A novel cathode for alkaline fuel cells based on a porous silver membrane

Porous silver membranes were investigated as potential substrates for alkaline fuel cell cathodes and as an approach for studying pore size effects in alkaline fuel cells. The silver membrane provides both the electrocatalytic function, mechanical support and a means of current collection. Relatively high active surface area (∼0.6 m² g⁻¹) results in good electrochemical performance (∼200 mA cm⁻² at 0.6 V and ∼400 mA cm⁻² at 0.4 V) in the presence of 6.9 M KOH. The electrode fabrication technique is described and polarization curves and impedance measurements are used to investigate the performance. The regular structure of the electrodes allows parametric studies of the performance of electrodes as a function of pore size. Impedance spectra have been fitted with a proposed equivalent circuit which was obtained following the study of impedance measurements under different experimental conditions (electrolyte concentration, oxygen concentration, temperature, and pore size). The typical impedance spectra consisted of one high frequency depressed semi-circle related to porosity and KOH wettability and one low-frequency semi-circle related to kinetics. A passive air-breathing hydrogen-air fuel cell constructed from the membranes in which they act as mechanical support, current collector and electrocatalyst achieves a peak power density of 50 mW cm⁻² at 0.40 V cell potential when operating at 25 °C.     

A new application for nickel foam in alkaline fuel cells

The use of nickel foam as an electrode substrate in alkaline fuel cells (AFCs) has been investigated for bi-polar cells incorporating an electrically conducting gas diffusion layer. This contribution focuses on the cathode, and draws comparisons between nickel foam and nickel mesh substrates. One of the principal electrocatalysts for the cathodic reduction of oxygen is silver, so an improvement in electrochemical performance was obtained by electroplating the nickel foam with silver. The electrodeposition process was optimised to maximise electrochemical performance with a minimum of silver deposited. Nickel foam, which is less expensive than the usual nickel mesh, appears to be a good substrate for AFC electrode fabrication, especially when incorporating a conducting gas diffusion layer. Silver deposited by electroplating onto the nickel foam was found to result in significantly enhanced electrochemical performance, with a reduction in both the Ohmic resistance of the electrode, as well as the oxygen reduction reaction charge transfer resistance.     

An improved cathode for alkaline fuel cells

The use of nickel foam as an electrode substrate in alkaline fuel cells (AFCs) has been investigated for bipolar cells incorporating an electrically conducting gas diffusion layer (GDL). Improved performance, compared to a previous design, was obtained by adding an extra active layer (AL) composed of manganese (IV) oxide (MnO₂) deposited onto carbon black. This new cathode design performed significantly better (130 mA cm⁻² at 0.8 V and 25 °C) than the previous design (35 mA cm⁻² under the same condition), especially at higher potential. It has been shown that the GDL is a key component of the gas diffusion electrode for both performance and durability, especially with liquid electrolytes. Electrochemical impedance spectroscopy was used to study the various losses in the cathode as a function of electrolyte concentration, oxygen concentration and cell voltage. The typical impedance spectra consisted of one high frequency depressed semi-circle which is believed to be related to porosity and KOH wettability, and one low frequency semi-circle related to electrode kinetics.     

High performance porous polybenzimidazole membrane for alkaline fuel cells

In this study, a highly ion-conductive and durable porous polymer electrolyte membrane based on ion solvating polybenzimidazole (PBI) was developed for anion exchange membrane fuel cells (AEMFCs). The introduction of porosity can increase the attraction of electrolytic solutions (e.g., potassium hydroxide (KOH)) and ion solvation, which results in the enhancement of PBI's ionic conductivity. The morphology, thermo-physico-chemical properties, ionic conductivity, alkaline stability, and the AEMFC performance of KOH-doped PBI membranes with different porosities were characterized. The ionic conductivity and AEMFC performance of 70 wt.% porous PBI was about 2 times higher than that of the commercially available Fumapem^® FAA. All KOH-doped porous PBI membranes maintained their ionic conductivity after accelerated alkaline stability testing over a period of 14 days, while the commercial FAA degraded just after 3 h. The excellent performance and good durability of KOH-doped porous PBI membrane makes it a promising candidate for AEMFCs.     

Review of gas diffusion cathodes for alkaline fuel cells

This paper gives a technical background to alkaline fuel cells (AFCs), introducing the advantages and drawbacks of the technology. AFCs offer the potential for low cost, mass producible fuel cells, without the dependency on platinum based catalysts and (currently) expensive membrane electrolytes. The AFC uses relatively low cost electrolytes based on aqueous bases such as potassium hydroxide. The inherent CO₂ sensitivity of the electrolyte can be addressed by filtering out the CO₂ from the air intake using a simple scrubber and periodically replacing the liquid electrolyte.     

Non-platinum cathode catalysts for alkaline membrane fuel cells

Hydrogen–oxygen fuel cells using an alkaline anion exchange membrane were prepared and evaluated. Various non-platinum catalyst materials were investigated by fabricating membrane-electrode assemblies (MEAs) using Tokuyama membrane (# A201) and compared with commercial noble metal catalysts. Co and Fe phthalocyanine catalyst materials were synthesized using multi-walled carbon nanotubes (MWCNTs) as support materials. X-ray photoelectron spectroscopic study was conducted in order to examine the surface composition. The electroreduction of oxygen has been investigated on Fe phthalocyanine/MWCNT, Co phthalocyanine/MWCNT and commercial Pt/C catalysts. The oxygen reduction reaction kinetics on these catalyst materials were evaluated using rotating disk electrodes in 0.1 M KOH solution and the current density values were consistently higher for Co phthalocyanine based electrodes compared to Fe phthalocyanine. The fuel cell performance of the MEAs with Co and Fe phthalocyanines and Tanaka Kikinzoku Kogyo Pt/C cathode catalysts were 100, 60 and 120 mW cm⁻² using H₂ and O₂ gases.     

Anionic membrane and ionomer based on poly(2,6-dimethyl-1,4-phenylene oxide) for alkaline membrane fuel cells

Hydroxyl-ion conductive poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) membranes with different characteristics were prepared via relatively simple bromination/amination serial reactions with reduced number of involved chemicals and shorter reaction time. The effects of reactants ratio, reaction atmosphere, polymer concentration, casting solvent, and hydroxylation treatment on reaction were investigated in details. The microstructure, water uptake, swelling ratio, ion-exchange capacity and ionic conductivity of the membranes were also studied. The obtained results demonstrate that, the ionic conductivity of the membrane is dependent on casting solvent. The N-methyl-2-pyrrolidonecast membrane exhibits the highest conductivity with the thinnest film. Although the membrane was prepared via a relatively simple preparation route with least toxic chemicals, a competitive ionic conductivity value of 1.64 × 10⁻² S cm⁻¹ was achieved at 60 °C. A power density of 19.5 mW cm⁻² has been demonstrated from the alkaline membrane fuel cell operated at 70 °C, assembled from the entirely homemade membrane electrode assembly without any hot-pressing.     

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.     

Anion-exchange membranes composed of quaternized-chitosan derivatives for alkaline fuel cells

A series of quaternized-chitosan derivatives (QCDs) with various degrees of quaternization was synthesized using glycidyltrimethylammonium chloride as a main quaternized reagent. These QCDs were then processed into hydroxide—form quaternary ammonium salts with aqueous potassium hydroxide solutions. The resultant hydroxide—form QCD gels were further crosslinked into anion-exchange membranes using ethylene glycol diglycidyl ether. The crosslinking density, crystallinity, swelling index, ion exchange capacity, ionic conductivity and thermal stability of the crosslinked membranes were subsequently investigated. It was found that properties of crosslinked membranes were modulated mainly by the degree of quaternization and crosslinking density of membranes. Some membranes exhibited promising characteristics and had the potential for applications in alkaline polymer electrolyte fuel cells in considering their integrative properties.     

High performance cathode based on carbon fiber felt for magnesium-air fuel cells

A series of carbon fiber felt/PTFE based gas diffusion layers (GDL) for Mg-air fuel cells were prepared by a simple method of immersing carbon fiber felt in PTFE suspension. Critical properties of the as-prepared GDL, including the surface morphology, electronic resistivity, porosity and gas permeability, have been characterized to investigate the effect of PTFE suspension concentration and PTFE content on the properties of the GDL. The micrographs indicated that the PTFE was homogenously dispersed on the carbon fiber felts and showed structure with a microporous layer. The as-prepared GDL exhibited good mechanical property, high electronic conductivity, sufficient water repellency and high gas permeability. Compared with the Mg-air fuel cell with a traditional carbon powder based cathode, the performance and the stability of Mg-air fuel cell with the carbon fiber felt based GDL are improved significantly.     

The effect of materials on proton exchange membrane fuel cell electrode performance

This paper describes the optimisation in the fabrication materials and techniques used in proton exchange membrane fuel cell (PEMFC) electrodes. The effect on the performance of membrane electrode assemblies (MEAs) from the solvents used in producing catalyst inks is reported. Comparison in MEA performances between various gas diffusion layers (GDLs) and the importance of microporous layers (MPLs) in gas diffusion electrodes (GDEs) are also shown. It was found that the best performances were achieved for GDEs using tetrahydrofuran (THF) as the solvent in the catalyst ink formulation and Sigracet 10BC as the GDL. The results also showed that our in-house painted GDEs were comparable to commercial ones (using Johnson Matthey HiSpec™ and E-TEK catalysts).     

Hydroxide exchange composite membrane based on soluble quaternized polyetherimide for potential applications in fuel cells

A series of soluble quaternized polyetherimides (QAPEIs) have been successfully synthesized by homogeneous quaternization in trimethylamine aqueous solution. ¹H NMR spectra confirm the successful synthesis of QAPEI. The QAPEIs exhibit good solubility in membrane-preparation solvents, making it possible to prepare the QAPEI composite membrane. Novel composite hydroxide exchange membranes have been prepared by incorporating QAPEIs with polytetrafluoroethylene (PTFE) membranes. The SEM images, gas permeation measurements and FTIR spectra show that the QAPEI is successfully filled in PTFE membrane and the resulted composite membrane is dense and smooth. The ion exchange capacity of composite membranes ranges from 0.35 to 0.58 mmol g⁻¹. The composite membranes have appropriate water uptake (≤154%) and moderate swelling ratio (≤42%) even at 60 °C. The hydroxide conductivity of the composite membrane reaches 11.9 mS cm⁻¹ at 20 °C that increases to 35.2 mS cm⁻¹ at 60 °C. TGA curve shows that the composite membrane possesses high thermal stability (T_OD: 210 °C). All these properties indicate that the QAPEI/PTFE composite membranes are good candidates for use as HEMs in HEM fuel cells.     

Preparation and operation of gas diffusion electrodes for high-temperature proton exchange membrane fuel cells

Gas diffusion electrodes for high-temperature PEMFC based on acid-doped polybenzimidazole membranes were prepared by a tape-casting method. The overall porosity of the electrodes was tailored in a range from 38% to 59% by introducing porogens into the supporting and/or catalyst layers. The investigated porogens include volatile ammonium oxalate, carbonate and acetate and acid-soluble zinc oxide, among which are ammonium oxalate and ZnO more effective in improving the overall electrode porosity. Effects of the electrode porosity on the fuel cell performance were investigated in terms of the cathodic limiting current density and minimum air stoichiometry, anodic limiting current and hydrogen utilization, as well as operations under different pressures and temperatures.     

Comparison of different types of membrane in alkaline direct ethanol fuel cells

It has been understood that the use of cation-exchange membranes (CEM) and alkali-doped polybenizimidazole membranes (APM) in alkaline direct ethanol fuel cells (DEFC) with an added base in the fuel exhibits performance similar to the use of anion-exchange membranes (AEM). The present work is to assess the suitability of the three types of membrane to alkaline DEFCs by measuring and comparing the membrane properties including the ionic conductivity, the species permeability, as well as the thermal and mechanical properties. The comparison shows that: (i) the AEM is still the most promising membrane for the alkaline DEFC, although the thermal stability needs to be further enhanced; (ii) before solving the problem of the poor thermal stability of AEMs, the CEM is another choice for the alkaline DEFC running at high temperatures (<90 °C); and (iii) the APM can also be applied to the alkaline DEFC operating at high temperatures, but its mechanical property needs to be substantially enhanced and the species permeability needs to be dramatically decreased.     

Carbonate resilience of flowing electrolyte-based alkaline fuel cells

Alkaline fuel cells (AFCs) are promising power sources due to superior kinetics and the ability to use inexpensive non-noble metal catalysts. However, carbonate formation from carbon dioxide in air has long been considered a significant hurdle for liquid electrolyte-based AFC technologies. Carbonate formation consumes hydroxyl anions, which leads to (i) reduced electrode performance if formed salts precipitate from solution and (ii) lowered electrolyte conductivity, which reduces cell performance and operating lifetime. Here, using a flowing electrolyte-based microfluidic fuel cell, we demonstrate that AFC performance can be resilient to a broad range of carbonate concentrations. Furthermore, we investigate the effects of carbonate formation rates on projected AFC operational lifetime. Results from this study will aid in the design of AFC-based power sources in light of the tradeoffs between performance, durability and cost.     

Imidazolium-functionalized polysulfone hydroxide exchange membranes for potential applications in alkaline membrane direct alcohol fuel cells

A series of imidazolium-functionalized polysulfones were successfully synthesized by chloromethylation-Menshutkin two-step method. PSf-ImOHs show the desired selective solubility: insoluble in alcohols (e.g., methanol and ethanol), and soluble in 50 vol.% aqueous solutions of acetone or tetrahydrofuran, implying their potential applications for both the alcohol-resistant membranes themselves and the ionomer solutions in low-boiling-point water-soluble solvents. PSf-ImOH also possesses very high thermal stability (T_OD: 258 °C), higher than quaternary ammonium and quaternary phosphonium functionalized polysulfones (T_OD: 120 °C and 186 °C, repsectively). Ion exchange capacity (IEC) of PSf-ImOH membranes ranges from 0.78 to 2.19 mmol g⁻¹ with degree of chloromethylation from 42% to 132% of original chloromethylated polysulfone. As expected, water uptake, swelling ratio, and hydroxide conductivity increase with IEC and temperatures. With 2.19 mmol g⁻¹ of IEC, the PSf-ImOH 132% membrane exhibits the highest hydroxide conductivity (53 mS cm⁻¹ at 20 °C), higher than those of all other reported polysulfone-based HEMs (1.6–45 mS cm⁻¹) and other imidazolium-functionalized HEMs (19.6–38.8 mS cm⁻¹). In addition, PSf-ImOH membranes have low methanol permeability of 0.8–4.7 × 10⁻⁷ cm² s⁻¹, one order of magnitude smaller than that of Nafion212 membrane. All these properties indicate imidazolium-functionalized polysulfone is very promising for potential applications in alkaline membrane direct alcohol fuel cells.     

Study of operating conditions and cell design on the performance of alkaline anion exchange membrane based direct methanol fuel cells

Direct methanol fuel cells using an alkaline anion exchange membrane (AAEM) were prepared, studied, and optimized. The effects of fuel composition and electrode materials were investigated. Membrane electrode assemblies fabricated with Tokuyama^® AAEM and commercial noble metal catalysts achieved peak power densities between 25 and 168 mW cm⁻² depending on the operating temperature, fuel composition, and electrode materials used. Good electrode wettability at the anode was found to be very important for achieving high power densities. The performance of the best AAEM cells was comparable to Nafion^®-based cells under similar conditions. Factors limiting the performance of AAEM MEAs were found to be different from those of Nafion^® MEAs. Improved electrode kinetics for methanol oxidation in alkaline electrolyte at Pt-Ru are apparent at low current densities. At high current densities, rapid CO₂ production converts the hydroxide anions, necessary for methanol oxidation, to bicarbonate and carbonate: consequently, the membrane and interfacial conductivity are drastically reduced. These phenomena necessitate the use of aqueous potassium hydroxide and wettable electrode materials for efficient hydroxide supply to the anode. However, aqueous hydroxide is not needed at the cathode. Compared to AAEM-based fuel cells, methanol fuel cells based on proton-conducting Nafion^® retain better performance at high current densities by providing the benefit of carbon dioxide rejection.     

Development of porous carbon foam polymer electrolyte membrane fuel cell

In order to prove the feasibility of using porous carbon foam material in a polymer electrolyte membrane fuel cell (PEMFC), a single PEMFC is constructed with a piece of 80PPI (pores per linear inch) Reticulated Vitreous Carbon (RVC) foam at a thickness of 3.5 mm employed in the cathode flow-field. The cell performance of such design is compared with that of a conventional fuel cell with serpentine channel design in the cathode and anode flow-fields. Experimental results show that the RVC foam fuel cell not only produces comparative power density to, but also offers interesting benefits over the conventional fuel cell. A 250 h long term test conducted on a RVC foam fuel cell shows that the durability and performance stability of the material is deemed to be acceptable. Furthermore, a parametric study is conducted on single RVC foam fuel cells. Effect of geometrical and material parameters of the RVC foam such as PPI and thickness and operating conditions such as pressure, temperature, and stoichiometric ratio of the reactant gases on the cell performance is experimentally investigated in detail. The single cell with the 80PPI RVC foam exhibits the best performance, especially if the thinnest foam (3.5 mm) is used. The cell performance improves with increasing the operating gauge pressure from 0 kPa to 80 kPa and the operating temperature from 40 °C to 60 °C, but deteriorates as it further increases to 80 °C. The cell performance improves as the stoichiometric ratio of air increases from 1.5 to 4.5; however, the improvement becomes marginal when it is raised above 3.0. On the other hand, changing the stoichiometric ratio of hydrogen does not have a significant impact on the cell performance.     

Water management in alkaline anion exchange membrane fuel cell anode

Water management is an important issue for alkaline anion exchange membrane fuel cell (AAEMFC) due to its significant role in the energy conversion processes. In this study, a numerical model is developed to investigate the water transport in AAEMFC anode. The gas and liquid transport characteristics in the gas diffusion layer (GDL) and catalyst layer (CL) with different designs and under various operating conditions are discussed. The results show that the current density affects the liquid water distribution in anode most significantly, and the temperature is the second considerable factor. The stoichiometry ratio of the supplied reactant has insignificant effect on the liquid water transport in anode. The change of liquid water amount in anode with cathode relative humidity follows a similar trend with anode inlet relative humidity. Some numerical results are also explained with published experimental and modeling data with reasonable agreement.