GdBaCo2O5+x layered perovskite as an intermediate temperature solid oxide fuel cell cathode

GdBaCo₂O_5+x (GBCO) was evaluated as a cathode for intermediate-temperature solid oxide fuel cells. A porous layer of GBCO was deposited on an anode-supported fuel cell consisting of a 15 μm thick electrolyte of yttria-stabilized zirconia (YSZ) prepared by dense screen-printing and a Ni–YSZ cermet as an anode (Ni–YSZ/YSZ/GBCO). Values of power density of 150 mW cm⁻² at 700 °C and ca. 250 mW cm⁻² at 800 °C are reported for this standard configuration using 5% of H₂ in nitrogen as fuel. An intermediate porous layer of YSZ was introduced between the electrolyte and the cathode improving the performance of the cell. Values for power density of 300 mW cm⁻² at 700 °C and ca. 500 mW cm⁻² at 800 °C in this configuration were achieved.     

A direct borohydride fuel cell employing Prussian Blue as mediated electron-transfer hydrogen peroxide reduction catalyst

A direct borohydride-hydrogen peroxide fuel cell employing carbon-supported Prussian Blue (PB) as mediated electron-transfer cathode catalyst is reported. While operating at 30 °C, the direct borohydride-hydrogen peroxide fuel cell employing carbon-supported PB cathode catalyst shows superior performance with the maximum output power density of 68 mW cm⁻² at an operating voltage of 1.1 V compared to direct borohydride-hydrogen peroxide fuel cell employing the conventional gold-based cathode with the maximum output power density of 47 mW cm⁻² at an operating voltage of 0.7 V. X-ray diffraction (XRD), Scanning Electron Microscopy (SEM), and Energy Dispersive X-ray Analysis (EDAX) suggest that anchoring of Cetyl-Trimethyl Ammonium Bromide (CTAB) as a surfactant moiety on carbon-supported PB affects the catalyst morphology. Polarization studies on direct borohydride-hydrogen peroxide fuel cell with carbon-supported CTAB-anchored PB cathode exhibit better performance with the maximum output power density of 50 mW cm⁻² at an operating voltage of 1 V than the direct borohydride-hydrogen peroxide fuel cell with carbon-supported Prussian Blue without CTAB with the maximum output power density of 29 mW cm⁻² at an operating voltage of 1 V.     

Performance of a direct methanol alkaline membrane fuel cell

A study of a direct methanol fuel cell (DMFC) operating with hydroxide ion conducting membranes is reported. Evaluation of the fuel cell was performed using membrane electrode assemblies incorporating carbon-supported platinum/ruthenium anode and platinum cathode catalysts and ADP alkaline membranes. Catalyst loadings used were 1 mg cm⁻² Pt for both anode and cathode. The effect of temperature, oxidant (air or oxygen) and methanol concentration on cell performance is reported. The cell achieved a power density of 16 mW cm⁻², at 60 °C using oxygen. The performance under near ambient conditions with air gave a peak power density of approximately 6 mW cm⁻².     

Development of high-performance planar borohydride fuel cell modules for portable applications

Direct borohydride fuel cell (DBFC) as a liquid type fuel cell is promising for portable applications. In this study, we report our recent progress in the micro-fuel cell development. A power density of 80 mW cm⁻² was achieved in passive mode at ambient conditions when using the anode containing nickel, carbon-supported Pd catalyst and Nafion ionomer. Current efficiency was also found to be greatly increased due to the use of Nafion rather than polytetrafluoroethylene (PTFE). Based on improvements on single cell performance, planar multi-cell power modules were assembled to study the feasibility of making high-performance and practical DBFC power units. A power of 2.5 W was achieved in a fully passive eight-cell module after significantly simplifying cell structure.     

Performance characteristics of a novel tubular-shaped passive direct methanol fuel cell

The present work consists of a tubular-shaped direct methanol fuel cell (DMFC) that is operated completely passively with methanol solution stored in a central fuel reservoir. The benefit of a tubular-shaped DMFC over a planar-shaped DMFC is the higher instantaneous volumetric power energy density (power/volume) associated with the larger active area provided by the tubular geometry. Membrane electrode assemblies (MEAs) with identical compositions were installed in both tubular and planar-shaped, passive DMFCs and tested with 1, 2, and 3 M methanol solutions at room temperature. The peak power density for the tubular DMFC was 19.0 mW cm⁻² and 24.5 mW cm⁻² while the peak power density for the planar DMFC was 20.0 mW cm⁻² and 23.0 mW cm⁻² with Nafion^® 212 and 115 MEAs, respectively. Even though the performance of the fuel cell improved with each increase in methanol concentration, the fuel and energy efficiencies decreased for both the tubular and planar geometries due to increased methanol crossover. The tubular DMFC experienced higher methanol crossover potentially due to a higher static fluid pressure in the anode fuel reservoir (AFR) caused by the vertical orientation of the tubular fuel reservoir. The performance of the tubular DMFC in this work represents an 870% improvement in power density from the previous best, passive, tubular DMFC found in the literature.     

Direct alkaline fuel cell for multiple liquid fuels: Anode electrode studies

A direct alkaline fuel cell with a liquid potassium hydroxide solution as an electrolyte is developed for the direct use of methanol, ethanol or sodium borohydride as fuel. Three different catalysts, e.g., Pt-black or Pt/Ru (40 wt.%:20 wt.%)/C or Pt/C (40 wt.%), with varying loads at the anode against a MnO₂ cathode are studied. The electrodes are prepared by spreading the catalyst slurry on a carbon paper substrate. Nickel mesh is used as a current-collector. The Pt–Ru/C produces the best cell performance for methanol, ethanol and sodium borohydride fuels. The performance improves with increase in anode catalyst loading, but beyond 1 mg cm⁻² does not change appreciably except in case of ethanol for which there is a slight improvement when using Pt–Ru/C at 1.5 mA cm⁻². The power density achieved with the Pt–Ru catalyst at 1 mg cm⁻² is 15.8 mW cm⁻² at 26.5 mA cm⁻² for methanol and 16 mW cm⁻² at 26 mA cm⁻² for ethanol. The power density achieved for NaBH₄ is 20 mW cm⁻² at 30 mA cm⁻² using Pt-black.     

Microstructural factors of electrodes affecting the performance of anode-supported thin film yttria-stabilized zirconia electrolyte (∼1 μm) solid oxide fuel cells

The effects of the microstructural factors of electrodes, such as the porosity and pore size of anode supports and the thickness of cathodes, on the performance of an anode-supported thin film solid oxide fuel cell (TF-SOFC) are investigated. The performance of the TF-SOFC with a 1 μm-thick yttria-stabilized zirconia (YSZ) electrolyte is significantly improved by employing anode supports with increased porosity and pore size. The maximum power density of the TF-SOFCs increases from 370 mW cm⁻² to 624 mW cm⁻² and then to over 900 mW cm⁻² at 600 °C with increasing gas transport at the anode support. Thicker cathodes also improve cell performance by increasing the active reaction sites. The maximum power density of the cell increases from 624 mW cm⁻² to over 830 mW cm⁻² at 600 °C by changing the thickness of the lanthanum strontium cobaltite (LSC) cathode from 1 to 2-3 μm.     

Design, fabrication and performance evaluation of a miniature air breathing direct formic acid fuel cell based on printed circuit board technology

A miniature air breathing compact direct formic acid fuel cell (DFAFC), with gold covered printed circuit board (PCB) as current collectors and back boards, is designed, fabricated and evaluated. Effects of formic acid concentration and catalyst loading (anodic palladium loading and cathodic platinum loading) on the cell performance are investigated and optimized fuel concentration and catalyst loading are obtained based on experimental results. A maximum power density of 19.6 mW cm⁻² is achieved at room temperature with passive operational mode when 5.0 M formic acid is fed and 1 mg cm⁻² catalyst at both electrodes is used. The home-made DFAFC also displays good long-term stability at constant current density.     

Highly fluorinated comb-shaped copolymer as proton exchange membranes (PEMs): Fuel cell performance

The fuel cell performance (DMFC and H₂/air) of highly fluorinated comb-shaped copolymer is reported. The initial performance of membrane electrode assemblies (MEAs) fabricated from comb-shaped copolymer containing a side-chain weight fraction of 22% are compared with those derived from Nafion and sulfonated polysulfone (BPSH-35) under DMFC conditions. The low water uptake of comb copolymer enabled an increase in proton exchange site concentrations in the hydrated polymer, which is a desirable membrane property for DMFC application. The comb-shaped copolymer architecture induces phase separated morphology between the hydrophobic fluoroaromatic backbone and the polysulfonic acid side chains. The initial performance of the MEAs using BPSH-35 and Comb 22 copolymer were comparable and higher than that of the Nafion MEA at all methanol concentrations. For example, the power density of the MEA using Comb 22 copolymer at 350 mA cm⁻² and 0.5 M methanol was 145 mW cm⁻², whereas the power densities of MEAs using BPSH-35 were 136 mW cm⁻². The power density of the MEA using Comb 22 copolymer at 350 mA cm⁻² and 2.0 M methanol was 144.5 mW cm⁻², whereas the power densities of MEAs using BPSH-35 were 143 mW cm⁻².     

Direct operation of cone-shaped anode-supported segmented-in-series solid oxide fuel cell stack with methane

Operation of cone-shaped anode-supported segmented-in-series solid oxide fuel cell (SIS-SOFC) stack directly on methane is studied. A cone-shaped solid oxide fuel cell stack is assembled by connecting 11 cone-shaped anode-supported single cells in series. The 11-cell-stack provides a maximum power output of about 8 W (421.4 mW cm⁻² calculated using active cathode area) at 800 °C and 6 W (310.8 mW cm⁻²) at 700 °C, when operated with humidified methane fuel. The maximum volumetric power density of the stack is 0.9 W cm⁻³ at 800 °C. Good stability is observed during 10 periods of thermal cycling test. SEM-EDX measurements are taken for analyzing the microstructures and the coking degrees.     

Effects of operation conditions on direct borohydride fuel cell performance

In this study, the influences of different operational conditions such as cell temperature, sodium hydroxide concentration, oxidant conditions and catalyst loading on the performance of direct borohydride fuel cell which consisted of Pd/C anode, Pt/C cathode and Na⁺ form Nafion membrane as the electrolyte were investigated. The experimental results showed that the power density increased by increasing the temperature and increasing the flow rate of oxidant. Furthermore, it was found that 20 wt.% of NaOH concentration was optimum for DBFC operation. When oxygen was used as oxidant instead of air, better performance was observed. Experiments also showed that electrochemical performance was not considerably affected by humidification levels. An enhanced power density was found by increasing the loading of anodic catalyst. In the present study, a maximum power density of 27.6 mW cm⁻² at a cell voltage of 0.85 V was achieved at 55 mA cm⁻² at 60 °C when humidified air was used.     

Carbon monoxide-fueled solid oxide fuel cell

This study explored CO as a primary fuel in anode-supported solid oxide fuel cells (SOFCs) of both tubular and planar geometries. Tubular single cells with active areas of 24 cm² generated power up to 16 W. Open circuit voltages for various CO/CO₂ mixture compositions agreed well with the expected values. In flowing dry CO, power densities up to 0.67 W cm⁻² were achieved at 1 A cm⁻² and 850 °C. This performance compared well with 0.74 W cm⁻² measured for pure H₂ in the same cell and under the same operating conditions. Performance stability of tubular cells was investigated by long-term testing in flowing CO during which no carbon deposition was observed. At a constant current of 9.96 A (or, 0.414 A cm⁻²) power output remained unchanged over 375 h of continuous operation at 850 °C. In addition, a 50-cell planar SOFC stack was operated at 800 °C on 95% CO (balance CO₂), which generated 1176 W of total power at a power density of 224 mW cm⁻². The results demonstrate that CO is a viable primary fuel for SOFCs.     

A plate-frame flow-through microfluidic fuel cell stack

We present a plate-frame microfluidic fuel cell architecture with porous flow-through electrodes. The architecture combines the advantages of recent microfluidic fuel cells with those of traditional plate-frame PEM fuel cells and enables vertical stacking with little dead volume. Peak current and power densities of 15.7 mA cm⁻² and 5.8 mW cm⁻² were observed. In addition to the new plate-frame architecture, microfabrication techniques have been used to create a new form of high performance electrode. Laser ablation of a polymer precursor followed by a pyrolysis process was used to create a thin, low-cost micro-porous electrode that provides for more rapid reactant transport. Here we show a 140% increase in power density compared to commercial carbon fiber paper.     

Performance increase of microfluidic formic acid fuel cell using Pd/MWCNTs as catalyst

This paper shows that the combination of an O₂ saturated acidic fluid setup (O₂-setup) and a composite of Pd nanoparticles supported on multiwalled-carbon nanotubes (Pd/MWCNTs) as anode catalyst material, results in the improvement of microfluidic fuel cell performance. Microfluidic fuel cells were constructed and evaluated at low HCOOH concentrations (0.1 and 0.5 M) using Pd/V XC-72 and Pd/MWCNTs as anode and Pt/V XC-72 as cathode electrode materials, respectively. The results show a higher power density (2.9 mW cm⁻²) for this cell when compared to the value reported in the literature that considers a commercial Pd/V XC-72 and 3.3 mW cm⁻² using a Pd/MWCNTs with a 50% less Pd loading than that commercial Pd/V XC-72.     

Effect of carbon black concentration in carbon fiber paper on the performance of low-temperature proton exchange membrane fuel cells

This study uses fuel cell gas diffusion layers (GDLs) made from carbon fiber paper containing carbon black in proton exchange membrane fuel cells (PEMFCs) in order to determine the relationship between carbon black content and fuel cell performance. The connection between fuel cell performance and the carbon black content of the carbon fiber paper is discussed, and the effects of carbon black on the carbon fiber paper's thickness, density, and surface resistivity are investigated. When a carbon fiber paper GDL contains 10 wt% phenolic resin and 2% carbon black, and reaction area was 25 cm² and operating temperature 40 °C, tests show that a carbon electrode fuel cell could achieve 1026.4 mA cm⁻² and maximum power of 612.8 mW cm⁻² under a 0.5 V load.     

Direct borohydride fuel cell using Ni-based composite anodes

In this study, nickel-based composite anode catalysts consisting of Ni with either Pd on carbon or Pt on carbon (the ratio of Ni:Pd or Ni:Pt being 25:1) were prepared for use in direct borohydride fuel cells (DBFCs). Cathode catalysts used were 1 mg cm⁻² Pt/C or Pd electrodeposited on activated carbon cloth. The oxidants were oxygen, oxygen in air, or acidified hydrogen peroxide. Alkaline solution of sodium borohydride was used as fuel in the cell. High power performance has been achieved by DBFC using non-precious metal, Ni-based composite anodes with relatively low anodic loading (e.g., 270 mW cm⁻² for NaBH₄/O₂ fuel cell at 60 °C, 665 mW cm⁻² for NaBH₄/H₂O₂ fuel cell at 60 °C). Effects of temperature, oxidant, and anode catalyst loading on the DBFC performance were investigated. The cell was operated for about 100 h and its performance stability was recorded.     

Improved fuel use efficiency in microchannel direct methanol fuel cells using a hydrophilic macroporous layer

We demonstrate state-of-the-art room temperature operation of silicon microchannel-based micro-direct methanol fuel cells (μDMFC) having a very high fuel use efficiency of 75.4% operating at an output power density of 9.25 mW cm⁻² for an input fuel (3 M aqueous methanol solution) flow rate as low as 0.55 μL min⁻¹. In addition, an output power density of 12.7 mW cm⁻² has been observed for a fuel flow rate of 2.76 μL min⁻¹. These results were obtained via the insertion of novel hydrophilic macroporous layer between the standard hydrophobic carbon gas diffusion layer (GDL) and the anode catalyst layer of a μDMFC; the hydrophilic macroporous layer acts to improve mass transport, as a wicking layer for the fuel, enhancing fuel supply to the anode at low flow rates. The results were obtained with the fuel being supplied to the anode catalyst layer via a network of microscopic microchannels etched in a silicon wafer.     

Polymer electrolytes based on sulfonated polysulfone for direct methanol fuel cells

This paper reports the development and characterization of sulfonated polysulfone (SPSf) polymer electrolytes for direct methanol fuel cells. The synthesis of sulfonated polysulfone was performed by a post sulfonation method using trimethyl silyl chlorosulfonate as a mild sulfonating agent. Bare polysulfone membranes were prepared with two different sulfonation levels (60%, SPSf-60 and 70%, SPSf-70), whereas, a composite membrane of SPSf-60 was prepared with 5 wt% silica filler. These membranes were investigated in direct methanol fuel cells (DMFCs) operating at low (30–40 °C) and high temperatures (100–120 °C). DMFC power densities were about 140 mW cm⁻² at 100 °C with the bare SPSf-60 membrane and 180 mW cm⁻² at 120 °C with the SPSf-60-SiO2 composite membrane. The best performance achieved at ambient temperature using a membrane with high degree of sulfonation (70%, SPSf-70) was 20 mW cm⁻² at atmospheric pressure. This makes the polysulfone-based DMFC suitable for application in portable devices.     

Fabrication and characterization of micro PEM fuel cells using pyrolyzed carbon current collector plates

A novel fabrication technique for micro proton exchange membrane fuel cells (μPEMFCs) based on carbon-MEMS (C-MEMS) was optimized to yield higher performance cells. Polymer manufacturing is relatively easy compared to directly patterning graphite as is typically done to make fuel cell bipolar plates. In a C-MEMS approach, fuel cell bipolar plates are fabricated by first patterning polymer Cirlex^® sheets. By subsequently pyrolyzing the machined polymer sheets at high temperature in an inert atmosphere, carbon bipolar plates with intricate groove structures to distribute the reactants are obtained. Using an improved assembly technique such as polishing the carbonized plates to minimize the contact resistance between gas diffusion layers (GDL) and bipolar plates, better pyrolysis temperature control and a better end plate design, a μPEMFC with a 0.64 cm² active surface was fabricated using the newly developed bipolar plates. At 1 atm and 25 °C a maximum power density of ∼76 mW cm⁻² was obtained, and at 2 atm and 25 °C ∼85 mW cm⁻² was achieved. These data are comparable with data reported in the literature for μPEMFCs and are a dramatic improvement over earlier results reported for the same C-MEMS based fuel cell. Electrochemical Impedance Spectroscopy (EIS) and cyclic voltammetry were carried out to characterize steady-state and transient characteristics of the novel C-MEMS fuel cell.     

Maltodextrin-powered enzymatic fuel cell through a non-natural enzymatic pathway

Enzymatic fuel cells (EFCs) use a variety of fuels to generate electricity through oxidoreductase enzymes, such as oxidases or dehydrogenases, as catalysts on electrodes. We have developed a novel synthetic enzymatic pathway containing two free enzymes (maltodextrin phosphorylase and phosphoglucomutase) and one immobilized glucose-6-phosphate dehydrogenase that can utilize an oligomeric substrate maltodextrin for producing electrons mediated via a diaphorase and vitamin K₃ electron shuttle system. Three different enzyme immobilization approaches were compared based on electrostatic force entrapment, chemical cross-linking, and cross-linking with the aid of carbon nanotubes. At 10 mM glucose-6-phosphate (G6P) as a substrate concentration, the maximum power density of 0.06 mW cm⁻² and retaining 42% of power output after 11 days were obtained through the method of chemical cross-linking with carbon nanotubes, approximately 6-fold and 3.5-fold better than those of the electrostatic force-based method, respectively. When changed to maltodextrin (degree of polymerization = 19) as the substrate, the EFC achieved a maximum power density of 0.085 mW cm⁻². With the advantages of stable, low cost, high energy density, non-inhibitor to enzymes, and environmental friendly, maltodextrin is suggested to be an ideal fuel to power enzymatic fuel cells.