Analysis of pressurized operation of 10 layer solid oxide electrolysis stacks

High temperature steam electrolysis using solid oxide electrolysis cell (SOEC) technology can provide hydrogen as fuel for transport or as base chemical for chemical or pharmaceutical industry. SOECs offer a great potential for high efficiencies due to low overpotentials and the possibility for waste heat use for water evaporation. For many industrial applications hydrogen has to be pressurized before being used or stored. Pressurized operation of SOECs can provide benefits on both cell and system level, due to enhanced electrode kinetics and downstream process requirements. Experimental results of water electrolysis in a pressurized SOEC stack consisting of 10 electrolyte supported cells are presented in this paper. The pressure ranges from 1.4 to 8 bar. Steady-state and dynamically recorded U(i)-curves as well as electrochemical impedance spectroscopy (EIS) were carried out to evaluate the performance of the stack under pressurized conditions. Furthermore a long-term test over 1000 h at 1.4 bar was performed to evaluate the degradation in exothermic steam electrolysis mode. It was observed that the open circuit voltage increases with higher pressure due to well-known thermodynamic relations. No increase of the limiting current density was observed with elevated pressure for the ESC-stacks (electrolyte supported cell) that were investigated in this study. The overall and the activation impedance were found to decrease slightly with higher pressure. Within the impedance studies, the ohmic resistance was found to be the most dominant part of the entire cell resistance of the studied electrolyte supported cells of the stack. A constant current degradation test over 1000 h at 1.4 bar with a second stack showed a voltage degradation rate of 0.56%/kh.     

Effect of operating conditions and gas flow patterns on the system performances of IIR-SOFC fueled by methanol

Mathematical models of an indirect internal reforming solid oxide fuel cells (IIR-SOFC) fueled by methanol were developed to analyze the thermal coupling of the internal endothermic steam reforming with exothermic electrochemical reactions and predict the system performance. The simulations indicated that IIR-SOFC fueled by methanol can be well performed as autothermal operation, although slight temperature gradient occurred at the entrance of the reformer chamber. Sensitivity analysis of five important parameters (i.e. operating voltage, reforming catalyst reactivity, inlet steam to carbon ratio, operating pressure and flow direction) was then performed. The increase of operating voltage lowered the average temperature along the reformer chamber and improved the electrical efficiency, but it oppositely reduced the average current density. Greater temperature profile along the system can be obtained by applying the catalyst with lower reforming reactivity; nevertheless, the current density and electrical efficiency slightly decreased. By using high inlet steam to carbon ratio, the cooling spot at the entrance of the reformer can be reduced but both current density and electrical efficiency were decreased. Lastly, with increasing operating pressure, the system efficiency increased and the temperature dropping at the reformer chamber was minimized.     

High-performance ceramic composite electrodes for electrochemical hydrogen pump using protonic ceramics

Ceramic composite electrodes comprising an electron-conducting ceramic (Sr-doped LaVO₃), a protonic ceramic [Cu and Y-doped Ba(Ce,Zr)O₃], and small amounts of CeO₂ and Pd as catalysts were fabricated using an infiltration method for use in an electrochemical hydrogen pump and the hydrogen fluxes and the faradaic efficiency were measured by analyzing the gas compositions. This composite electrode performed well; the area-specific resistance of the electrode polarization at 1 A cm^?2 was just 0.15 Ω cm² at 973 K in hydrogen pumping mode, and the overpotential at a large current density of 2 A cm^?2 was only about 1.1 V at 973 K. To optimize the operating conditions, the effects of the steam vapor pressure and hydrogen partial pressure on the electrochemical performance of the hydrogen pump were investigated. The steam in the sweep side was consumed by the steam electrolysis due to the partial oxygen conductivity. Therefore, supplying insufficient steam to the cathode was found to cause a steep increase in the voltage at high currents owing to a decrease in the proton conductivity.     

基于新能源发电的电解水制氢直流电源研究

利用新能源发电进行电解水制氢是实现新能源就地消纳和氢能利用的重要途径,以匹配电解水制氢工作特性的制氢电源为研究对象,通过分析质子交换膜电解槽电解电流、温度与电解槽端口电压、能量效率、制氢速度之间的关系,得出制氢电源需具备输出低电流纹波、输出大电流、宽范围电压输出的特性。为满足新能源电解制氢系统需求,提出一种基于Y型三相交错并联LLC拓扑结构的制氢电源方案,该方案谐振腔三相交错并联输出,满足电解槽大电流低纹波工作特性,并采用脉冲频率控制实现谐振软开关,提高变换效率。最后,搭建仿真模型和6 kW模块化实验样机,验证所提出方案的合理性与可行性。     

Pressurized PEM water electrolysis: Efficiency and gas crossover

In this study the influence of cathodic and anodic pressures during polymer electrolyte membrane water electrolysis on the gas crossover is simulated and compared to in-situ measurements of the anodic hydrogen content at differential and balanced pressure operation. The efficiency losses due to the reduced Faraday efficiency caused by crossover, ohmic loss of the membrane and pressurized hydrogen and oxygen evolution are estimated. Therefore, the correlated dependencies on the current density, membrane thickness, anodic and cathodic pressures, membrane conductivity and permeabilities are quantified. In addition, pressurized electrolysis is compared to adiabatic and isothermal subsequent compression in focus of efficiency. The outcome of this study can be utilized as a powerful computational tool to optimize the membrane thickness with respect to the operating pressures.     

隔膜特性对氢气电化学压缩能耗的影响

文章选取Nafion 211和Nafion 117膜作为电化学压缩隔膜,通过仿真建模方法研究了隔膜特性对电化学压缩机压缩性能的影响。研究结果表明:隔膜欧姆阻抗对电压效率影响显著,隔膜气密性对电流效率影响显著;与Nafion 117膜相比,将Nafion 211膜应用于电化学压缩机时,电化学压缩机能够获得更高的能量效率,当工作电流为0.5 A/cm^2,压缩比为3时,等温压缩效率最高可达到50%。将以Nafion 211膜为隔膜,工作电流为2 A/cm^2的电化学压缩机用于制氢厂的管束车充装,当气源压力为2 MPa,管束车压力由5 MPa充装至20 MPa时,平均能量效率为35%,综合平均电耗为2 kW·h/kg,比目前常用的机械往复式压缩机的电耗降低了30%以上。     

太阳能驱动的固体氧化物电解池制氢系统性能研究

采用太阳能驱动电解水制氢是实现将太阳能转换为氢能来存储的最佳方式。该文提出一种采用光伏、光热协同驱动固体氧化物电解池（SOEC）进行高温蒸汽电解的制氢系统。建立各子系统数学模型,选取北京地区夏至日气象参数,分析太阳辐照度对制氢系统的性能影响,最后对整个系统进行能量及火用分析。结果表明,电流密度和温度是影响SOEC工作的重要因素。在电流密度较大的情况下升高温度,将有利于提高电解效率。耦合太阳能后系统最大能量及火用效率分别达到19.1%和20.3%。火用分析结果表明系统最大有用功损失发生在光电转换过程,火用损比例为87%。提升光电效率,将成为提高太阳能-氢能转换效率的关键。     

Predicting efficiency of solar powered hydrogen generation using photovoltaic-electrolysis devices

Hydrogen fuel for fuel cell vehicles can be produced by using solar electric energy from photovoltaic (PV) modules for the electrolysis of water without emitting carbon dioxide or requiring fossil fuels. In the past, this renewable means of hydrogen production has suffered from low efficiency (2–6%), which increased the area of the PV array required and therefore, the cost of generating hydrogen. A comprehensive mathematical model was developed that can predict the efficiency of a PV-electrolyzer combination based on operating parameters including voltage, current, temperature, and gas output pressure. This model has been used to design optimized PV-electrolyzer systems with maximum solar energy to hydrogen efficiency. In this research, the electrical efficiency of the PV-electrolysis system was increased by matching the maximum power output and voltage of the photovoltaics to the operating voltage of a proton exchange membrane (PEM) electrolyzer, and optimizing the effects of electrolyzer operating current, and temperature. The operating temperature of the PV modules was also an important factor studied in this research to increase efficiency. The optimized PV-electrolysis system increased the hydrogen generation efficiency to 12.4% for a solar powered PV-PEM electrolyzer that could supply enough hydrogen to operate a fuel cell vehicle.     

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。     

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.     

Direct electrolysis of bunsen reaction product for hydrogen production: The continuous-flow operation

Hydrogen sulfide (H₂S) emitted from oil industry's hydrotreating processes can be converted into hydrogen and used back to the same processes through a H₂S splitting cycle, where the Bunsen reaction and HI decomposition are two participating reactions. To overcome the difficulties and complications posted in the scaling up of the cycle, direct electrolysis of the Bunsen reaction product solution was proposed and has been studied in a batch electrolysis cell in our earlier work. This paper studies the direct electrolysis using a customer-made, continuous-flow electrolysis cell. The effects of the operating parameters including the current density, the entering HI concentration and flow rate of the anolyte, the toluene to aqueous phase ratio and stirring speed in anolyte cell, the H₂SO₄ concentration and circulation rate of the catholyte on the performing parameters such as the conversion of iodide ions, the yield of iodine transferred to toluene, and the anodic and cathodic current efficiencies for iodide conversion and hydrogen production were carefully investigated. The results show that the cathodic current efficiency for hydrogen production is nearly 100% for all the runs and that the anodic current efficiency for iodide ion conversion to iodine is relatively low (20%–70%) and varies with the changes in operating parameters. Running at high levels of the current density, the volumetric ratio of toluene to aqueous phase in anolyte, or the stirring speed in anolyte, and low levels of the entering concentration of I^? in anolyte or the flow rate of anolyte in electrolysis operation are in favor of having a high iodide conversion and high I₂-toluene yield. Iodide anions at a few mmol L^?1 level (a few thousandths of the entering concentration) are found in the cathodic chamber caused by its diffuse against the electric field and the proton exchange membrane. The continuous, direct electrolysis of the Bunsen product solution can be considered being adapted in the sulfur-iodine (S–I) water splitting cycle for hydrogen production.     

Configuration design and performance optimum analysis of a solar-driven high temperature steam electrolysis system for hydrogen production

A new solar-driven high temperature steam electrolysis system for hydrogen production is presented, in which the main energy consumption processes such as steam electrolysis processes, heat transfer processes, and product compression processes are included. The detailed thermodynamic-electrochemical modeling of the solid oxide steam electrolysis (SOSE) is carried out, and consequently, the electrical and thermal energy required by every energy consumption process are determined. The efficiency of the system is derived, from which the effects of some of the important parameters such as the operating temperature, component thickness of the SOSE, leakage resistance, effectiveness of heat exchangers, and inlet rate of water on the performance of the system are discussed. It is found that the efficiency attains its maximum when a proper current density is chosen. The ratio of the required electric energy to the total energy input of the system is calculated, and consequently, the problem how to rationally operate the solar concentrating beam splitting device is investigated. The results obtained will be helpful for further understanding the optimal design and performance improvement of a practical solar-driven high temperature steam electrolysis system for hydrogen production.     

Compact and fast-response 150 kW hydrogen-oxygen steam generator

Burning of hydrogen produces high-grade heat, which can be used for zero-carbon power generation and/or heating and in heat-intensive industries. In combination with water electrolysis, hydrogen combustion provides efficient energy storage method for variable renewables. Hydrogen combustion systems are compact, powerful and highly maneuverable in comparison with fuel cells. We present experimental results of fire tests of a water-cooled hydrogen-oxygen steam generator (HOSG). This fast-response device has start-up time less than 15 s to thermal capacity of 147 kW. Temperature of generated steam is within 1173–1273 K, parameters of steam and energy conversion efficiency can be adjusted the water-to-hydrogen ratio. The maximum efficiency of conversion of chemical energy of hydrogen into enthalpy of steam is 98.7%.     

Parameter study of a porous solar-based propane steam reformer using computational fluid dynamics and response surface methodology

Solar-driven steam reforming of fossil fuels is a promising renewable method for hydrogen production that reduces emissions compared with traditional approaches such as combustion-based technologies. In the present study, a steady-state computational fluid dynamic (CFD) model is developed to investigate a porous solar propane steam reformer (PSR). P1 approximation for radiation heat transfer is coupled with the CFD model, employing User-Defined Functions (UDFs). Innovative propane steam reformers have received less attention in terms of optimization and sensitivity analysis to improve their performance and efficiency. Hence, the effects of porosity, pore diameter, inlet velocity, solar irradiation flux, inlet temperature, and foam thermal conductivity on the propane conversion, hydrogen production rate, and pressure drop are studied using response surface methodology (RSM). The inlet velocity, solar irradiation flux, and pore diameter are found to be the most influential parameters, among those mentioned, on propane conversion, hydrogen productivity, and pressure drop, respectively. Furthermore, optimization is carried out in order to minimize pressure drop and maximize hydrogen production. The reformer with the 70% propane conversion provides the lowest pressure drop maintaining the same hydrogen productivity compared with 80% and 90% propane conversions.     

Reduced internal resistance of microbial electrolysis cell (MEC) as factors of configuration and stuffing with granular activated carbon

With limited external applied voltage, the microbial electrolysis cell (MEC) could produce hydrogen by exoelectrogenic microorganisms. The present study revealed that a cubiod-shaped chamber effectively reduces the distance between electrodes and thereby reduces the internal resistance of the entire cell. With 0.6 V of applied voltage, the cuboid MEC had a columbic efficiency of 33.7%, much higher than that achieved in the H-shaped MEC test (ca. 15%) of comparable size. Filling the anode chamber with granular activated carbon further enhanced the columbic efficiency to 45%. The corresponding hydrogen conversion rate could reach 35%.     

Aspen Plus model of an alkaline electrolysis system for hydrogen production

A model of an alkaline electrolysis plant is proposed in this paper, including both stack and balance of plant, with the objective of analyzing the performance of a complete electrolysis system. For this purpose, Aspen Plus has been used in this work due to its great potential and flexibility. Since this software does not include codes for modelling the electrolysis cells, a custom model for the stack has been integrated as a subroutine, using a tool called Aspen Custom Modeler. This stack model is based on semi-empirical equations which describe the voltage cell, Faraday efficiency and gas purity as a function of the current. The rest of the components in the electrolysis plant have been modelled with standard operation units included in Aspen Plus. Simulations have been carried out in order to evaluate and optimize the balance of the plant of an alkaline electrolysis system for hydrogen production. Also, a parametric study has been conducted. The results show that increasing the operation temperature and reducing the pressure can improve the overall performance of the system. The proposed model in this work for the alkaline electrolyzer can be used in the future to develop a useful tool to carry out techno-economic studies of alkaline electrolysis systems integrated with other process.     

Transport properties of proton conducting phosphate glass: An electrochemical hydrogen pump enabling the formation of dry hydrogen gas

In this study, a proton conducting phosphate glass, which was fabricated by alkali-proton substitution, was clearly demonstrated to transport protons over a wide oxygen partial pressure in air and hydrogen atmospheres, and in both dry and wet conditions. The proton transport number was confirmed to be unity, regardless of whether the glass was exposed to air or hydrogen atmospheres, in both hydrogen concentration and water vapor concentration cells. The dry hydrogen can be formed by electrochemical hydrogen pumping without current leakage as a result of the presence of other charge carriers. These properties derive from a proton incorporation mechanism, which is non-dependent on defect equilibria, unlike acceptor-doped perovskite-type proton conducting oxides. The advantages of applying this proton conducting glass electrolyte to fuel cells and steam electrolysis is also discussed.     

Electrochemical high temperature technology for hydrogen production or direct electricity generation

Solid oxide electrolyte cells have been developed for hydrogen production from water vapour with high overall efficiency. On the basis of experimental data plant concepts have been designed for autothermal electrolysis operation with low temperature (∼150°C) steam input at a mean cell voltage of ∼1.3 V. The production of modules of series connected cells is under way in order to demonstrate the vapour electrolysis in a small, but complete plant for ∼3.5 kW hydrogen output. The actual state of the art of this technology will be described. It has been demonstrated that the solid oxide cells can be operated also in the reverse mode acting as fuel cells for hydrogen as well as for carbon monoxide. Calculations of overall performance of high temperature fuel cell plants show that, e.g. for electricity generation from natural gas, efficiencies of about 60% can be achieved with the additional advantage of extremely low NO_x emission.     

Comparison of electrical energy efficiency of atmospheric and high-pressure electrolysers

Efforts are being made to produce highly pressurised electrolysers to increase the overall energy efficiency by eliminating mechanical compression. However, in-depth modelling of electrolysers suggests that electrolysis at atmospheric pressure is electrically more energy efficient if parasitic energy consumption and gas losses are incorporated in both cases. The reversible cell voltage increases with increasing pressures. The electrode activation and Ohmic losses, leakage current and inevitable heat losses increase the electrolysis voltage beyond the thermoneutral voltage and consequently heat removal from the stack becomes essential. The expected gas loss at various operating pressures is incorporated to reveal the energy consumption that would occur in practice. Comparison of total energy consumption at various operating pressure up to 700 atm is performed and atmospheric electrolysers are found more efficient at all levels. Practical considerations such as corrosion, hydrogen embrittlement, operational complexity, dynamic response and cost are less favourable for pressurised electrolysers.     

Combustion efficiency measurements and burner characterization in a hydrogen-oxyfuel combustor

Energy generation from renewable sources in the power sector keeps constantly increasing. This raises the demand for fast and flexible large-scale storage technologies. Steam generation via stoichiometric combustion of hydrogen and oxygen within a steam cycle is a promising way to recombine both gases, which can be generated by electrolysis utilizing excess renewable energy. At the same time, this technology could provide balancing and spinning network reserves. A crucial parameter of this approach is the combustion efficiency, since residual hydrogen or oxygen can damage downstream components of the power plant steam cycle. The current paper investigates the combustion of hydrogen and oxygen under steam diluted conditions. Flow field, mixing, flame types and combustion efficiency are assessed. The combustion efficiency measurement is very challenging in this case, as the combustor products consist mostly of pure steam and cannot be dried for conventional gas analysis. This is solved by an in-situ measurement method to quantify the combustion efficiency. Initial results of this approach are also presented in the current work.