Reversible operation performance of microtubular solid oxide cells with a nickelate-based oxygen electrode

This paper describes the reversible operation of a highly efficient microtubular solid oxide cell (SOC) with a nickelate-based oxygen electrode. The fuel cell was composed of a microtubular support of nickel and yttria stabilized zirconia (Ni-YSZ), an YSZ dense electrolyte, and a double oxygen electrode formed by a first composite layer of praseodymium nickelate (PNO) and gadolinium-doped ceria (CGO) and a second one of PNO. A good performance of the cell was obtained at temperatures up to 800 °C for both fuel cell (SOFC) and electrolysis (SOEC) operation modes, specially promising in electrolysis mode. The current density in SOEC mode at 800 °C is about −980 mA cm⁻² at 1.2V with 50% steam. Current density versus voltage curves (j-V) present a linear behavior in the electrolysis mode, with a specific cell area resistance (ASR) of 0.32 Ω cm⁻². Durability experiments were carried out switching the voltage from 0.7V to 1.2V. No apparent degradation was observed in fuel cell mode and SOEC mode up to a period of about 100 h. However, after this period especially in electrolysis mode there is an accumulated degradation associated to nickel coarsening, as confirmed by SEM and EIS experiments. Those results confirm that nickelate based oxygen electrodes are excellent candidates for reversible SOCs.     

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.     

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.     

A new anode material for solid oxide electrolyser: The neodymium nickelate Nd2NiO4+δ

Neodymium nickelate, with composition Nd₂NiO_4+δ is integrated as oxygen electrode in a solid oxide electrolyte supported cell made of a TZ3Y electrolyte and a Ni-CGO hydrogen electrode. This cell is tested in both fuel cell (SOFC) and electrolysis (SOEC) mode and the reversible operation is proven, ASR values being slightly lower in electrolysis mode. Performances in SOEC mode are compared with a commercial cell based on the same electrolyte and cathode, but with lanthanum strontium manganite (LSM) as anode. For a voltage of 1.3 V, current densities of 0.40, 0.64 and 0.87 A cm⁻² are measured at 750, 800 and 850 °C, respectively; they are much higher than the ones measured in the same conditions for the LSM-containing cell. Indeed, for a voltage of 1.3 V, current densities are respectively 1.7, 3 and 4.2 times higher for the Nd₂NiO_4+δ cell than for the LSM one at 850, 800 and 750 °C, respectively. Consequently, Nd₂NiO_4+δ can be considered as a good candidate for operating below 800 °C as oxygen electrode for high temperature steam electrolysis.     

The properties of the fuel electrode of solid oxide cells under simulated seawater electrolysis

In this study, electrolysis of seawater in flat-tube nickel-yttria-stabilized zirconia (Ni-YSZ) electrode-supported solid oxide electrolysis cells (SOECs) were modeled and the effects of variations in electrical conductivity and microstructure of Ni-YSZ electrode support were investigated. When the current density was greater than 700 mA·cm⁻², the conductivity of the electrode support decreased slightly with an increase in current density at 800 °C in hydrogen reduction environment; the conductivity of the electrode support decreased with an increase in the current density when the current density was greater than 400 mA·cm⁻² at 800 °C in the seawater electrolysis environment. During long-term durability experiment of seawater electrolysis, the degradation rates in area specific resistance (ASR) were 0.096 mΩ·cm²/100 h and 0.207 mΩ·cm²/100 h with a current density of 300 mA·cm⁻² (i.e., ≤400 mA·cm⁻²) and 1000 mA·cm⁻² (i.e., ≥400 mA·cm⁻²), respectively. Besides, the various ions commonly present in seawater did not contaminate the Ni-YSZ support during the long-term durability test. The degradation mechanism of seawater electrolysis in flat-tube SOECs is discussed and clarified.     

Efficient and stable heterostructured air electrode for solid oxide steam electrolysis

High performance and excellent durability are essential for the practical application of solid oxide electrolysis cell (SOEC). Here we have demonstrated efficient and durable solid oxide steam electrolysis by constructing active La_0.8Sr_0.2CoO_3-δ/Gd_0.2Ce_0.8O_2-δ (LSC/GDC) heterointerface in air electrode using a simple co-impregnation method. The heterostructured air electrode exhibits the outstanding activity for oxygen evolution reaction, and its exchange current density (557 mA cm^?2) is 69 times higher than that of the traditional LSM-YSZ. The resulting cell reaches ?1.86 A cm^?2 @1.3 V and ?2.30 A cm^?2 @1.5 V at 800 °C and 50% absolute humidity (A.H), and the polarization resistance from the oxygen electrode only is 0.02 Ω cm². Most importantly, the heterostructured cell presents excellent long-term stability for the 1035 h steam electrolysis operation and excellent durability for 100 times charge-discharge cycles. In the heterostructured air electrode, the problem of electrode delamination is avoided due to the reduced oxygen partial pressure at anode/electrolyte resulting from easy diffusion of O^2? at the interphase, and the coarsening of LSC and GDC nanoparticles is limited because of the LSC/GDC percolative interfaces from phase segregation process. This work proposes a simple and effective strategy to design heterointerface for efficient and durable solid oxide steam electrolysis.     

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.     

23,000 h steam electrolysis with an electrolyte supported solid oxide cell

An electrolyte supported solid oxide cell of 45 cm² area was operated in the steam-electrolysis mode during more than 23,000 h before scheduled shutdown, of which 20,000 h with a current density of j = ?0.9 A cm^?2. The cell consisted of a scandia/ceria doped zirconia electrolyte (6Sc1CeSZ), CGO diffusion-barrier/adhesion layers between electrolyte and electrodes, a lanthanum strontium cobaltite ferrite (LSCF) oxygen electrode, and a nickel/gadolinia-doped ceria (Ni/GDC) steam/hydrogen electrode. Voltage degradation in the operation period with j = ?0.9 A cm^?2 was 7.4 mV/1000 h (0.57%/1000 h) and the increase in the area specific resistance 8 mΩ cm²/1000 h. The final cell voltage was 1.33 V (at 851 °C cell temperature). After dismantling, the cell showed no mechanical damage at electrolyte and H₂/H₂O electrode; a small fraction of the oxygen electrode was delaminated. Impedance spectroscopy applied at the steady state DC current density confirmed a degradation dominated by an increasing ohmic term, mainly due to ionic conductivity decay in the electrolyte. In addition, a small non-ohmic and at least partly reversible O₂ electrode contribution to degradation was identified, affected by a pollution from the (compressor) purge air.     

Effects of TiO2 and SDC addition on the properties of YSZ electrolyte

In this study, we investigate the effects of adding titanium dioxide (TiO₂) and samarium doped cerium oxide (SDC) on the properties of yttrium-stabilized zirconia (YSZ) electrolyte. The microstructure, mechanical, and electrochemical properties of the electrolyte are investigated. The performance in CO₂ electrolysis is measured by supplying carbon dioxide to Ni-YSZ electrode and nitrogen to LSM electrode. Results show that TiO₂ and SDC addition can reduce the sintering temperature and increase grain size. The ionic conductivity is 0.123 S cm⁻¹ at 1000 °C. In addition, the thermal expansion coefficient at 1000 °C is 8.25 × 10⁻⁶ K⁻¹. The current density of the cell is 439 mA cm⁻² at 1.3 V and 1000 °C in solid oxide electrolysis cell.     

Development,manufacture and validation of an open cathode LT-PEMFC stack at HySA systems

This work presents open cathode low temperature polymer electrolyte membrane fuel cell stack development and validation process project performed at HySA Systems as a part of a long-term programme funded by Department of Science and Innovation in South Africa. A detailed explanation of the stack design, manufacturing, assembly and validation is given as well as detailed analysis of results is presented. Prototype stack has an electrode active area of 50 cm², bipolar plates made of graphite composite material (Eisenhuth) and membrane electrode assemblies manufactured in South Africa - HyPlat (Pty) Ltd. A short 10-cell stack is validated using FuelCon Evaluator stack test station and custom designed stack control system integrated with complete balance of plant components. The stack maximum current and power densities are 1.2 Acm⁻² at 0.5 V and 0.6 Wcm⁻², respectively. Performed current hold (300 h) and open circuit voltage (60 h) durability tests resulted in degradation rates of 0.64 mVh⁻¹ and 3.83 mVh⁻¹, respectively.     

Highly active and stable A-site Pr-doped LaSrCrMnO-based fuel electrode for direct CO2 solid oxide electrolyzer cells

Direct CO₂ electrolysis has been explored as a means to store renewable energy and produce renewable fuels. La chromate-based perovskite oxides have attracted great attention as fuel electrode materials for solid oxide electrolyzer cells. However, the electrochemical catalytic activity of such oxides is relatively low, and their stability has not been confirmed. In this study, Pr is doped into La_0.75Sr_0.25Cr_0.5Mn_0.5O_3-δ (LSCM) and the applicability of the resulting fuel electrode to direct CO₂ electrolysis is investigated. The polarization resistance of the resulting electrode at 800 °C is decreased by 25%. Distribution function of relaxation times analysis indicates that the observed improvements may be attributed to increased oxygen ion conductivity. A full cell of Pr-doped LSCM-gadolinium-doped ceria (GDC)|scandia-stabilized zirconia|La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ-GDC achieves an electrolysis current of 0.5 A cm⁻² at 1.36 V and a Faradaic efficiency close to 100%. Short-term (210 h) stability testing of the cell under an electrolysis current of 0.5 A cm⁻² at 800 °C with pure CO₂ as the feedstock reveals a decrease in applied voltage at a rate of 7 mV kh⁻¹, thereby indicating excellent stability. Thus, given its satisfactory performance and stability, the Pr-doped LSCM electrode may be considered a promising candidate material for direct CO₂ electrolysis.     

Application of CuNi–CeO2 fuel electrode in oxygen electrode supported reversible solid oxide cell

The oxygen electrode-supported reversible solid oxide cell (RSOC) has demonstrated distinguishing advantages of fuel flexibility, shorter gas diffusion path and more choices for fuel electrode materials. However, there are serious drawbacks including the difficulty of co-firing the oxygen electrode and electrolyte, and the inefficient electrochemical performance. In this study, a (La_0.8Sr_0.2)_0.95MnO_3-δ (LSM) supported RSOC with the configuration of La_0.6Sr_0.4Fe_0.9Sc_0.1O_3-δ (LSFSc)-YSZ/YSZ/CuNi–CeO₂-YSZ is fabricated by tape casting, co-sintering and impregnation technologies. The single cell is evaluated at both fuel cell (FC) and electrolysis cell (EC) mode. Significant maximum power density of 436.0 and 377 mW cm^?2 is obtained at 750 °C in H₂ and CH₄ fuel atmospheres, respectively. At electrolysis voltage of 1.3 V and 50% steam content, current density of ?0.718, ?0.397, ?0.198 and ?0.081 A cm^?2 is obtained at 750, 700, 650 and 600 °C respectively. Much higher electrolysis performance than FC mode is exhibited probably due to the optimized electrodes with increased triple phase boundary (TPB) area and faster gas diffusion (oxygen and steam) and electrochemical reactions for water splitting. Additionally, the short-term stability of single cell in H₂ and CH₄ are also studied.     

High temperature water electrolysis in solid oxide cells

Hydrogen production via high temperature steam electrolysis is a promising technology as it involves less electrical energy consumption compared to conventional low temperature water electrolysis, as consequence of the more favourable thermodynamic and electrochemical kinetic conditions for the reaction. This paper reports on the Solid Oxide Electrolyser Cell (SOEC) performance as function of the operating parameters temperature, humidity and current density. Current–voltage measurements are coupled with impedance spectroscopy, in order to identify the different loss terms in the cell behaviour coming from the electrolyte resistance and the electrode processes. Remarkably high electrical-to-hydrogen energy conversion efficiencies are achieved (e.g., cell voltages of 1.0 and 1.25 V at −1 A cm⁻² and 900 and 800 °C, respectively). Results obtained, moreover, show that an important limitation for the electrolysis reaction, at least at moderate absolute humidity values below about 70 vol.% can be the steam diffusion in the hydrogen/steam electrode.     

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.     

Hydrogen production performance of 3-cell flat-tubular solid oxide electrolysis stack

Flat-tubular solid oxide electrolysis cells have been manufactured with a ceramic interconnector in a body to minimize the stack volume and eliminate metallic components. The NiO-YSZ supports are prepared by the extrusion method, and the other cell components, which included the electrolyte, air electrode, and ceramic interconnector, are fabricated by slurry coating methods. The active area of a single cell is 30 cm², and the gas tightness of the stack is checked in the range below 2 bars. The effects of the operating conditions on the performance of a solid oxide electrolysis stack are investigated using electrochemical impedance spectroscopy and the I-V test. Consequently, it is confirmed that sufficient steam content stimulates the electrochemical splitting of water and decreases the activation energy for water electrolysis at a high temperature. In our 3-cell stack test, the hydrogen production rate is 4.1 lh⁻¹, and the total hydrogen production was 144.32 l during 37.1 h of operation.     

Deficiency of hydrogen production in commercialized planar Ni-YSZ/YSZ/LSM-YSZ steam electrolysis cells

Deficient, or non-linear hydrogen production is for the first time experimentally observed in large-scale planar Ni-YSZ/YSZ/LSM-YSZ steam electrolysis cells. The apparent coinciding of the concentration polarization and Faraday efficiency decrease at certain current density (?0.5Acm^?2 or -0.6Acm^?2 for selected steam content) indicates that steam starvation appears to affect the hydrogen production's linearity, which in essence the Ni/NiO redox process is believed to play a role in such normal SOEC operations. The SOEC survives 10h extreme polarization through electric conduction and oxygen vacancy transportation. Rational SOEC working mode is recommended accordingly. The present work is complementary for the general application of the Faraday's Law to estimate hydrogen production, and to further evaluate the SOEC's overall characteristics.     

La0.75Sr0.25Cr0.5Mn0.5O3 as hydrogen electrode for solid oxide electrolysis cells

La_0.75Sr_0.25Cr_0.5Mn_0.5O₃ (LSCM) has been applied as hydrogen electrode (cathode) material in solid oxide electrolysis cells operating with different steam concentrations (20, 40, 60 and 80 vol.% absolute humidity (AH)) using 40 sccm H₂ carrier gas at 800, 850 and 900 °C, respectively. Impedance spectra and voltage-current curves were measured as a function of cell electrolysis current density and steam concentration to characterize the cell performance. The cell resistance decreased with the increase in electrolysis current density while increased with the increase in steam concentration under the same electrolysis current density. At 1.6 V applied electrolysis voltage, the maximum consumed current density increased from 431 mA cm⁻² for 20 vol.% AH to 593 mA cm⁻² for 80 vol.% AH at 850 °C. Polarization and impedance spectra experiments revealed that LSCM-YSZ hydrogen electrode played a major role in the electrolysis reaction.     

Gadolinium doped ceria-impregnated nickel-yttria stabilised zirconia cathode for solid oxide electrolysis cell

Steam electrolysis (H₂O → H₂ + 0.5O₂) was investigated in solid oxide electrolysis cells (SOECs). The electrochemical performance of GDC-impregnated Ni-YSZ and 0.5% wt Rh-GDC-impregnated Ni-YSZ was compared to a composite Ni-YSZ and Ni-GDC electrode using a three-electrode set-up. The electrocatalytic activity in electrolysis mode of the Ni-YSZ electrode was enhanced by GDC impregnation. The Rh-GDC-impregnated Ni-YSZ exhibited significantly improved performance, and the electrode exhibited comparable performance between the SOEC and SOFC modes, close to the performance of the composite Ni-GDC electrode. The performance and durability of a single cell GDC-impregnated Ni-YSZ/YSZ/LSM-YSZ with an H₂ electrode support were investigated. The cell performance increased with increasing temperature (700 °C-800 °C) and exhibited comparable performance with variation of the steam-to-hydrogen ratio (50/50 to 90/10). The durability in the electrolysis mode of the Ni-YSZ/YSZ/LSM-YSZ cell was also significantly improved by the GDC impregnation (200 h, 0.1 A/cm², 800 °C, H₂O/H₂ = 70/30).     

Study of catalyst sprayed membrane under irradiation method to prepare high performance membrane electrode assemblies for solid polymer electrolyte water electrolysis

In this work, a catalyst sprayed membrane under irradiation (CSMUI) method was investigated to develop high performance membrane electrode assembly (MEA) for solid polymer electrolyte (SPE) water electrolysis. The water electrolysis performance and properties of the prepared MEA were evaluated and analyzed by polarization curves, electrochemistry impedance spectroscopy (EIS) and scanning electron microscopy (SEM). The characterizations revealed that the CSMUI method is very effective for preparing high performance MEA for SPE water electrolysis: the cell voltage can be as low as 1.564 V at 1 A cm⁻² and the terminal voltage is only 1.669 V at 2 A cm⁻², which are among the best results yet reported for SPE water electrolysis with IrO₂ catalyst. Also, it is found that the noble metal catalysts loadings of the MEA prepared by this method can be greatly decreased without significant performance degradation. At a current density of 1 A cm⁻², the MEA showed good stability for water electrolysis operating: the cell voltage remained at 1.60 V without obvious deterioration after 105 h operation under atmosphere pressure and 80 °C.     

Systematic constructing CoFe-Prussian blue analogue on NiCo-layered double hydroxide to obtain heterostructure two-bimetallic phosphide composite as efficient self-supported eletrocatalyst for overall water and urea electrolysis

It remains a challenge to explore economical, high-effective and long term stability electrocatalysts toward large-scale hydrogen production. This work utilizes surface engineering strategy to in-situ CoFe-Prussian Blue Analogues on NiCo-layered double hydroxides to obtain 3D hierarchical heterostructure precursor (NiCo–CoFe-PBA). After phosphatization, this precursor can be further transform into tri-metallic phosphide (NiCoP/CoFeP@NF) and directly act as efficient self-supported electrode for Water and Urea Electrolysis. Impressively, the obtained NiCoP/CoFeP@NF-12 (±) electrode shows excellent catalytic performance with only requires the cell voltage of 1.61 and 1.46 V to deliver 10 mA cm⁻² in overall water splitting and urea electrolysis, respectively, which benefiting from the porous Ni foam (NF) substrate, large catalytic activity area, remarkable mass/electron transfer property, the synergistic effect of components as well as superhydrophilicity and superaerophobicity of electrode surface. In addition, the experimental results also confirm that urea-assisted system has energy saving advantage superior than traditional water splitting in alkaline electrolyte. Moreover, the hierarchical strategy can also be introduced to the construction of other intricate composites for the utilization in energy conversion and storage.