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.     

Comparison of ceria and zirconia based electrolytes for solid oxide electrolysis cells

Steam electrolysis for hydrogen production is investigated in solid oxide electrolysis cell (SOEC). Sc³⁺, Ce⁴⁺, and Gd³⁺ are doped in zirconia (SCGZ) and compared with yttria stabilized zirconia (YSZ) and gadolinium doped ceria (GDC) electrolyte. Electrolyte-supported cells are fabricated. The SCGZ and YSZ electrolytes are dense with >95% relative density while GDC is less densified. The activation energy of conduction of the SCGZ electrolyte is the lowest at 65.58 kJ mol^?1 although phase transformation is detected after electrolyte fabrication process. Cathode-supported cell having SCGZ electrolyte (Ni-SCGZ/SCGZ/BSCF) shows the highest electrochemical performance. Durability test of the cells in electrolysis mode is carried out over 60 h (0.3 A cm^?2, 1073 K, H₂O to H₂ ratio of 70:30). Significant performance degradation of Ni-GDC/YSZ/GDC/BSCF cell is observed (0.0057 V h^?1) whereas the performance of Ni-YSZ/YSZ/BSCF and Ni-SCGZ/SCGZ/BSCF are rather stable under the same operating conditions. The BSCF remains attaching to the SCGZ electrolyte and additional phase transformation is not observed after prolong operation.     

Achieving high-efficiency hydrogen production using planar solid-oxide electrolysis stacks

Steam electrolysis in solid oxide electrolysis cells (SOECs) is considered as an effective method to achieve high-efficiency hydrogen production. In the present investigation, samples of 1-cell, 2-cell and 30-cell SOEC stacks were tested under electrolysis of steam to give a practical evaluation of the SOEC system efficiency of hydrogen production. The samples were tested at 800 °C under various operating conditions up to 500 h without significant degradation, and obtained steam conversion rates of 12.4%, 23% and 82.2% for the 1-cell, 2-cell and 30-cell stacks, respectively. System efficiencies of hydrogen production were calculated for the samples based on their real performance. A maximum efficiency value of 52.7% was achieved in the 30-cell stack.     

System level heat integration and efficiency analysis of hydrogen production process based on solid oxide electrolysis cells

The solid oxide electrolysis cells (SOEC) technology is a promising solution for hydrogen production with the highest electrolysis efficiency. Compared with its counterparts, operating at high temperature means that SOEC requires both power and heat. To investigate the possibility of coupling external waste heat with the SOEC system, and the temperature & quantity requirement for the external waste heat, a universal SOEC system operating at atmospheric pressure is proposed, modeled and analyzed, without specific waste heat source assumption such as solar, geothermal or industrial waste heat. The SOEC system flow sheet is designed to create opportunity for external waste heat coupling. The results show that external waste heat is required for feed stock heating, while the recommended coupling location is the water evaporator. The temperature of the external waste heat should be above 130 °C. For an SOEC system with 1 MW electrolysis power input, the required external waste heat is about 200 kW. When the stack operates at thermoneutral state and 800 °C, the specific energy consumption is 3.77 kWh/Nm³-H₂, of which electric power accounts for 84% (3.16 kWh/Nm³-H₂) and external waste heat accounts for 16% (0.61 kWh/Nm³-H₂). The total specific energy consumption remains almost unchanged when operating the SOEC stack around the thermoneutral condition.     

Multidimensional and transient modeling of an alkaline water electrolysis cell

A three-dimensional (3-D) transient numerical model of an alkaline water electrolysis (AWE) cell with potassium hydroxide solution is developed by rigorously accounting for the hydrogen and oxygen evolution reactions and resulting species and charge transport through various AWE components. First, the AWE model is experimentally validated against a polarization curve corresponding to a wide range of currents as high as 2.0 A·cm^?2. In general, the simulation results compare well with the measured data and further reveal the operating characteristics of AWE cells, showing key distributions of solid/electrolyte potentials and multidimensional contours of reactant and product concentrations at various current densities. In particular, the contribution of hydroxide ion (OH^?) diffusion to the ohmic losses through porous electrodes and a porous separator are quantitatively examined at low and high electrolyte flow rates. The present full 3-D AWE model provides a basic understanding of the electrochemical and transport phenomena and can be further applied to practical large-scale AWE cell and stack geometries for grid-scale hydrogen production.     

The effect of gas compositions on the performance and durability of solid oxide electrolysis cells

High-temperature electrolysis with various gas compositions has been performed to investigate the effects of the hydrogen partial pressure and the humidity generated by the steam electrode on the performance and durability of solid oxide electrolysis cells. The power density of the button cell used in this research is 0.48 W cm⁻² at 750 °C, and the flow rates of the air and humidified hydrogen are 100 cc min⁻¹. By changing the flow ratio of H₂:Ar:H₂O(g) from 10:0:4 to 1:9:4, the cell's OCV decreases from 0.973 V to 0.877 V, and the charge transfer resistance increases from 1.126 Ω cm² to 1.645 Ω cm². The close relationship between the conversion efficiency of high-temperature electrolysis and steam composition is evident in the increase in the cell's charge transfer resistance from 0.381 Ω cm² to 1.056 Ω cm² as the steam content changed from 40 vol% to 3 vol%. Although the electrochemical splitting of water is stimulated in the short term by excessive steam flow, the Ni-YSZ electrodes have been damaged by the steam electrode's low H₂ partial pressure. Consequently, the steam electrode's gas composition must be optimized in the long-term because of the trade-off between performance and durability, which depends on the water concentration of the steam electrodes.     

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.     

Long-term tests of a Jülich planar short stack with reversible solid oxide cells in both fuel cell and electrolysis modes

A two-cell planar stack in the Jülich F-design with solid oxide cells has been built and the reversible operation between fuel cell and electrolysis modes has been demonstrated. The cells were anode supported cells (ASC) with yttria-stabilized zirconia (YSZ) electrolytes, Ni/YSZ hydrogen electrodes and perovskite oxygen electrodes with lanthanum strontium cobalt ferrite (LSCF). This paper summarizes and discusses the preliminary experimental results on the long-term aging tests of the reversible solid oxide planar short stack for fuel cell operation (4000 h) at a current density of 0.5 A cm⁻² which shows a degradation of 0.6% per 1000 h and for steam electrolysis operation (3450 h) and co-electrolysis operation of CO₂ and H₂O (640 h) under different current densities from −0.3 to −0.875 A cm⁻² which show different degradation rates depending on current density and on steam or co-electrolysis.     

Development and preliminary testing of a unitized regenerative fuel cell based on PEM technology

Results related to the development and testing of a unitized regenerative fuel cell (URFC) based on proton-exchange membrane (PEM) technology are reported. A URFC is an electrochemical device which can operate either as an electrolyser for the production of hydrogen and oxygen (water electrolysis mode) or as a H₂/O₂ fuel cell for the production of electricity and heat (fuel cell mode). The URFC stack described in this paper is made of seven electrochemical cells (256 cм² active area each). The nominal electric power consumption in electrolysis mode is of 1.5 kW and the nominal electric power production in fuel cell mode is 0.5 kW. A mean cell voltage of 1.74 V has been measured during water electrolysis at 0.5 A cm⁻² (85% efficiency based on the thermoneutral voltage of the water splitting reaction) and a mean cell voltage of 0.55 V has been measured during fuel cell operation at the same current density (37% electric efficiency based on the thermoneutral voltage). Preliminary stability tests are satisfactory. Further tests are scheduled to assess the potentialities of the stack on the long term.     

A promising strontium and cobalt-free air electrode Pr1-xCaxFeO3-δ for solid oxide electrolysis cell

A promising strontium and cobalt-free ferrite Pr_1-xCa_xFeO_3-δ (PCF, x = 0, 0.1, 0.2, 0.3, 0.4, 0.5) has been synthesized successfully by glycine-nitrate combustion method and used as the air electrode of solid oxide electrolysis cell (SOEC) for steam electrolysis. The crystal structure and electricity conductivity of PCF are investigated in detail. According to the conductivity test, Pr_0.6Ca_0.4FeO_3-δ (PCF64) with higher conductivity is selected as the air electrode to preparing the single cell with structure of PCF64|GDC|SSZ|YSZ-NiO. Under SOFC mode, the maximum power density of the single cell is 462.93 mW cm⁻² at 800 °C with hydrogen as fuel. Under SOEC mode, the current density reaches 277.14 mA cm⁻² and the corresponding hydrogen production rates is 115.84 mL cm⁻² h⁻¹ at 800 °C at 1.3 V. In the 10 h short-term stability test, the cell shows good electrolysis stability.     

Preparation and electrochemical behavior of dense YSZ film for SOEC

High Temperature Electrolysis (HTE) through a solid oxide electrolytic cell (SOEC) had been receiving more and more attentions recently because of its high conversion efficiency (45–59%) and its potential usage for large-scale hydrogen or synthetic fuels production. One of the key technologies associated with SOEC fabrication was to prepare dense yttria-stabilized zirconia (YSZ) electrolyte film on the surface of hydrogen electrode. A novel screen-printing method was developed to fabricate gas-tight YSZ films on porous NiO-YSZ to reduce ohmic resistance of electrolytes and improve electrochemical performance of cells in this paper. The effects of pre-calcining temperature of cathodes, numbers of printing layers and sintering temperature of YSZ films were investigated in detail. SEM and EIS analyses revealed that the selected process parameters had significant influences on the microscopic morphology of YSZ electrolyte film, the OCVs and power density of the prepared cells. After optimization, a 10 μm dense YSZ film was prepared successfully on porous NiO-YSZ support with an OCV of 1.069 V and the electrolysis current density up to 0.681 A/cm² at 1.50 V and 850 °C.     

Optimization of the electrode-supported tubular solid oxide cells for application on fuel cell and steam electrolysis

Hydrogen electrode-supported tubular solid oxide cells (SOCs) were fabricated by dip-coating and co-sintering method. The electrochemical properties of tubular SOCs were investigated both in fuel cell and electrolysis modes. Ni-YSZ was employed as hydrogen electrode support. The pore ratio of Ni-YSZ support strongly affected the performance of tubular SOCs, especially in steam electrolysis mode. The pore ratio was adjusted by the content of pore-former in support slurry. The results showed that 3 wt.% pore former content is the proper value to produce high performance both in fuel cell and electrolysis modes. In fuel cell mode, the maximum power density reached 743.1 mW cm⁻² with H₂ (105 sccm) and O₂ (70 sccm) as working gases at 850 °C. In electrolysis mode, as the electrolysis voltage was 1.3 V, the electrolysis current density reached 425 mA cm⁻² with H₂ (35 sccm) and N₂ (70 sccm) adsorbed 47% steam as working gases in hydrogen electrode at 850 °C. The stability of tubular SOCs was related to the ratio of NiO/YSZ in the support. The sample with NiO/YSZ = 60/40 shows a better performance than the sample with NiO/YSZ = 50/50.     

Microstructural modification of the anode/electrolyte interface of SOEC for hydrogen production

The microstructure of the anode/electrolyte interface is one of key factors to affect the hydrogen production performance of solid oxide electrolysis cells (SOECs). In this paper, a novel interfacial modification method was developed to enhance the active electrode area and the electrolysis performance via preparing a porous YSZ layer on the surface of the dense electrolyte. The effects of YSZ electrolyte pre-sintering temperature, the thickness of dense YSZ electrolyte film and preparation of porous YSZ on the microscopic morphology and the performance of single button cells were investigated. After optimization, a 9 μm porous YSZ layer was successfully prepared on the surface of a 4 μm dense YSZ electrolyte film with an OCV of 1.072 V at the temperature of 850 °C. The results of electrochemical tests showed that the current densities could elevate from −0.681 A cm⁻² to −1.118 A cm⁻² when electrolyzed at 1.5 V under SOEC mode after microstructural modification.     

A novel two‐part interconnector for solid oxide fuel cells

A two‐part interconnector is developed for solid oxide fuel cell stacks to reduce cost and to improve sealing. The novel interconnector involves a metallic core for current collection and gas distribution and a ceramic support to house the metallic core and to separate two short stacks. The new interconnector reduces usage of expensive metallic alloys and substantially reduce mismatch between stack components due to higher expansion coefficient of metals. The new interconnectors also improve sealing with glass–ceramics eliminating chromium evaporation which is a major reason for sealing failure in fully metallic interconnectors. A proof of concept short stack is manufactured and tested in this study. A comparable performance with a convectional interconnector is obtained with new interconnector, while substantially improving the sealing quality. Copyright © 2014 John Wiley & Sons, Ltd.     

Theoretical study and performance evaluation of hydrogen production by 200 W solid oxide electrolyzer stack

High temperature steam electrolyzers, taking advantage of high temperature heat, can produce more hydrogen by using less electrical energy than low temperature electrolyzers. This paper presents an experimental study on hydrogen production by using a 200 W solid oxide stack working in reverse mode. A thermodynamic study of the process was performed by measuring the heat and mass balance of stack at different operating conditions. Different definitions of efficiency were used to highlight the limit and potential of the process. The I–V curve, the flow rate measurements and the GC analysis on outlet flows were used to calculate the hydrogen and oxygen productions. In addition, the influence of steam dilution, water utilization and operating temperature on conversion efficiency and stack's thermal balance was evaluated. With this aim, the tests were performed at three operating temperature (700 °C, 750 °C and 800 °C) over a range of steam inlet concentration from 50% to 90% and water utilization up to 70%. The hydrogen and oxygen flows produced by electrolysis, at different loads, were directly measured after water condensation: net flows up to 2.4 ml/(min cm²) of hydrogen and 1.2 ml/(min cm²) of oxygen were measured and compared to the theoretical ones, showing a good agreement.     

Trends in research and development of protonic ceramic electrolysis cells

Hydrogen energy is an ever-growing and increasingly crucial current field of science and technology aimed at solving many global problems, such as world-wide pollution, the greenhouse effect, inefficient energy conversion, as well as the depletion of non-renewable energy sources. The present work highlights the most recent achievements of hydrogen production utilising protonic ceramic electrolysis cells (PCECs) as energy conversion systems. PCECs are a promising technology, capable of combining high efficiency, flexibility under diverse working conditions and excellent performance. Special attention is paid to novel functional materials, technological strategies and modes of optimising PCECs to allow their excellent electrochemical characteristics to be maintained at relatively low operation temperatures (400–800 °C) compared with traditional solid oxide electrolysis cells based on oxygen-conducting electrolytes.     

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.     

Graphene oxide modified non-noble metal electrode for alkaline anion exchange membrane water electrolyzers

This paper reports the performance of a graphene oxide modified non noble metal based electrode in alkaline anion exchange water electrolyzer. The electrolytic cell was fabricated using a polystyrene based anion exchange membrane and a ternary alloy electrode of Ni as cathode and oxidized Ni electrode coated with graphene oxide as anode. The electrochemical activity of the graphene oxide modified electrode was higher than the uncoated electrode. The anion exchange membrane water electrolyzer (AEMWE) with the modified electrode gave 50% higher current density at 30 °C with deionised water compared to that of an uncoated electrode at 2 V. Performance was found to increase with increase in temperature and with the use of alkaline solutions. The results of the solid state water electrolysis cell are promising method of producing low cost hydrogen.     

80,000 current on/off cycles in a one year long steam electrolysis test with a solid oxide cell

The current ON/OFF switching of a solid oxide electrolysis cell is treated as elementary step for power variation in a steam electrolyser system. If the cell voltage in the ON mode is adjusted to the thermal neutral voltage, heat generation remains zero in both modes, which largely facilitates the thermal management. To verify whether the cells withstand the switching, an electrolysis durability test with an electrolyte supported solid oxide cell was performed during one year at about 850 °C. The cell consisted of a 3YSZ electrolyte, CGO diffusion-barrier/adhesion layers, a lanthanum strontium cobaltite ferrite (LSCF) oxygen electrode, and a nickel/gadolinia-doped ceria (Ni/GDC) steam/hydrogen electrode. The test included two operation blocks with each 40,000 cycles of 2 min duration and a current density of −0.7 Acm⁻² in the ON mode (−0.07 Acm⁻² in OFF mode), as well as steady-state ON periods with 5800 h duration. Voltage degradation was 5 mV/1000 h (0.4%/1000 h) and the increase in the area specific resistance 7 mΩcm²/1000 h, without notable dependence on current cycling. Impedance spectroscopic results were in agreement with the only small switching transients seen in the cell voltage; moreover, they confirmed a dominating ohmic degradation together with minor contributions from gas conversion and reaction, respectively. No electrode delamination was detectable after scheduled test completion.     

Monolithic flat tubular types of solid oxide fuel cells with integrated electrode and gas channels

A new monolithic solid oxide fuel cell (SOFC) design stacked with flatten tubes of unit cells without using metallic interconnector plate is introduced and evaluated in this study. The anode support is manufactured in a flat tubular shape with fuel channel inside and air gas channel on the cathode surface. This design allows all-ceramic stack to provide flow channels and electrical connection between unit cells without needing metal plates. This structure not only greatly reduces the production cost of SOFC stack, but also fundamentally avoids chromium poisoning originated from a metal plate, thereby improving stack stability. The fuel channel was created in the extrusion process by using the outlet shape of mold. The air channel was created by grinding the surface of pre-sintered support. The anode functional layer and electrolyte were dip-coated on the support. The cathode layer and ceramic interconnector were then spray coated. The maximum power density and total resistance of unit cell with an active area of 30 cm² at 800 °C were 498 mW/cm² and 0.67 Ωcm², respectively. A 5-cell stack was assembled with ceramic components only without metal plates. Its maximum power output at 750 °C was 46 W with degradation rate of 0.69%/kh during severe operation condition for more than 1000 h, proving that such all-ceramic stack is a strong candidate as novel SOFC stack design.