Microstructure optimization of fuel electrodes for high-efficiency reversible proton ceramic cells

Reversible protonic ceramic cells (R-PCCs) are efficient energy storage and conversion devices that can operate in two modes, namely, in the fuel cell mode for the conversion of fuel to electricity, and in the electrolysis (EC) mode for the EC of water into hydrogen and oxygen. Fuel electrode is a critical component of fuel-electrode-supported R-PCCs, and its pore structure directly affects the electrochemical performance of the R-PCCs, but it has not been fully studied yet. Herein, the pore structure of Ni–BaZr_0.1Ce_0.7Y_0.1Yb_0.1O_3−δ (Ni–BZCYYb) fuel electrodes was systematically modulated by varying the weight ratio (0, 5, 10, and 15 wt.%) of the pore-former added to Ni–BZCYYb, and the electrochemical performance characteristics in the fuel cell and EC modes were investigated. The cell with 10 wt.% pore-former in the Ni–BZCYYb electrode achieved a remarkable peak power density of 540.7 mW cm⁻² and a high current density of –2.28 A cm⁻² at 1.3 V at 700°C in the fuel cell and EC modes, respectively, and showing excellent durability for over 100 h. These results further highlight the critical role of the microstructure of fuel electrodes, which can be modified to achieve exceptional performance, particularly in EC operations.     

Investigation of 30-cell solid oxide electrolyzer stack modules for hydrogen production

The 30-cell nickel-yttria stabilized zirconia (Ni-YSZ) hydrogen electrode-supported planar solid oxide electrolyzer (SOE) stack modules were manufactured and tested at 800 °C in steam electrolysis mode for hydrogen production. The electrolysis efficiency of the stack modules was higher than 100% at a total steam and hydrogen flow of 2.1 sccm cm⁻², a H₂O/H₂ ratio of 80/20, and a current density of <0.2 A cm⁻². The electrolysis efficiency, current efficiency, and actual hydrogen production rate of the stack modules increased with increasing H₂O/H₂ ratio at a constant current density. However, the electrolysis and current efficiencies decreased steadily at high current densities. During hydrogen production, the stack modules were operated at 800 °C and a constant current density of 0.15 A cm⁻² for up to 1100 h. A steam conversion rate of 62% and current efficiency of 87.4% were obtained; the actual hydrogen production rate reached as high as 103.6 N L h⁻¹. Post-mortem analysis showed that delamination of the LSM–YSZ oxygen electrode mainly occurred in the steam and air inlet area of the 10×10 cm² cells.     

Performances of micro-tubular solid oxide cell with novel asymmetric porous hydrogen electrode

Asymmetric-porous hollow-fiber has been fabricated by a phase-inversion process and employed as the hydrogen electrode for micro-tubular solid oxide cell (MT-SOC). The microstructure and electrochemical properties of MT-SOC were investigated in detail. The asymmetric-porous hydrogen electrode possesses unique two layer finger-like porous micro-structure with a thin functional layer and a thick fuel delivery layer. When the MT-SOC was operated in fuel cell mode, maximum power densities of 0.54, 0.71 and 1.25 W/cm² were obtained at 800, 850 and 900 °C, respectively. On the other hand, when the MT-SOC was operated in electrolysis mode at 900 °C with an applied voltage of 1.3 V, current densities of 0.68 A/cm² and 2.57 A/cm² were obtained at 30 vol.% and 80 vol.% absolute humidity (AH), respectively. These results indicate that novel-microstructured MT-SOC can be effectively fabricated towards high performance fuel cell and electrolysis cell.     

Effect of fabrication methods of bifunctional catalyst layers on unitized regenerative fuel cell performance

In order to understand the origins of performance variations in unitized regenerative fuel cells (URFCs), bifunctional catalyst layers (BCLs) fabricated with two different methods, i.e., ink deposition on membrane or GDL, were designed in this paper. The performances of the two different methods were evaluated, and their reaction dynamics were measured by electrochemical impedance spectra. The different BCLs, caused by the different preparation processes, were found to influence the fuel cell performance. The cell potentials of the URFCs using platinum sprayed onto the gas diffusion layer (GDL) are above 0.100 V higher than those with platinum sprayed onto the membrane at 800 mA cm⁻² in fuel cell (FC) mode. The mass transport resistances of the URFCs at different operation modes were also compared. It was proved that the platinum layer formed by applying platinum onto the GDL could prevent the cell from water flooding in FC mode. However, it was found that the cell performance changed slightly in water electrolysis mode with different BCLs. The electron conduction path was also found to be hindered by an IrO₂ agglomerate, which led to a decrease in cell performance. The highest and lowest round-trip efficiencies of the URFC with different BCLs were 42.1% and 22.3%, respectively, at 800 mA cm⁻².     

Cell/stack electrochemical performance and stability of cone-shaped tubular SOFCs for direct operation with methane

Cone-shaped tubular anode-supported solid oxide fuel cells (SOFCs) and two-cell-stack based on NiO-YSZ traditional anodes direct utilization methane as fuel were successfully developed in this study. The single cell exhibited maximum power densities of 1.255 W cm⁻² for hydrogen and 1.099 W cm⁻² for methane at 800 °C, respectively. A stability test of the single cell was performed with different constant current densities at 700 °C in methane. The results indicated that the single cell can be operated stable at high current density in methane. And EDX results showed that there is no measurable coking effect of operation in methane at relatively high current density.A two-cell-stack based on the above-mentioned SOFCs was fabricated and tested by direct utilization of methane. Its typical electrochemical performance was investigated. The two-cell-stack provided a maximum power output of about 3.5 W (350 mW cm⁻² calculated using effective cathode area) by directly using methane at 800 °C. The stack experienced 20 h durability testing. The results demonstrated that the stack was kept at around 1 V (J = 0.05 A cm⁻²) at 700 °C. The stack presented basically stably during the whole test, and the performance of the stack is acceptable for application.     

High temperature water electrolysis using metal supported solid oxide electrolyser cells (SOEC)

Metal supported cells as developed according to the DLR SOFC concept by applying plasma deposition technologies were investigated for use as solid oxide electrolyser cells (SOEC) for high temperature steam electrolysis. Cells consisting of a porous ferritic steel support, a diffusion barrier layer, a Ni/YSZ hydrogen electrode, a YSZ electrolyte and a LSCF oxygen electrode were electrochemically characterised by means of i-V characteristics and electrochemical impedance spectroscopy measurements including a long-term test over 2000 h. The cell voltage during electrolysis operation at a current density of −1.0 A cm⁻² was 1.28 V at an operating temperature of 850 °C and 1.4 V at 800 °C. A long-term test run over 2000 h with a steam content of 43% at 800 °C and a current density of −0.3 A cm⁻² showed a degradation rate of 3.2% per 1000 h. The impedance spectra revealed a significantly enhanced polarisation resistance during electrolysis operation compared to fuel cell operation which was mainly attributed to the hydrogen electrode.     

Comparative study on the electrochemical performance of Sr(Ti0.3Fe0.7)O3-δ fuel electrode in different fuel gases for solid oxide electrochemical cells

Sr(Ti_1-xFe_x)O_3?δ (STF) perovskite has been developed as one of the alternatives to Nickel-base fuel electrodes for solid oxide electrochemical cells (SOCs) that can provide good tolerance to redox cycling and fuel impurities. Recent results on STF fuel electrodes present excellent electrochemical performance and outstand stability both under H₂ fuel cell mode and H₂O electrolysis mode, however, the electrochemical characteristics in other fuel gases, such as CO, CO–H₂ mixture, CH₄, and CO–CO₂ mixture have not been investigated. Herein, we report the electrochemical performance of Sr(Ti_0.3Fe_0.7)O_3?δ fuel electrode on La_0.8Sr_0.2MnO_3?δ-Zr_0.92Y_0.16O_2?δ (LSM-YSZ) oxygen electrode supported SOCs with thin YSZ electrolyte using different fuel gases. At 800 °C, the peak power density slightly decreased from 0.9 W/cm² in wet H₂ to 0.68 W/cm² in wet CO under fuel cell mode. However, the cell only showed a peak power density of 0.27 W/cm² at 800 °C in wet CH₄, reaching 0.75 W/cm² at 850 °C, when the open-circuit voltage increased from 0.9 V to 1.02 V. STF fuel electrode exhibited much worse CO₂ electrolysis performance than steam electrolysis, especially in high CO₂ concentration due to the increased ohmic resistance and electrode polarization resistance.     

Sustainable Syngas Production by High-Temperature Co-electrolysis

High-temperature co-electrolysis shows comparable performance to steam electrolysis. Current densities above 1 A cm⁻² can be reached between 700 °C and 800 °C. Tailor-made syngas is produced, mainly determined by the reactant ratio. The experimental results are supported by modeling. Durability tests with cathode-supported cells show increased voltage degradation rates during electrolysis compared to fuel cell operation. Nickel depletion is found to be the main cause.     

Improving performance of proton ceramic electrolysis cell perovskite anode by Zn doping

As an important renewable energy, hydrogen energy becomes an important part of the future energy system. Proton ceramic electrolysis cell (PCEC) enables the efficient, clean, large-scale preparation of hydrogen, which is a new type of energy conversion device, attracting the attention of many researchers. Sr₂Fe_1.4Zn_0.1Mo_0.5O_6-δ (SFZM) anode materials were developed to investigate the effect of B-site doping of Zn on the electrochemical properties of the Sr₂Fe_1.5Mo_0.5O_6-δ (SFM) materials. The results reveal that the doping of Zn increases the concentration of oxygen vacancies and improves the electrocatalytic activity, which in turn improves the performance of the material. A current density of 408 mA cm⁻² has been achieved at 1.3 V when the SFZM-based single cell was operated in an electrolysis mode (50% H₂O in air) at 600 °C, higher than SFM-based single cells (286 mA cm⁻² at 1.3 V). In addition, the SFZM-based single cell exhibited good durability in a stability test at an electrolysis current density of 408 mA cm⁻². This work confirms that SFZM is a promising material for proton ceramic electrolysis cell anode.     

A novel BaCe0.5Fe0.3Bi0.2O3–δ perovskite-type cathode for proton-conducting solid oxide fuel cells

A perovskite-type BaCe_0.5Fe_0.3Bi_0.2O_3-δ (BCFB) was employed as a novel cathode material for proton-conducting solid oxide fuel cells (SOFCs). The single cells with the structure of NiO-BaZr_0.1Ce_0.7Y_0.2O_3-δ (BZCY7) anode substrate|NiO-BZCY7 anode functional layer|BZCY7 electrolyte membrane|BCFB cathode layer were fabricated by a dry-pressing method and investigated from 550 to 700 °C with humidified hydrogen (~3% H₂O) as the fuel and the static air as the oxidant. The low interfacial polarization resistance of 0.098 Ω cm² and the maximum power density of 736 mW cm⁻² are achieved at 700 °C. The excellent electrochemical performance indicates that BCFB may be a promising cathode material for proton-conducting SOFCs.     

Electrochemical pretreatment of distillery wastewater using aluminum electrode

Electrochemical (EC) oxidation of distillery wastewater with low (BOD₅/COD) ratio was investigated using aluminum plates as electrodes. The effects of operating parameters such as pH, electrolysis duration, and current density on COD removal were studied. At a current density of 0.03 A cm⁻² and at pH 3, the COD removal was found to be 72.3%. The BOD₅/COD ratio increased from 0.15 to 0.68 for an optimum of 120-min electrolysis duration indicating improvement of biodegradability of wastewater. The maximum anodic efficiency observed was 21.58 kg COD h⁻¹ A⁻¹ m⁻², and the minimum energy consumption observed was 0.084 kWh kg⁻¹ COD. The kinetic study results revealed that reaction rate (k) decreased from 0.011 to 0.0063 min⁻¹ with increase in pH from 3 to 9 while the k value increased from 0.0035 to 0.0102 min⁻¹ with increase in current density from 0.01 to 0.03 A cm⁻². This study showed that the COD reduction is more influenced by the current density. The linear and the nonlinear regression models reveal that the COD reduction is influenced by the applied current density.     

Rationally structuring proton-conducting solid oxide fuel cell anode with Ni metal catalyst and porous skeleton

In response to the urgent demand for highly active anodes for lower-temperature proton-conducting solid oxide fuel cells (H–SOFCs) and with the aim to explore the function of metal catalysts and porous skeletons, two series anode-supported BaZr_0.1Ce_0.7Y_0.2O_3-δ (BZCY)-based single cells with varying Ni catalysts and pore formers were assembled and evaluated comparably. In the exploration of Ni catalyst variable, the NBZCY65-35-20SS (65 wt% NiO in NiO-BZCY prepared with adding 20 wt% starch) anode possesses the highest performance, for the 1:1 vol ratio of Ni-BZCY could offer the maximum effective triple-phase boundary (TPB) area in the prerequisite of having abundant pores achieved with the 20 wt% pore former as a constant. In addition, both the NBZCY65-35-15SS and NBZCY65-35-25SS anode demonstrate inferior electrochemical properties separately due to the inadequate reducing gas transmission channels and reaction sites. The BZCY cell assembled with NBZCY65-35-20SS reveals an excellent performance, in which the peak power densities (PPDs) were 660, 539, 413, 272 mW cm⁻² and the polarization resistances (R_P) were 0.061, 0.126, 0.28, 0.652 Ω cm² at 700, 650, 600, 550 °C, respectively. NBZCY65-35-20SS, which has both a superior TPB area and a fine porous anode skeleton, is a preferable option for anode-supported H–SOFCs. On the whole, the scientific regulations governing metal catalysis and pore-forming could be beneficial to the architecture of fine H–SOFC anode structures.     

Evaluation of Cu-substituted La1.5Sr0.5NiO4+δ as air electrode for CO2 electrolysis in solid oxide electrolysis cells

Ruddlesden-Popper oxide, Cu-substituted La_1.5Sr_0.5NiO_4+δ series materials (La_1.5Sr_0.5Ni_1-xCu_xO_4+δ; denoted as LSNCu_x; x = 0, 0.1, 0.25, 0.5) are investigated as air electrodes in solid oxide electrolysis cells (SOECs) for electrolysis of CO₂. Room temperature crystal structure, electrical conductivity and oxygen exchange capacity, as well as electrochemical performance of LSNCu_x are comprehensively investigated. Among the series of samples, LSNCu_0.25 half-cell exhibits the lowest polarization resistance value of 0.179 Ω cm² at 800 °C, which decreases by approximately 86.07% compared with that of LSN. In addition, the fuel electrode-supported single cell with LSNCu_0.25 air electrode presents a high current density of 1.2 A cm^?2 at 1.5 V under 30% CO–70% CO₂ condition at 800 °C, which is 207% of LSN (0.58 A cm^?2) under the same condition. Results show that the impressive catalytic activity for oxygen evolution reaction (OER) is ascribed to the improved electronic conductivity and oxygen exchange capacity. With Cu substitution for Ni-site, the contraction of Ni–O bond in NiO₆ octahedron and increased concentration of charge carries owing to the oxidation of Ni²⁺ to Ni³⁺ are beneficial to the electron conduction. The formation of more interstitial oxygen as ionic compensation also favors the oxygen ion diffusion/exchange and greatly accelerates the charge transfer process. Furthermore, no degradation is observed for the single cell durability test at 750 °C for 50 h, which demonstrates the highly stable performance of LSNCu_0.25 air electrode for electrolysis of CO₂.     

A-site deficient Sr0.9Ti0.3Fe0.7O3−δ perovskite：A high stable cobalt-free oxygen electrode material for solid oxide electrochemical cells with excellent electrocatalytic activity and CO2 tolerance

Recently, SrTi_0.3Fe_0.7O_3?δ (STF) has been investigated as a highly stable oxygen electrode material for solid oxide electrochemical cells (SOCs) with a sufficiently low resistance for cell operation at temperatures of > 700 °C. However, in general, the STF electrode performance is limited at temperatures of ≤ 700 °C due to the low oxygen surface exchange coefficient, which is mainly caused by high Sr surface segregation. To improve the electrode performance, Sr_0.9(Ti_0.3Fe_0.7)O_3?δ (A-STF) with an A-site-deficient design is developed to reduce the Sr content and thus reduce the Sr surface segregation, thereby providing a unique combination of excellent oxygen electrode performance and long-term stability. The A-site deficiency reduces the electrode polarization resistance by > 3 times at 600 °C and clearly improves the oxygen diffusion and surface exchange coefficients due to the decrease of Sr surface segregation. The A-STF electrode exhibits stable performance in the fuel cell and electrolysis modes at 1 A cm^?2 > 1200 h. The stability of STF-based oxygen electrodes in a CO₂-enriched atmosphere is investigated, and the results indicate that A-STF exhibits excellent CO₂ tolerance.     

Na+ doping activates and stabilizes layered perovskite cathodes for high-performance fuel cells

A highly active mixed conductive cathode is required for solid oxide fuel cells (SOFCs) based on yttria-stabilized zirconia (YSZ) at reduced temperatures, which is one of the most important factors for their commercialization. Herein, we propose a Na⁺ doping strategy to activate and stabilize the triple-conducting (H⁺/O²⁻/e⁻) layered perovskite oxide of representative NdBa_0.5Sr_0.5Co_1.5Fe_0.5O_5+δ (NBSCF) for high-performance YSZ fuel cells. The results show that Na⁺ doping enhances the electrochemical properties of the NBSCF cathode, with polarization impedance decreasing from 0.105 to 0.080 Ω cm² at 750 °C and output power increasing from 946.05 to 1435.75 mW cm⁻² at 800 °C. Furthermore, high-temperature XRD (HT-XRD) and the oxygen temperature-programmed desorption (O₂-TPD) further confirm that Na⁺ doping can improve the structural stability of NBSCF. The single cell with a Na-doped NBSCF cathode showed no degradation of current density for more than 120 h at 700 °C and exhibited good stability. This work demonstrates the promise of Na⁺ doping for layered perovskite cathodes and an effective way to promote fuel cell performance.     

SDC-SCFZ dual-phase ceramics: Structure,electrical conductivity,thermal expansion,and O2 permeability

New CO₂-resistant dual-phase Sm_0.2Ce_0.8O_1.925–SrCo_0.4Fe_0.55Zr_0.05O_3-δ (SDC-SCFZ) ceramics present a promising outlook for potential future applications in membrane reactors and solid oxide fuel cells. Their high oxygen permeation flux and stability in CO₂ sweep gas also allow their integration in oxyfuel combustion. Here the structural characteristics, electrical conductivities, thermal expansion behaviors, and oxygen permeabilities of four different SDC-SCFZ membranes with weight ratios of 10:90, 25:75, 50:50, and 75:25 (SDC:SCFZ) are systematically studied. Among these four SDC-SCFZ compositions, 0.6 mm-thick 25 wt% SDC-75 wt% SCFZ displayed the highest oxygen permeation fluxes that reach 1.26 mL min⁻¹ cm⁻² at 950°C and retained its phase integrity under alternating He and CO₂ sweep gas over 72 hours of operation. This composite also showed a moderate thermal expansion coefficient of 1.90 × 10⁻⁵ K⁻¹ between 30°C and 1000°C and an electrical conductivity of at least 16 S cm⁻¹ at 550°C and above. Modeling studies revealed that the oxygen permeation fluxes through 25SDC-75SCFZ are limited by surface exchange reactions from 700°C to 800°C and mixed bulk diffusion and surface exchange reactions above 800°C.     

Boosting catalytic and CO2 adsorption ability by in situ Cu nanoparticle exsolution for solid oxide electrolysis cell cathode

Solid oxide electrolysis cell (SOEC) is recognized as an effective means to accomplish sustainable development since it is an efficient electrochemical technology for CO₂ emission reduction. However, the electrocatalytic reduction activity of the cathode for CO₂ restricts the development of SOEC. Herein, an A-site deficient perovskite Sr_1·9Fe_1·3Cu_0·2Mo_0·4Ti_0·1O_6-δ (SFCMT) was proposed as cathode material that can in situ exsolve uniform Cu nanoparticles. The exsolution of Cu increases the concentration of oxygen vacancies and provides abundant adsorption sites for CO₂, resulting in excellent electrochemical catalytic capacity. Cu@SFCMT-based single cells exhibit excellent electrolytic performance under pure CO₂, with current densities up to 3.21 A cm⁻² at 1.8 V and 800 °C and interface polarization resistance (Rp) as low as 0.20 Ω cm² at 800 °C. Furthermore, the current density changes slightly after the 140 h stability test at 1.2 V. Cu@SFCMT exhibits outstanding electrochemical activity and durability, making it a viable SOEC cathode material.     

Novel Sr1.95Fe1.4Co0.1Mo0.5O6-δ anode heterostructure for efficient electrochemical oxidative dehydrogenation of ethane to ethylene by solid oxide electrolysis cells

The electrocatalytic conversion of ethane to ethylene is an important industrial process since ethylene is useful for the production of various chemical intermediates and polymers. However, this process often requires high temperatures. Metal-oxide heterogeneous interfaces constructed by in-situ exsolved process under reducing conditions would be favorable for promoting the catalyst activity, selectivity, and stability of ethane conversion to ethylene. Herein, Sr_1.95Fe_1.4Co_0.1Mo_0.5O_6-δ (abbreviated as SFCoM) was prepared as a novel anode material of solid oxide electrolysis cells (SOECs) for green ethylene production by electrochemical oxidative dehydrogenation of ethane. After reduction, nano CoFe particles were in-situ exsolved on SFCoM oxides to form a nano alloy-oxide heterostructure (CoFe@SFCoM) with large numbers of reactive sites, relevant for improving the conversion rate of ethane and the yield of ethylene. At 800 °C, the single cell based on CoFe@SFCoM anode exhibited a current density of 1.89 A cm⁻² at 1.6 V with an ethane conversion rate of 36.4% and corresponding ethylene selectivity of 94.5%. After 50 h of testing, the electrolysis current density(∼0.5 A cm⁻²) and ethylene yield(∼18.43%) of the single cell did not change significantly, showing good stability. In sum, CoFe@SFCoM looks very promising for future use as a SOECs anode for the electro-catalytic conversion of ethane to ethylene.     

Tubular solid oxide fuel cells fabricated by a novel freeze casting method

Freeze casting is an established method for fabricating porous ceramic structures with controlled porosity and pore geometries. Herein, we developed a novel freeze casting and freeze drying process to fabricate tubular anode supports for solid oxide fuel cells (SOFCs). Freeze casting was performed by injecting aqueous anode slurry to a dual-purpose freeze casting and freeze drying mold wrapped with peripheral coils for flowing a coolant. With the use of an ice barrier layer, proper control of the experimental setup, and adjustments in the drying temperature profile, complete drying of the individual anode tubes was achieved in 4 hours. The freeze-cast anode tubes contained radially aligned columnar pore channels, thus significantly enhancing the gaseous diffusion. SOFC single cells with conventional Ni/yttria-stabilized zirconia/strontium-doped lanthanum manganite materials were prepared by dip coating the thin functional layers onto the anode support. Single cell tests showed that the concentration polarization was low owing to the highly porous anode support with directional pores. With H₂/N₂ (1:1) fuel, maximum power densities of 0.47, 0.36, and 0.27 W/cm² were recorded at 800°C, 750°C, and 700°C, respectively. Our results demonstrate the feasibility of using freeze casting to obtain tubular SOFCs with desired microstructures and fast turn-around times.     

Effects of lanthanum strontium cobalt ferrite (LSCF) cathode properties on hollow fibre micro-tubular SOFC performances

Single layer La_0.6Sr_0.4Co_0.2Fe_0.8O₃ hollow fibre (HF) precursors (<1 mm ID) produced by phase inversion (PI) were sintered at 1,200, 1,350 and 1,400 °C. The increase in sintering temperature resulted in microstructural changes in the LSCF fibres, reflected in their electrical conductivities. LSCF-based cathodes with different designs were brushed onto co-extruded nickel–gadolinium-doped ceria (CGO) anode/CGO electrolyte dual-layer HFs (<1 mm ID) fabricated by PI. The effect of cathode layers on the overall performance of the fuel cells (FCs) was assessed using nearly identical anode and electrolyte compositions, thicknesses, and microstructures. Cathode microstructure design caused cells to perform differently producing peak power densities of 0.35–0.7 W cm⁻² at 600 °C. Impedance spectroscopy analysis at 600 °C on the FCs produced 0.12–0.24 Ω cm² confirming the cathode’s structural effect on the overall area-specific resistance of the FCs. The best performing FC with a brush-deposited cathode was compared to a similar FC where cathode was deposited by dip coating; at 600 °C the first produced 0.6 W cm⁻² while the second cell 0.7 W cm⁻². Co-extruding anodes and electrolytes by using PI and combining dip coating for cathode deposition could lead to the fabrication of FCs with enhanced microstructures and improved performances.