ElectrochemicalPropertiesofMagnesiumBasedHydrogenStorageAlloysImprovedbyTransitionMetalBoridesandSilicidesadditives.doc

精品论文electrochemical properties of magnesium based hydrogen storage alloys improved by transition metal borides and silicides additivesjiao lifang, yuan huatanginstitute of new energy material chemistry,engineering research center of energy storage & conversion (ministry of education) and key laboratory of energy-material chemistry(tianjin),nankai university, tianjin, p.r. china （300071）e-mail: abstracttransition metal borides and silicides prepared by mechanical alloying (ma) and chemical reductionmethods (cr) were introduced to improve the corrosion resistance of magnesium based hydrogen storage alloys. the additive of feb prepared by ma can remarkably enhance the discharge capacity and cycling stability which has initial discharge capacity of 355.9 mah.g-1 and keeps 224 mah.g-1 after100 cycles, and the exchange density i0 of mgni-nib(cr) electrodes is 344.80 mag-1 but mgni is only 67.6 mag-1 which leading to the better rate capability of the composite alloys. the results of semcharacterization, cyclic chargedischarge tests, potentiodynamic polarization, linear polarization andac impedance experiment shows that the corrosion inhibition property of mgni in alkaline are improved by transition metal borides and silicides additives.keywords: mgni, hydrogen storage alloys, transition metal, borides, silicides1. introductionmagnesium-based alloys have attracted much attention and extensive research as negative electrode materials for ni/mh batteries, due to their high discharge capacity, richmineralresources,lowcostandgoodinitialactivationpropertyin electrochemical processes 1-3. nevertheless, the alloys are not satisfactory for practical applications as negative materials for ni/mh batteries because of their slow kinetics of h-sorption, rapid degradation during cycling in koh electrolyte 4. a large amount of work, such as the optimization of alloy composition and surface modifications has been done to solve these problems 5-10. it is known that borides are used as the ceramic materials because of the strong hardness and excellent corrosion inhibition property in alkaline or acid solution by forming a thin film on the surface of the as-protected composites 11-14. in addition, borides such as nib and cob are important catalysts for the selective hydrogenation in organic reactions 15. transition metal silicides-based alloys represent the largest family of intermetallic compounds having many attractive attributes for corrosion resistant properties and are, from tribology and surface engineering points of view, a prospective new class of advanced tribological coating materials for applications under corrosive and oxidative hostile service conditions because of their intrinsic high hardness, covalent-dominated strong atomic bonds and excellent chemical and thermodynamic stability 16-18.the influence of transition metal borides on magnesium based hydrogen alloys have been extensively studied in our group 19-22. in this work, transition metal borides and silicides prepared by mechanical alloying (ma) and chemical reduction methods (cr) were introduced to improve the corrosion resistance of magnesium- 9 -based hydrogen storage alloys.2. experimental2.1. preparationthe amorphous mb(m=fe, co, ni) was synthesized by chemical reduction method (cr). a typical experimental procedure is as follows: 250 dm3 of kbh4 solution (2.0 mol dm3) was adjusted to ph = 12 with potassium hydroxide to prevent violent hydrolysis.the aqueous solution of kbh4 was dropwise added to a 0.1 mol dm3 mso4 (m=fe, co, ni) aqueous solution under vigorous stir in argon atmosphere. an ice bath was used to control the reaction temperature. after that, the solution wasstirred for about 1 h to release the hydrogen to prevent burning in the consequent steps. in filtration procedure, the precipitate was washed with distilled water to clear the reaction residues, then with acetone to make the sample easy to be dried. finally, thesample was dried in vacuum at 80 c for 24 h for the removal of evaporable contents.all the reagents were of analytical grade (tianjin, north tianyi chemical reagentcompany) and used as received without further purification.transition metal borides and silicides mx (m=fe, co, ni; x=b, si) were prepared by mechanical alloying of pure m and b or si powders for certain hours. mgni alloy was prepared from the mixed powders of pure magnesium and nickel power at a molar ratio of 1:1 by ma for 100 h.2.2. characterization and electrochemical measurementsthe surface patterns of the alloys were characterized by scanning electron microscopy (sem) on a hitachi x-650 scanning electron microscope. electrodes for tests were prepared as follows: 0.8 g of a mixture of as-prepared powder and nickel powder (mass ration 1:3) were pressed into pellet (10mm in diameter) at 30mpa. a sandwich of the pellet between two foam nickel disks (20mm in diameter) was pressed at 20 mpa, on which a nickel strip was soldered. electrochemical tests employed a three-electrode system, as-prepared electrode as working electrode,niooh/ni(oh)2 as counter electrode, hgo/hg as reference electrode and 5 mol dm3koh aqueous solution as electrolyte. charge-discharge cycle tests were performed using a land ct2001a battery testing system. chi 660b electrochemical workstation was used for potentiodynamic polarization (50% dod (depth of discharge); scan rate:1mvs1, 1.2 0.2v vs. hgo/hg), electrochemicalimpedance spectroscopy (eis)(50% dod, open circuit potential, amplitude: 5mv, 104 101 hz), linear polarization (50% dod, scan rate: 0.1mvs1 5 5mv vs. open circuit potential).all the experiments were conducted at room temperature.3. results and discussion3.1. characterization of compoundsfigure 1 shows scanning electron microscopy (sem) images of the compounds whichprepared at its optimum conditions. nib (fig.1b) and cob (fig.1d) prepared by chemical reduction method have an ultrafine amorphous structure, leading to the lower surface energy of the composite alloys mgni-nib (fig.1b1) and mgni-cob (fig.1d1) than that of pure mgni(fig.1h). feb (fig.1e) prepared by ma method hassmall particle size, though the composite mgnifeb (fig.1e1) particle size wasbigger than that of pure mgni alloy, the layered structures may be benefit its electrochemical performances. all the composite alloys with large interfaces and defects have various sizes it can bee seen that the mgni alloy consists of particles with irregular shape and rough surface, the grain size varies from 2 to 4 m in diameter. for composite mgni-cosi, the particles are small, regular and smooth, the size of them is between 1 and 2.5 m. in addition, the interspaces of mgni-cosi are more uniform than pure mgni, which will be benefit for the electrode performance. as can be seen in fig.1 (f1) and (h), the pulverization of mgni alloy was more serious than that of mgni-feb, indication that amorphous feb was very helpful to prevent pulverization during mgni electrode charge-discharge cycles.the experimental conditions were listed as footnote in figure 1. mgni+nib(ma 120 h)(100:10, 10 h) means the composite alloy were prepared by ma mgni and nib(which prepared by ma for 120 h) for 10 h in a weight ratio of 100:10, and the rest may be deduced by analogy.3.2.electrochemical performance of compoundsas shown in fig. 2, all composites show better cycling stability than mgni, and it can be clearly seen that the additive of feb prepared by ma can remarkably enhance the discharge capacity and cycling stability. ma-feb-mgni has initial discharge capacity of 355.9 mah.g-1 and keeps 224 mah.g-1 after 100 cycles.the potentiodynamic polarization curves of mgni and its composites are shown in fig. 3 and the results obtained by tafel fitting are listed in table 1. for the composite alloys, the corrosion potential ecorr shifts toward positive direction and the corrosion current icorr is lower comparing with the mgni alloy. these results suggest that the addition of transition metal borides and silicides improve the anticorrosion behavior of the alloy to a certain degree. also it can be seen that the additive of feb prepared by ma can remarkably enhance the discharge capacity and cycling stability. the cycliing stability of discharge experiment (shown in fig. 2) also supports the conclusion.exchange current density is an important kinetic parameter for the charge/discharge reaction. it is the rate of hydriding/dehydriding at the equilibrium state and can be used to evaluate the kinetics of the reactions 23. the exchange current density is determined from linear micropolarization curve. fig. 4 shows typical corrected linear polarization curves for mgni and composite electrode alloys obtained at low overpotential (6 mv). a good linear relationship between overpotential and current is observed. the exchange current density i0 can be calculated by the following formula24:rti0 = id f（1）in which r is the gas constant, t the absolute temperature, id the applied current density, f the faraday constant and the total overpotential. the i0 value calculated according to above equation, which indicates that the kinetics of the electrochemical hydrogen reaction is improved in composite electrode alloys. table 2 give the exchange density i0 of mgni and composite electrodes, as is shown in table 2 that mgni-nib electrodes have larger exchange density i0 which means they have the fastest rate of hydriding/dehydriding at the equilibrium state and the best high rate capability among these alloys.it can be seen clearly from fig. 5 that the curves consisted of semicircles and linear warburg impedance. it showed the rate control step was the charge transfer process at the interface between the alloy and electrolyte. it is well known that mg on the alloy surface was easily oxidized into mg(oh)2 in koh solution, and ni could be partially oxidized at the ending period of discharge. it can be concluded from table 3 that the charge transfer resistance of all the composite electrodes is smaller thanmgni(1.792), indicating the addition of transition metal borides and silicidesprevent mg from being oxidized into mg(oh)2 in koh solution to some extent.4. conclusiontransition metal borides and silicides mx (m=fe, co, ni; x=b, si) were prepared by mechanical alloying (ma) and chemical reduction methods (cr). all composites show better cycling stability than mgni, the additive of feb prepared by ma can remarkably enhance the discharge capacity and cycling stability which has initial discharge capacity of 355.9 mah.g-1 and keeps 224 mah.g-1 afeter 100 cycles. for the composite alloys, the corrosion potential ecorr shifts toward positive direction and the corrosion current icorr is lower comparing with the mgni alloy, especially for mgni-nib(cr), the exchange density i0 of the electrodes is 344.80 mag-1 but mgniis only 67.6 mag-1 which leading to the better rate capability of the composite alloys.the eis results shows that the charge transfer resistance of all the composite electrodes is smaller than mgni(1.792), indicating the addition of transition metalborides and silicides prevent mg from being oxidized into mg(oh)2 in koh solution to some extent. electrochemical performance of magnesium based hydrogen alloys can be remarkably improved by addition of transition metal borides and silicides.acknowledgementsthe work was supported by nsfc (50701025), doctroal fundation of ministry ofeducation (20070055064).references1wang mh, zhang y, zhang lz, sun lx, tan zh, xu f, yuan ht, zhang t. the effects of partial substitution of cr for ni on the electrochemical properties of mg1.75al0.25ni1-xcrx(0x0.3). j power sources 2006;159:159-162.2wang mh, zhang lz, zhang y, sun lx, tan zh, xu f, yuan ht, zhang t. effects of partial substitution by fe and co for ni in the mg1.75al0.25ni electrode alloy on their electrochemical performances. int j hydrogen energy 2006;31:775-779.3liu wh. the electrolyte temperature dependence of the electrochemical hydrogen storage property ofmg-ni alloy codeposited from aqueous solution. j alloys compd 2005;404-406:694-698.4rongeat c, rou l. effect of particle size on the electrode performance of mgni hydrogen storage alloy. j power sources 2004;132:302-308.5iwakura c, nohara s, inoue h. preparation and characterization of mgni0.9ti0.06v0.04ni-carbon material composites for electrochemical use. solid state ionics 2002,148:499-502.6si tz, zhao gp, zhang qa. phase structures and electrochemical properties of ca0.4mg0.6(ni0.9al0.05m0.05)2(m=cu, mn, cr or co) alloys. int j hydrogen energy 2007;32:600-605.7guo zp, huang zg, konstantinov k, liu hk, dou sx. electrochemical hydrogen storage properties of nonstoichiometric amorphous mgni1+xcarbon composites (x=0.050.3). int j hydrogen energy2006;31:2032-2039.8liu jw, yuan ht, cao js, wang yj. effect of tial substitution on the electrochemical properties of amorphous mgni-based secondary hydride electrodes. j alloy compd 2005;392:300305.9kim js, lee cr, choi jw, kang sg. effects of f-treatment on degradation of mg2ni electrode fabricated by mechanical alloying. j power sources 2002;104:201207.10 zhou zx, zhang ys. properties of the ternary mg2ni0.75co0.25 hydrogen storage alloy after fluorination treatment. j electrochem soc 2001;148:a554558.11 narayanan tsns, krishnaveni k, seshadri sk. electroless ni-p/ni-b duplex coatings: preparation and evaluation of microhardness, wear and corrosion resistance. mater chem phys 2003; 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(b) cr-nib(prepared by chemical reduction method); (a1) ma-nib + mgni (weight ratio 100:10 ma for 10 h); (b1) cr-nib + mgni (weight ratio 100:10ma for15 h); (c) ma-cob(ma for 120 h); (d) cr-cob; (c1) ma-cob + mgni (weight ratio 100:10 ma for 15 h); (d1) cr-cob + mgni (weight ratio 100:15 ma for10 h); (e) ma-feb(ma for 120 h); (f) cr-feb; (e1)ma-feb + mgni (weight ratio 100:15 ma for 15 h); (f1) cr-feb + mgni (weight ratio 100:15 ma for15 h); (g)ma-cosi (ma for 80 h) + mgni (weight ratio 90:10 ma for 10 h); (h) mgni (ma for 100 h); (f1) cr-feb + mgni electrode after 100 chargedischarge cycles; (h) mgni electrode after 100 chargedischarge cycles.450400350300250mgni(ma 100 h)mgni+nib(ma 120 h)(100:10, 10 h) mgni+nib(cr)(100:10, 15