Koichiro Asazawa

Noble Metal-Free Hydrazine Fuel Cell Catalysts: EPOC Effect in Competing Chemical and Electrochemical Reaction Pathways

We report the discovery of a highly active Ni-Co alloy electrocatalyst for the oxidation of hydrazine (N(2)H(4)) and provide evidence for competing electrochemical (faradaic) and chemical (nonfaradaic) reaction pathways. The electrochemical conversion of hydrazine on catalytic surfaces in fuel cells is of great scientific and technological interest, because it offers multiple redox states, complex reaction pathways, and significantly more favorable energy and power densities compared to hydrogen fuel. Structure-reactivity relations of a Ni(60)Co(40) alloy electrocatalyst are presented with a 6-fold increase in catalytic N(2)H(4) oxidation activity over todays benchmark catalysts. We further study the mechanistic pathways of the catalytic N(2)H(4) conversion as function of the applied electrode potential using differentially pumped electrochemical mass spectrometry (DEMS). At positive overpotentials, N(2)H(4) is electrooxidized into nitrogen consuming hydroxide ions, which is the fuel cell-relevant faradaic reaction pathway. In parallel, N(2)H(4) decomposes chemically into molecular nitrogen and hydrogen over a broad range of electrode potentials. The electroless chemical decomposition rate was controlled by the electrode potential, suggesting a rare example of a liquid-phase electrochemical promotion effect of a chemical catalytic reaction (EPOC). The coexisting electrocatalytic (faradaic) and heterogeneous catalytic (electroless, nonfaradaic) reaction pathways have important implications for the efficiency of hydrazine fuel cells.

Investigation of PEM type direct hydrazine fuel cell

Abstract Hydrazine was examined as a fuel in a direct-liquid-fueled fuel cell employing proton exchange membrane (PEM) for the electrolyte. Hydrazine showed better performance than methanol in the direct fuel cell; the cell using hydrazine gave voltage twice as high as that using methanol in the low-current density region. The I – V characteristics were drastically changed depending on the surface area of the anode catalyst. Compositions of the exhaust materials from each electrode were analyzed in order to investigate the reaction that occurred at the electrodes. The analysis revealed that the catalytic decomposition reaction of hydrazine proceeded further than the electro-oxidation reaction on the anode side using a high specific surface area catalyst. The crossover of hydrazine and ammonia through the PEM was confirmed and the reduction of the hydrazine crossover is important in developing further high performance.

Study of Anode Catalysts and Fuel Concentration on Direct Hydrazine Alkaline Anion-Exchange Membrane Fuel Cells

A platinum-free fuel cell using liquid hydrazine hydrate (N 2 H 4 ·H 2 O) as the fuel and comprised of a cobalt or nickel anode and a cobalt cathode exhibits high performance. In this study, the fuel cell performances using nickel, cobalt, and platinum as anode catalysts are evaluated and compared. It is found that fuel cell performance in the case of nickel and cobalt is higher than that in the case of platinum. Further, anodic reactions are discussed by comparing the hydrazine consumption and ammonia generation when cobalt and nickel are used as anode catalyst. Cobalt exhibits a higher rate of decomposition than nickel. Nickel is found to be the most suitable anode catalyst among the above mentioned anode catalysts for this fuel cell. The influence of hydrazine hydrate and KOH concentrations in the fuel on cell performance is also discussed. Cell performance is the highest at a hydrazine hydrate concentration of 4 M and a KOH concentration of 1 M. The maximum power density of the alkaline anion-exchange membrane fuel cell, comprised of a nickel anode and a Co-PPY-C (cobalt polypyrrole carbon) cathode, is 617 mW cm -2 .

Anode Catalysts for Direct Hydrazine Fuel Cells: From Laboratory Test to an Electric Vehicle

Novel highly active electrocatalysts for hydrazine hydrate fuel cell application were developed, synthesized and integrated into an operation vehicle prototype. The materials show in both rotating disc electrode (RDE) and membrane electrode assembly (MEA) tests the world highest activity with peak current density of 16,000u2005Au2009g(-1) (RDE) and 450u2005mWu2009cm(-2) operated in air (MEA).

Anion Conductive Aromatic Block Copolymers Containing Diphenyl Ether or Sulfide Groups for Application to Alkaline Fuel Cells

A novel series of aromatic block copolymers composed of fluorinated phenylene and biphenylene groups and diphenyl ether (QPE-bl-5) or diphenyl sulfide (QPE-bl-6) groups as a scaffold for quaternized ammonium groups is reported. The block copolymers were synthesized via aromatic nucleophilic substitution polycondensation, chloromethylation, quaternization, and ion exchange reactions. The block copolymers were soluble in organic solvents and provided thin and bendable membranes by solution casting. The membranes exhibited well-developed phase-separated morphology based on the hydrophilic/hydrophobic block copolymer structure. The membranes exhibited mechanical stability as confirmed by DMA (dynamic mechanical analyses) and low gas and hydrazine permeability. The QPE-bl-5 membrane with the highest ion exchange capacity (IEC = 2.1 mequiv g(-1)) exhibited high hydroxide ion conductivity (62 mS cm(-1)) in water at 80 °C. A noble metal-free fuel cell was fabricated with the QPE-bl-5 as the membrane and electrode binder. The fuel cell operated with hydrazine as a fuel exhibited a maximum power density of 176 mW cm(-2) at a current density of 451 mA cm(-2).

Comparative Study on the Catalytic Activity of the TM–N2 Active Sites (TM = Mn, Fe, Co, Ni) in the Oxygen Reduction Reaction: Density Functional Theory Study

We investigate the interaction of transition metal–nitrogen (TM–N2; TM = Mn, Fe, Co, and Ni) active sites with oxygen reduction reaction (ORR) related molecules using the density functional theory (DFT) calculations. Generally, the trend of molecular adsorption energy in TM–N2 systems is Mn–N2 > Fe–N2 > Co–N2 > NiN2. In the case of O2 adsorption, the O2 molecule is adsorbed with a symmetric side-on configuration on the TM–N2 systems, regardless the type of TM atom. We also find that when the reaction of the adsorbed HO2 molecule and an H atom takes place, instead of forming H2O2 molecule the reaction produces two OH radicals. From the evaluation of the potential energy surface profiles of the oxygen reduction reaction (ORR), we find that a direct four-electron reduction pathway could be facilitated on the TM–N2 active sites. However, all of the TM–N2 systems share the same main rate-limiting reaction, which is the OH reduction step. Generally, the edge-like TM–N2 active site has stronger molecular adsorpt...

Robust anion conductive polymers containing perfluoroalkylene and pendant ammonium groups for high performance fuel cells

A novel series of ammonium-containing copolymers (QPAF-4) were designed and synthesized as anion exchange membranes for alkaline fuel cell applications. The copolymers were prepared via a nickel promoted polycondensation reaction with high molecular weights (Mw = 72.7–276.4 kDa as precursors) and were composed of perfluoroalkylene and fluorenyl groups with pendant ammonium groups. The QPAF-4 membrane with optimized copolymer composition and ion exchange capacity exhibited high hydroxide ion conductivity (86.2 mS cm−1 in water at 80 °C) and excellent mechanical properties (large elongation at break = 269%). A severe alkaline stability test of the QPAF-4 membranes in 1 M KOH at 80 °C for 1000 h and the post-test analyses of the 1H NMR spectra, solubility, and mechanical properties revealed minor, or no, changes in the chemical structure and properties. Alkaline fuel cells using the QPAF-4 membrane were operated using hydrazine as a fuel and oxygen or air as oxidant to achieve the high maximum power density of 515 mW cm−2. The durability of the membrane was also confirmed in the operating fuel cell at a constant current density for longer than 1000 h.

XAFS Analysis of Unpyrolyzed CoPPyC Oxygen Reduction Catalysts for Anion-Exchange Membrane Fuel Cells (AMFC)

CoPPyC were analyzed with synchrotron X-ray adsorption fine structure (XAFS) measurements, and the correlations between electrochemical properties and structure of electrocatalysts are discussed. Electrochemical properties have been analyzed by using rotating-ring disk electrode in 1M KOH. Acid-treated CoPPyC (CoPPyC-AT) has higher activity than PPyC for oxygen reduction reaction. Concentration of Co in CoPPyC-AT is as low as 0.1 atomic %. From the analysis of EXAFS of Co, CoPPyC electrocatalysts as synthesized consist of two peaks. The peak around 1.6 Aa was assigned to Co-N and/or Co-O shells. The second peak around 2.6 Aa was assigned to Co-O-Co shells originated from cobalt hydroxide. CoPPyC-AT showed only one peak of assigned to Co-N and/or Co-O, and it indicates that cobalt hydroxide is removed by acid treatment. So, it is clear that a co-existence of cobalt and nitrogen in CoPPyC-AT shows specific performance, and pyrolysis is not necessary to make correlation of Co-N.

Adsorption of Carbohydrazide on Au(111) and Au 3 Ni(111) Surfaces

Carbohydrazide (CH6N4O) is a potential substitute to hydrazine in fuel cell applications. This paper presents a theoretical study on the adsorption of carbohydrazide on Au(111) and Au3Ni(111) surfaces using first principles calculations based on density functional theory. Results show that without van der Waals correction in the calculations, carbohydrazide weakly physisorbs on Au(111), corroborating the experimentally observed high overpotential requirement for carbohydrazide oxidation on Au catalyst. An enhanced reactivity is observed by alloying Au with Ni due to the emergence of a localized d-band near the Fermi level that interacts strongly with the HOMO of carbohydrazide. On Au3Ni(111), a N–Ni bond between carbohydrazide and the surface is formed, characterized by the hybridization of N–pz and Ni–dzz states. These results pose insights into the use of 3d transition metals as alloying components in enhancing the reactivity of Au catalyst for carbohydrazide oxidation.Graphical Abstract

Study of Catalytic Reaction at Electrode–Electrolyte Interfaces by a CV-XAFS Method

A method combining cyclic voltammetry (CV) with x-ray absorption fine structure (XAFS) spectroscopy, viz. CV-XAFS, has been developed to enable inxa0situ real-time investigation of atomic and electronic structures related to electrochemical reactions. We use this method to study the reaction of a Pt/C cathode catalyst in the oxygen reduction reaction (ORR) in an alkaline electrolyte, using x-ray energies near the Pt LIII edge for XAFS measurements. It was found that the current induced by the ORR was first observed at approximately 0.08xa0V versus Hg/HgO, although the Pt valence, which is reflected in the oxidation states, remained almost unchanged. The electronic structure of the catalytic surface in the ORR was observed to be different in the negative and positive scan directions of CV measurements. Hydrogen adsorption is also discussed on the basis of the observation of this spectral change. We have demonstrated that CV-XAFS provides dynamical structural and electronic information related to electrochemical reactions and can be used for inxa0situ real-time measurements of a catalyst.