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.     

Tungsten(VI) oxide as a versatile support for Pd nanocatalysts for alkaline direct alcohol fuel cells

The instability of carbon support materials has motivated the development of metal oxides supports which are stable under the fuel cell environment. In this study, tungsten (VI) oxide (WO₃) is utilized as a secondary support and cocatalyst for the electrooxidation of methanol and ethanol. Functionalized carbon nanodots employed as primary supports were blended with WO₃ nanoparticles to form a composite support onto which Pd nanoparticles were deposited by a borohydride reduction method. The synthesized Pd/fCNDs-WO₃ electrocatalysts were characterized by Transmission Electron microscopy (TEM), X-ray diffractometry (XRD) and X-ray photoelectron spectroscopy. XRD results proved that incorporating WO₃ into Pd/fCNDs electrocatalyst shifts the Pd diffraction peaks to lower 2Ɵ value due to lattice relaxation. XPS results revealed that W exist in oxidised form and confirmed the strong interaction between the support material and the catalyst. The Pd/fCNDs-WO₃ electrocatalysts exhibited a remarkable catalytic activity towards methanol and ethanol oxidation. High current densities of 87.24 mA cm⁻² and 44.23 mA cm⁻² were obtained for ethanol and methanol oxidation, respectively, using a catalyst with 2.5% Pd loading. EIS, CA and stability tests revealed that the presence of WO₃ in Pd/fCNDs electrocatalyst improves the kinetics, tolerance to poisoning and long-term durability in alkaline conditions. This superior performance is attributed to the electronic coupling between Pd and WO₃ nanoparticles.     

Pulsed plasma-polymerized alkaline anion-exchange membranes for potential application in direct alcohol fuel cells

Pulsed plasma polymerization is adopted to synthesize alkaline anion-exchange membranes (AAEMs) with high contents of functional groups. The attenuated total reflection Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy and thermo gravimetric analysis demonstrate that the benzyltrimethylammonium cationic groups can be successfully introduced into the polymer matrix. The content of the quaternary nitrogen in pulsed plasma-polymerized membrane is up to 1.93 atom%. The ultra-flat and undamaged morphology structure of the AAEMs indicates a low plasma ablation effect in the pulsed plasma polymerization. The excellent properties of the pulsed plasma-polymerized AAEMs, including good adhesion to the substrate, acceptable chemical stability and thermal stability, high ion-exchange capacity (1.42 mmol g⁻¹) and water uptake (59.73 wt%), interesting ionic conductivity (0.0205 S cm⁻¹ in deionized water at 20 °C) and ethanol permeability (3.37 × 10⁻¹¹ m² s⁻¹), suggest a great potential for application in direct alcohol fuel cells.     

Comparative exergy analysis of direct alcohol fuel cells using fuel mixtures

Within the last years there has been increasing interest in direct liquid fuel cells as power sources for portable devices and, in the future, power plants for electric vehicles and other transport media as ships will join those applications. Methanol is considerably more convenient and easy to use than gaseous hydrogen and a considerable work is devoted to the development of direct methanol fuel cells. But ethanol has much lower toxicity and from an ecological viewpoint ethanol is exceptional among all other types of fuel as is the only chemical fuel in renewable supply. The aim of this study is to investigate the possibility of using direct alcohol fuel cells fed with alcohol mixtures. For this purpose, a comparative exergy analysis of a direct alcohol fuel cell fed with alcohol mixtures against the same fuel cell fed with single alcohols is performed. The exergetic efficiency and the exergy loss and destruction are calculated and compared in each case. When alcohol mixtures are fed to the fuel cell, the contribution of each fuel to the fuel cell performance is weighted attending to their relative proportion in the aqueous solution. The optimum alcohol composition for methanol/ethanol mixtures has been determined.     

Synthesis of PdNi catalysts for the oxidation of ethanol in alkaline direct ethanol fuel cells

Carbon-supported PdNi catalysts for the ethanol oxidation reaction in alkaline direct ethanol fuel cells are successfully synthesized by the simultaneous reduction method using NaBH₄ as reductant. X-ray diffraction characterization confirms the formation of the face-centered cubic crystalline Pd and Ni(OH)₂ on the carbon powder for the PdNi/C catalysts. Transmission electron microscopy images show that the metal particles are well-dispersed on the carbon powder, while energy-dispersive X-ray spectrometer results indicate the uniform distribution of Ni around Pd. X-ray photoelectron spectroscopy analyses reveal the chemical states of Ni, including metallic Ni, NiO, Ni(OH)₂ and NiOOH. Cyclic voltammetry and chronopotentiometry tests demonstrate that the Pd₂Ni₃/C catalyst exhibits higher activity and stability for the ethanol oxidation reaction in an alkaline medium than does the Pd/C catalyst. Fuel cell performance tests show that the application of Pd₂Ni₃/C as the anode catalyst of an alkaline direct ethanol fuel cell with an anion-exchange membrane can yield a maximum power density of 90 mW cm⁻² at 60 °C.     

Nickel nanorods over nickel foam as standalone anode for direct alkaline methanol and ethanol fuel cell

A highly electroactive nickel nanorod (NNR)/nickel foam (NF) electrode was fabricated for direct alcohol fuel cells (DAFCs) using a simple and cost-effective hydrothermal process. The Ni/NiO nanorods were successfully grown on the surface of an NF electrode, which strongly enhanced the anode wettability and increased surface area by 18 times (11.9 m² g⁻¹), resulting in interfacial polarization resistance reduction. The NNR/NF electrode shows high electro-catalytic activity and great stability during alcohol oxidation. The current densities obtained for NNR/NF were four (479 mA cm⁻²) and six (543 mA cm⁻²) times higher than that for pristine NF in the cases of methanol and ethanol oxidations, respectively. This high current density can be attributed to the superhydrophilic surface of the Ni/NiO nanorods and corresponding high mass transfer capability between the electrolytes and Ni/NiO nanorods embedded on the surface of the electrodes. This study presents a new approach for using the novel NNR/NF as a cheap and high performance anode in DAFCs.     

Synthesis and characterization of anion exchange membranes for alkaline direct methanol fuel cells

Alkaline fuel cells suggest solution for the problems of low methanol oxidation kinetics and methanol crossover, which are limiting the development of direct methanol fuel cells. In this work, a novel anion exchange membrane, quaternized poly(aryl ether oxadiazole), was prepared through polycondensation, grafting and quaternization. The ionic conductivity of as-synthesized anion exchange membrane can reach up to 2.79 × 10⁻² S/cm at 70 °C. The physical and chemical stability of the anion exchange membranes could also meet the requirement for alkaline direct methanol fuel cells.     

Quaternized poly(vinyl alcohol)/alumina composite polymer membranes for alkaline direct methanol fuel cells

The quaternized poly(vinyl alcohol)/alumina (designated as QPVA/Al₂O₃) nanocomposite polymer membrane was prepared by a solution casting method. The characteristic properties of the QPVA/Al₂O₃ nanocomposite polymer membranes were investigated using thermal gravimetric analysis (TGA), scanning electron microscopy (SEM), dynamic mechanical analysis (DMA), micro-Raman spectroscopy, and AC impedance method. Alkaline direct methanol fuel cell (ADMFC) comprised of the QPVA/Al₂O₃ nanocomposite polymer membrane were assembled and examined. Experimental results indicate that the DMFC employing a cheap non-perfluorinated (QPVA/Al₂O₃) nanocomposite polymer membrane shows excellent electrochemical performances. The peak power densities of the DMFC with 4 M KOH + 1 M CH₃OH, 2 M CH₃OH, and 4 M CH₃OH solutions are 28.33, 32.40, and 36.15 mW cm⁻², respectively, at room temperature and in ambient air. The QPVA/Al₂O₃ nanocomposite polymer membranes constitute a viable candidate for applications on alkaline DMFC.     

Sodium borohydride as an additive to enhance the performance of direct ethanol fuel cells

The effect of adding small quantities (0.1-1 wt.%) of sodium borohydride (NaBH₄) to the anolyte solution of direct ethanol fuel cells (DEFCs) with membrane-electrode assemblies constituted by nanosized Pd/C anode, Fe-Co cathode and anion-exchange membrane (Tokuyama A006) was investigated by means of various techniques. These include cyclic voltammetry, in situ FTIR spectroelectrochemistry, a study of the performance of monoplanar fuel cells and an analysis of the ethanol oxidation products. A comparison with fuel cells fed with aqueous solutions of ethanol proved unambiguously the existence of a promoting effect of NaBH₄ on the ethanol oxidation. Indeed, the potentiodynamic curves of the ethanol-NaBH₄ mixtures showed higher power and current densities, accompanied by a remarkable increase in the fuel consumption at comparable working time of the cell. A ¹³C and ¹¹B {¹H}NMR analysis of the cell exhausts and an in situ FTIR spectroelectrochemical study showed that ethanol is converted selectively to acetate while the oxidation product of NaBH₄ is sodium metaborate (NaBO₂). The enhancement of the overall cell performance has been explained in terms of the ability of NaBH₄ to reduce the PdO layer on the catalyst surface.     

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.     

Addition of rhenium (Re) to Pt-Ru/f-MWCNT anode electrocatalysts for enhancement of ethanol electrooxidation in half cell and single direct ethanol fuel cell

Pt-Ru and Pt–Re and Pt-Ru-Re nanoparticles supported on functionalized multi-walled carbon nanotubes (f-MWCNT) were synthesized via modified polyol reduction method and tested thoroughly in a half cell and single direct ethanol fuel cell for ethanol electrooxidation in acidic medium. The MWCNTs were functionalized in a mixture of HNO₃/H₂SO₄ solution for depositing a more active metal alloy nanoparticle on support material. The alloy formation of bi-metallic and tri-metallic electrocatalysts were examined by XRD analysis and more clearly explained by FE-SEM element mapping. The TEM analyses reveal that electrocatalysts nanoparticles are well dispersed on f-MWCNT, with spherical shapes and nano sizes range of 1.5–4 nm. The electrochemical analyses by cyclic voltammetry and chronoamperometry measurements reveal that tri-metallic electrocatalyst Pt-Ru-Re (1:1:0.5)/f-MWCNT exhibits the highest electrocatalytic activity and stability towards ethanol electrooxidation among all the synthesized electrocatalysts. The same electrocatalyst as anode in single DEFC results in excellent performance in comparison to all other synthesized electrocatalysts, with a maximum power density of 9.52 mW/cm² at a cell temperature of 30 °C. The bi-metallic Pt-Ru (1:1)/f-MWCNT and Pt–Re (1:1)/f-MWCNT produced power density of 7.48 mW/cm² and 4.74 mW/cm² at room temperature of 30 °C. The power density of DEFC enhanced 2.44 times, when cell operating temperature was increased from 30 °C to 80 °C using anode electrocatalyst Pt-Ru-Re (1:1:0.5)/f-MWCNT and keeping other parameters constant. The best result obtained in half cell and single DEFC using Pt-Ru-Re (1:1:0.5)/f-MWCNT electrocatalyst may be attributed to the synergistic effect of Pt, Ru and Re combined with bi-functional and ligand effects.     

Facile synthesis of a Bi-modified PtRu catalyst for methanol and ethanol electro-oxidation in alkaline medium

In this work a facile synthesis for a high-performance PtRuBi/C catalyst was presented through a simple mixture of commercial PtRu/C and Bi(NO₃)₃. Most of the Bimodified the PtRu particle surface via irreversible adsorption and deposition processes. X-ray photoelectron spectroscopy analysis indicated that Bi₂O₃ was the main form in the catalyst and that there exists an interaction between Bi₂O₃ and Pt. The current density of PtRuBi/C (1:1:0.2 for Pt:Ru:Bi) in the cyclic voltammograms for methanol or ethanol oxidation is over 2.6 times higher than that of PtRu/C. The anti-poisoning ability of this catalyst was also greatly improved. The Bi-containing catalyst had abundant oxygenated species and facilitated removal of poisonous intermediate species.     

Insights on the electrooxidation of ethanol with Pd-based catalysts in alkaline electrolyte

In this work, we report a facile method of synthesis of carbon supported Pd, PdRu, and PdNi nanoparticles, and a comparative study of their catalytic behavior for the electrooxidation of ethanol in alkaline media. The addition of metals such as Ru or Ni increases the oxophilicity of the Pd surface, as observed from the shifting of the Pd oxide reduction peaks. As a consequence, the onset potential for the electrooxidation of ethanol shifts to less positive values on the bimetallic catalysts. The nature and evolution of the species formed during the electrooxidation of ethanol over the catalysts under study has been monitored using in situ infrared spectroscopy. In order to assess properly the evolution of the species formed during the electrooxidation of ethanol, infrared spectra have been recorded in both H₂O and D₂O electrolytes. The results presented in this work demonstrate that the scission of the C–C bond of ethanol takes place at the surface of Pd/C and PdM/C (M = Ni and Ru) at potentials as low as 30 mV. However, at potentials above E ≥ 400 mV, acetates are the main species formed during the electrooxidation of ethanol.     

In and ex situ characterization of an anion-exchange membrane for alkaline direct methanol fuel cell (ADMFC)

Anion exchange membrane fumasep^® FAA-2 was characterized with ex and in situ methods in order to estimate the membranes’ suitability as an electrolyte for an alkaline direct methanol fuel cell (ADMFC). The interactions of this membrane with water, hydroxyl ions and methanol were studied with both calorimetry and NMR and compared with the widely used proton exchange membrane Nafion^® 115. The results indicate that FAA-2 has a tighter structure and more homogeneous distribution of ionic groups in contrast to the clustered structure of Nafion, moreover, the diffusion of OH⁻ ions through this membrane is clearly slower compared to water molecules. The permeability of methanol through the FAA-2 membrane was found to be an order of magnitude lower than for Nafion. Fuel cell experiments in 1 mol dm⁻³ methanol with FAA-2 resulted in OCV of 0.58 V and maximum power density of 0.32 mW cm⁻². However, even higher current densities were obtained with highly concentrated fuels. This implies that less water is needed for fuel dilution, thereby decreasing the mass of the fuel cell system. In addition, electrochemical impedance spectroscopy for the ADMFC was used to determine ohmic resistance of the cell facilitating the further membrane development.     

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.     

Poly (vinyl alcohol)/3-(trimethylammonium) propyl-functionalized silica hybrid membranes for alkaline direct ethanol fuel cells

A novel hybrid membrane based on poly (vinyl alcohol)/3-(trimethylammonium) propyl-functionalized silica (PVA-TMAPS) is prepared by a simple solution-casting method. The properties of the PVA-TMAPS membranes are characterized by Fourier transform infrared (FT-IR) spectroscopy, scanning electron microscopy (SEM), thermal gravimetric analysis (TGA), and X-ray diffraction (XRD). It is found that the thermal stability of the membranes increases with the addition of TMAPS particles. Moreover, the study of the effect of different weight ratios of PVA to TMAPS on the OH⁻ conductivity shows that the membrane with a ratio of PVA:TMAPS = 90:10 exhibited the highest OH⁻ conductivity. Finally, it is shown that the application of the alkaline membrane (PVA:TMAPS = 90:10) to an direct ethanol fuel cell can yield a peak power density of about 50 mW cm⁻² at 60 °C.     

Pt support of multidimensional active sites and radial channels formed by SnO2 flower-like crystals for methanol and ethanol oxidation

SnO₂ nanoflowers and nanorods have been synthesized by the hydrothermal method without using any capping agent. Both types of SnO₂ nanostructures are selected as a support of Pt catalyst for methanol and ethanol electrooxidation. The synthesized SnO₂ nanostructures and SnO₂ supported platinum (Pt/SnO₂) catalysts are characterized by X-ray diffraction, scanning electron microscope and high resolution transmission electron microscope. The electrocatalytic properties of the Pt/SnO₂ and Pt/C catalysts for methanol and ethanol oxidation have been investigated systematically by typical electrochemical methods. The influence of SnO₂ morphology on its electrocatalytic activity is comparatively investigated. The Pt/SnO₂ flower-shaped catalyst shows higher electrocatalytic activity and better long-term cycle stability compared with other electrocatalysts owing to the multidimensional active sites and radial channels of liquid diffusion.     

Highly stable hydroxyl anion conducting membranes poly(vinyl alcohol)/poly(acrylamide-co-diallyldimethylammonium chloride) (PVA/PAADDA) for alkaline fuel cells: Effect of cross-linking

Highly stable hydroxyl anion conducting membranes have been developed using poly(vinyl alcohol) (PVA) as matrix by incorporation of poly(acrylamide-co-diallyldimethylammonium chloride) (PAADDA) as anion charge carriers. In order to clarifying the cross-linking effect on membrane performances, two series of PVA/PAADDA membranes were prepared by direct and indirect chemical cross-linking ways, and have been characterized in detail at structural and hydroxyl ion (OH⁻) conducting property by FTIR spectroscopy, thermal gravity analysis (TG), scanning electron microscopy (SEM), water sorption, ion exchange capacity and alkaline resistance stability. The OH⁻ conductivity of the membranes increased with increasing the content of PAADDA in polymer and temperature, and reached 0.74–12 mS cm⁻¹ with direct cross-linking way and 0.66–7.1 mS cm⁻¹ with indirect cross-linking way in the temperature range 30–90 °C. The membranes are found to have the same IEC values but the membranes with direct cross-linking way showed higher water uptake than that with indirect cross-link one. Both membranes showed the thermal stability above 200 °C, and can integrity in 100 °C hot water and methanol solution, where the swelling are better suppressed as high dense chemical cross-linkages in PVA network. Very low methanol permeability (from 1.82 × 10⁻⁷ to 3.03 × 10⁻⁷ cm² s⁻¹) in 50% methanol solution was obtained at 30 °C. Besides, the chemical stability in 80 °C, 6 M hot alkali conditions and long-term stability of 350 h in 60 °C hot water revealed that the PVA/PAADDA membranes are promising for potential application in alkaline fuel cells.     

Carbon supported PtRh catalysts for ethanol oxidation in alkaline direct ethanol fuel cell

Owing to the formation of an oxametallacyclic conformation, the C–C bond cleavage is the preferential channel for the ethanol dissociation on the Rh surface, the addition of Rh to Pt can increase the CO₂ yield during the ethanol oxidation. However, in acidic media the slow oxidation kinetics of CO_ads to CO₂ limits the overall reaction rate. In this work, we prepare carbon supported PtRh catalysts and compare their catalytic activities with that of Pt/C in alkaline media. Cyclic voltammetry tests demonstrate that the Pt₂Rh/C catalyst exhibits a higher activity for the ethanol oxidation than Pt/C does. Linear sweep voltammetry tests show that the peak current density on Pt₂Rh/C is about 2.4 times of that on Pt/C. The enhanced electro-activity can be ascribed not only to the improved C–C bond cleavage in the presence of Rh, but also to the accelerated oxidation kinetics of CO_ads to CO₂ in alkaline media.     

Performance comparison of modified poly(vinyl alcohol) based membranes in alkaline fuel cells

There are several problems which are holding back the use of fuel cells. The utilization of fuel cells depends on the start-up costs which are very high due to the use of expensive materials for their construction. In that respect, we describe a cost-effective alkaline fuel cell (AFC) that uses solid, polymer based, membrane instead of conventionaly used, highly concentrated, corrosive, liquid alkaline electrolyte. This approach to AFC is potentially the basis of a simple, low-cost system, that can solve one of the problems of the highly-efficient and environment-friendly AFC.The focus of this paper are low cost composite alkaline membranes, based on poly(vinyl alcohol) (PVA). The PVA matrix is made by solution cast method and gamma irradiation crosslinking. Three different types of membranes are obtained in this manner - plain PVA membrane, PVA membrane cross-linked using gamma irradiation (γ-PVA) and composite PVA membrane doped with Mo (PVA-Mo). These membranes are immersed in the alkaline solution and investigated as anion exchange membranes. The performance of the solid alkaline fuel cells (SAFCs) containing these PVA membranes has been studied under hydrogen and oxygen gas flow on the Pt/C catalyst. Both, γ-PVA and PVA-Mo membranes are modified to absorb larger amounts of alkaline solution than the PVA membrane, thus greatly improving the performance of the SAFC, in terms of output power. This is clearly indicated in the polarisation curves. The electrochemical impedance spectroscopy measurements during the SAFC operation were also performed to give better insight in the effect observed. Investigation presented in this paper clearly indicates that solid alkaline PVA membranes can be used for the construction of the SAFCs.