High temperature electrolyzer based on solid oxide co-ionic electrolyte: A theoretical model

In the present work a theoretical model of a solid oxide electrolyzer based on an electrolyte having both oxygen ion and proton conductivity is considered. The main parameters of the electrolytic process and an electrolyzer (distribution of gas components, electromotive forces and current densities along the electrolyzer channel, average values of electromotive forces and current densities) were calculated depending on a proton transport number and mode of the reactants’ feeding (co- and counter-flow). The performed analysis demonstrates considerable influence of the mode of feeding on all parameters of the electrolyzer: operation under the counter-flow mode is preferable as regards the specific characteristics and uniformity of their distribution along the electrolyzer. It is shown that the electrolyser's specific characteristics increase with the increase of the proton transport number.     

Process modelling of an alkaline water electrolyzer

In this paper a model for the prediction of the product gas purity in alkaline water electrolysis is proposed. For the estimation of the exhaust gas compositions the operating conditions, such as current density, electrolyte flow rate, concentration and temperature as well as process management possibilities are considered. The development of the model relies on a classical process engineering approach and depicts the electrolysis cell through coupled continuously stirred-tank reactors. Furthermore, the mass transport phenomena between the phases are considered through the application of Reynolds and Sherwood correlations. Finally, the validation of the model is performed through experiments, which are carried out in a lab-scale electrolyzer with a 150 cm² zero-gap cell and KOH electrolyte at atmospheric pressure. This investigation reveals that gas purity in alkaline water electrolysis is mainly affected by mixing the anodic and cathodic electrolyte cycles, which transport dissolved electrolysis products into the opposite half cell compartments. However, this transport mechanism can be significantly reduced by adjustment of the operating conditions of the electrolyzer.     

Influence of process conditions on gas purity in alkaline water electrolysis

In this paper the influence of operating conditions on the product gas purity of a zero-gap alkaline water electrolyzer was examined. Precise knowledge of the resulting gas purity is of special importance to prevent safety shutdown when the electrolyzer is dynamically operated using a renewable energy source. The investigation in this study involves variation of temperature, electrolyte concentration and flow rate as well as different electrolyte management concepts. The experiments were carried out in a fully automated lab-scale electrolyzer with a 150 cm² zero-gap cell and approximately 31 wt% KOH at ambient and balanced cathodic and anodic pressure. The purity of the evolved gases was measured via online gas chromatography. It can be seen from the experiments that a temperature increase and flow rate decrease reduces the gas impurity when mixing catholyte and anolyte. A further reduction of gas impurity can be achieved when both cycles are being separated and a dynamic cycling strategy is applied.     

Hybrid PV/fuel cell system design and simulation

In this paper, a hybrid Photovoltaic (PV)-fuel cell generation system employing an electrolyzer for hydrogen generation is designed and simulated. The system is applicable for remote areas or isolated loads. Fuzzy regression model (FRM) is applied for maximum power point tracking to extract maximum available solar power from PV arrays under variable insolation conditions. The system incorporates a controller designed to achieve permanent power supply to the load via the PV array or the fuel cell, or both according to the power available from the sun. Also, to prevent corrosion of the electrolyzer electrodes after sunset, i.e. when its current drops to zero, the electric storage device is designed so as to isolate the electrolyte from the electrolysis cell.     

Evaluation of materials and components degradation of a PEM electrolyzer for marine applications

An investigation of components degradation in a polymer electrolyte membrane (PEM) electrolyzer technology for naval application is carried out. In naval applications, an electrolyzer has to work using treated seawater obtained on board. Electrolyzer performance is evaluated in terms of hydrogen production per unit of electrical energy input starting from a seawater desalination plant simulating the conditions on board of the ship. A desalinator plant, tailored for the specific application, is designed to produce water having a low ionic conductivity (<5 μS) compatible with the PEM stack electrolyzer requirements. Electrochemical characterizations are carried out on an in-house developed 9-cells PEM electrolyzer by using treated water. Several aspects related with the degradation of materials and components electrolyzer are investigated under oxidizing atmosphere. Specific tests are carried out in a corrosive ambient that simulates the sea environment in terms of humidity, salinity, corrosive conditions etc… In order to simulate highly oxidizing atmosphere, a Dry Corrosion Test Cabinet (DCTC^®) is used to test MEAs and electrolyzer components on the basis of specific international norms (ASTM standard). Both materials and components are characterized by electrochemical and physico-chemical analysis before and after the DCTC^® treatment.     

预磁极化条件下PEM水电解制氢特性与效率研究

为提高质子交换膜（proton exchange membrane,PEM）水电解制氢速率、降低电解所需能耗,针对磁场预极化条件下蒸馏水的分子极性和应力特性进行研究,通过构建磁场环境下氢质子的能级跃迁微观物理模型与磁化矢量——极化氢质子浓度对应的宏观数学模型,对不同磁场强度下电解液的离子电导率、电流密度和制氢速率进行定性和定量分析,并利用自主搭建的可调节预磁极化PEM水电解制氢试验平台对所提出方法的有效性进行重复试验。试验结果表明,经过预磁极化处理的蒸馏水电导率提高了2~3倍,且随着磁场强度的增加,PEM电解电流密度不断增大,极间电圧不断减小,制氢速率明显提升。     

Samarium doped ceria-(Li/Na)2CO3 composite electrolyte and its electrochemical properties in low temperature solid oxide fuel cell

A composite of samarium doped ceria (SDC) and a binary carbonate eutectic (52 mol% Li₂CO₃/48 mol% Na₂CO₃) is investigated with respect to its morphology, conductivity and fuel cell performances. The morphology study shows the composition could prevent SDC particles from agglomeration. The conductivity is measured under air, argon and hydrogen, respectively. A sharp increase in conductivity occurs under all the atmospheres, which relates to the superionic phase transition in the interface phases between SDC and carbonates. Single cells with the composite electrolyte are fabricated by a uniaxial die-press method using NiO/electrolyte as anode and lithiated NiO/electrolyte as cathode. The cell shows a maximum power density of 590 mW cm⁻² at 600 °C, using hydrogen as the fuel and air as the oxidant. Unlike that of cells based on pure oxygen ionic conductor or pure protonic conductor, the open circuit voltage of the SDC-carbonate based fuel cell decreases with an increase in water content of either anodic or cathodic inlet gas, indicating the electrolyte is a co-ionic (H⁺/O²⁻) conductor. The results also exhibit that oxygen ionic conductivity contributes to the major part of the whole conductivity under fuel cell circumstances.     

Analytically investigating the characteristics of a high‐temperature unitized regenerative solid oxide fuel cell

A unitized regenerative solid oxide fuel cell (URSOFC) can be considered as a next‐generation power source and a storage device in the future since it can generate electricity in the SOFC mode and also produce H₂/O₂ in the solid oxide electrolyzer cell (SOEC) mode. In this paper, a two‐dimensional axisymmetric model is developed to simulate the characteristics of a URSOFC. The performance curves for an in‐house button URSOFC under different operating temperatures of 600, 700, and 800 °C are measured to validate the present model. Both the measured data and the prediction results reveal the beneficial effects of higher temperature on the cell performance. Based on the results of numerical simulations, the majority of the fuel gas is consumed at the interface of the electrolyte and the electrode, causing a great fuel concentration gradient near the interface. In addition, the predicted cell performance curves in both the SOFC and the SOEC modes correspond well with the measured data, demonstrating the applicability of this model in a button URSOFC. Copyright © 2013 John Wiley & Sons, Ltd.     

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.     

In-situ measurement of hydrogen crossover in polymer electrolyte membrane water electrolysis

An in-situ method for determination of hydrogen crossover in polymer electrolyte membrane (PEM) water electrolysis cells is discussed. The measurement principle is based on the electrochemical compensation of the crossover flux, which translates the mass flux determination into an electric current measurement. The proposed method features an extremely simple set-up and measurement procedure, as well as high accuracy. It allows for measurement with a fully assembled cell at normal water electrolysis conditions by use of standard equipment, also installed in industrial electrolyzer plants. The technique is especially suitable for high-pressure PEM electrolyzers operated under asymmetric pressure conditions. The applicability of the suggested method for a broad pressure range is briefly illustrated with a laboratory scale electrolyzer plant and by comparison of the measured data with available literature values.     

Prevention of the water flooding by micronizing the pore structure of gas diffusion layer for polymer electrolyte fuel cell

In polymer electrolyte fuel cells, high humidity must be established to maintain high proton conductivity in the polymer electrolyte. However, the water that is produced electrochemically at the cathode catalyst layer can condense in the cell and cause an obstruction to the diffusion of reaction gas in the gas diffusion layer and the gas channel. This leads to a sudden decrease of the cell voltage. To combat this, strict water management techniques are required, which usually focus on the gas diffusion layer. In this study, the use of specially treated carbon paper as a flood-proof gas diffusion layer under extremely high humidity conditions was investigated experimentally. The results indicated that flooding originates at the interface between the gas diffusion layer and the catalyst layer, and that such flooding could be eliminated by control of the pore size in the gas diffusion layer at this interface.     

Performance evaluation and optimum design strategies of an acid water electrolyzer system for hydrogen production

The performance of a new acid water electrolyzer system for hydrogen production is investigated, based on semi-empirical equations of a phosphoric acid water electrolyzer. The circulating electrolyte concentrations under differently operating temperatures are optimized so that the minimum input voltages of the electrolyzer are determined for other given conditions. The optimum electrochemical characteristics of the electrolyzer are revealed. Moreover, it is expounded that the Joule heat resulting from the irreversibilities inside the electrolyzer is larger than the thermal energy needed in the water splitting process. The general performance characteristics of the phosphoric acid water electrolyzer system are discussed, from which the lower bound of the operating current density is determined. The upper bound of the operating current density is further determined by introducing a multi-objective function including the system efficiency and hydrogen production rate. Consequently, some optimum design strategies of a phosphoric acid water electrolyzer system are obtained and may be chosen according to different practical requirements.     

Continuous operation in an electric and hydrogen hybrid energy storage system for renewable power generation and autonomous emergency power supply

Under the background of extensive improvement of renewable resources and demand for reliable emergency power supply, we proposed a hybrid energy storage system including an electric double-layer capacitor bank and a hydrogen system which is composed of fuel cell, electrolyzer, gas tank and metal hydride tank. Through its integration with photovoltaic power sources in a local direct current grid, we expect to obtain both of stable energy source at ordinary times and long-time reliable autonomous emergency power supply when outages happen. A three-day demonstration of the proposed system was performed. The fluctuation compensation performance of the components and the long-time stable power supply obtained by the entire system were evaluated at first, hence the configuration and the management methods of the proposed system were verified in the autonomous emergency power supply application. Meanwhile, the performance of the hybrid use of the gas tank and the metal hydride tank in the system was preliminarily evaluated, for its effectiveness verification on reducing auxiliary power for temperature condition of the metal hydride tank. Moreover, we investigated the distribution characteristics of the power and energy loss in the electric double-layer capacitor, electrolyzer and fuel cell, and their correlation to the efficiency characteristics under different conditions during the operation. The investigation results showed that the continual low-load-ratio state of the electrolyzer and fuel cell led to the low efficiency, the rare high-power occurrence of the electrolyzer and fuel cell led their demanded excessive power capacity. Thus, we proposed a solution method of shifting the electrolyzer and fuel cell's load to the EDLC, when the electrolyzer and fuel cell are in low-load-ratio and excessive high-power state, in order for efficiency increase and facility capacity reduction.     

An analytical model for solid oxide fuel cells with bi-layer electrolyte

A theoretical model for a solid oxide fuel cell (SOFC) with a bi-layer electrolyte is developed and analytical solutions of various important relationships, such as I–V relationship, distribution of oxygen partial pressure in the bi-layer electrolyte, leakage current density etc. are obtained. Based on the assumptions of constant ionic conductivity and reversible electrodes, the model takes into considerations of transports of both ions and electrons in the electrolyte. The modeling results are compared with both experimental data and results from other models in the literature and very good agreements are obtained.     

Semi-empirical model and experimental validation for the performance evaluation of a 15 kW alkaline water electrolyzer

This paper presents a semi-empirical mathematical model for predicting the electrochemical behavior of an alkaline water electrolysis system, based on the polarization curve and Faraday efficiency as a function of the current density under different operating conditions, such as, temperature and pressure. Also, the gas impurities of hydrogen in oxygen have been modeled for safety reasons due to its importance when the electrolyzer is dynamically operated using renewable energy sources. The different parameters defined in the model have been calculated by MATLAB, using a non-linear regression, on the basis of experimental data obtained in a 15 kW alkaline test bench. The simulated and measured values have been compared to ensure the accuracy and validity of the proposed model. In this sense, the error has been evaluated for the voltage with an average result of 5.67 mV per cell and for the Faraday efficiency and the gas impurities of hydrogen in oxygen with a value lower than 1%. These results show an excellent correlation between experimental and modeled data, so the model is a useful design and optimization tool for alkaline electrolyzers. Also, a sensitivity analysis has been used to determine the most influential operating variables in the performance of the electrolyzer.     

Parametric study of large-scale production of syngas via high-temperature co-electrolysis

A process model has been developed to evaluate the potential performance of a large-scale high-temperature co-electrolysis plant for the production of syngas from steam and carbon dioxide. The co-electrolysis process allows for direct electrochemical reduction of the steam/carbon dioxide gas mixture, yielding hydrogen and carbon monoxide, or syngas. The process model has been developed using the UniSim system-analysis code. Using this code, a detailed process flow sheet has been defined that includes all the components that would be present in an actual plant such as pumps, compressors, heat exchangers, turbines, and the electrolyzer. Since the electrolyzer is not a standard UniSim component, a custom integral co-electrolysis model was developed for incorporation into the overall UniSim process flow sheet. The integral co-electrolysis model assumes local chemical equilibrium among the four process-gas species via the water-gas shift reaction. The electrolyzer model allows for the determination of co-electrolysis outlet temperature, composition (anode and cathode sides); mean Nernst potential, operating voltage and electrolyzer power based on specified inlet gas flow rates, heat loss or gain, current density, and cell area-specific resistance. The integral electrolyzer model was validated by comparison with results obtained from a fully three-dimensional computational fluid dynamics model developed using FLUENT, and by comparison with experimental data. This paper provides representative results obtained from the UniSim flow sheet model for a 300 MW_e co-electrolysis plant, coupled to a high-temperature gas-cooled nuclear reactor. The co-electrolysis process, coupled to a nuclear reactor, provides a means of recycling carbon dioxide back into a useful liquid fuel. If the carbon dioxide source is based on biomass, the overall process, from production through utilization, would be climate-neutral.     

One-dimensional dynamic modeling of a high-pressure water electrolysis system for hydrogen production

Research on high-pressure water electrolyzers is under way worldwide as the economic production of hydrogen from renewable energy sources becomes more important. With increases in operating pressures, new safety issues have emerged, for which a reliable dynamic model of the electrolyzers is important for predicting their behavior. In this paper, a one-dimensional dynamic model of a high-pressure proton exchange membrane water electrolyzer is proposed. The model integrates various important physico-chemical phenomena inside the electrochemical cell that have been investigated individually into a dynamic model framework. Water transport, gas permeation, gas volume variation in anode/cathode channels, gas compressibility, and water vaporization are considered to formulate the model. Numerical procedures to handle and solve the model and the model performance for the prediction of steady and dynamic state behaviors are also presented.     

Dynamic modelling of alkaline self-pressurized electrolyzers: a phenomenological-based semiphysical approach

This paper proposes a phenomenological based semiphysical model (PBSM) for a self-pressurized alkaline electrolyzer. The model, based on mass and energy balances, represents the dynamic behaviour of hydrogen and oxygen production using electrolysis. The model allows to anticipate operational variables as dynamic responses in the concentrations of the electrolytic cell, and variations in both, level and pressure, at the gas separation chambers due to the change in electric current. The model parameters have been adjusted based on experimental measurements taken from an available prototype and through a suitable identification process. Simulation results replicate the current dynamic response of the experimental self-pressurized electrolyzer assembly. This model proves to be useful in the improvement of the control of gas production rate in this kind of assemblies, both as a validated simulation platform and as a source of reduced order models for model-based control design.     

A parametric study of multi-phase and multi-species transport in the cathode of PEM fuel cells

In this study, a mathematical model is developed for the cathode of PEM fuel cells, including multi-phase and multi-species transport and electrochemical reaction under the isothermal and steady-state conditions. The conservation equations for mass, momentum, species and charge are solved using the commercial software COMSOL Multiphysics. The catalyst layer is modeled as a finite domain and assumed to be composed of a uniform distribution of supported catalyst, liquid water, electrolyte and void space. The Stefan–Maxwell equation is used to model the multi-species diffusion in the gas diffusion and catalyst layers. Owing to the low relative species' velocity, Darcy's law is used to describe the transport of gas and liquid phases in the gas diffusion and catalyst layers. A serpentine flow field is considered to distribute the oxidant over the active cathode electrode surface, with pressure loss in the flow direction along the channel. The dependency of the capillary pressure on the saturation is modeled using the Leverette function and the Brooks and Corey relation. A parametric study is carried out to investigate the effects of pressure drop in the flow channel, permeability, inlet relative humidity and shoulder/channel width ratio on the performance of the cell and the transport of liquid water. An inlet relative humidity of 90 and 80% leads to the highest performance in the cathode. Owing to liquid water evaporation, the relative humidity in the catalyst layer reaches 100% with an inlet relative humidity of 90 and 80%, resulting in a high electrolyte conductivity. The electrolyte conductivity plays a significant role in determining the overall performance up to a point. Further, the catalyst layer is found to be important in controlling the water concentration in the cell. The cross-flow phenomenon is shown to enhance the removal of liquid water from the cell. Moreover, a shoulder/channel width ratio of 1:2 is found to be an optimal ratio. A decrease in the shoulder/channel ratio results in an increase in performance and an increase in cross flow. Finally, the Leverette function leads to lower liquid water saturations in the backing and catalyst layers than the Brooks and Corey relation. The overall trend, however, is similar for both functions. Copyright © 2007 John Wiley & Sons, Ltd.