PBI‐Based Polymer Membranes for High Temperature Fuel Cells – Preparation,Characterization and Fuel Cell Demonstration

Proton exchange membrane fuel cell (PEMFC) technology based on perfluorosulfonic acid (PFSA) polymer membranes is briefly reviewed. The newest development in alternative polymer electrolytes for operation above 100 °C is summarized and discussed. As one of the successful approaches to high operational temperatures, the development and evaluation of acid doped polybenzimidazole (PBI) membranes are reviewed, covering polymer synthesis, membrane casting, acid doping, physicochemical characterization and fuel cell testing. A high temperature PEMFC system, operational at up to 200 °C based on phosphoric acid‐doped PBI membranes, is demonstrated. It requires little or no gas humidification and has a CO tolerance of up to several percent. The direct use of reformed hydrogen from a simple methanol reformer, without the need for any further CO removal, has been demonstrated. A lifetime of continuous operation, for over 5000 h at 150 °C, and shutdown‐restart thermal cycle testing for 47 cycles has been achieved. Other issues such as cooling, heat recovery, possible integration with fuel processing units, associated problems and further development are discussed.     

A Preliminary Study on WO3‐Infiltrated W–Cu–ScYSZ Anodes for Low Temperature Solid Oxide Fuel Cells

Preparation and electrochemical characterization of WO₃‐infiltrated 0.48W–0.52Cu–ScYSZ (WCS) anode for solid oxide fuel cell are reported. The DC conductivity of a WO₃ ceramic was 1,200 and 24 S cm^–1 in reducing and oxidizing atmospheres, respectively, at 650 °C. WCS porous backbones in the form of symmetric cells were prepared by screen printing of WO₃–CuO–ScYSZ ink and subsequent sintering at 1,300 °C for 1 h in 9% H₂/N₂. Analysis of the sintered backbone by X‐ray diffraction showed the metallic W and Cu phases. Precursor solutions of WO₃ or CuO were infiltrated into porous WCS backbones to form the anode. The electrochemical performance of these anodes measured by impedance spectroscopy showed polarization resistances of 11 and 6.5 Ω cm² for WO₃ and CuO infiltrated anodes, respectively, at 600 °C in humidified hydrogen. Activation energy values of 86.8 and 96.5 kJ mol^–1 were obtained for WO₃ and CuO infiltrated WCS anodes, respectively. The microstructure of the tested anodes showed well‐dispersed sub‐micron particles of WO₃ in the WCS backbone whereas CuO infiltration resulted in a dense microstructure.     

Expanded Graphite–Epoxy–Flexible Silica Composite Bipolar Plates for PEM Fuel Cells

Silica impregnated expanded graphite–epoxy composites are developed as bipolar plates for proton exchange membrane (PEM) fuel cells. These composite plates were prepared by solution impregnation, followed by compression molding and curing. Mechanical properties, electrical conductivities, corrosion resistance, and contact angles were determined as a function of impregnated content. The plates show high flexural strength with 5% methyltrimethoxysilane (MTMS) addition (20 MPa) and in‐plane conductivity of 131 S cm⁻¹ that meet the DOE target (>100 S cm⁻¹). Corrosion current values as low as 1.09 μA cm⁻² were obtained. The contact angle was found to be 80°. Power density of 1 W cm⁻² was achieved with custom made expanded graphite–polymer composite plates. High efficiency values were obtained at low current regions.     

Electrochemical Performance of Cobalt‐free Nd0.5Ba0.5Fe1–xNixO3–δ Cathode Materials for Intermediate Temperature Solid Oxide Fuel Cells

A new series of cobalt‐free perovskite‐type oxides, Nd_0.5Ba_0.5Fe_1–xNi_xO_3–δ (0 ≤ x ≤ 0.15), have been prepared by a citric acid‐nitrate process and investigated as cathode materials for proton conducting intermediate temperature solid oxide fuel cells (IT‐SOFCs). The conductivity of the oxides was measured at 300–800 °C in air. It is discovered that partial substitution of Ni for Fe‐sites in Nd_0.5Ba_0.5Fe_1–xNi_xO_3–δ obviously enhances the conductivity of the oxides. Among the series of oxides, the Nd_0.5Ba_0.5Fe_0.9Ni_0.1O_3–δ (NBFNi10) exhibits the highest conductivity of 140 S cm⁻¹ in air at 550 °C. A single H₂/air fuel cell with proton‐conducting BaZr_0.1Ce_0.7Y_0.2O_3–δ (BZCY) electrolyte membrane (ca. 40 μm thickness) and NBFNi10‐BZCY composite cathode and NiO‐BZCY composite anode was fabricated and tested at 600–700 °C. The peak power density and the interfacial polarization resistance (R_p) of the cell are 490 mW cm⁻² and 0.15 Ω cm² at 700 °C, respectively. The experimental results indicate that NBFNi10 is a promising cathode material for the proton‐conducting IT‐SOFCs.     

Novel Ag–Glass Composite Interconnect Materials for Anode‐Supported Flat‐Tubular Solid Oxide Fuel Cells Operated at an Intermediate Temperature

We developed novel Ag–glass composite interconnect materials for anode‐supported flat‐tubular solid oxide fuel cells (SOFCs) operated at 700 °C by optimization of the glass content. For this purpose, the variations of phase stability, area specific resistance (ASR), microstructure, gas leak rate, cell performance, and open circuit voltage (OCV) were determined for the Ag–glass composite materials with respect to the glass content. The Ag–glass composite materials maintain phase stability without chemical reactions. The ASR increased as the glass content increases due to glass existing as an insulator between the Ag phases. All the composite materials showed dense coating layers on the anode support and had a low gas leak. The cell performance and OCV were measured to identify the optimum composition of the Ag–glass composites. Our results confirm that Ag–glass composites are suitable for high performance interconnects in anode‐supported flat‐tubular fuel cells operated below 700 °C.     

Operational Inhomogeneities in La0.9Sr0.1Ga0.8Mg0.2O3–δ Electrolytes and La0.8Sr0.2Cr0.82Ru0.18O3–δ–Ce0.9Gd0.1O2–δ Composite Anodes for Solid Oxide Fuel Cells

The electrolyte/anode interface in solid oxide fuel cells with La_0.9Sr_0.1Ga_0.8Mg_0.2O_3–δ electrolytes and composite anodes containing La_0.8Sr_0.2Cr_0.82Ru_0.18O_3–δ and Ce_0.9Gd_0.1O_2–δ (GDC) was studied using transmission electron microscope Z‐contrast imaging and energy dispersive X‐ray spectroscopy. The anode/electrolyte interface of an operated cell had numerous defective regions in the electrolyte, immediately adjacent to anode GDC particles. These areas had a different chemical composition than other electrolyte regions and were crystallographically inhomogeneous. These regions were not observed in a cell reduced in hydrogen that was not operated, suggesting that they were the result of combined electrical and chemical potential gradients present during cell operation. Ru nanoparticles were observed on the chromite surfaces of the operated.     

Nickel–Iron Anode Substrate for Smart Solid Oxide Fuel Cells with a Self‐Protecting Function Against Reoxidation

Development of highly reliable solid oxide fuel cells (SOFCs) is strongly requested, and the introduction of a self‐protecting function is an ideal approach to increase the reliability of SOFCs. A highly porous (>33%) Ni–Fe metal substrate, which has well‐developed nanopores, is prepared by reduction of NiO–Fe₂O₃. In an oxidizing atmosphere, a thin layer of Fe₂O₃ forms on the surface of the substrate. As a result, the porous morphology changes at the surface and becomes denser. This morphological change occurs only at the surface and prevents oxidation of Ni in the bulk of the substrate. Furthermore, the surface morphology returns to its original state following reduction. Therefore, despite the fact that Ni is readily oxidized, Ni metal phase is sustained in the Ni–Fe bimetallic alloy substrate even after 480 h oxidation in air. The cell power density is also stably sustained after a few reduction–reoxidation cycles. Here, we report that Ni–Fe bimetal alloy substrate exhibits a self‐protecting function against reoxidation of the substrate, which would otherwise lead to a permanent failure of the cell.     

Preparation of BaZr0.1Ce0.7Y0.2O3–δ Based Solid Oxide Fuel Cells with Anode Functional Layers by Tape Casting

Solid oxide fuel cells (SOFCs) based on the proton conducting BaZr_0.1Ce_0.7Y_0.2O_3–δ (BZCY) electrolyte were prepared and tested in 500–700 °C using humidified H₂ as fuel (100 cm³ min^–1 with 3% H₂O) and dry O₂ (50 cm³ min^–1) as oxidant. Thin NiO‐BZCY anode functional layers (AFL) with 0, 5, 10 and 15 wt.% carbon pore former were inserted between the NiO‐BZCY anode and BZCY electrolyte to enhance the cell performance. The anode/AFL/BZCY half cells were prepared by tape casting and co‐sintering (1,300 °C/8 h), while the Sm_0.5Sr_0.5CoO_3–δ (SSC) cathodes were prepared by thermal spray deposition. Well adhered planar SOFCs were obtained and the test results indicated that the SOFC with an AFL containing 10 wt.% pore former content showed the best performance: area specific resistance as low as 0.39 Ω cm² and peak power density as high as 0.863 W cm^–2 were obtained at 700 °C. High open circuit voltages ranging from 1.00 to 1.12 V in 700–500 °C also indicated negligible leakage of fuel gas through the electrolyte.     

Design of BaZr0.8Y0.2O3–δ Protonic Conductor to Improve the Electrochemical Performance in Intermediate Temperature Solid Oxide Fuel Cells (IT‐SOFCs)

BaZr_0.8Y_0.2O_3–δ, (BZY), a protonic conductor candidate as an electrolyte for intermediate temperature (500–700 °C) solid oxide fuel cells (IT‐SOFCs), was prepared using a sol–gel technique to control stoichiometry and microstructural properties. Several synthetic parameters were investigated: the metal cation precursors were dissolved in two solvents (water and ethylene glycol), and different molar ratios of citric acid with respect to the total metal content were used. A single phase was obtained at a temperature as low as 1,100 °C. The powders were sintered between 1,450 and 1,600 °C. The phase composition of the resulting specimens was investigated using X‐ray diffraction (XRD) analysis. Microstructural characterisation was performed using field emission scanning electron microscopy (FE‐SEM). Chemical stability of the BZY oxide was evaluated upon exposure to CO₂ for 3 h at 900 °C, and BZY showed no degradation in the testing conditions. Fuel cell polarisation curves on symmetric Pt/BZY/Pt cells of different thicknesses were measured at 500–700 °C. Improvements in the electrochemical performance were obtained using alternative materials for electrodes, such as NiO‐BZY cermet and LSCF (La_0.8Sr_0.2Co_0.8Fe_0.2O₃), and reducing the thickness of the BZY electrolyte, reaching a maximum value of power density of 7.0 mW cm^–2 at 700 °C.     

In Situ Measurements on Solid Oxide Fuel Cell Cathodes – Simultaneous X‐Ray Absorption and AC Impedance Spectroscopy on Symmetrical Cells

A solid oxide fuel cell in operando is a complex multiphasic entity under electrical polarization and operating at high temperatures. In this work, we reproduce these conditions while studying transition metal redox chemistry in situ at the cathode. This was achieved by building a furnace that allowed for X‐ray absorption near‐edge structure and AC impedance spectroscopy data to be obtained simultaneously on symmetrical cells while at operating temperatures. The cell electrodes consisted of phases from the Ruddlesden–Popper family; La₂NiO_4+δ, La₄Ni₃O_10–δ, and composites thereof. The redox chemistry of nickel in these cathodes was probed in situ through investigation of changes in the position of the X‐ray absorption K‐edge. An oxidation state reduction (Ni³⁺ to Ni²⁺) was observed on heating the cells; this was correlated to changing concentrations of ionic charge carriers in the electrode. Polarizing the cells resulted in dramatic changes to their electrical performance but not to the bulk redox chemistry of the electrode. The implications of this with respect to explaining the polarization behavior are discussed.     

Forecast of Performance Parameters of Automotive Fuel Cell Systems – Delphi Study Results

Fuel cells are a promising propulsion technology option in sustainable and zero‐emission drivetrain strategies as they offer a high potential to significantly reduce well‐to‐wheel greenhouse gas emissions and the dependency on fossil energy resources. At the same time, the current technological performance of automotive fuel cell systems is not yet sufficient to meet market demands. Therefore, the technical development of fuel cells is a critical factor for a successful market introduction of fuel cell electric vehicles (FCEV). This paper describes the methodology and results of a two‐round Delphi Survey conducted by the Institut für Kraftfahrzeuge of RWTH Aachen University to assess the technological potential of polymer electrolyte membrane fuel cell (PEMFC) systems in automotive applications by 2030. The analysis of the current and future performance level of key performance indicators (KPI) of automotive fuel cell systems helps to identify critical performance parameters and to prioritize research and development demands. KPI analyzed in the Delphi Survey as forecast parameters include system efficiency, durability, power density, and specific power.     

Performance of Cobalt‐free La0.6Sr0.4Fe0.9Ni0.1O3–δ Cathode Material for Intermediate Temperature Solid Oxide Fuel Cells

A series of cobalt‐free perovskite‐type cathode materials La_0.6Sr_0.4Fe_1–xNi_xO_3–δ (0 ≤ x ≤ 0.15) for intermediate temperature solid oxide fuel cells (IT‐SOFCs) are prepared by a citric‐nitrate process. The conductivities of the cathode materials are measured as functions of temperature (300–800 °C) in air by AC impedance method, and the La_0.6Sr_0.4Fe_0.9Ni_0.1O_3–δ (LSFN10) has the highest conductivity to be 160 S cm^–1 at 400 °C. A single IT‐SOFC based on LSFN10 cathode, BaZr_0.1Ce_0.7Y_0.2O_3–δ electrolyte membrane and Ni–BaZr_0.1Ce_0.7Y_0.2O_3–δ anode substrate was fabricated by a simple spin‐coating process, and the performances of the cell using hydrogen as fuel and air as the oxidant were researched by electrochemical methods at 600–700 °C. The maximum power densities of the cell are 405 mW cm^–2 at 700 °C, 238 mW cm^–2 at 650 °C, and 140 mW cm^–2 at 600 °C, respectively. The results indicate that the LSFN10 is a promising cathode material for proton conducting IT‐SOFCs.     

Direct Formic Acid Fuel Cells with 600 mA cm–2 at 0.4 V and 22 °C

A demonstration of direct formic acid fuel cells (DFAFCs) generating very high power density at ambient temperature is reported. In particular, the performance of the Pd black as an anode catalyst for DFAFCs with different formic acid feed concentrations at different operating temperatures has been evaluated. The Pd black based DFAFCs with dry air and zero backpressure can generate a maximum power density of 248 and 271 mW cm^–2 at 22 °C and 30 °C respectively. The open cell potential is 0.90 V. These results show that DFAFCs are potentially excellent alternative power sources for small portable electronic devices.     

A PBI‐Sb0.2Sn0.8P2O7‐H3PO4 Composite Membrane for Intermediate Temperature Fuel Cells

Fine particles of a solid proton conductor Sb_0.2Sn_0.8P₂O₇ were incorporated in PBI‐H₃PO₄ membranes with 20 wt.%. In SEM figures, the Sb_0.2Sn_0.8P₂O₇ particles exhibited even and uniform distribution in the PBI‐Sb_0.2Sn_0.8P₂O₇ membrane. Influences of the immersing time and the concentration of H₃PO₄ solution for immersion on H₃PO₄ loading level were investigated. H₃PO₄ loading level was found an important factor on membrane conductivity. Incorporation of Sb_0.2Sn_0.8P₂O₇ in the PBI‐H₃PO₄ membrane resulted in greater membrane conductivities. In the single cell tests, the peak power density of the membrane electrode assembly (MEA) with the PBI‐Sb_0.2Sn_0.8P₂O₇‐H₃PO₄ membrane was also greater than that of a MEA with PBI‐H₃PO₄ membrane. One MEA using PBI‐Sb_0.2Sn_0.8P₂O₇‐H₃PO₄ membrane achieved a peak power density of 0.67 W cm^–2 at 175 °C with H₂/O₂ and exhibited satisfactory stability.     

Electrochemical Performance of Nd1.95NiO4+δ Cathode supported Microtubular Solid Oxide Fuel Cells

Nd_1.95NiO_4+δ (NNO) cathode supported microtubular cells were fabricated and characterized. This material presents superior oxygen transport properties in comparison with other commonly used cathode materials. The supporting tubes were fabricated by cold isostatic pressing (CIP) using NNO powders and corn starch as pore former. The electrolyte (GDC, gadolinia doped ceria based) was deposited by wet powder spraying (WPS) on top of pre‐sintered tubes and then co‐sintered. Finally, a NiO/GDC suspension was dip‐coated and sintered as the anode. Optimization of the cell fabrication process is shown. Power densities at 750 °C of ∼40 mWcm⁻² at 0.5V were achieved. These results are the first electrochemical measurements reported using NNO cathode‐supported microtubular cells. Further developments of the fabrication process are needed for this type of cells in order to compete with the standard microtubular solid oxide fuel cells (SOFC).     

Flame Spray Synthesis of Nanoscale La0.6Sr0.4Co0.2Fe0.8O3–δ and Ba0.5Sr0.5Co0.8Fe0.2O3–δ as Cathode Materials for Intermediate Temperature Solid Oxide Fuel Cells

Electrochemical performance and degradation was analysed by conductivity measurements as well as thermogravimetric analysis (TGA) under different atmospheres. CO₂ was identified as a critical parameter in terms of carbonate formation from Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3–δ and causes a strong increase in the material resistivity, whereas La_0.6Sr_0.4Co_0.2Fe_0.8O_3–δ is unaffected. The oxygen exchange kinetic of both compositions is affected by CO₂ containing atmospheres.     

Study of Carbon Deposition Behavior on Cu–Co/CeO2–YSZ Anodes for Direct Butane Solid Oxide Fuel Cells

Electro‐catalytic activity of Cu–Co/CeO₂–YSZ anodes towards oxidation of H₂ and n‐C₄H₁₀ fuels and carbon depositions are investigated using different Cu–Co loadings. Cu–Co/CeO₂–YSZ anode based SOFCs with YSZ as electrolyte and LSM/YSZ as cathode were prepared by tape casting and wet impregnation methods and performance was analyzed using I–V characteristics and impedance spectroscopy. The Cu–Co/CeO₂–YSZ anodes with Cu–Co loading of 10, 15, and 25 wt.% produced power density of 60, 197, and 400 mW cm^–2 in H₂ and 190, 225, and 275 mW cm^–2 in n‐C₄H₁₀ at 800 °C. The power density is increased with the increase in Cu–Co loading in Cu–Co/CeO₂–YSZ anodes. The electrochemical impedance spectra shows less ohmic and polarization resistance for 25 wt.% Cu–Co loading in comparison to 10 and 15 wt.% Cu–Co. Scanning electron microscopy and high resolution transmission electron microscopy shows that the carbon fibers formed are hollow in nature with 70 nm size, whereas, thermal gravimetric analysis and X‐ray diffraction points out that they are amorphous in nature. The performance degradation of Cu–Co/CeO₂–YSZ anodes in n‐C₄H₁₀ in 16 h is attributed to increasing amount of carbon deposition with time, which is contrary to our earlier observation in Cu‐Fe/CeO₂–YSZ anode.     

Optimisation and Evaluation of La0.6Sr0.4CoO3 – δ Cathode for Intermediate Temperature Solid Oxide Fuel Cells

In this work, La_0.6Sr_0.4CoO_{3 – δ}/Ce_{1 – x}Gd_xO_{2 – δ} (LSC/GDC) composite cathodes are investigated for SOFC application at intermediate temperatures, especially below 700 °C. The symmetrical cells are prepared by spraying LSC/GDC composite cathodes on a GDC tape, and the lowest polarisation resistance (R_p) of 0.11 Ω cm² at 700 °C is obtained for the cathode containing 30 wt.‐% GDC. For the application on YSZ electrolyte, symmetrical LSC cathodes are fabricated on a YSZ tape coated on a GDC interlayer. The impact of the sintering temperature on the microstructure and electrochemical properties is investigated. The optimum temperature is determined to be 950 °C; the corresponding R_p of 0.24 Ω cm² at 600 °C and 0.06 Ω cm² at 700 °C are achieved, respectively. An YSZ‐based anode‐supported solid oxide fuel cell is fabricated by employing LSC/GDC composite cathode sintered at 950 °C. The cell with an active electrode area of 4 × 4 cm² exhibits the maximum power density of 0.42 W cm^–2 at 650 °C and 0.54 W cm^–2 at 700 °C. More than 300 h operating at 650 °C is carried out for an estimate of performance and degradation of a single cell. Despite a decline at the beginning, the stable performance during the later term suggests a potential application.     

Special Materials in Pyrotechnics: VI. Silicon – An Old Fuel with New Perspectives

The application of silicon as fuel in common pyrotechnic and explosive compositions is reviewed. For part V see Ref. [56].