Fuel cells are the energy conversion system where the direct conversion from chemical energy to electric energy takes place using an electrochemical system. We can theoretically use many kind of fuels, such as methanol, ethanol, hydrazine and even carbon. Their theoretical efficiencies are very high. However, the electrochemical oxidation rates of fuels other than hydrogen is far small compared to that of hydrogen especially at low temperatures. To improve their activities the anode catalyst is a key. And the analysis of the reaction products is important to get a complete oxidation. In high temperature fuel cells such as molten carbonate fuel cells and solid oxide fuel cells, we can use many kinds of fuels including carbon and carbon monoxide. Fuel cells could be applied in wide variety of fuels and wide range of temperatures.

For reduction of CO₂ in a transportation sector, the biggest problem is the sequestration of CO₂, since it is emitted and diffused from a huge number of automobiles in spatial diversity. It will be one of the effective solutions to spread zero-CO₂ emission vehicles, such as EVs and FCVs, and to concentrate CO₂ on the energy supply side. The vision of our research-and-development “the CAFE project” for realization of the low carbon society is described here. Key technologies are: (1) Platinum-free anode/cathode catalysts. (2) Anion exchange membrane. (3) Electrochemical reaction of taking out an electron directly from the liquid fuel as a high energy density carrier. Especially, hydrazine hydrate (N₂H₄・H₂O)is hopeful as an on-carbon liquid fuel. For the fuel supply infrastructure, it is expectable to use the existing 40, 000 gas stations all over Japan practically by taking advantage of a liquid fuel. Many laboratories of universities, public research institutes, and private enterprises in and outside Japan are taking part in this CAFE project.

Abstract: Rh porphyrin catalysts, which can oxidize CO at low overpotentials, w ere developed for the use of CO and CO-contaminated hydrogen gas (CO/H₂) as fuels for proton exchange membrane fuel cells. The CO oxidation activity significantly depended on the structure of porphyrin ligands, and the best catalyst catalyzed CO electro-oxidation even below 0.05 V at 60°C. A membrane electrode assembly that contains a Rh porphyrin as an anode catalyst generated considerable electricity when pure CO is supplied as a fuel. A combined catalyst of Rh porphyrin and Pt-Ru catalysts exhibited high activity toward CO(2%)-contaminated H₂・CO electro-oxidation by Rh porphyrins would counteract CO poisoning of Pt-Ru catalysts.

Rechargeable Direct Carbon Fuel Cell (RDCFC) uses solid carbon fuel and operates at high temperature. The RDCFC generates electric-power using carbon fuel that is charged by the decomposition of a hydrocarbon. The RDCFC utilizes unique reactions, namely, electrochemical oxidation of not only carbon but also CO produced by the reaction of CO₂ with C. Because no gas is supplied to RDCFC during power generation, t he anode of RDCFC is almost a closed system. The power generation characteristics are stable because the partial pressure of CO and CO₂, which are produced by electrochemical reactions, can be kept constant at thermal equilibrium composition while carbon fuel is present. In principle, 100% fuel utilization can be achieved, and the theoretical efficiency (DG/DH, determined by thermodynamics) is about 100%. Because the energy density of solid carbon is higher than that of either gas or liquid fuel, the RDCFC is expected to be a compact, high efficiency electric power generator. Expected applications include portable power sources for PCs and power sources for electric vehicles.

Abstract: Direct methanol fuel cell (DMFC) attracts much attention as a light weight and small-size power generation system due to its simple fueling system design. Methanol crossover (MCO) is a phenomenon,i n which methanol fuel permeation from the anode to the cathode occurs,that causes a loss of methanol fuel and a suppression of DMFC power. To tackle the MCO is one of the central issues to realize the DMFC for practical applications. Mixed reactant DMFC (MRFC), in which MCO does not occur,c an be a solution,since a mixture of O₂-containing methanol solution is fed to the cathode and anode of the stacked cells without using any separator. The MRFC requires reaction-selective electrocatalysts; the anode only reacts methanol and the cathode only oxygen. The presented materials have not shown sufficient reaction selectivity thus far. This article introduces new electrocatalysts developed for the MRFC electrodes.

Abstract: Fuel cells using glucose as a fuel are promising for portable and implantable energy devices because glucose is used as an energy source in most organisms, and not hazardous to human health. Several catalysts have been used to oxidize glucose such as noble metals, metal ad-atoms, redox dyes, microorganisms and enzymes. In this article, we focus on enzymatic fuel cells. For effective electron transfer between an enzyme and an electrode, mediators are generally used. However, when mediators are attached to polymers to form redox polymers, electron conduction through the redox polymer becomes the rate-limiting step in the overall electrode reactions, and limits the current density. To obtain high current density, we have proposed a high-surface-area three-dimensional enzyme electrode made of redox-polymer-grafted carbon black. The effectiveness of the electrode was verified by experiments and a mathematical model. The model calculation suggests that an increase in the surface coverage of the enzyme allows an increase in the current density to the order of 10² mA/cm², which allows biofuel cells to be used in portable energy devices. In addition, the performance of a membrane-electrode assembly-style biofuel cell using the electrode was demonstrated.

Abstract: Direct fuel cells using a polymer electrolyte membrane are promising power sources for portable electric appliances due to their suitability for miniaturization and rapid start-stop operation. The electrochemical oxidation of fuel compounds was investigated on several electrodes to identify novel fuels for use in direct fuel cells alternative to methanol. Biomass fuels, including L-ascorbic acid, ethanol and D-glucose, were studied as fuels in two-types of direct fuel cells with a cation-exchange membrane (CEM) or an anion-exchange membrane (AEM). A direct L-ascorbic acid fuel cell could be operated even without an anode catalyst. The maximum power density of direct ethanol and glucose fuel cells was significantly increased by the use of an AEM.

The feasibility of the direct ammonia fuel cells operating at low temperatures was investigated. In this study, the anion-exchange membrane (AEM) was used as an electrolyte. The performance of the fuel cells employing three-different anodes was evaluated by feeding NH₃

fuel. The open circuit voltage (OCV) was lower than the ideal electromotive force of 1.17 V and significantly dependent on the electrocatalysts. The obtained OCV was in the following sequence; Pt-Ru > C > Pt/C > Ru/C. This tendency agreed with the order of electrocatalytic activity of anodes evaluated in an alkaline aqueous solution with NH₃. The OCV and performance continuously decreased during the consecutive 1-V characteristic measurements due to the N_ad poisoning over the electrocatalyst surface when the Pt/C was applied as an anode. Furthermore, a noticeable amount of ammonia fuel was permeated across the AEM, and the nitrogen species were detected as N₂ and NO in the cathode exhaust gas. This phenomenon also led to the reduction in OCV of the direct ammonia fuel cell.

In order to develop an anode without precious metal oxide for polymer electrolyte water electrolysis, oxygen evolution reactions (OER) on Zr and Ta compounds (film or powder) have been investigated in sulfuric acid. The pseudo-current density was defined as i* (= I・Q_A^-1), where Q_A was the anodic electric charge of the cyclic voltammogram, and applied to determine for the pseudo-OER specific activity. The Zr compound film prepared under partial pressure of oxygen (po₂) of 8.6 mPa and partial pressure of oxygen (pn₂) of 0.22 Pa at base temperature (θ_Base) of 200°C for 80 min during the sputtering had the largest i* in this study. It was also the largest i* of Zr compounds in this study. The Ta compound film prepared under po₂ of 6 mPa and pn₂ of 0.22 Pa at θ_Base of 100°C for 100 min during the sputtering had the largest i* in this study. The i* of Ta compound film became closer to that of IrO₂ powder at higher temperature of sulfuric acid.

Heat-treated Pt catalysts followed by the acid treatment to introduce the surface functional groups on the support material were prepared by applying high temperature heat-treatment and repetition treatment with strong acid solution onto the Pt catalyst. The amount of the surface functional groups were evaluated by Boehm method. Heat-treated and acid treated Pt catalyst could show better I-V performance than heat-treated catalyst because it contains much surface functional groups on the surface of the support material. Heat-treated and acid treated Pt catalyst could also show good stability against potential sweep.