Effect of sulfur contaminants on MCFC performance

Molten carbonate fuel cells (MCFC) used as carbon dioxide separation units in integrated fuel cell and conventional power generation can potentially reduce carbon emission from fossil fuel power production. The MCFC can utilize CO₂ in combustion flue gas at the cathode as oxidant and concentrate it at the anode through the cell reaction and thereby simplifying capture and storage. However, combustion flue gas often contains sulfur dioxide which, if entering the cathode, causes performance degradation by corrosion and by poisoning of the fuel cell. The effect of contaminating an MCFC with low concentrations of both SO₂ at the cathode and H₂S at the anode was studied. The poisoning mechanism of SO₂ is believed to be that of sulfur transfer through the electrolyte and formation of H₂S at the anode. By using a small button cell setup in which the anode and cathode behavior can be studied separately, the anodic poisoning from SO₂ in oxidant gas can be directly compared to that of H₂S in fuel gas. Measurements were performed with SO₂ added to oxidant gas in concentrations up to 24 ppm, both for short-term (90 min) and for long-term (100 h) contaminant exposure. The poisoning effect of H₂S was studied for gas compositions with high- and low concentration of H₂ in fuel gas. The H₂S was added to the fuel gas stream in concentrations of 1, 2 and 4 ppm. Results show that the effect of SO₂ in oxidant gas was significant after 100 h exposure with 8 ppm, and for short-term exposure above 12 ppm. The effect of SO₂ was also seen on the anode side, supporting the theory of a sulfur transfer mechanism and H₂S poisoning. The effect on anode polarization of H₂S in fuel gas was equivalent to that of SO₂ in oxidant gas.     

A fuel cell with selective electrocatalysts using hydrogen peroxide as both an electron acceptor and a fuel

We have realized a novel hydrogen peroxide fuel cell that uses hydrogen peroxide (H₂O₂) as both an electron acceptor (oxidant) and a fuel. H₂O₂ is oxidized at the anode and reduced at the cathode. Power generation is based on the difference in catalysis toward H₂O₂ between the anode and cathode. The anode catalyst oxidizes H₂O₂ at a more negative potential than that at which the cathode catalyst reduces H₂O₂. We found that Ag is suitable for use as a cathode catalyst, and that Au, Pt, Pd, and Ni are desirable for use as anode catalysts. Alkaline electrolyte is necessary for power generation. The performance of this cell is clearly explained by cyclic voltammograms of H₂O₂ at these electrodes. This cell does not require a membrane to separate the anode and cathode compartments. Furthermore, separate paths are not needed for the fuel and electron acceptor (oxidant). These properties make it possible to construct fuel cells with a one-compartment structure.     

Direct NaBH4/H2O2 fuel cells

A fuel cell (FC) using liquid fuel and oxidizer is under investigation. H₂O₂ is used in this FC directly at the cathode. Either of two types of reactant, namely a gas-phase hydrogen or an aqueous NaBH₄ solution, are utilized as fuel at the anode. Experiments demonstrate that the direct utilization of H₂O₂ and NaBH₄ at the electrodes results in >30% higher voltage output compared to the ordinary H₂/O₂ FC. Further, the use of this combination of all liquid fuels, provides numerous advantages (ease of storage, reduced pumping requirements, simplified heat removal, etc.) from an operational point of view. This design is inherently compact compared to other cells that use gas phase reactants. Further, regeneration is possible using an electrical input, e.g. from power lines or a solar panel. While the peroxide-based FC is ideally suited for applications such as space power where air is not available and a high energy density fuel is essential, other distributed and mobile power uses are of interest.     

Influence of operation conditions on direct NaBH4/H2O2 fuel cell performance

Although hydrogen fuel cells have attracted so much attentions in these years because of the application prospect in electric vehicles, some obstacles have not been solved yet, among which hydrogen storage is one of the biggest. Direct borohydride fuel cell (DBFC) is another choice without hydrogen storage problem because borohydride is used as reactant directly in the fuel cell. In this paper, DBFC performance under different operation conditions was studied including electrolyte membrane type, operation temperature, borohydride concentration, supporting electrolyte and oxidant. Results showed that, with Pt/C and MnO₂ as anode and cathode electrocatalyst, respectively, Nafion^® 117 membrane as electrolyte, 1.0 M, 3.0 M and 6.0 M NaBH₄ and H₂O₂ solution in NaOH as reactant solution, 80 °C operation, the peak power density could reach 130 mW/cm².     

Exploring MnCr2O4–Gd0.1Ce0.9O2-δ as a composite electrode material for solid oxide fuel cell

Symmetrical solid oxide fuel cell (SOFC) adopting the same material at both electrodes is potentially capable of promoting thermomechanical compatibility between near components and lowering stack costs. In this paper, MnCr₂O₄–Gd_0.1Ce_0.9O_2-δ (MCO-GDC) composite electrodes prepared by co-infiltration method for symmetrical electrolyte supported and anode supported solid oxide fuel cells are evaluated at a temperature range of 650–800 °C in wet (3% H₂O) hydrogen and air atmospheres. Without any alkaline earth elements and cobalt, the co-infiltrated MCO-GDC composite electrode shows excellent activity for oxygen reduction reaction but mediocre activity for hydrogen oxidation reaction. With MCO-GDC as the cathode, the Ni-YSZ (Y₂O₃ stabilized ZrO₂) anode supported asymmetrical cell demonstrates a peak power density of 665 mW cm⁻² at 800 °C. The above results suggest MCO-GDC is a promising candidate cathode material for solid oxide fuel cells.     

Mixed ionic electronic conducting perovskite anode for direct carbon fuel cells

The conversion of carbonaceous materials to electricity in a Direct Carbon Fuel Cell (DCFC) offers the most efficient process with theoretical electric efficiency close to 100%. One of the key issues for fuel cells is the continuous availability of the fuel at the triple phase boundaries between fuel, electrode and electrolyte. While this can be easily achieved with the use of a porous fuel electrode (anode) in the case of gaseous fuels, there are serious challenges for the delivery of solid fuels to the triple junctions. In this paper, a novel concept of using mixed ionic electronic conductors (MIEC) as anode materials for DCFCs has been discussed. The lanthanum strontium cobalt ferrite, La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ (LSCF) was chosen as the first generation anode material due to its well known high mixed ionic and electronic conductivities in air. This material has been investigated in detail with respect to its conductivity, phase and microstructural stability in DCFC operating environments. When used both as the anode and cathode in a DCFC, power densities in excess of 50 mW/cm² were obtained at 804 °C in electrolyte supported small button cells with solid carbon as the fuel. The concept of using the same anode and cathode material has also been evaluated in electrolyte supported thick wall tubular cells where power densities around 25 mW/cm² were obtained with carbon fuel at 820 °C in the presence of helium as the purging gas. The concept of using a mixed ionic electronic conducting anode for a solid fuel, to extend the reaction zone for carbon oxidation from anode/electrolyte interface to anode/solid fuel interface, has been demonstrated.     

Electrocatalysts supported on multiwalled carbon nanotubes for direct borohydride–hydrogen peroxide fuel cell

Electrocatalysts of Rh, Ru, Pt, Au, Ag, Pd, Ni, and Cu supported on multiwalled carbon nanotubes for direct borohydride–hydrogen peroxide fuel cells are investigated. Metal/γ-Al₂O₃ catalysts for NaBH₄ and H₂O₂ decomposition tests are manufactured and their catalytic activities upon decomposition are compared. Also, the effects of XC-72 and multiwalled carbon nanotube (MWCNT) carbon supports on fuel cell performance are determined. The performance of the catalyst with MWCNTs is better than that of the catalyst with XC-72 owing to a large amount of reduced Pd and the good electrical conductivity of MWCNTs. Finally, the effect of electrodes with various catalysts on fuel cell performance is investigated. Based on test results, Pd (anode) and Au (cathode) are selected as catalysts for the electrodes. When Pd and Au are used together for electrodes, the maximum power density obtained is 170.9 mW/cm² (25 °C).     

A possible future fuel cell: the peroxide/peroxide fuel cell

Hydrogen peroxide (H₂O₂) and the reduction/oxidation by‐products of peroxide are non‐toxic to humans and the environment. Simple, low‐concentration hydrogen‐peroxide solutions used as fuel and direct peroxide/peroxide fuel cells (DPPFCs) face significant challenges in the development of a new class of power generators. A power density of 10 mWcm^?2 at a cell potential of 0.55 V have been achieved with a DPPFC composed of carbon‐paper‐supported nickel as the anode catalyst and carbon‐paper PbSO₄ as the cathode catalyst. The catalysts have been prepared by electroless deposition. Using non‐precious metals rather than platinum in our FC makes the cell cost effective comparable to that of PEMFCs. Additionally, as a low‐price fuel, H₂O₂ reduces the cost of this FC. Copyright © 2012 John Wiley & Sons, Ltd.     

Alkaline polymer electrolyte fuel cell with Ni-based anode and Co-based cathode

Alkaline polymer electrolyte fuel cells (APEFCs) are a new class of fuel cell that has been expected to combine the advantages of alkaline fuel cells (AFCs) and polymer electrolyte fuel cells (PEFCs). In recent decade, APEFCs have drawn much attention in the fuel cell world. While great efforts have been devoted to the development of high-performance alkaline polymer electrolytes (APEs), prototypes of APEFC using nonprecious metal catalysts in both the anode and the cathode have not been well implemented, except for our previous report where Ni–Cr was used as the anode catalyst and Ag was employed as the cathode catalyst. In the present work, we report our recent progress in this regard. The self-crosslinked quaternary ammonia polysulfone (xQAPS), a high-performance APE that possesses both good ionic conductivity and extremely high dimensional stability, is applied as both the electrolyte membrane and the ionomer impregnated in the electrodes. Carbon-supported Co-polypyrrole (CoPPY/C) is employed as the cathode catalyst and a new Ni-based catalyst, W-doped Ni, is used as the anode catalyst, which features in high oxidation tolerance. H₂–O₂ and H₂-air APEFCs are thus fabricated and show a decent performance with peak power density being 40 and 27.5 mW/cm² at 60 °C, respectively.     

Optimization of a direct carbon fuel cell for operation below 700 °C

Direct carbon fuel cells (DCFCs) are the most efficient technology to convert solid carbon energy to electricity and thus could have a major impact on reducing fuel consumption and CO₂ emissions. The development of DCFCs to commercialisation stage is largely prohibited by their poor power densities due to the high resistive loss from anode. Here, we report a high-performance Sm_0.2Ce_0.8O_1.9 electrolyte-supported hybrid DCFC with Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ cathode and optimised anode configuration. The catalytic oxidation of carbon is improved, which results in an area specific resistance of only 0.41 Ω cm² at 650 °C at the anode. The hybrid DCFC achieves a peak power density of 113.1 mW cm⁻² at 650 °C operating on activated carbon. The stability of the fuel cell has also been improved due to the optimised current collection.     

Combined generation and separation of hydrogen in an electrochemical water gas shift reactor (EWGSR)

Hydrogen rich gas, originating from fossil fuel reforming processes or biomass gasification, contains a significant amount of CO. Typically, the yield of H₂ is increased with subsequent water gas shift units, converting CO to CO₂ and additional H₂. This study describes a new reactor concept enabling the water gas shift reaction and the separation of the generated hydrogen in one process step by using electrical energy. This electrochemical water gas shift reactor applies a H₃PO₄-doped Poly(2,5-benzimidazole) membrane as electrolyte and carbon supported Pt or PtRu as anode catalyst. The reactor operation was investigated at 130 °C and 150 °C with a H₂ free anode feed stream of humidified CO and N₂. The experimental results show the feasibility of the reactor concept, as H₂ was generated at the cathode according to Faradays Law. Anodic PtRu led to lower power demands than Pt. The operation at the two temperatures showed that 130 °C results in a lower electrical power demand while generating an equal amount of H₂. The feasibility of the reactor was assessed using exergy efficiency analysis.     

Electrochemical performances of LiNiCoMn bifunctional electrode for low temperature solid oxide fuel cells

Lithium transition metal oxides LiNi_0.83Co_0.11Mn_0.06O₂ (NCM-83) and LiNi_0.8Co_0.1Mn_0.1O₂ (NCM-811) are prepared and acted as cathodes and bifunctional electrodes for low temperature solid oxide fuel cells with H₂ and CH₄ fuels. The Ni anode-supported cell with NCM-83 cathode exhibits maximum power density (P_max) of 0.72 W cm⁻² with H₂ fuel at 600 °C. The symmetric cell with NCM-83 electrodes shows high P_max of 0.465 W cm⁻² with H₂ fuel and 0.354 W cm⁻² with CH₄ fuel at 600 °C. And the P_max of the cell with NCM-811 as anode and NCM-83 as cathode is 0.204W cm⁻² with H₂ fuel at 600 °C. The oxygen vacancies in NCM materials are conducive to the rapid oxygen ion conduction of the cathode, and in the anodic reduction atmosphere, the NCM materials will generate Ni/Co active particles in situ, proving the NCM materials can be advanced bifunctional electrode materials for hydrogen oxidation reaction and oxygen reduction reaction at low temperature.     

Optimization of BaZr0.1Ce0.7Y0.2O3−δ-based proton-conducting solid oxide fuel cells with a cobalt-free proton-blocking La0.7Sr0.3FeO3−δ-Ce0.8Sm0.2O2−δ composite cathode

BaZr_0.1Ce_0.7Y_0.2O_3−δ (BZCY)-based proton-conducting solid oxide fuel cells (H-SOFC) with a cobalt-free proton-blocking La_0.7Sr_0.3FeO_3−δ-Ce_0.8Sm_0.2O_2-δ (LSF-SDC) composite cathode were fabricated and evaluated. The effect of firing temperature of the cathode layer on the chemical compatibility, microstructure of the cathode and cathode-electrolyte interface, as well as electrochemical performance of single cells was investigated in detail. The results indicated that the cell exhibited the most desirable performance when the cathode was fired at 1000 °C; moreover, at the same firing temperature, the power performance had the least temperature dependence. With humidified hydrogen (∼2% H₂O) as the fuel and ambient air as the oxidant, the polarization resistance of the cell with LSF-SDC cathode fired at 1000 °C for 3 h was as low as 0.074 Ω cm² at 650 °C after optimizing microstructures of the anode and anode-electrolyte interface, and correspondingly the maximum power density achieved as high as 542 mW cm⁻², which was the highest power output ever reported for BZCY-based H-SOFC with a cobalt-free cathode at 650 °C.     

Electrode reaction properties using a reactant gas addition method in a commercial 100 cm2 class solid oxide fuel cell

This work investigates the reaction characteristics of the anode and cathode by overpotential analyses in 100 cm² class planar anode-supported SOFCs. The reactant gas addition (RA) technique was applied to analyse the overpotential, which uses the reactant gas flow rate and partial pressure as parameters due to their variation upon adding a reactant species to an electrode. The anodic overpotential was determined to be made up of mass transfer-induced overpotentials of H₂ and H₂O species. The H₂O species account for the majority of the anodic overpotential at the measured current range i.e., 0–150 mA cm^?2. Thus, the anodic reaction is under an extreme H₂O-induced mass-transfer resistance compared with H₂. The RA method showed that the cathodic overpotential was mainly due to a deficiency of O₂ species in the mass transfer through the gas phase rather than the solid phase. Furthermore, both cathodic and anodic overpotentials depended on gas flow rate and utilisation, indicating a significant gas-phase mass transfer effect.     

High-performance anode for solid acid fuel cells prepared by mixing carbon substances with anode catalysts

Optimization of Pt-based electrode structure is a key to enhance power generation performance of fuel cells and to reduce the Pt loading. This paper presents a new methodology for anode fabrication for solid acid fuel cells (SAFCs) operating at ca. 200 °C. Our membrane electrode assembly for SAFCs consisted of a CsH₂PO₄/SiP₂O₇ composite electrolyte and Pt-based electrodes. To obtain the anode, a commercial Pt/C catalyst and carbon substance, such as carbon black and carbon nanofiber, were mixed. The composite anode with Pt loading = 0.5 mg cm⁻² demonstrated superior current-voltage characteristics to a benchmark Pt/C anode with Pt loading = 1 mg cm⁻². We consider that the mixing of Pt/C catalyst and carbon substrate facilitated H₂ mass transfer and increased the number of active sites.     

Synthesis and characterization of NiO/GDC–GDC dual nano-composite powders for high-performance methane fueled solid oxide fuel cells

GDC (gadolinium-doped ceria) is well known as a high oxygen ionic conductor and is a catalyst for the electrochemical reaction with methane fuel leading to the oxidation of deposited carbon that can clog the pores of the anode and break the microstructure of the anode. NiO/GDC–GDC dual nano-composite powders were synthesized by the Pechini process, which were used as an AFL (anode functional layer) or anode substrates along with a GDC electrolyte and LSCF–GDC cathode. The anodes, AFL, and electrolyte were fabricated by a tape-casting/lamination/co-firing. NiO–GDC anode and NiO/GDC–GDC anode-supported unit cells were evaluated in terms of their power density and durability. As a result, the NiO/GDC–GDC dual nano-composite demonstrated an improved power density from 0.4 W/cm² to 0.56 W/cm² with H₂ fuel/air and from 0.3 W/cm² to 0.56 W/cm² with CH₄ fuel/air at 650 °C. In addition, it could be operated for over 500 h without any degradation with CH₄ fuel.     

Highly carbon– and sulfur–tolerant Sr2TiMoO6−δ double perovskite anode for solid oxide fuel cells

Ni-based cermets are most commonly used anode materials for solid-oxide fuel cells (SOFCs), but poor stability operating on hydrocarbon fuels seriously hampers their commercialization due to carbon deposition and sulfur poisoning. Here, we report a carbon– and sulfur–tolerant double perovskite anode Sr₂TiMoO_6−δ (STMO) combining the characteristics of two simple perovskites of SrTiO₃ and SrMoO₃. The STMO anode exhibits excellent thermal and chemical compatibility with La_0.9Sr_0.1Ga_0.8Mg_0.2O_3–δ (LSGM) and Ce_0.8Sm_0.2O_1.9 (SDC) electrolytes in 5% H₂/Ar. The single cell with STMO anode demonstrates good stability and excellent coking resistance and sulfur tolerance in H₂S-containing syngas during a 60-h period. The maximum power density (P_max) values of a LSGM-electrolyte-supported single cell with STMO anode are 505 and 275 mW cm⁻²at 850 °C in H₂ and H₂S-containing syngas, respectively. The electrochemical performance is further improved by impregnation of Pd nanoparticles, where the P_max values achieve 1009 and 586 mW cm⁻² at 850 °C under the same conditions, respectively, showing great potential as an anode material for SOFCs operating on H₂S-containing syngas. Our study provides a strategy to develop versatile double perovskite materials by combining the relevant characteristics of two separate perovskites.     

Multifunctional layer-perovskite oxide La2-xCexCuO4 for solid oxide fuel cell applications

Materials are always among the first considerations to the development of low temperature solid oxide fuel cells (SOFCs). In this study, we investigate the multifunctionality of a layer-perovskite oxide La_2-xCe_xCuO₄ (LCCO) for its applications in SOFC as cathode, anode and electrolyte. The performances of the LCCO cathode and anode fuel cells are characterized by I–V–P and electrochemical impedance spectra (EIS). Results suggest that LCCO is a good cathode material and it can also deliver impressive anode performance. Though LCCO is noticed to be reduced by H₂ in the anode, the cell performance is relatively stable under multiple times of operation. The existing of ceria and reduced Cu in it may be a reason for its anode catalytic activation. For the application in electrolyte, LCCO is mixed with ionic conductor Ce_0.8Sm_0.2O_2-δ (SDC) in different weight ratios. Differences in power output and open circuit voltage for the cells containing various ratios of LCCO under normal and reverse operation conditions are highlighted. The electronic conductivity of LCCO doesn't bring in electronic leakage if it is kept in a certain range. The multifunctionality of LCCO would enable it to be potentially applied in single layer fuel cell to simplify the structure and fabrication process of SOFC.     

Performance of an anode-supported tubular solid oxide fuel cells stack with two single cells connected by a co-sintered ceramic interconnector

Two anode-supported tubular solid oxide fuel cells (SOFCs) have been connected by a co-sintered ceramic interconnector to form a stack. This novel bilayered ceramic interconnector consists of La-doped SrTiO₃ (La_0.4Sr_0.6TiO₃) and Sr-doped lanthanum manganite (La_0.8Sr_0.2MnO₃), which is fabricated by co-sintering with green anode at 1380 °C for 3 h. La_0.4Sr_0.6TiO₃ (LST) acts as a barrier avoiding the outward diffusion of H₂ to the cathode; while La_0.8Sr_0.2MnO₃ (LSM) prevents O₂ from diffusing inward to the anode. The compatibility of LST and LSM, as well as their microstructure which co-sintered with anode are both studied. The resistances between anode and LST/LSM interconnector at different temperatures are determined by AC impedance spectra. The results have showed that the bilayered LST/LSM is adequate for SOFC interconnector application. The active area is 2 cm² for interconnector and 16 cm² for the total cathode of the stack. When operating at 900 °C, 850 °C, 800 °C with H₂ as fuel and O₂ as oxidant, the maximum power density of the stack are 353 mW cm⁻², 285 mW cm⁻² and 237.5 mW cm⁻², respectively, i.e., approximately 80% power output efficiency can be achieved compared with the total of the two single cells.     

A novel doped CeO2–LaFeO3 composite oxide as both anode and cathode for solid oxide fuel cells

A novel composite oxide Ce(Mn,Fe)O₂-La(Sr)Fe(Mn)O₃ (CFM-LSFM) was synthesized and evaluated as both anode and cathode materials for solid oxide fuel cells. The cell with CFM-LSFM electrodes was fabricated by tape-casting and screen printing technique. The power-generating performance of this cell was comparable to that of the cell with Ni-SSZ anode and LSM-SSZ cathode. During the 120 h long-term test in hydrogen at 800 °C, the performance increased by 8.6% from 256 to 278 mW cm⁻². This was attributed to the decrease of polarization resistance and ohmic resistance during the test. The XRD results showed the presence of Fe, MnO and some unknown second phases after heat-treating the electrode materials in H₂ which may be beneficial to the anode electrochemical process. The gradual decrease of polarization resistance as increasing the current density possibly resulted from the increasing content of water in the anode.