l-Ascorbic acid as an alternative fuel for direct oxidation fuel cells

l-Ascorbic acid (AA) was directly supplied to polymer electrolyte fuel cells (PEFCs) as an alternative fuel. Only dehydroascorbic acid (DHAA) was detected as a product released by the electrochemical oxidation of AA via a two-electron transfer process regardless of the anode catalyst used. The ionomer in the anode may inhibit the mass transfer of AA to the reaction sites by electrostatic repulsion. In addition, polymer resins without an ionic group such as poly(vinylidene fluoride) and poly(vinyl butyral) were also useful for reducing the contact resistance between Nafion membrane and carbon black used as an anode, although an ionomer like Nafion is needed for typical PEFCs. A reaction mechanism at the two-phase boundaries between AA and carbon black was proposed for the anode structure of DAAFCs, since lack of the proton conductivity was compensated by AA. There was too little crossover of AA through a Nafion membrane to cause a serious technical problem. The best performance (maximum power density of 16 mW cm⁻²) was attained with a Vulcan XC72 anode that included 5 wt.% Nafion at room temperature, which was about one-third of that for a DMFC with a PtRu anode.     

Preparation of platinum-ruthenium onto solid polymer electrolyte membrane and the application to a DMFC anode

The ‘impregnation-reduction method’ has been investigated as a tool for the preparation of a direct methanol fuel cell (DMFC) anode. In this method, PtRu electrocatalysts were directly bonded onto a polymer electrolyte membrane by the chemical reduction of a mixture of Pt and Ru complexes impregnated in the membrane. The deposited PtRu particles were embedded in the 3-4 μm region of the membrane surface to form a porous and hydrophilic layer. The PtRu layers turned out to be applicable to the DMFC anode, despite their small active surface areas compared to PtRu nanoparticles used in the conventional method. Approximately, 3 mg cm⁻² of the PtRu layer exhibited better catalyst utilization and facilitated the release of evolving CO₂. This preparation technique is attractive for the application of various solid polymer electrolyte materials with low heat-resistance or various shapes, etc.     

Transient phenomena in a PEMFC during the start-up of gas feeding observed with a 97-fold segmented cell

To measure local phenomena in a PEMFC during a transitional state induced by changing of the feeding gas, a segmented cell was fabricated and the local current and local potential distribution were measured under open-circuit conditions. The anode or cathode was divided into 97 segments of 1.5 mm each. A change in the anode gas from nitrogen or oxygen to hydrogen induced momentary internal currents among the segments. The potential distribution in the electrolyte was observed simultaneously using three quasi-reference electrodes located locally. The results supported the reverse-current decay mechanism, which is known to be a mechanism of cathode degradation. Furthermore, internal currents were observed when the cathode gas was changed from nitrogen to oxygen. While the cathode was not subjected to a harmful potential, a large potential distribution was induced in the anode.     

A fuel cell with selective electrocatalysts using hydrogen peroxide as both an electron acceptor and a fuel

We have realized a novel hydrogen peroxide fuel cell that uses hydrogen peroxide (H₂O₂) as both an electron acceptor (oxidant) and a fuel. H₂O₂ is oxidized at the anode and reduced at the cathode. Power generation is based on the difference in catalysis toward H₂O₂ between the anode and cathode. The anode catalyst oxidizes H₂O₂ at a more negative potential than that at which the cathode catalyst reduces H₂O₂. We found that Ag is suitable for use as a cathode catalyst, and that Au, Pt, Pd, and Ni are desirable for use as anode catalysts. Alkaline electrolyte is necessary for power generation. The performance of this cell is clearly explained by cyclic voltammograms of H₂O₂ at these electrodes. This cell does not require a membrane to separate the anode and cathode compartments. Furthermore, separate paths are not needed for the fuel and electron acceptor (oxidant). These properties make it possible to construct fuel cells with a one-compartment structure.     

A two-compartment cell for using soluble benzoquinone derivatives as active materials in lithium secondary batteries

In this study, soluble redox couples were used as active materials for an electrode using a newly designed two-compartment cell. In this cell, liquid electrolyte was separated by a solid electrolyte diaphragm, which prevents dissolved active materials from reaching the counter electrode. To balance the apparent current density and the apparent energy density, a porous sheet made of carbon paper as a current collector was set on the side of the positive electrode with an active material impregnated into it, and Li foil was set on the side of the negative electrode. Some soluble benzoquinone derivatives were examined by charge/discharge cycling for use as active materials of the positive electrode in lithium secondary batteries. Some of them showed specific capacities close to the theoretical values, assuming two-electron reduction. Among them, 2,5-dipropoxy-1,4-benzoquinone (DPBQ) could be cycled regardless of whether the amount used exceeded the solubility (with precipitate in the electrolyte) or not (all is dissolved). This implies that the active material reacts at the surface of the current collector in the dissolved state, and the precipitated fraction also participates by dissolution into the electrolyte. The results also suggest that a good cycle performance using our two-compartment cell requires an active material with relatively high solubility.     

A fundamental study on electrochemical hydrogen generation from borohydrides

Borohydrides (LiBH₄, NaBH₄, KBH₄, etc.) are the most attractive materials for hydrogen storage due to their high-volumetric and -gravimetric hydrogen density as well as safety issues. Although H₂ for fuel cells is generated by the hydrolysis of borohydrides, it is very difficult to control the rate of H₂ generation due to the nature of the catalytic reaction. In addition, the change in enthalpy (ΔH) of the reaction is directly wasted as heat generation. We propose a method for the electrochemical generation of hydrogen, in which a borohydride in an alkaline solution is oxidized at the anode while water is reduced at the cathode to generate H₂ gas. The cell has a cation exchange polymer electrolyte membrane between a precious metal anode and a Pt cathode to inhibit the crossover of BH₄⁻ anion. The open circuit voltage of the cell is positive, which raises the possibility of spontaneous operation with electrical generation as an alternative to the heat generation in hydrolysis. At the cathode, the rate of H₂ generation coincides well with the current density, indicating that H₂ generation from borohydrides can be electrochemically controlled by means of this hydrogen generator.     

High-capacity organic positive-electrode material based on a benzoquinone derivative for use in rechargeable lithium batteries

The performance of 2,5-dimethoxy-1,4-benzoquinone (DMBQ) as an active material for rechargeable lithium batteries was investigated. A positive-electrode that incorporated DMBQ showed an initial discharge capacity of 312 mAh g⁻¹ with an average voltage of 2.6 V vs. Li⁺/Li. This discharge capacity corresponds to a benzoquinone-based two-electron redox behavior, and is more than twice that of the conventional positive-electrode material lithium cobalt oxide (LiCoO₂). Furthermore, the positive-electrode with DMBQ showed fair cycle-life performance. Theoretical quantum calculations based on the density functional theory (DFT) were also performed to clarify the mechanism of the electrochemical properties of the solid state of DMBQ.     

Depression of proton conductivity in recast Nafion film measured on flat substrate

The proton conductivity of recast Nafion^® thin films in the lateral direction (parallel to the interface) was measured in humidified atmospheres. The conductivity decreased with a decrease in the thickness of the film: e.g., the conductivity of a film with a thickness of about 100 nm was about an order of magnitude less than that of the bulk material. The dependence of the conductivity on temperature was also measured, and a thinner film showed a higher apparent activation energy for conduction. Since both the conductivity and the apparent activation energy for conduction were affected by the thickness, these phenomena may be due to an intrinsic change in the material. Based on the fact that the apparent activation energy for conduction in the bulk membrane under dry conditions is high, the high apparent activation energy for conduction in thin films may be due to the hindrance of water adsorption.     

Evaluation of the number of carboxyl groups on glassy carbon after modification by 3,4-dihydroxybenzylamine

An electrochemical method for determining the number of carboxyl groups on glassy carbon (GC) has been developed. In this method, a carboxyl group is modified with 3,4-dihydroxybenzylamine (DHBA) by the action of carbodiimide. The number of carboxyl groups can be determined from the peak area of DHBA in cyclic voltammograms of GC. The physical adsorption of DHBA on GC is not desirable for this quantification. Thus, physical adsorption is minimized by thorough washing and optimization of the modification conditions. Physical adsorption was assessed by experiments on DHBA-treatment without carbodiimide. The minimization and assessment of the physical adsorption of DHBA enables the use of this modification to determine the number of carboxyl groups on GC. The method is quantitative and much more rapid than other methods that have been used to evaluate surface oxides. Using this method, the numbers of carboxyl groups on several electrochemically oxidized GCs were determined. The results revealed that strong electrochemical oxidation of GC increases not only the total number of carboxyl groups but also the surface area of the GC. Strong oxidation does not greatly increase the number of carboxyl groups per surface area.     

Comparative study of carbon-supported Pt/Mo-oxide and PtRu for use as CO-tolerant anode catalysts

Carbon-supported Pt/Mo-oxide catalysts were prepared, and the reformate tolerances of Pt/MoOx/C and conventional PtRu/C anodes were examined to clarify the features and differences between these catalysts. Fuel cell performance was evaluated under various reformate compositions and operating conditions, and the CO concentrations at the anode outlet were analyzed simultaneously using on-line gas chromatography. Pt/MoOx showed better CO tolerance than PtRu with CO(80 ppm)/H₂ mixtures, especially at higher fuel utilization conditions, which is mainly due to the higher catalytic activity of Pt/MoOx for the water-gas shift (WGS) reaction and electro-oxidation of CO. In contrast, the CO₂ tolerance of Pt/MoOx was much worse than that of PtRu with a CO₂(20%)/H₂ mixture. The results of voltammetry indicated that the coverage of adsorbates generated by CO₂ reduction on Pt/MoOx was higher than that on PtRu, and therefore, the electro-oxidation of H₂ is partly inhibited on Pt/MoOx in the presence of 20% CO₂. With CO(80 ppm)/CO₂(20%)/H₂, the voltage losses of Pt/MoOx and PtRu are almost equal to the sum of the losses with each contaminant component. Although the adsorbate coverage on Pt/MoOx increases in the presence of 20% CO₂, CO molecules in the gas phase could still adsorb on Pt through an adsorbate ‘hole’ to promote WGS or electro-oxidation reactions, which leads to a reduction in the CO concentration under CO/CO₂/H₂ feeding conditions.