Zhou Peng Li

A Fuel Cell Development for Using Borohydrides as the Fuel

A fuel cell was developed using borohydride solutions as the fuel. The cell consisted of an anode made of a Zr-Ni alloy, a cathode made of Pt/C, and a Na + form Nafion membrane as the electrolyte. The borohydride-fueled cell showed an open-circuit voltage of 1.3 V, compared with 1.0 V for a hydrogen gas-fueled one. The anode exhibited a small polarization property compared with the cathode. The cathode polarization was the main reason for the cell voltage drop with increasing currents. When a Nafion membrane was used as the electrolyte, it was confirmed that cations (Na + ) were the charge carrier in it. Compared with Nation 112 membrane, Nafion 117 membrane demonstrated a considerable resistance to borohydride crossover and resulted in acceptable cell performance. However, there are several problems such as H 2 evolution during operation, BH 4 crossover, NaOH accumulation at the cathode, and NaBO 2 accumulation at the anode in recent systems. Further effort is needed to develop the fuel cell using borohydrides as the fuel.

Anodic Oxidation of Alkali Borohydrides Catalyzed by Nickel

Borohydrides in alkaline media are potential fuels for fuel cells due to their high energy and power density. In this work, we studied the anodic oxidation of borohydrides on a nickel electrode. The open-circuit potential was found to be about 0.15-0.2 V more negative than the hydrogen potential, depending on the concentration of BH - 4 . The results of polarization measurements indicated that a high power density can be achieved for the BH - 4 /Ni system. However, the coulombic efficiency was found to be 50% or less due to hydrogen evolution. A further investigation showed that hydrogen gas was not only generated from the hydrolysis reaction, but also from the electrochemical reaction. The actual anodic reaction of borohydride on the Ni electrode was proved to be a four-electron process rather than an eight-electron one.

Preparation of sodium borohydride by the reaction of MgH2 with dehydrated borax through ball milling at room temperature

Abstract A convenient method was developed to synthesize NaBH4 by the reaction of MgH2 with Na2B4O7 through ball milling at room temperature. In order to improve the sodium borohydride yield, Na compounds were added to compensate the Na insufficiency in reactants when MgH2 instead of NaH was used as the reducing agent. It was found that Na2CO3 addition was better than NaOH or Na2O2 addition in increasing the borohydride yield.

Protide compounds in hydrogen storage systems

Based on three chemical states of hydrogen: protide (H−), protium (H0) and proton (H+), a triangular hydrogen energy system was proposed. The transfers among protide, protium and proton attracted our attention. We experimentally proved that NaBH4 as a protide carrier can release its energy through a fuel cell (borohydride fuel cell) directly or generate hydrogen gas for polymer electrolyte membrane fuel cell application. The used fuel (meta-borate) can be reverted to NaBH4 through a reaction with a saline hydride (MgH2).

Preparation of potassium borohydride by a mechano-chemical reaction of saline hydrides with dehydrated borate through ball milling

A convenient method was developed to synthesize potassium borohydride by a mechano-chemical reaction of saline hydrides with dehydrated borates with a planetary ball mill. Among the tested saline hydrides, MgH2 was the most effective reactant for borohydride formation. In order to improve the borohydride yield, it was necessary to add an excessive amount of MgH2. When a 35% excess of MgH2 was added, potassium borohydride yield reached 100%. The borohydride formation was strongly dependent on the water content in reactants. When the water content in KBO2 samples was over 24.8 wt.% (equivalent to KBO2·1.5H2O), no borate was converted into borohydride by the mechano-chemical reaction.

Electrochemical properties and characteristics of a fluorinated AB2-alloy

Abstract A series of fluorination techniques have been developed in this laboratory to improve the surface condition of AB 2 alloy. The fluorination was found effective for removing oxide layer and implanting catalytic Ni to the alloy surfaces. The developed fluorination techniques improved considerably the initial activation characteristics, decreased the electrochemical reaction impedance and increased the rate capacity of AB 2 electrodes.

Fluorination mechanism and its effects on the electrochemical properties of metal hydrides

Abstract Fluorination of hydriding alloys has been found effective for improving durability and initial discharge characteristics of hydride electrodes in Ni–MH rechargeable batteries. However, it has also been found that it decreases the discharge capacity because of the fluoride formed on the surface which decreases the electrical conductivity of the electrode. A more advanced technique has been developed in this laboratory to implant metallic Ni in the fluoride layer to form a functionally graded surface layer. The proposed fluorination technique considerably improves the durability and initial activation characteristics of AB 5 -types of hydride, electrodes and also the initial activation characteristics of AB 2 electrodes. The fluorination was found effective for removing the oxide layer which acts as an impedance to electrochemical hydrogen uptake.

Improvement of the electrochemical properties of Zr-based AB2 alloys by an advanced fluorination technique

An advanced fluorination technique has been developed for improving the electrochemical performances of Zr-based AB2 alloys. During the procedure, alloys were ball milled in a complex fluoride solution. It was found that the electrode of the treated alloy exhibited excellent electrochemical performance. Its activation property and high-rate discharge capability were found to be satisfactory compared with the conventional AB5-based electrodes. The electrode demonstrated a significant decrease of the reaction resistance measured by an impedance spectrometric method. The F-treated alloy was found to give a larger specific surface area and to form a highly Ni-covered surface.

Electrochemical cycle life of Zr-based Laves phase alloys influenced by alloy stoichiometry and composition

Nonstoichiometric Zr-Mn-Ni-based Laves phase alloys were prepared and their electrochemical cycle life was examined in order to develop suitable alloys for the nickel-metal hydride battery. Both the stoichiometry and chemical composition of alloys were found to influence the electrochemical cyclic stability of alloys by changing their mechanical stability. In most cases the alloys with better mechanical stability exhibited longer cycle life, except the alloy with a high vanadium content. Also, the electrochemical cyclic stability of alloys was found to he closely related to their hysteresis factors in this alloy system.

Improvement of electrochemical cyclic durability of Zr-based AB2 alloy electrodes

Abstract Zr-based AB 2 Laves phase alloys are promising materials for the negative electrode in Ni–MH batteries. In this work, we attempted to improve their cyclic durability in the electrolyte by using hyper-stoichiometric and Cr-substituted alloys. It was found that the hyper-stoichiometric alloys exhibited improved cycle life as a result of increased mechanical stability during the cycling. The Cr-incorporated alloys showed a low level of vanadium dissolution and small change in surface area after the cycling, but their overall electrochemical performances were discouraging.