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[en] Highlights: • A simple hot-mould-modified Nafion membrane is used for direct methanol fuel cells. • Regular spindle-type groove array is uniformly distributed on the membrane surface. • Modification lowers methanol crossover, swelling, and improves proton conductivity. • Power density and long-time running performance of fuel cells are improved visibly. • This method provides a practicable way for direct methanol fuel cell applications. - Abstract: To lower methanol crossover and volume swelling degree, and to improve proton conductivity, a simple hot-mould-modifying method has been introduced to modify Nafion membrane for the direct methanol fuel cell application. To evaluate effect of the modification on properties of the Nafion membrane and fuel cell performance, a series of measurements of membranes and fuel cells have been carried out. The results show that, compared with the normal membrane, the modified Nafion membrane with regular spindle-type groove array possesses higher proton conductivity and methanol diffusion resistance, and 31.9% better dimensional stability, owing to its larger electrical double-layer capacitance come from the higher contact area between electron-electrode and ion electrolyte, and its more compact internal structure. And also, the direct methanol fuel cell based on the modified Nafion membrane shows 13.3% higher discharge power density and better long-time running performance than the normal one. Furthermore, this hot-mould-modifying method could be introduced into doping/coating-modified membranes reported in the current literature to further modify Nafion membranes, because this method is compatible with the current modifications.
[en] The current direct methanol fuel cells are still largely limited to the neat methanol operation due to the high methanol permeation of the currently used Nafion membranes. To achieve high methanol concentration DMFC application of the sulfonated poly(ether ether ketone) (SPEEK)-based proton exchange membrane, the neat SPEEK membrane is here modified by blending with the designed cross-linked fully aromatic sulfonated polyamide (Cr-fa-SPA). As the similar fully aromatic features of the SPEEK and the Cr-fa-SPA and the formed hydrogen bonding interactions between the SPEEK and the Cr-fa-SPA, the SPEEK/Cr-fa-SPA membranes show good compatibility, which is beneficial for the construction of the uniform and contact morphology of the membrane. In addition, the cross-linking network and the hydrogen bonding interactions in the SPEEK/Cr-fa-SPA membranes are favorable for enhancing the dimensional stability and reducing the methanol permeation. In particular, the lowest methanol permeability of 0.46 × 10−7 cm2/s is successfully obtained by blending 40 wt% Cr-fa-SPA in the SPEEK/Cr-fa-SPA membrane, which is almost two magnitudes lower than that of the Nafion 117 membrane. Finally, the SPEEK/Cr-fa-SPA DMFC devices were successfully demonstrated with the highly concentrated 20 M methanol fuels with remarkable performance.
[en] Highlights: • Pt nanoparticles were uniformly deposited on graphene with MoO_3. Their size can be tuned by controlling MoO_3 loading. These Pt catalysts are high active on methanol oxidation. They also show high tolerance to CO poisoning. - Abstract: Pt nanoparticles (NPs) were uniformly deposited on the reduced graphene oxides (RGOs) by one-pot thermoreduction strategy with assist of MoO_3. MoO_3 can significantly reduce the size of Pt NPs on RGOs. These Pt NPs can be averaged to be 3.0 to 4.1 nm with MoO_3 loading from 27.4 to 8.8%. Without MoO_3, the size of Pt NPs can reach up to 15.2 nm. In addition, MoO_3 in Pt-MoO_3/RGO catalysts conducts a surface-confined reversible electron transfer. And the Pt-MoO_3/RGO catalysts show strong resistance to CO poisoning and high activity towards methanol oxidation reaction (MOR). Among these Pt-based catalysts, Pt-MoO_3/RGO catalysts with 16.5% MoO_3 loading possess a largest MOR current up to 610 mA mg"−"1 Pt with a smallest deteriorate rate of 0.000425 s"−"1 polarizing for 5000 s at 0.65 V. These results demonstrate commercial feasibility for Pt catalysts to reduce significantly the amount of precious metals Pt in parallel to maintain a high MOR activity and CO tolerance.
[en] Highlights: • Heat-treated CoPc/C supported W18O49 has the potential to replace platinum in DMFCs. • It has similar characteristics to platinum but cheaper in price. • Single cell shows an improvements compared with other non-platinum catalysts. - Abstract: Currently, platinum is used as the cathode catalyst in direct methanol fuel cells (DMFCs). However, platinum reduces the number of active sites for the oxygen reduction reaction (ORR) due to the reaction between platinum and methanol and the strong adsorption of carbon monoxide molecules on the platinum surface, and in addition, platinum is very expensive. Hence, this study proposes cobalt phthalocyanine/carbon-tungsten oxide nanowires (W18O49) as a non-platinum catalyst for the cathode side of a DMFC. We determined the kinetic parameters of the catalyst and investigated the factors affecting the reaction. The factors involved in the examination of the significant parameters were the synthetic method, the pyrolysis temperature, the mass ratio of CoPc to carbon, the mass ratio of CoPc/C to tungsten hexachloride and the molarity of tungsten hexachloride. The optimum parameters were determined to be a pyrolysis temperature of 600 °C and a mass ratio of CoPc/C:WCl6 of 1.90. These conditions produced a peak potential at 0.63 V vs. RHE, a mass activity of 1.76 A gcatalyst−1 at 0.65 V, an average electron transfer number of 3.9 with water as the main product, and an electron transfer number of 3.9 at 0.65 V. Finally, the results showed that the non-platinum catalyst (which is less expensive than platinum) has similar characteristics to platinum. The single cell performance test showed that the power density is 9.0 mW cm−2. The catalyst has comparable performance with other macrocycle catalysts with modified structures.
[en] Highlights: • The dependence of current on flow rate has been measured in a methanol fuel cell. • The average number of electrons released per methanol molecule is obtained. • Methanol crossover was measured by analysis of methanol in the cell exhaust. • A method for accounting for crossover losses is presented. • The efficiency loss due to crossover has been quantified. - Abstract: The energy efficiency of a direct methanol fuel cell (DMFC) is dependent on the cell potential, crossover of methanol and oxygen, and the average number of electrons released per methanol molecule (n_a_v) at the anode. The n_a_v value is determined by the product distribution (CO_2:formic acid:formaldehyde), and in the absence of methanol loss due to crossover can be determined from the dependence of the cell current on the flow rate of the methanol solution. The effects of crossover have been assessed here by analysis of methanol in the cell exhaust, and a method to correct for crossover is presented. Under mass transport limited conditions, the effect of crossover is shown to be negligible, and accurate n_a_v values are obtained from the flow rate dependence of the current alone. However, analysis of methanol in the cell exhaust is essential for determining n_a_v under mixed kinetic and mass transport control. The exhaust concentration, combined with the flow rate dependence of the current, also provides a measure of efficiency losses due to crossover.
[en] For the purpose of a direct methanol fuel cell (DMFC), this research investigates the kinetics of methanol oxidation in a porous layer consisting of external hierarchical nanostructures through 2D COMSOL simulation. Three different lengths of nanowires (L) were considered in simulations. The investigation showed that specific activity was reversely proportional to nanowires length and roughness factor (Rf). However, the current density increased by increasing Rf. Although the current density in case of L = 200 nm and 500 nm was identical with respect to Rf, there was a slight deviation when L = 1000 nm due to the changing from kinetic to diffusion controlled regime, which was identified by investigation of Thiele modulus. The catalytic efficiency for L = 1000 nm dropped to 50% at Rf = 140, whereas the high efficiency with no mass-transport limitation was achieved by shorter nanowires. Therefore, increasing Rf within the simulation range resulted in increasing the total catalytic activity but simultaneously decreasing the specific activity because of the decrease in pore accessibility and catalytic efficiency of nanostructures. (paper)
[en] Phosphotungstic acid (H3PW12O40, HPW) molecules were anchored onto carbon nanotubes (CNTs) by electrostatic self-assembly using poly(diallyldimethylammonium chloride) (PDDA); subsequent incorporation of these particles into poly(vinyl alcohol) (PVA) afforded a methanol-blocking membrane for direct methanol fuel cell (DMFC) application. The prepared membrane exhibited a significantly higher proton conductivity of 9.4 mS cm−1 at 60 °C and satisfactory proton conductivity stability over a 120-h test than a PVA membrane. Moreover, the composite membrane showed a decrease in the methanol permeability by ∼40% compared to a PVA membrane (6.10 × 10−7 cm2 s−1) and much better proton-to-methanol selectivity because of a higher dimensional stability after the incorporation of CNTs. A single DMFC based on the prepared membrane exhibited a maximum power density of 16 mW cm−2 at 60 °C. Thus, CNT-PDDA-HPW/PVA membranes have a great potential as an alternative proton exchange membrane for DMFC application.
[en] Proton exchange membrane fuel cells fed with H_2/CO mixtures at the anode have a considerably lower performance than fuel cells fed with pure hydrogen. However, when operated in an autonomous oscillatory regime, the overall voltage loss decreases due to a self-cleaning mechanism. Another molecule, also widely used as feed in the fuel cell and susceptible to kinetic instabilities, is methanol. To the best of our knowledge, there are no reports on autonomous voltage oscillations in the direct methanol fuel cell (DMFC). The purpose of this work was to explore if such instabilities also occur in the DMFC system. Initially, half-cell experiments with a gas diffusion electrode were performed. Then, a DMFC was operated under current control and studied by means of electrochemical impedance spectroscopy. The half-cell measurements revealed that the induction period for oscillations depends on the mass transfer conditions, where on stagnant electrode the induction time was shorter than in the case of forced convection. The DMFC showed also autonomous voltage oscillations above a certain threshold current. The results obtained by electrochemical impedance spectroscopy give evidence of a negative differential resistance in the fuel cell, hitherto not described in the literature, which can be related to the appearance of oscillations during galvanostatic methanol electro-oxidation. These results open the possibility to evaluate the performance of low-temperature fuel cells fed with carbon-containing fuels under oscillatory operating conditions.
[en] A passive, air-breathing 4-cell micro direct methanol fuel cell (μDMFC) stack is presented featured by a fuel delivery structure for a long-term and stable power supply. The fuel is reserved in a T shape tank and diffuses through the porous diffusion layer to the catalyst at anode. The stack has a maximum power output of 110mW with 3M methanol at room temperature and output a stable power even thought 5% fuel is the remained in reservoir. Its performance decreases less than 3% for 100 hours continuous work. As such, it is believed to be more applicable for powering the wireless sensor nodes
[en] The effect of the organic solvent polarity on the properties of unsupported PtRu catalyst inks and on the performance of the catalyst layers prepared with them for the methanol electrooxidation, has been studied. The light scattering results indicate that the PtRu-Nafion"® aggregates in the inks prepared with n-butyl acetate (NBA) are larger than those prepared with 2-propanol (IPA). The lower polarity of the former favours the aggregation of Nafion"® and nanoparticles. The electron microscopy images and porosimetry measurements of the catalyst layers show that the secondary pore volume between the agglomerates is larger for NBA. The linear sweep voltammetry and eis results for the methanol electrooxidation in the three-electrode cell denote the higher active surface area for NBA and comparable specific oxidation rates of the intermediates in both catalysts layers. The current densities for PtRu anode catalyst layers in single DMFC are higher when the solvent is NBA, the mass transport limitations being much more apparent with IPA. The adapted transmission line equivalent circuit to interpret the impedance results in single DMFC indicates that the proton resistance for NBA is significantly lower than for IPA, thus suggesting that the greater number of accessible active sites for methanol oxidation in the former are well connected to the Nafion"® ionomers and easier transported to the membrane.