Pore structure improvement in cermet for anode-supported protonic ceramic fuel cells

The porous cermet of Yb-doped BaZrO₃ and nickel (BZYb–Ni) for anode-supported protonic ceramic fuel cells (PCFCs) was fabricated by compression molding following the liquid condensation process (LCP). The gas permeability of BZYb–Ni produced by LCP (BZYb–Ni_LCP) was greater than that of BZYb–Ni produced by a conventional drying method (BZYb–Ni_CDM) although their porosities were similar. The greater permeability of BZYb–Ni_LCP than that of BZYb–Ni_CDM is consistent with more efficient structures for gas flow; a smaller specific surface area, and a larger critical diameter for pores in BZYb–Ni_LCP than those for pores in BZYb–Ni_CDM.     

Fabrication of ceramic composite anode at low temperature for high performance protonic ceramic fuel cells

A ceramic composite anode composed of (La_0.8Sr_0.2) (Cr_0.5Mn_0.5)O_3-δ (LSCM), Ba(Zr_0.75Y_0.15)O_3-δ (BZY), and catalysts was applied in hydrocarbon fuels for protonic ceramics fuel cells. LSCM and BZY served as an electronic conductor and a protonic ceramic, respectively. The single phase of LSCM, a promising electronically conductive ceramic, could be obtained by performing calcination when exposed to air and hydrogen reduction at 973 K, which was much lower than the conventional calcination temperature (approximately 1273 K). The LSCM-BZY composite anode was fabricated successfully at such a low temperature using the infiltration method. By testing the composite electrodes at different temperatures, namely 973, 873, 1223, and 1373 K, the effect of the calcination temperature of LSCM on anode performance in hydrogen and methane fuels was successfully investigated. The composite anode with LSCM calcined at 973 and 1073 K showed three-fold improved performance in H₂ fuel and two-fold in CH₄ fuel than that of the composite anode with LSCM calcined at 1373 K.     

Improvement of Ni-Cermet performance of protonic ceramic fuel cells by catalyst

Generally, the NiO composite anode becomes porous after reduction. To infiltrate additional catalysts such as Pd into the NiO-composite anode before reducing NiO to Ni, a porous NiO composite anode for protonic ceramic fuel cells (PCFCs) was fabricated in this study. The porous NiO composite was fabricated by adding graphite as a pore former along with CuO as a sintering agent. The addition of graphite increased the porosity of the NiO composite anode but resulted in poor sinterability, which was addressed by adding CuO as a sintering agent to the NiO composite anode. The Pd catalyst was added to the NiO-composite anode before reducing NiO to Ni. The composite anode for PCFC with three components, namely Ni, protonic ceramics, and a Pd catalyst, was obtained by reducing NiO to Ni during the measurement. The addition of the Pd catalyst improved the anode performance in methane fuel and hydrogen fuel by enhancing the catalytic activity for the electrochemical reaction on the surface.     

Stable perovskite-fluorite dual-phase composites synthesized by one-pot solid-state reactive sintering for protonic ceramic fuel cells

This work synthesized a series of oxide composites with nominal compositions of BaCe₀₅Zr_0.4Y_0.1O_3-δ (BCZY)-Ce_0.5Y_0.5O_2-δ (YDC) based on the BCZY/YDC molar ratios of 0.5:1, 1:1, 2:1, and 4:1 (BCZY-YDC-0.5–1, BCZY-YDC-1-1, BCZY-YDC-2-1, and BCZY-YDC-4-1) using a one-pot solid state reactive sintering (SSRS) method. The X-ray diffraction (XRD) patterns and refinement proved that the one-pot SSRS at 1450 °C for 12 h could achieve the perovskite-fluorite dual-phase composites (DPCs) with the desired structure compositions. The scanning electron microscopy (SEM) images showed that the two DPCs of BCZY-YDC-1-1 and BCZY-YDC-2-1 formed excellent percolation for perovskite and fluorite phases. The conductivity measurement by electrochemical impedance spectroscopy (EIS) and the transference number measurement by the electromotive force (EMF) test proved that changing the BCZY/YDC molar ratio and operating condition could adjust the DPC's conduction property to make them suitable for electrolytes and electrode scaffolds for protonic ceramic fuel cells (PCFCs). The long-term conductivity testing and the crystal structure analysis after EMF testing under versatile conditions indicated that the DPCs of BCZY-YDC-2-1 and BCZY-YDC-1-1 were durable PCFC component materials. The PCFC button cells with the as-discovered BCZY-YDC-2-1 and BCZY-YDC-1-1 as an electrolyte and an anode scaffold, respectively, and the reported Ba–Ce–Fe–Co–O perovskite-perovskite composite as a cathode showed promising performance under H₂/air gradient.     

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.     

Large-area anode-supported protonic ceramic fuel cells combining with multilayer-tape casting and hot-pressing lamination technology

Anode-supported protonic ceramic fuel cells (PCFCs) have many advantages such as the excellent performance and durability at lower temperatures. However, the fabrication of large-area PCFCs is still a big challenge. In this work, the anode supported 10 × 10 cm² PCFCs with BaZr_0.1Ce_0.7Y_0.1Yb_0.1O_3-δ (BZCYYb) electrolyte are fabricated by an optimized sintering process combining with multilayer-tape casting and hot-pressing lamination technology. Through optimizing the sintering shrinkage and thermal expansion behavior, anode supported large-area PCFCs with dense BZCYYb electrolyte are obtained with optimal sintering process. The maximum power density of the cell using H₂ as fuel reaches 400 mW/cm² at 700 ℃. The cell also shows a good thermal cycling performance and durability in 425 h test. Preliminary experimental results indicate that the fabrication technology with multilayer-tape casting and hot-pressing lamination is successful for large-area PCFCs, which can improve the cell fabrication efficiency and provide positive reference for large-area PCFCs.     

Gas phase relations for protonic solid-electrolyte fuel cells

Gas phase relations are applied to a protonic solid-electrolyte fuel cell operating at 300°C. Kinetic theory and diffusion are considered with the electrodes subjected to a static gas pressure. For hydrogen and oxygen flow to the anode and cathode, respectively, gas flow parameters are investigated for this dynamic system. The relative effects of the static and dynamic gases are assessed and it is found that the rate of molecular incidence at the electrode surface from gas flow is insignificant relative to that caused by thermal motion during laminar flow. The implications of this result for fuel cell design are discussed.Molecular mean free paths are discussed in relation to the electrode morphological parameter of pore size. Diffusion coefficients and corresponding limiting current densities are evaluated for three processes. It is found that for electrodes with 10% porosity the rate-determining steps are caused by activation or ohmic polarization rather than by concentration polarization.Nomenclature A area - D Fick's law diffusion coefficient - D _1,1 coefficient for the self-diffusion of like molecules - D _1,2 coefficient for the interdiffusion of two gas species - D _k Knudsen diffusion coefficient - d diameter of gas flow area - F Faraday's constant - f volume flow rate at 300°C, 1 atm - f volume flow rate as measured by mass flow controller - k Boltzmann's constant - I current - J current density - J _L limiting current density - J _m molar flux - M ₁ gram molecular weight of gas species one - M ₂ gram molecular weight of gas species two - m molecular mass - N _v number of moles per unit volume - n number of molecules per unit volume - n _e number of electron transfers - P gas pressure - P _i partial pressure of gas species - p porosity - Q total charge - r electrode pore radius - R electrical resistance - Re Reynolds number - R _g gas constant - T gas temperature - t time - v linear velocity of gas - average molecular velocity - average molecular velocity of species one - average molecular velocity of species two - V gas volume - V _cell cell voltage - V _c equilibrium voltage - V _act activation polarization - V _conc concentration polarization - z number of molecules striking a surface perpendicular to the flow direction per unit area and time - z number of molecules striking a surface perpendicular to the incident direction per unit time resulting from thermal velocity - ₁ molecular diameter of species one - ₂ molecular diameter of species two - _1.2 average molecular diameter of species one and two - _1.1 mean free path between collisions of like molecules - _1.2, _2.1 mean free path of one species between collisions with a second species - _1.2, ₁, _1.2, ₂ mean free path for one species when colliding with either species one or two - mass density - tortuosity     

Development of La(CrCoFeNi)O3 system perovskites as interconnect and cathode materials for solid oxide fuel cells

The purpose of this research is to develop interconnect and cathode materials for use in solid oxide fuel cells (SOFCs) which demonstrate desired properties of outstanding sintering properties, high electrical conductivity, and excellent chemical stability at high temperatures. Five different perovskite oxides of lanthanum in combination with chromium, iron, cobalt and nickel oxides powders, i.e. LaCr_0.7Co_0.1Fe_0.1Ni_0.1O₃(LCr7CFN), LaCo_0.7Cr_0.1Fe_0.1 Ni_0.1O₃(LCo7CFN), LaFe_0.7Cr_0.1Co_0.1Ni_0.1O₃(LFe7CCN), LaNi_0.7Cr_0.1Co_0.1Fe_0.1O₃(LNi7CCF), and LaCr_0.25Co_0.25Fe_0.25Ni_0.25O₃(LCCFN), were synthesized through the Pechini method. XRD results show that all materials are in single phase, either rhombohedral or orthorhombic crystal structure. The resulting powders were able to be sintered to a high relative density at a temperature of 1400 °C for 2 h in air. The electrical conductivity of the sintered sample was measured and evaluated from 300 °C to 800 °C. The LCCFN sample appears to have the best combination of sintering property (approximate 94% relative density) and electrical conductivity (88.13 Scm⁻¹ at 800 °C).     

Preparation and performance of Na-doped La2Ce2O7 electrolytes for protonic ceramic fuel cells

The protonic material La₂Ce₂O₇ exhibits good tolerance to H₂O and CO₂ compared to BaCeO₃-based materials and has become increasingly popular for operation at low-to-intermediate temperatures in protonic ceramic fuel cells. In this work, doping La₂Ce₂O₇ with Na in a series with varying compositions is studied. All of the precursors are prepared by a common citrate-nitrate combustion method. X-ray diffraction images reveal that all of the La_2-xNa_xCe₂O_7-δ samples have a cubic structure. The La_2-xNa_xCe₂O_7-δ pellets are characterized by scanning electron microscopy and are observed to be dense without holes. The effects of Na-doping on the La₂Ce₂O₇ electrical conductivity are carefully investigated in air at 350–800 °C and 5%H₂-95% Ar environments at 350–700 °C. It is found that different levels of Na doping in La₂Ce₂O₇ are conducive to improving the electrical conductivity and sinterability. Among the pellets, La_1.85Na_0.15Ce₂O_7-δ exhibited the highest electrical conductivity in air and 5% H₂-95% Ar atmospheres. Anode-supported half cells with La_1.85Na_0.15Ce₂O_7-δ electrolyte are also fabricated via a dry-pressing process, and the corresponding single cell exhibited a desirable power performance of 501 mW cm⁻² at 700 °C. The results demonstrate that La_1.85Na_0.15Ce₂O_7-δ is a promising proton electrolyte with high conductivity and sufficient sinterability for use in protonic ceramic fuel cells operating at reduced temperatures.     

The rapid one-step fabrication of bilayer anode for protonic ceramic fuel cells by phase inversion tape casting

The bilayer anode fabricated by phase inversion tape casting has an excellent microstructure for protonic ceramic fuel cell compared with the dry pressing method. The large diameter and straight hole structure facilitates the fuel gas transportation thus eliminates the concentration polarization loss. But a dense skin layer (70 μm) results in a power density of only 150 mWcm⁻² at 600 ℃. The anode added with 10 wt% corn starch could eliminate the skin layer, but cause a mismatch between electrolyte suspension and anode. To resolve the mismatch between electrolyte and anode, an anode functional layer (AFL) is employed with different content of corn starch (10−40 wt%). Finally, the single cell with optimized bilayer anode has a maximum power density of 574 mWcm⁻² at 650 ℃. This work provides an easy and rapid method for the preparation of planar protonic ceramic fuel cell with satisfactory performance at intermediate temperature range.     

Polysulfone ionomers for proton-conducting fuel cell membranes: 2. Sulfophenylated polysulfones and polyphenylsulfones

Polysulfones and polyphenylsulfones having pendant phenyl groups with sulfonic acid units have been prepared by lithiation of the respective polymer, followed by reaction with 2-sulfobenzoic acid cyclic anhydride. The resulting ionomers were cast into membranes and properties such as thermal stability, ion-exchange capacity, water sorption and proton conductivity were evaluated. These membranes proved to have a high thermal stability, with a decomposition temperature between 300 and 350 °C, and a high proton conductivity, 60 mS/cm at 70 °C for a polyphenylsulfone with 0.9 sulfonic acid group per repeating unit measured at 100% relative humidity. Moreover, some of the membranes endured immersion in water at temperatures ranging from 20 to 150 °C without swelling extensively, and therefore kept their mechanical stability under these conditions. It was also shown that these membranes retained a high conductivity up to 150 °C under humidifying conditions. The combination of properties make these membranes potential candidates for fuel cells operating at temperatures above 100 °C.     

Scanning electron microscopic studies of porous carbon electrodes used in alkaline fuel cells

Multilayer, polytetrafluoroethylene (PTFE)-bonded gas diffusion-type electrodes were prepared by the rolling method. Changing the electrode structure and manufacturing method improved alkaline fuel cell performance. Activated carbon or carbon black was used as the support material, with platinum as a catalyst and nickel screen as the backing material. Double-layer electrodes possessed both active and diffusion layers on the backing layer. However, the single-layer electrodes had only the active layer on the backing layer. The electrodes were prepared by using different PTFE contents and platinum loadings. In this study the surface photographs of the electrodes were taken with a scanning electron microscope. Elemental analyses of the surface elements were performed by energy dispersive X-ray spectroscopy (EDXS). Electrodes having activated carbon on their surfaces were observed to possess a nonuniform and porous structure. These electrodes showed better performance than electrodes having carbon black, which presented a uniform and nonporous structure.     

Highly durable Sr-doped LaMnO3-based cathode modified with Pr6O11 nano-catalyst for protonic ceramic fuel cells based on Y-doped BaZrO3 electrolyte

Sr-doped LaMnO₃ (LSM) is one of the state-of-the-art cathodes with outstanding chemical stability but suffers from poor electrochemical activity. It is promising that LSM can be used as good cathode for protonic ceramic fuel cells (PCFCs), provided that its catalytic activity can be improved. Herein, we have developed a high-performance and durable LSM-based cathode used for PCFCs with stable Y-doped BaZrO₃ electrolyte. In this work, commercial LSM cathode is mixed with high-performance BaZr_0.85Y_0.15O_3-δ particles. Pr₆O₁₁ nano-catalysts are homogeneously dispersed into the composite cathode, resulting in a significant decrease in the cathode polarization resistance from 0.51 to 0.12 Ω·cm² at 600 °C. The power output improves by ~96% at 600 °C. Distribution of relaxation time analysis indicates that the processes of oxygen adsorption/dissociation and oxygen species diffuse to triple phase boundary sites are significantly promoted by Pr₆O₁₁. Furthermore, the 100 h stability tests show that the modified cathode is extremely stable.     

Functionalized carbon support with sulfonated polymer for direct methanol fuel cells

Recently electrodes for direct methanol fuel cell (DMFC) have been developed for getting high fuel cell performances by controlling composition of catalysts and sulfonated polymers, developing catalyst particles, modifying carbon supports, etc. The electrodes in DMFCs are porous layers, which are composed of catalyst, which is black or carbon supported, and sulfonated polymers in a blended form. In the present study, carbon support for catalysts was functionalized to play dual roles of a mass transport and a catalyst support. The functionalized carbon support was characterized and compared with pristine one by thermal and spectroscopic analysis, and loading of platinum (Pt) catalysts on modified support was performed by gas reduction. The electrodes with Pt on functionalized carbon support were fabricated, though the conventional electrodes were prepared with sulfonated polymer and Pt catalysts. Membrane electrode assembly with Pt catalyst on functionalized support showed a higher DMFC performance of 30 mW cm⁻² at 50 °C without using additional sulfonated polymer. Integration of electrode components in one body has another advantage of easier and simpler process in preparing electrodes for DMFCs. Improved DMFC performance of the electrode containing functionalized carbon was be attributed to a better mass transport which maximize the catalytic activities.     

Two new siloxanic proton-conducting membranes: Part I. Synthesis and structural characterization

The development of stable polymer electrolytes having good proton conductivity, low cost and operating at medium temperatures represent a crucial step in the evolution of polymer electrolyte fuel cells. We describe two new siloxanic proton-conducting membranes that were synthesized through a two-stage protocol. In the first stage, a poly(methyl hydrosiloxane) precursor (P) bearing siloxane side chains with sulfonic acid groups was prepared. In the second step, the hydrolysis of pristine precursor or its derivative obtained by grafting siloxane chains on P yielded two types of membranes with the formulas {Si(CH₃)₃O[Si(CH₃)HO]_21.26[Si(CH₃)((CH₂)₃SO₃H)O]_1.8[Si(CH₃)((CH₂)₃Si(CH₃)₂O)O]₁₄Si(CH₃)₃}_n (A) and {Si(CH₃)₃O[Si(CH₃)HO]_21.26[Si(CH₃)((CH₂)₃SO₃H)O]_1.8[Si(CH₃)((CH₂)₃(Si(CH₃)₂O)_w)O]_v[Si(CH₃)((CH₂)₃Si(CH₃)₂O-)O]_14 − vSi(CH₃)₃}_n (B), with w = 20.31. Polymer membranes of A and B were prepared by means of a hot-pressing process at 80 °C and 10 t/cm². Scanning electron microscopy showed that A and B are rubbery materials with rough and transparent surfaces. Thermogravimetric investigations performed under air atmosphere disclosed that A and B are thermally stable up to at least 198 °C. DSC measurements yielded T_g(s) of −44 and −60 °C for A and B, respectively. The polymers exhibit ionic exchange capacities of 0.33 (A) and 0.15 meq/g (B). FT-IR and FT-Raman investigations revealed that the polymers consist of reticulated siloxane networks with pendant silicone chains having sulfonic acid groups.     

Bifunctional electrodes for unitised regenerative fuel cells

The effects of different configurations and compositions of platinum and iridium oxide electrodes for the oxygen reaction of unitised regenerative fuel cells (URFC) are reported. Bifunctional oxygen electrodes are important for URFC development because favourable properties for the fuel cell and the electrolysis modes must be combined into a single electrode. The bifunctional electrodes were studied under different combinations of catalyst mixtures, multilayer arrangements and segmented configurations with single catalyst areas. Distinct electrochemical behaviour was observed for both modes and can be explained on the basis of impedance spectroscopy. The mixture of both catalysts performs best for the present stage of electrode development. Also, the multilayer electrodes yielded good results with the potential for optimisation. The influence of ionic and electronic resistances on the relative performance is demonstrated. However, penalties due to cross currents in the heterogeneous electrodes were identified and explained by comparing the performance curves with electrodes composed of a single catalyst. Potential improvements for the different compositions are discussed.     

Cu-based amorphous alloy electrodes for fuel cells

The present work describes the characteristics of Cu–Zr and Cu–Ti amorphous alloys as catalysts for oxidation of methanol and formaldehyde in alkaline solutions. Two Cu-based amorphous alloys, Cu₆₀Zr₄₀ and Cu₆₀Ti₄₀ both prepared by melt spinning were investigated. Two types of electrode were used: as-quenched amorphous alloys and surface-activated amorphous alloys. The surface-activation treatment consisted in the immersion of the ribbons in 1 M HF solution for 30 s. The determination of the oxidation activity of methanol and formaldehyde was carried out by galvanostatic and by steady-state polarization measurements and cyclic voltammetry experiments in 1 M NaOH solutions containing CH₃OH or CH₂O, deoxygenated by nitrogen bubbling for 30 min at 30 °C. The HF-treated amorphous alloys exhibit catalytic activity only for formaldehyde oxidation; Zr-based alloys show higher current densities than pure crystalline copper and Ti-based alloys.     

Perovskite and A2MO4-type oxides as new cathode materials for protonic solid oxide fuel cells

Solid state ionic devices based on high-temperature proton conductors can be used for various applications, especially in a new class of fuel cells, the Protonic Ceramic Fuel Cell (SOFC-H⁺). These systems are currently operating at intermediate temperatures (500-600 °C) and one of the major problems is the overpotential at the cathode side. In this context, various perovskite oxides AMO_3 − δ (A = La, Ba, Sr; M = Mn; Fe, Co, Ni) and A₂MO₄-type compounds (A = La, Nd, Pr or Sr; M = Ni) have been investigated. Their properties under moist cathodic atmosphere have been studied. Actually, they are stable and exhibit high electrical conductivity (σ > 100 S cm⁻¹) as well as good electrocatalytic properties towards oxygen reduction.The electrochemical properties of these oxides deposited on the protonic electrolyte BaCe_0.9Y_0.1O_3 − δ have been studied and the Area Specific Resistances have been measured under air/H₂O (3%) atmosphere. The obtained values at 600 °C, especially for Ba_0.5Sr_0.5Fe_0.8Co_0.2O_3 − δ and Pr₂NiO_4 + δ show to be promising cathode materials for Protonic Ceramic Fuel Cell applications.     

Thermo-mechanical analysis of 3D manufactured electrodes for solid oxide fuel cells

Additive manufacturing has widened the scope for designing more performing microstructures for solid oxide fuel cells (SOFCs). Structural modifications, such as the insertion of ceramic pillars within the electrode, facilitate ion transport and boost the electrochemical performance. However, questions still remain on the related mechanical requirements during operation. This study presents a comprehensive thermal-electrochemical-mechanical model targeted to assess the stress distribution in 3D manufactured electrodes. Simulations show that a dense pillar increases the stress distribution by ca. 10 % compared to a flat electrode benchmark. The stress is generated by the material thermal contraction and intensifies at the pillar-electrolyte junction while external loads have negligible effects. An analysis on manufacturing inaccuracies indicates that sharp edges, surface roughness and tilted pillars intensify the stress; nonetheless, the corresponding stress increase is narrow, suggesting that manufacturing inaccuracies can be easily tolerated. The model points towards robust design criteria for 3D manufactured electrodes.