Performance of 2,4-dinitrophenol as a positive electrode in magnesium reserve batteries q

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1 . Journl of Power Sources Performnce of 2,4-dinitrophenol s positive electrode in mgnesium reserve btteries q S. Gopukumr ), S. Ntrjn, R. Thirunkrn, M. Skthivel Centrl Electrochemicl Reserch Institute, Krikudi , Tmil Ndu, Indi Received 3 December 1999; ccepted 31 December 1999 Abstrct Mgnesium is n interesting node bttery mteril with mny dvntges such s its high stndrd potentil of y2.37 V., low cost nd good low-temperture performnce due to n exothermic corrosion rection during dischrge. Orgnic romtic nitro compounds undergo multi-electron trnsfers of up to 18 during dischrge, nd hence, give high specific energies up to 2 A hrg. in comprison with conventionl inorgnic bttery depolrizers such s MnO 2, HgO, CuO nd AgO. Thus, it is worthwhile fbricting nd studying the performnce of bttery system combining mgnesium nd 2,4-dinitrophenol DNP. using queous hlide electrolytes like MgCl 2, MgBr nd Mg ClO This pper describes the preprtion of DNP cthodes fter stndrdiztion of the cthode mixture. MgrDNP cells 1 V, 1 A h. hve been ssembled using the bove cthode in conjunction with mgnesium lloy AZ 31. nodes nd dischrged t different current densities 1.7, 3.3, 5, 6.6 marcm 2. in 2 M MgCl, MgBr nd Mg Cl Cyclic voltmmogrms of DNP hve been recorded in 2 M Mg ClO. 4 2t vrious sweep rtes nd concentrtions in order to understnd the reduction behvior. The study suggests tht DNP is suitble orgnic compound for use s positive electrode mteril in mgnesium reserve btteries. q 2000 Elsevier Science S.A. All rights reserved. Keywords: Mgnesium; Orgnic cthodes; 2,4-Dinitrophenol; Reserve btteries 1. Introduction Mgnesium-bsed btteries w1,2x re of immense technicl interest for vriety of resons. The metl is redily vilble, esily mchinble nd possesses high negtive thermodynmic potentil. The components re eco-friendly nd chep. Further, the btteries operte over wide temperture rnge y408c to q608c. nd re gining importnce due to the possible substitution for costly lithium-bsed btteries w3,4x in severl pplictions. Severl orgnic, s well s inorgnic compounds, hve been investigted s possible cndidtes for use s positive electrode cthode. ctive mterils in mgnesium btteries q Presented t 21st Interntionl Power Sources Symposium, Brighton, UK, My, ) Corresponding uthor. Fx: q E-mil ddress: gkumr41@hotmil.com S. Gopukumr.. w5 7 x. Among these, orgnic compounds contining nitro groups exhibit multi-electron trnsfers, nd hence, re prticulrly recommended for use in bttery pplictions. A comprehensive study of different orgnic compounds for use in reserve btteries with high specific energies ws crried out by Glicksmn nd Morehouse w8,9 x. It ws concluded tht dinitro compounds re promising cthode mterils for use in mgnesium-bsed systems s they involve s much s 12 electron trnsfer during reduction. A study on the utility of met-dinitrobenzene s well s substituted dinitrobenzene ws reported by Sivsmy et l. w10 x. In view of the bove, it is useful to study the effect of substituents on the performnce of substituted metdinitrobenzene in conjunction with mgnesium nodes in different queous mgnesium hlidesrperhltes s electrolyte solutions. This pper reports the performnce of DNP in mgnesium reserve btteries t different current densities nd with different electrolytes, viz., 2 M queous solutions of mgnesium chloride, mgnesium bromide nd mgnesium perchlorte. Cyclic voltmmetric studies hve r00r$ - see front mtter q 2000 Elsevier Science S.A. All rights reserved.. PII: S

2 ( ) S. Gopukumr et l.rjournl of Power Sources Fig. 1. Dischrge curve of MgrDNP cells: optimiztion of cetylene blck. been crried out on DNP to scertin the nture of the reduction tking plce in different queous mgnesium electrolyte solutions. 2. Experimentl 2.1. Chemicls 2,4-DNP, mgnesium perchlorte, E. Merck, Germny., mgnesium bromide, mgnesium chloride Lob Chemie, AR. were used Cell ssembly nd dischrge studies AZ 31 mgnesium lloy sheets of 1.5 mm thickness nd 6cm 2 re were used s nodes. The cthode current-collector ws copper mesh nd ws of the sme size s the node. The cthode consists of mixture of 2,4-DNP initilly optimized with vrying mounts of cetylene blck nd 2 wt.% of queous crboxymethylcellulose CMC. binder. The cthode mixture ws spred uniformly over the current-collector nd compcted t n optimized pressure of 300 kgrcm 2. The electrodes were then wrpped with cellophne sheets tht served s seprtors Cyclic Õoltmmetric studies The DNP test solution ws plced in n electrochemicl cell nd bubbled with nitrogen gs to give n inert tmosphere. A BAS-100A electrochemicl nlyser Bio Anlyticl Systems, USA. ws used for cyclic voltmmetric studies. The electrochemicl cell consisted of glssycrbon working electrode re s 0.7 cm 2. nd n AgrAgCl reference electrode. Voltmmogrms were recorded in the potentil rnge from y200 to y500 mv using scn rte of mvrs. The experiments were repeted for concordncy. The cells were ssembled with one cthode plced between two nodes. The cells were ctivted t room Fig. 2. Dischrge curve of Mgr2,4-DNP cells: optimiztion of binder. CMC. Fig. 3. Dischrge curve of MgrDNP cells: optimiztion of pressure.

3 108 ( ) S. Gopukumr et l.rjournl of Power Sources Tble 1 Optimiztion of cetylene blck in cthode mix Cell prmeter Acetylene blck wt.% Open-circuit voltge V Cpcity A hrg Tble 2 Optimiztion of binder in cthode mix Cell prmeter CMC binder ml Cpcity A hrg Fig. 5. Dischrge curve of MgrDNP cells in different electrolytes t 3.33 Tble 3 Optimiztion of pressure in cthode mix 2 Cell prmeter Pressure kgrcm Cpcity A hrg temperture using the required volume of 2-M queous solution of MgCl, MgBr or Mg ClO s the electrolyte nd the performnce chrcteristics were evluted. 3. Results nd discussion Figs. 1 3 present the dischrge behvior of MgrDNP for optimiztion of cetylene blck, binder nd pressure, respectively, in the cthode mixtures t 20 ma in 2 M queous Mg ClO. 4 2solutions t 308C. It is evident from Fig. 1 nd Tble 1, tht the open-circuit voltge remins constnt V. with vrying composition of cetylene blck. This observtion is in greement with the findings of Tye w11 x. Moreover, the cpcity output increses with increse in cetylene blck content up to 50 wt.%, nd therefter mrginlly decreses. This my be due to the reduction of electroctive species nd hence to decrese in the rection sites. Accordingly, 50 wt.% AB ws tken s n optimized quntity for further investigtion. The binder composition of 0.5 cm 3 is found to be the optimum, s cn be seen from Fig. 2 nd Tble 2. This fct could be due to the fll in resistnce of the electrode with decrese in binder content. It is observed however, tht minimum of cm 3 is necessry for the mechnicl strength of the cthodes. Tble 3 nd Fig. 3 show tht the mximum cpcity obtined t 200 kgrcm 2 pressure cn be ssigned to mximum porosity of the cthode. Beyond this pressure, the cpcity of the cell declines, which my be due to decrese in porosity. The dischrge behvior of MgrDNP cells follows flt dischrge plteu with n initil decy in voltge. The verge operting voltge is in the order Mg ClO. 4 2) MgCl ) MgBr Figs The internl resistnce 2 2 Fig. 4. Dischrge curve of MgrDNP cells in different electrolytes t 1.67 Fig. 6. Dischrge curve of MgrDNP cells in different electrolytes t 5

4 ( ) S. Gopukumr et l.rjournl of Power Sources Fig. 7. Dischrge curve of MgrDNP cells in different electrolytes t 6.67 Fig. 8. Operting potentil of MgrDNP cells t vrious current densities nd electrolytes. Tble 4. of the cells in different electrolytes indicted tht the lower vlue is obtined in the cse of Mg ClO. 4 2nd hence enhnces the operting voltge. The verge voltge of the cells t vrious current densities nd in different electrolytes is presented in Fig. 8. Tble 5 presents the cpcity of MgrDNP cells obtined t different current densities nd electrolytes. It is seen tht Mg ClO. 4 2ex- hibits superior behvior up to the highest current density investigted. It is observed tht the performnce in different electrolytes, viz., Mg ClO. 4 2, MgBr 2, nd MgCl 2, shows lmost the sme cpcity up to the highest current density, even though the corrosion of Mg node is differw12 x. Comprison of the present results with our erlier stud- ent in ech electrolyte ies on 3,5-DNT Tble 6. shows tht the cpcity obtined in the cse of DNP is higher throughout the investigted rnge of current density. This cn be ttributed to the ese of electron cceptnce by the nitro group. Further, the higher efficiency of DNP cn lso be due to greter degree of electrode reduction of the nitro groups due to n inductive effect. The number of electrons trnsferred is mximum in the cse of Mg ClO. 4 2t ll current densities investigted Tble 7.. At higher current densities, however, the electron reduction is less due to incresed polriztion. Cyclic voltmmetric studies of DNP revel only well-defined cthodic pek the reduction process hs n irreversible nture, s depicted in Fig. 9. The studies were crried out in the potentil rnge V for Tble 4 Internl resistnce of MgrDNP cells in different electrolytes Electrolyte Mg ClO MgBr MgCl Internl resistnce V. vrious concentrtions nd sweep rtes. Further, with incresing sweep rte n. nd concentrtion C., the pek potentil E. p shifts to more negtive vlue, thereby confirming the irreversible nture. All the studies were crried out in 2 M Mg ClO. electrolyte Fig A plot of i vs. 6n gives liner reltionship Fig These p Tble 5 Cpcity A hrg. t vrious current densities for different electrolytes Electrolyte Cpcity A hrg Mg ClO MgBr MgCl y2. Current density ma cm. Tble 6 Comprison of 2,4-DNP with 3,5-DNT Orgnic cthode Cpcity A hrg ,5-DNT ,4-DNP y2. Current density ma cm. Tble 7 Cpcity efficiency %. nd number of electrons trnsferred n. in MgrDNP cells Electrolytes 2. Current density marcm n % n % n % n % Mg ClO MgBr MgCl

5 110 ( ) S. Gopukumr et l.rjournl of Power Sources Fig. 9. Reduction mechnism of 2,4-DNP. Fig. 12. Vrition of pek current i. with concentrtion C. of DNP. p Fig. 10. Cyclic voltmmogrm of 7.5 mm 2,4-DNP in 2 M Mg ClO. 4 2t vrious sweep rtes. results suggest tht reduction of DNP is diffusion-controlled... Fig. 11. Vrition of pek current i with sweep rte n. p A plot of i p vs. C lso indictes liner reltionship Fig A constnt vlue is observed for i rac6n p As re of the working electrode. from Tble 8. These fctors further confirm tht the reduction of DNP proceeds vi diffusion-controlled process. Qulittive nlysis ws crried out on the end product obtined fter the dischrge of the cells. The following procedure ws performed to test for the presence of mine. The mteril ws dissolved in dilute HCl nd sodium nitrte solution ws dded. A cler solution ws obtined. This ws treted with b-npthol in NOH solution nd plced in ice-cold wter for 5 min. A scrlet red dye ws obtined, which thereby confirmed the presence of mine. Tble 8 Effect of concentrtion C. nd sweep rte n. on the reduction of DNP in Mg ClO. solution p p p p y5 Concen- Sweep i =10 y E i r6n i rac6n trtion rte, n ma. mv. =10 mm. mvrs

6 ( ) S. Gopukumr et l.rjournl of Power Sources Conclusion 1. A stedy operting voltge is observed for DNP in comprison to 3,5-DNT. 2. The cthode efficiency of DNP is 55% t low current densities 1.7 marcm 2. nd is 19% even when the current density is incresed to The electrochemicl reduction of DNP is irreversible nd diffusion-controlled. References wx. 1 G.W. Vinl Ed., Primry Btteries, Wiley, New York, 1950, pp wx 2 C.D.S. Tuck, Modern Bttery Technology, Ellis Horwood, New York, wx 3 L. Jrvis, J. Power Sources wx 4 T.D. Gregory, R.J. Hoffmn, R.C. Winterton, J. Electrochem. Soc wx 5 G. Kumr, A. Sivshnmugm, N. Muniyndi, J. Power Sources wx 6 G. Kumr, A. Sivshnmugm, R. Sridhrn, J. Electrochem. Soc wx 7 G. Kumr, A. Sivshnmugm, N. Muniyndi, J. Appl. Electrochem wx 8 R. Glicksmn, C.K. Morehouse, J. Electrochem wx 9 R. Glicksmn, C.K. Morehouse, J. Electrochem. Soc w10x K. Sivsmy, S. Rjeswri, K. Dkshinmurthy, J. Power Sources w11x G.W. Heise, N.C. Choon, Primry Bttery II Wiley, New York, w12x R. Udyn, N. Muniyndi, P.B. Mthur, Br. Corros. J