.676 $/kg, respectively.