Methanol and proton transport through chitosan‐phosphotungstic acid membranes for direct methanol fuel cell

Abstract

Composite chitosan-phosphotungstic acid membranes were synthesized by ionotropic gelation. Their liquid uptake is higher for thin membranes (23 ± 2 μm), while it is lower ($70%) for thicker membranes (50-70 μm). Polarization curves recorded using single module fuel cell at 70 C allowed to estimate a peak power density of 60 mW cm −2 by using 1 M as methanol and low Pt and Pt/Ru loadings (0.5 and 3 mg cm −2) at the cathode and at the anode, respectively. Electrochemical impedance spectroscopy was used to estimate the membrane conductivity and to model the electrochemical behavior of methanol electrooxidation inside the fuel cell revealing a two-step mechanism mainly responsible of overall kinetic losses. Transport of methanol inside the membrane was studied by potentiostatic measurements, allowing to estimate a methanol diffusivity of 3.6 × 10 −6 cm 2 s −1 .

Effect of TiO2 and Al2O3 Addition on the Performance of Chitosan/Phosphotungstic Composite Membranes for Direct Methanol Fuel Cells

“…CS/PTA hybrid inorganic-organic membranes were morphologically characterized at the microscale by SEM. In previous works, it has been demonstrated that PEMs for hydrogen-fed and methanol-fed low-T fuel cells can be successfully synthesized through ionotropic gelation method [ 21 , 22 , 24 ]. These PEMs were compact and flaw-free, without the presence of micro/macro voids that can negatively affect cell performance.…”

“…This result can be explained by considering that, at 250 mV, both electrode reactions have enough overpotential to be fully activated and, therefore, voltage drop is essentially due to the ohmic losses. To understand the reason why it is important to insert an inductance in the EEC to model the electrochemical behavior of the DMFC depending on the impedance spectrum cell voltage, we can consider the MOR as a two-step reaction, based on the following steps [ 24 ]: with k 1 and k 2 the rate constants of the two steps considered for the overall MOR. In particular, Reaction (6) is the methanol oxidation, involving four moles of electrons, which leads to the adsorption of intermediate CO species, whilst Reaction (7) involves the oxidation of CO ads to CO 2 .…”

“…In particular, Reaction (6) is the methanol oxidation, involving four moles of electrons, which leads to the adsorption of intermediate CO species, whilst Reaction (7) involves the oxidation of CO ads to CO 2 . It can be demonstrated that, if only one intermediate reaction is involved in the overall MOR (i.e., CO ads ), the admittance, Y (i.e., impedance reciprocal) related to methanol oxidation is expressed by the following equation [ 24 ]: that is, corresponding to an EEC that depends on the A value. The latter depends on many factors, such as the Tafel slopes of Reactions (6) and (7), but also on k 1 and k 2 .…”

“…Phosphotungstic acid (PTA) is one of the HPAs that were successfully employed to form composite CS/HPA membranes to be used as proton exchange membranes in hydrogen-fed FCs [ 19 , 20 , 21 , 22 , 23 ] and in DMFCs. In particular, CS/PTA membranes have been successfully tested as proton exchange membranes, reaching high performances at 70 °C with relatively low catalyst loadings at both anode and cathode [ 24 ]. Nevertheless, they showed quite high methanol permeability values, demonstrating that there is still room for improving their performances and decreasing methanol crossover phenomenon.…”

Tuning of the Mg Alloy AZ31 Anodizing Process for Biodegradable Implants

“…It is interesting to note, by looking at the inset of Figure , that an inductive loop is present in the low-frequency range of the EIS spectrum of as-polished AZ31. The presence of an inductive loop in the EIS spectrum is typically related to the electrochemical processes that involve more than one reaction step, where one step comprises the formation of adsorbed species on the surface leading to a dependence of reaction kinetics on the electrode surface coverage. ,, This behavior is usually observed in the case of pure Mg and Mg-alloy electrodes, for which the oxide/hydroxide film that spontaneously grows on the surface is quite incoherent and does not cover all of the substrate surface, not offering good corrosion protection to the metal, mostly in acidic or neutral environment, as HS. Moreover, the cathodic process usually coupled to the Mg oxidation is the water reduction that leads to gaseous HE that typically occurs following a multistep kinetic path, involving the presence of adsorbed species. , …”