Electrocatalytic activity of perovskite-based cathodes for solid oxide fuel cells

A very active cathode material for intermediate temperature - solid oxide fuel cells (IT-SOFCs) is obtained by mixing La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ (LSCF) and Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3-δ (BSCF) powders. Three different volume ratios are considered: BSCF-LSCF 50-50 v/v% (BL50), BSCF-LSCF 70-30 v/v% (BL70) and 30–70 v/v% (BL30).The electrodes are slurry coated on Ce_0.8Sm_0.2O_2-δ electrolyte and sintered at 1100 °C. After the sintering step XRD-analyses highlight relevant cation inter-diffusion within the mixed powders. As a result, an enhanced activity of BL30-BL70 electrodes towards oxygen reduction reaction is detected in comparison to LSCF or BSCF pure powders. A polarization resistance of 0.021 Ω cm2 at 650 °C for BL70 is obtained, one of the lowest value reported in literature for SOFC cathodes. Furthermore, all the electrodes show lower activation energy than the two reference materials in the considered temperature range (500–650 °C) and two different kinetic regimes are identified at the extremes of this range. Effect of the applied overpotential (0–0.3 V) on the electrode kinetic is also investigated.After a preliminary ageing, performed at 650 °C for 200 h by applying a current density of 200 mA cm-2, the electrodes preserve a remarkable performance as IT-SOFC cathodes, despite an initial degradation. A stable value of 0.048 Ω cm2 of polarization resistance for the sample richer in BSCF is recorded.     

Oxygen reduction and transportation mechanisms in solid oxide fuel cell cathodes

In recent years, various models have been developed for describing the reaction mechanisms in solid oxide fuel cell (SOFC) especially for the cathode electrode. However, many fundamental issues regarding the transport of oxygen and electrode kinetics have not been fully understood. This review tried to summarize the present status of the SOFC cathode modeling efforts, and associated experimental approaches on this topic. In addition, unsolved problems and possible future research directions for SOFC cathode kinetics had been discussed.     

Thermostable ionomeric filled membrane for H2/O2 fuel cell

Dispersion of submicronic particles of phosphatoantimonic acid fillers (H3) with a 4.3 H⁺/kg (4.3 eq/kg) cationic exchange capacity (cec), in a solution of sulfonated polysulfone (PSS) of 1.07 H⁺/kg gives viscous suspension allowing the `filled' material to be shaped in thin films. From the conductivity measurements, a synergic effect between the acidity of PSS and H3 has been highlighted. Conductivity values close to those of the Nafion 117 have been determined in the same experimental conditions, i.e., 96% relative humidity at 80°C. Furthermore, the inorganic filler improves both the mechanical strength and the gas impermeability of the filled membrane as compared to an unfilled PSS membrane. A PSS–H3 membrane of 1.07 H⁺/kg cec filled with 7.1% in H3 provided 80% of Nafion performances in a H₂/O₂ fuel cell for 500 h at 80°C and 4 bars pressure of H₂ and O₂.     

Mesoporous silica template-derived nickel-cobalt bimetallic catalyst for urea oxidation and its application in a direct urea/H2O2 fuel cell

Ni-based catalysts are considered as an efficient anode material for urea fuel cells due to the low cost and high activity in alkaline media. Herein, we demonstrate that Ni-Co bimetallic hydroxide particles with highly porous nanostructures can be synthesized using mesoporous silica nanoparticles as templates. The replicated nanostructures of the Ni-Co hydroxide samples from the mesoporous silica templates are observed. The porous Ni_0.8Co_0.2(OH)₂ particles exhibited considerably enhanced electro-catalytic activity for urea oxidation reaction by providing a high surface area and fast mass transport for urea oxidation reaction. It is also found that the Co-doping at 20% significantly reduce the overpotential and increase the peak current of urea oxidation reaction. A direct urea/H₂O₂ fuel cell with the porous Ni_0.8Co_0.2(OH)₂ as anode material shows an excellent performance with maximum power densities of 11.2 and 25.6 mW cm⁻² at 20 °C and 70 °C with 0.5 M urea in 5 M KOH, respectively. Thus, this work suggests that the highly porous Ni_0.8Co_0.2(OH)₂-derived from the mesoporous silica templates can be used for urea oxidation and as an efficient anode material for urea-based fuel cells.     

Deactivation of Pt/VC proton exchange membrane fuel cell cathodes by SO2, H2S and COS

Sulfur contaminants in air pose a threat to the successful operation of proton exchange membrane fuel cells (PEMFCs) via poisoning of the Pt-based cathodes. The deactivation behavior of commercial Pt on Vulcan carbon (Pt/VC) membrane electrode assemblies (MEAs) is determined when exposed to 1 ppm (dry) of SO₂, H₂S, or COS in air for 3, 12, and 24 h while held at a constant potential of 0.6 V. All the three sulfur compounds cause the same deactivation behavior in the fuel cell cathodes, and the polarization curves of the poisoned MEAs have the same decrease in performance. Sulfur coverages after multiple exposure times (3, 12, and 24 h) are determined by cyclic voltammetry (CV). As the exposure time to sulfur contaminants increases from 12 to 24 h, the sulfur coverage of the platinum saturates at 0.45. The sulfur is removed from the cathodes and their activity is partially restored both by cyclic voltammetry, as shown by others, and by successive polarization curves. Complete recovery of fuel cell performance is not achieved with either technique, suggesting that sulfur species permanently affect the surface of the catalyst.     

Highly active robust oxide solid solution electro-catalysts for oxygen reduction reaction for proton exchange membrane fuel cell and direct methanol fuel cell cathodes

The identification and development of novel non-noble metals based electro-catalyst exhibiting excellent electrochemical activity and stability than noble metal electro-catalyst is important for commercial development of proton exchange membrane fuel cells (PEMFCs). Such non-noble electro-catalyst with unique electronic structure and superior electrochemical performance will immensely contribute to lowering the capital cost of PEMFCs. Herein, we have identified solid solution electro-catalysts of WO₃ and IrO₂ for oxygen reduction reaction (ORR) in PEMFCs exploiting theoretical first principles approaches. The theoretical results were experimentally validated by generation of nanostructured (W_1-xIr_x)O_y (x = 0.2, 0.3; y = 2.7–2.8) electro-catalysts for ORR. (W_0.7Ir_0.3)O_y demonstrated ～43% improved electrochemical activity than Pt/C with similar loading at 0.9 V (vs RHE), respectively. Moreover, single full cell PEMFC study revealed an acceptable ～81% improved maximum power density for (W_0.7Ir_0.3)O_y than Pt/C combined with excellent long term stability. These results thus, show the potential of (W_0.7Ir_0.3)O_y as ORR electro-catalyst for replacing of Pt/C in PEMFCs and direct methanol fuel cells on the additional grounds of superior methanol tolerance.     

Nanotubes of rare earth cobalt oxides for cathodes of intermediate-temperature solid oxide fuel cells

In this work we studied the electrochemical properties of cathodes for intermediate-temperature solid oxide fuel cells (IT-SOFCs) prepared with nanotubes of La_0.6Sr_0.4CoO₃ (LSCO). Their nanostructures consist of agglomerated nanoparticles in tubular structures of sub-micrometric diameter. The resulting cathodes are highly porous both at the micro- and the nanoscale. This fact increases significantly the access to active sites for the oxygen reduction.We investigated the influence of the diameter of the precursor nanotubes on the polarization resistance of the LSCO cathodes on CeO₂-10 mol.% Sm₂O₃ (SDC) electrolytes under air atmosphere, evaluated in symmetrical [LSCO/SDC/LSCO] cells. Our results indicate an optimized performance when the diameter of precursor nanotubes is sufficiently small to become dense nanorods after cathode sintering.We present a phenomenological model that successfully explains the behavior observed and considers that a small starting diameter acts as a barrier that prevents grains growth. This is directly related with the lack of contact points between nanotubes in the precursor, which are the only path for the growth of ceramic grains.We also observed that a conventional sintering process (of 1 h at 1000 °C with heating and cooling rates of 10 °C min⁻¹) has to be preferred against a fast firing one (1 or 2 min at 1100 °C with heating and cooling rates of 100 °C min⁻¹) in order to reach a higher performance. However, a good adhesion of the cathode can be achieved with both methods.Our results suggest that oxygen vacancy diffusion is enhanced while decreasing LSCO particle size. This indicates that the high performance of our nanostructured cathodes is not only related with the increase of the number of active sites for oxygen reduction but also to the fact that the nanotubes are formed by nanoparticles.     

Performance studies of solid oxide fuel cell cathodes in the presence of bare and cobalt coated E-brite alloy interconnects

The electrochemical performances of La_0.6Sr_0.4Co_0.2Fe_0.8O₃ (LSCF) electrodes were studied by half-cell measurements in the absence of chromia-forming alloy, in the presence of bare and Co coated E-brite alloy interconnects, respectively. The surface and cross-section properties of the bare and Co coated E-brite alloys, and LSCF electrodes were characterized by scanning electron microscopy (SEM), energy dispersive X-ray (EDX) analysis, and electron probe microanalysis (EPMA). The results showed a rapid degradation in LSCF performance when the bare E-brite alloy was used as interconnect. The growth of chromia scale on the E-brite alloy and the increase of Cr content throughout the LSCF electrode were observed. The uniform and dense Co coating process was developed to coat the E-brite alloy by using both electroless and electrodeposition methods. It was demonstrated that the Co layer effectively mitigates the Cr migration, leading to improved electrochemical stability of LSCF electrodes.     

A flexible on-fiber H2O2 microfluidic fuel cell with high power density

To promote the simplification and integration of membraneless microfluidic fuel cell (MMFC) system and combine with flexible portable devices, a flexible on-fiber MMFC exploiting H₂O₂ as sole reactant is presented, eliminating the separation requirement of fuel and oxidant. Nickel (Ni) nano-particles and Prussian blue with multiwalled carbon nanotube (PB-MWCNT) are coated on hydrophilic braided carbon fibers (BCFs) to serve as the anode and cathode, respectively. The three-dimensional (3D) flow-through anode and cathode with a wealth of exposed electroactive sites improve reactant mass transfer. The anode and cathode are respectively wound on both sides of the middle cotton thread-based flow channel for separation. Under the combination of capillary force and gravity, reactants flow continuously through the fiber-based microchannels without external pumps. Importantly, the H₂O₂ MMFC achieves the highest maximum power density (MPD) of 14.41 mW cm^?2 so far in one-chamber or single-stream H₂O₂ fuel cells. Besides, no serious deterioration in the power-generation performance is observed in complex practical operating conditions including bending with various angles, repeated folding and dropping. Three presented flexible MMFCs are connected to power a handheld calculator, indicating the tremendous potential of developing micro power supplies based on abundant flexible materials as well as green and sustainable energy.     

Pd-YSZ composite cathodes for oxygen reduction reaction of intermediate-temperature solid oxide fuel cells

Pd-Y₂O₃ stabilized ZrO₂ (YSZ) composite cathodes are prepared by conventional mechanical mixing and infiltration methods. In the case of infiltration, thermal decomposition and chemical reduction processes are used to form Pd particles on the YSZ scaffold. The phase structure, morphology and electrochemical performance of the Pd-YSZ composite cathodes are characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM) and electrochemical impedance spectroscopy (EIS). The performance of mechanically mixed Pd-YSZ composite cathodes is inadequate due to significant growth and sporadical distribution of Pd particles. The 5 wt.% Pd-loaded cathode prepared by infiltration-thermal decomposition process shows the lowest polarization resistance, i.e. between 0.042 Ω cm² and 1.5 Ω cm² in the temperature range of 850-600 °C, benefited from the formation of nano-sized Pd particles and the presence of well connected Pd network. The effect of Pd loading on the performance of the infiltrated-thermal decomposed Pd-YSZ composite cathodes is also evaluated, 5 wt.% Pd loading results in the lowest polarization resistances.     

Influence of fuel ratio on the performance of combustion synthesized bifunctional cobalt oxide catalysts for fuel cell application

Solution combustion synthesis was used to prepare cobalt oxide nanoparticles at different fuel ratio (φ = 0.5, 1, and 1.75). The synthesized particles were characterized using XRD, SEM, TEM, FTIR and XPS to study the morphological and structural features. The fuel rich condition provides a reducing atmosphere limiting further oxidation of synthesized nanoparticles but produces more carbon residue on the catalyst surface compared to fuel lean conditions. Increasing the fuel ratio (φ value) from 0.5 to 1.75 increases the crystallite size and lowers the surface area. The electrocatalytic performance studies conducted by cyclic voltammetry (CV) and linear sweep voltammetry (LSV) indicate significant changes in catalytic activities due to variation in synthesis conditions. The LSV results obtained between potential of −1.2 V and 0.75 V shows all the three cobalt oxide catalysts to have bifunctional properties of being active for both oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), with Co synthesized at lower fuel ratio (φ = 0.5) displaying the highest current density. The onset potential for Co (φ = 0.5) is more positive than Co (φ = 1) and Co (φ = 1.75). The kinetic current density for Co (φ = 0.5) is 6.45 mA cm⁻² and decreases with increase in fuel ratio. The OER current starts at ∼0.45 V for all the catalysts showing maximum density for Co (φ = 0.5) and gradually decreasing for catalysts synthesized at higher fuel ratio.     

Electrocatalytic reduction of dioxygen on PdCu for polymer electrolyte membrane fuel cells

The present research is aimed to study the oxygen reduction reaction (ORR) on a PdCu electrocatalyst synthesized through reduction of PdCl₂ and CuCl with NaBH₄ in a THF solution. Characterization of PdCu electrocatalyst was performed by X-ray diffraction (XRD), transmission electron microscopy (TEM) and energy-dispersive X-ray (EDX) spectroscopy. Characterization results showed that the synthesis method produced spherical agglomerated nanocrystalline PdCu particles of about 10 nm size. The electrochemical activity was evaluated using cyclic voltammetry (CV), rotating disc electrode (RDE) and electrochemical impedance spectroscopy (EIS) in a 0.5 M H₂SO₄ electrolyte at 25 °C. The onset potential for ORR on PdCu is shifted by ca. 30 mV to more positive values and enhanced catalytic current densities were observed, compared to that of pure Pd catalyst. The synthesized PdCu electrocatalyst dispersed on a carbon black support was tested as cathode electrode in a membrane-electrode assembly (MEA) achieving a power density of 150 mW cm⁻² at 0.38 V and 80 °C.     

Raney-platinum film electrodes for potentially implantable glucose fuel cells. Part 2: Glucose-tolerant oxygen reduction cathodes

We report the fabrication and characterization of glucose-tolerant Raney-platinum cathodes for oxygen reduction in potentially implantable glucose fuel. Fabricated by extraction of aluminum from 1 μm thin platinum-aluminum bi-layers annealed at 300 °C, the novel cathodes show excellent resistance against hydrolytic and oxidative attack. This renders them superior over previous cathodes fabricated from hydrogel-bound catalyst particles. Annealing times of 60, 120, and 240 min result in approximately 400-550 nm thin porous films (roughness factors ∼100-150), which contain platinum and aluminum in a ratio of ∼9:1. Aluminum release during electrode operation can be expected to have no significant effect on physiological normal levels, which promises good biocompatibility. Annealing time has a distinct influence on the density of trenches formed in the cathode. Higher trench densities lead to lower electrode potentials in the presence of glucose. This suggests that glucose sensitivity is governed by mixed potential formation resulting from oxygen depletion within the trenches. During performance characterization the diffusion resistance to be expected from tissue capsule formation upon electrode implantation was taken into account by placing a membrane in front of the cathode. Despite the resulting limited oxygen supply, cathodes prepared by annealing for 60 min show more positive electrode potentials than previous cathodes fabricated from hydrogel-bound activated carbon. Compared to operation in phosphate buffered saline containing 3.0 mM glucose, a potential loss of approximately 120 mV occurs in artificial tissue fluid. This can be reduced to approximately 90 mV with a protective Nafion layer that is easily electro-coated onto the Raney-platinum film.     

Characterisation of Co-based electrocatalytic materials for O2 reduction in fuel cells

Different cobalt containing materials obtained from the thermal decomposition of two organic precursors, cobalt–acetylacetonate and cobalt–tetrametoxyphenylporphyrin, and Co₃O₄ prepared by thermal decomposition of cobalt carbonate have been characterised by thermogravimetric analysis (TGA), X-ray diffraction (XRD) and high resolution transmission microscopy (HRTEM). It has been found that the cobalt precursor, the organic macrocycle and the active carbon are simultaneously needed during the heat treatment in inhert atmosphere. Their contemporaneous presence is useful to obtain a material on which the dispersed metal is able to catalyse reactions that produce pores or tunnels inside the carbon, thus resulting in an enhanced contact area. The organic macrocycle is only partially decomposed and the residual fraction could enhance the conductivity.     

Contribution of fuel cell systems to CO2 emission reduction in their application fields

Fuel cells (FCs) and their hybrid systems can play a key role in reducing carbon dioxide (CO₂) emissions. The present paper analyzes the contributions of the FC system to CO₂ emission reduction in three application fields.In the mobile application field, the direct methanol FC system has little or no influence on CO₂ emission reduction.The benefit of the FC in CO₂ emission reduction in the transportation field is directly dependant on the H₂ production method. Pre-combustion technology (with carbon capture) represents one of the best mid-term solutions for H₂ production. If FC vehicles (FCVs) use the H₂ produced by this process, the CO₂ emissions in this field could be decreased to 70–80% of the traditional CO₂ emissions.In the stationary application field, the FC system can be effectively operated as the distributed generation (DG) in terms of CO₂ emission reduction. Among the various types of FC or FC hybrid system used for DG, the solid oxide FC (SOFC) hybrid system with a CO₂ capture unit is the best option as it doubled the electricity efficiency compared to the traditional combustion cycle and decreases the CO₂ emission to 13.4% of the traditional CO₂ emission.However, the FC and carbon capture and sequestration (CCS) technologies need to be fully developed before the FC can contribute to reducing CO₂ emissions.     

Electrocatalytic reduction of platinum phosphate blue on carbon surfaces: A novel method for preparing fuel cell electrodes

An eletrodeposition process for platinum metal onto carbon surfaces through an electrocatalyzed reduction of a mixed valence oligomeric platinum phosphate blue is described. Cyclic voltammograms of the platinum phosphate blue revealed adsorption of the platinum substrate at the electrode surface followed by reduction first to Pt(II) and then to metallic platinum. These reductions appear to be electrocatalyzed processes as evidence from the almost featureless voltammograms at the first cycle followed by initial continuous growth of both reduction waves and then a leveling off after a few cycles. At the end of 5–10 cycles depending on the concentrations of the precursor, a thin platinum metal film was observed on the carbon. When pure platinum metal was used as the working electrode, both the reduction waves showed steady current over multiple scans indicating an absence of catalysis. The platinum-coated carbon electrodes function like pure platinum metal electrodes as demonstrated by comparing the cyclic voltammograms of the potassium hexacyanoferrate(III/II) redox system recorded with platinum working electrodes and platinum-coated carbon electrodes. The platinum-coated carbon electrode with a coating of 0.03 mg cm⁻² (geometric area) did not lose its properties even when the electrodes were kept at 2.5 M perchloric acid solution over several days. At the low coating level, a monolayer platinum loading was observed by scanning electron microscopy. These ultra-low platinum loaded electrodes exhibit large active surface areas and have potential for applications in PEM and PAF fuel cells.     

Preparation of honeycomb porous solid oxide fuel cell cathodes by breath figures method

Honeycomb porous LSM/YSZ composite cathodes are prepared using the breath figures (BFs) method with nontoxic and easily available water droplets as templates. They were characterized by scanning electron microscopy (SEM) and electrochemical impedance spectroscopy (EIS). Experimental results indicate that 1) ambient temperature, relative humidity, and thickness of the slurry have significant influence on the morphology of porous membrane; 2) the membrane with a thickness of 35 μm prepared at 35 °C and relative humidity of 70-75% shows the lowest polarization resistance between the LSM/YSZ composite cathode and YSZ electrolyte; and 3) the honeycomb structure of composite cathodes is favorable for lowering the low-frequency resistance concerning diffusion processes at relatively low operating temperatures of 650 and 700 °C.     

Comparison of nanostructured composite cathodes synthesized by liquid self-assembly and nanosolid mechanical-mixing for solid oxide fuel cell

The composite perovskites are promising low/intermediate-temperature solid oxide fuel cell (LT/IT-SOFC) cathode materials, which can combine the complementary advantages of multi-components. Here, novel liquid self-assembly (SA) is compared with nanosolid mechanical-mixing (MM) for the high-performance nanostructured composite. The same composition of the active BaCo_0.96Zr_0.04O_2.6+δ (12hBC, 38 mol%) with the stable BaZr_0.82Co_0.18O_3-δ (BZC, 62 mol%) is designed to compare the effects of different syntheses on composite properties. The BZC-12hBC (SA) can achieve molecular-level contact of multiple phases, which exhibits superior electrical conductivity, surface and heterointerface activity to those of BZC-12hBC (MM). When further applied on LT/IT-SOFC, the cell with BZC-12hBC (SA) cathode achieves the remarkable peak power densities of 2.06–0.24 W cm^?2 at 700–500 °C, while the cell with BZC-12hBC (MM) cathode only exhibits 0.63–0.11 W cm^?2. The electrochemical analysis reveals that BZC-12hBC (SA) cathode possesses better oxygen dissociative adsorption and species transportation than those of the BZC-12hBC (MM) cathode.     

Branched poly(ether ether ketone) based anion exchange membrane for H2/O2 fuel cell

High-performance anion exchange membranes (AEMs) are in need for practical application of AEM fuel cells. Novel branched poly(ether ether ketone) (BPEEK) based AEMs were prepared by the copolymerization of phloroglucinol, methylhydroquinone and 4,4′-difluorobenzophenone and following functionalization. The effects of the branched polymer structures and functional groups on the membrane's properties were investigated. The swelling ratios of all the membranes were kept below 15% at room temperature and had good dimensional stability at elevated temperatures. The branching degree has almost no effect on the dimensional change, but plays a great role in tuning the nanophase separation structure. The cyclic ammonium functionalized membrane showed a lower conductivity but a much better stability than imidazolium one. The BPEEK-3-Pip-53 membrane with the branching degree of 3% and piperidine functionalization degree of 53% showed the best performances. The ionic conductivity was 43 mS cm⁻¹ at 60 °C. The ionic conductivity in 1 M KOH at 60 °C after 336 h was 75% of its initial value (25% loss of conductivity), and the IEC was 83% of its initial value (17% loss of IEC), suggesting good alkaline stability. The peak energy density (60 °C) of the single H₂/O₂ fuel cell with BPEEK-3-Pip-53 membrane reached 133 mW cm⁻² at 260 mA cm⁻².