Study Of Monolithic Porous Carbon As Substrates For Nickel Hydroxide Electrodes

Single flow zinc nickel battery is suitable for large scale energystorage applications. In this battery, nickel hydroxide electrode as positiveelectrode, conductive inert material as negative electrode and flowingzincates in alkali solutions as electrolyte were used, respectively. Owingto the capacity and cycle life of this battery is restricted by the nickelhydroxide electrode, it is important to study a new-type nickel oxideelectrode with high capacity. In the dissertation, monolithic porouscarbon （MPC） as substrates for nickel hydroxide electrodes is proposedby our team. A novel MPC-Ni（OH）₂electrode has been obtained throughthe experimental study of preparation of MPC, electrochemicalimpregnation of Ni（OH）₂, modification of nickel hydroxide and MPC.The preparation method and the principle of MPC were studied. In thepaper, adopting phenolic resin （PF） as carbon precursors and binders,polyvinyl butyral （PVB） as pore former, and active carbon （AC） asconducting additives and underprop for bating contractility to makepolymer blend, sub-micron/micron monolithic porous carbon （MPC）were prepared by blend carbonization. The single factor and orthogonal methods were used to study the effects of solidified temperature,carbonization temperature, and content of AC and PVB on the porestructure, mechanical strength and conductivity of MPC. When the ratioof PF, PVB and AC is4:2:1, stabilized temperature is185℃, andcarbonization temperature is1000℃, a optimized MPC is prepared. Theoptimized MPC has a good mechanical strength, a high pore volume(0.766mL g^-1), a suitable pore diameter distribution （0.1-5μm） and a goodconductivity (11.24S cm^-1).MPC-Ni（OH）₂electrode is fabricated by electrochemicalimpregnation Ni（OH）₂from metal nitrate solutions on to the monolithicporous carbon （MPC）, which is used for the substrates of nickel electrode.The impregnation principle was studied and the impregnation reactionfactors were optimized. When the reaction temperature is80℃, theconcentration of nickel nitrate is2M, the initial pH of solution is3.5, thecurrent density is80mA cm^-2, the impregnation time is3h and thesolution is string, the prepared MPC-Ni（OH）₂electrode has the bestelectrochemical performance. In this condition, the MPC-Ni（OH）₂electrode reach a specific capacity of active material （SCAM） of259mAh g^-1, a specific capacity of electrode weight （SCEW） of136mAh g^-1and a specific capacity of electrode Area （SCEA） of24mAh cm^-2.MPC-Ni（OH）₂electrode with Co and Zn additives is fabricated by electrodepositing from metal nitrate solutions on to MPC. It is found thatthe prepared Co and Zn additives are substituted for nickel in the solidstate of Ni_1-x-yCo_xZn_y（OH）₂. Compared to pure MPC-Ni（OH）₂electrodes,the electrochemical performance and cycle life of MPC-Ni（OH）₂electrodes with Co and Zn additives improve. SCAM, SCEW and SCEAof MPC-Ni（OH）₂electrode with Co and Zn additives is271mAh g^-1,145mAh g^-1and26mAh cm^-2, respectively.The appearance of two potential plateaus during the discharge ofMPC-Ni（OH）₂electrode and the oxygen evolution characteristic on MPCin alkali solutions was studied. To restrain the oxygen evolution reactionand improve the cycle life of MPC-Ni（OH）₂electrode, two ways wereused. One was plating nickel on the surface of the Ni（OH）₂, the other wasplating nickel on the surface of the MPC. The former improved thecontact intensity between Ni（OH）₂and MPC. The latter improved theoverpotential of the oxygen evolution reaction on MPC and restrained theoxygen evolution reaction during the charge process. It is evident that thetwo ways all could enhance the cycle life of MPC-Ni（OH）₂electrodes.After30cycles, the two modified electrodes still keep83%and90%ofthe highest discharge capacity, respectively.