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1 Oxygen evlutin and recmbinatin kinetics inside sealed rechargeable, Ni-based batteries Ntten, P.H.L.; Verbitskiy, E.A.; Kruijt, W.S.; Bergveld, H.J. Published in: Jurnal f the Electrchemical Sciety DOI: 0.49/ Published: 0/0/2005 Dcument Versin Publisher s PDF, als knwn as Versin f Recrd (includes final page, issue and vlume numbers) Please check the dcument versin f this publicatin: A submitted manuscript is the authr's versin f the article upn submissin and befre peer-review. There can be imprtant differences between the submitted versin and the fficial published versin f recrd. Peple interested in the research are advised t cntact the authr fr the final versin f the publicatin, r visit the DOI t the publisher's website. The final authr versin and the galley prf are versins f the publicatin after peer review. The final published versin features the final layut f the paper including the vlume, issue and page numbers. Link t publicatin Citatin fr published versin (APA): Ntten, P. H. L., Verbitskiy, E. A., Kruijt, W. S., & Bergveld, H. J. (2005). Oxygen evlutin and recmbinatin kinetics inside sealed rechargeable, Ni-based batteries. Jurnal f the Electrchemical Sciety, 52(7), A423- A433. DOI: 0.49/ General rights Cpyright and mral rights fr the publicatins made accessible in the public prtal are retained by the authrs and/r ther cpyright wners and it is a cnditin f accessing publicatins that users recgnise and abide by the legal requirements assciated with these rights. Users may dwnlad and print ne cpy f any publicatin frm the public prtal fr the purpse f private study r research. Yu may nt further distribute the material r use it fr any prfit-making activity r cmmercial gain Yu may freely distribute the URL identifying the publicatin in the public prtal? Take dwn plicy If yu believe that this dcument breaches cpyright please cntact us prviding details, and we will remve access t the wrk immediately and investigate yur claim. Dwnlad date: 6. Jan. 209

2 /2005/52 7 /A423//$7.00 The Electrchemical Sciety, Inc. Oxygen Evlutin and Recmbinatin Kinetics Inside Sealed Rechargeable, Ni-Based Batteries P. H. L. Ntten, a,b, *,z E. Verbitskiy, a W. S. Kruijt, a and H. J. Bergveld a a Philips Research Labratries, 5656 AA Eindhven, The Netherlands b Eindhven University f Technlgy, 562 AZ Eindhven, The Netherlands A423 The xygen evlutin and recmbinatin kinetics in Ni-based battery systems have been investigated under temperaturecntrlled, steady-state, vercharging cnditins within a temperature range f 0-60 C. Theretical relatinships fr bth reactins have been derived. The mathematical relatinships have been cnfirmed by experiments n sealed rechargeable NiCd batteries. Under steady-state vercharging cnditins, the Cd electrde ptential was prven t be cnstant, thereby significantly reducing the cmplexity f the interpretatin f the results. The xygen evlutin reactin was fund t be a tw-step prcess at the Ni electrde: tw linear Tafel dependencies were fund at lw and high currents, separated by a transitin regin at intermediate currents. The xygen recmbinatin kinetics was fund t be first-rder in partial xygen pressure, strngly suggesting that the recmbinatin reactin is purely diffusin-cntrlled by xygen, redisslved int the electrlyte, t the Cd electrde surface. Fr all reactins, activatin energies have been determined. The results can directly be applied t aqueus battery mdels and give mre insight int bth the vercharging and self-discharge prcesses ccurring in these Ni-based systems The Electrchemical Sciety. DOI: 0.49/ All rights reserved. Manuscript submitted August 2, 2004; revised manuscript received January 7, Available electrnically June 6, * Electrchemical Sciety Active Member. z Peter.Ntten@Philips.cm The use f prtable electrnic equipment such as, fr example, cmputers, telephnes, shavers, and pwer tls, has been rapidly grwing during the last few decades. Obviusly, t pwer these prtable devices, small rechargeable batteries are indispensable. In additin, the use f secndary batteries in the autmtive sectr is als grwing sharply. Large batteries and battery packs are increasingly used t pwer hybrid electrical vehicles EVs and HEVs. Varius sealed rechargeable battery types are presently available fr energy strage. The three mst frequently applied battery systems in these applicatins are the aqueus NiCd and nickel-metal hydride NiMH 2,3 systems and the nnaqueus Li-in system, 4 each f them having specific advantages and disadvantages. While peratin f the mre expensive Li-in batteries requires quite a few electrnic safety precautins, 5 Ni-based systems can be perated in a relatively simple way, making the mature and rbust NiCd and NiMH batteries ecnmically attractive fr a wide range f applicatins. One f the mst imprtant requirements fr aqueus battery systems is that they must be resistant t prlnged vercharging. It is well knwn that at the end f the charging prcess and during vercharging, a severe cmpetitin ccurs between, n the ne hand, the main electrchemical energy strage reactins and, n the ther hand, the parasitic xygen evlutin and recmbinatin reactins. -3 The kinetics f these reactins determine the internal gas pressure, which will be established inside the batteries during ver charging and ultimately determine at which rate these batteries can be charged. Evidently, t perate Ni-based batteries under a wide variety f charging cnditins, the xygen gas pressure must remain within reasnable limits. This puts sme serius demands n the kinetics f the xygen evlutin and recmbinatin reactins. Nwadays, a lt f effrt has been devted t mdel rechargeable batteries, including bth aqueus and nnaqueus systems. 5-8 In recent wrk, it has been shwn that the vltage characteristics, including the accmpanying pressure and temperature behavir f bth sealed rechargeable NiCd 5,9,0 and NiMH batteries, can be successfully mdeled. Using these mdels, it has been illustrated that this way f mdeling is helpful t simulate nt nly these cmplex battery characteristics, but als the mutual electrical and thermal interactin between the battery r battery pack and the electrnics used in the applicatin. 5,2 In rder t intrduce the apprpriate prcesses and crrespnding parameters int these mdels, mre insight in the xygen kinetics is required. The aim f the present paper is t investigate the kinetics f bth the xygen evlutin and xygen recmbinatin reactin inside sealed Ni-based battery systems under practical battery perating cnditins rather than t clarify the detailed mechanism f the xygen recmbinatin reactins. As shwn in this paper, the electrde ptential f the individual electrdes plays an imprtant rle in the xygen reactin kinetics. In cntrast t the metal hydride MH electrde, the cadmium electrde is based n a disslutin/precipitatin mechanism, ultimately resulting in a tw-phase, slid-state, redx system. 3 This implies that the cadmium electrde ptential, in cntrast t the MH electrde ptential, is independent f its state-f-charge SOC and can be cnsidered as cnstant under certain cnditins, thereby significantly simplifying the theretical and experimental cmplexity fr the xygen kinetics. In this paper, we therefre cncentrate n unraveling the xygen evlutin and recmbinatin kinetics in NiCd batteries. The impact f these results n the crrespnding kinetics in NiMH is discussed. Theretical Cnsideratins When NiCd batteries are charged ch, electrical energy is cnverted int chemical energy and the reverse prcess takes place during discharging d. The main electrchemical energy strage reactins can, in their mst simplified frm, be represented by ch Ni OH 2 +OH NiOOH + H 2 O+e d ch Cd OH 2 +2e Cd + 2 OH 2 d Since the Ni electrde is designed t be the capacity-determining electrde in NiCd batteries, 0 it is generally accepted that an xygen recmbinatin cycle is initiated at the end f the charging prcess: O 2 is evlved at the Ni electrde and will recmbine at the Cd electrde. The verall xygen evlutin reactin at the Ni electrde can, in its mst simplified frm, be represented by k a 4OH l +2H 2 O+4e 3 k c where it is assumed that the prduced xygen disslves int the liquid electrlyte l. A mre detailed reactin sequence is treated in the Discussin. Equilibrium exists between xygen disslved in the electrlyte, O 2 l, and that present in the gas phase, O 2 g. This equilibrium can be represented by

3 A424 k O 2 l g 4 k 2 where k and k 2 are the respective rate cnstants. Because Cd OH 2 is present in excess in the Cd electrde, 0 reductin f Cd OH 2 still cntinues, accrding t Reactin 2. As a result, the partial xygen pressure within the cell starts t rise and electrchemical cnversin f O 2 will slwly be initiated at the Cd electrde, accrding t k c O 2 l +2H 2 O+4e 4OH 5 k a A general expressin fr the external current supplied t the battery I NiCd can nw be given I NiCd = I Ni + I = I Cd + I rec 6 where the partial currents flwing thrugh the Ni electrde, which are cmpsed f the main electrchemical reactin I Ni and the xygen evlutin reactin I, must f curse equalize the partial currents flwing thrugh the Cd electrde, which are cmpsed f the Cd energy strage reactin I Cd and the xygen recmbinatin reactin I rec. Frm Eq. 5 it is clear that water is required t facilitate the recmbinatin reactin. The mst likely recmbinatin mechanism is that xygen cming frm the Ni electrde is transprted via the gas phase tward the Cd electrde. Here, xygen redisslves int the electrlyte, and transprt tward the Cd-electrde/electrlyte interface takes place by means f diffusin where it is, subsequently, electrchemically cnverted. The presence f three-phase bundary regins is generally accepted t play a crucial rle in the recmbinatin reactin. The interplay between the xygen evlutin and xygen recmbinatin kinetics ultimately determines the develpment f the partial xygen pressure inside NiCd batteries. The gas law represents the relatinship between the partial xygen pressure p, the battery temperature T, and the free gas vlume V g inside the battery, accrding t p = m O 2 7 V g where R is the gas cnstant and m is the mlar amunt f O 2 in bth the gas phase and disslved in the electrlyte. Since the slubility f O 2 in strng alkaline slutins is very lw cmpared t that stred in the gas phase, 4 the cntributin f the electrlyte phase t m may be neglected. Using Faraday s law, m can simply be calculated frm the amunt f O 2 prduced at the Ni electrde I and the amunt f O 2 recmbining at the Cd electrde I rec dm = I O 2 I rec 8 dt nf where n, the number f electrns invlved, is 4 accrding t Eq. 3 and 5 and F is the Faraday cnstant. Cmbining Eq. 7 and 8 leads t the general expressin fr the pressure develpment inside NiCd dp = I I rec 9 dt nfv g A special case can nw be cnsidered when the supplied current t the battery is entirely used t drive the xygen evlutin and recmbinatin cycle. In this case, a steady-state situatin is reached in the vercharging regime, where and hence that dp dt =0 0 I NiCd = I = I rec Under these cnditins, electrchemical cnversin f Ni and Cd species n lnger takes place. As pinted ut belw, this steady-state cnditin has a lt f favrable interpretatinal advantages. Oxygen evlutin. Since xygen evlutin at a fully charged Ni electrde takes place at relatively psitive ptentials, the kinetic descriptin f this reactin can be cnsiderably simplified by cnsidering nly the andic branch f the Butler-Vlmer equatin, 0 accrding t I = I exp O 2 nf 2 where I represents the partial xygen evlutin current, is the charge-transfer cefficient, and the exchange current, I, is given by I = nfa Ni k a x ah2 y aoh z kc a l O 3 see derivatins given in Ref. 2 and 0. In Eq. 3, A Ni represents the Ni electrde surface area at which xygen evlutin takes place, k a and k c represent the respective andic and cathdic reactin rate cnstants, a i is the bulk activities f the varius electractive species invlved, and x, y, z are the crrespnding reactin rders. The verptential,, in Eq. 2 is defined as the difference between the Ni electrde ptential E Ni and the xygen equilibrium ptential E eq eq = E Ni E 4 where, accrding t the Nernst equatin, E eq is dependent n the xygen activity at the interface E eq = E + ln a x l ref 5 nf a in which E is the standard redx ptential f the O 2 /OH redx cuple and a ref is the xygen activity in the reference state. Bth a OH and a H2 O have been included in E /OH, as they are cnsidered t be cnstant in the cncentrated battery electrlyte 8 M. Assuming that xygen behaves like an ideal gas, Eq. 4 reveals that the activity f xygen disslved in the electrlyte relates t the partial xygen pressure p, accrding t a l = k 2 a g = O 2 k 2 p = p 6 k k where dentes the fugacity cnstant and = k 2 /k the slubility cnstant. The lw value fr ml/m 3 Pa is determined by the lw xygen slubility in the strng alkaline battery electrlyte. 4 Cnsidering a similar equatin fr p ref, i.e., a ref = p ref, Eq. 5 transfrms int E eq = E + ln p 7 ref x nf p ref in which p refers t the standard cnditins f atmsphere 0 5 Pa and 298 K. Replacing in Eq. 2 by means f Eq. 4 and 7 and eliminating a l in Eq. 3 fr p Eq. 6 yields I = nfa Ni k x p ref kc a O x 2 a y aoh z H2 O p exp O 2 nf E Ni E 8 It shuld be nted that Eq. 8 reveals that the xygen evlutin

4 A425 kinetics is independent f the partial xygen gas pressure, as expected frm Eq. 3. Since the battery vltage is much mre easily accessible than the individual electrde ptentials in sealed batteries, E NiCd =E Ni -E Cd is intrduced in Eq. 8, which ultimately leads t I = K exp O 2 nf E Cd E exp O 2 nf NiCd E 9 in which can be cnsidered as a mdified frm f the exchange current see Eq. 3 K = nfa Ni k x p ref kc a O x 2 a y aoh z H2 O 20 Cnsidering the special cnditin f steady state when dp /dt =0 Eq. 0, the electrchemical reductin f Cd OH 2 n lnger takes place and I NiCd = I = I rec. Cnsequently, the verptential at the Cd electrde is 0 and, as will be experimentally cnfirmed, E Cd can be cnsidered as cnstant fr the Cd tw-phase system. Perfrming the experiments under these steady-state cnditins and pltting the semilgarithmic frm f the supplied current vs. the battery vltage, accrding t ln I NiCd =lni =lnk + O 2 nf E Cd E + O 2 nf E NiCd 2 yields essential kinetic infrmatin fr the xygen evlutin reactin. It shuld, hwever, be nted that bth E Cd and E are temperature-dependent via the change in entrpy by E i T = E i T 298 S i n i F 22 where the value fr the entrpy change S i can be either psitive r negative depending n the thermdynamics f the reactin species i. 0 Oxygen recmbinatin. Fr a purely kinetically cntrlled xygen reductin reactin, fr which n transprt limitatins have t be cnsidered, the partial recmbinatin current I kin can be expressed by the cathdic branch f the Butler-Vlmer equatin I kin = I rec exp rec nf rec 23 where I rec, the exchange current fr the recmbinatin reactin, is given by I rec = nfa Cd k rec a k rec c a rec x rec a rec y rec H2 O a rec z rec OH 24 It shuld, hwever, be nted that k a and k c d nt necessarily need t be the same as thse fr the xygen evlutin reactin in Eq. 3. Cnsidering similar eliminatin steps t thse used t derive Eq. 9, we btain where x I rec = K rec P rec exp rec nf E Cd E K rec = nfa Cd k rec a k rec c rec x rec p ref x rec rec a rec y rec z H2 O a rec rec OH is cmpsed f the varius cnstants, which are nw specified fr the recmbinatin reactin at the Cd electrde. Pltting the lgarithmic frm f Eq. 25 gives ln I rec =lnk rec + rec nf E Cd E + x rec ln p 27 Again, under steady-state cnditins, E Cd can be cnsidered as cnstant and bth x rec and K rec can be deduced frm the slpe and intercept, respectively, f the expected linear dependence. In cntrast t the xygen evlutin kinetics, the recmbinatin kinetics is, accrding t Eq. 5, indeed dependent n the partial xygen pressure. Since the xygen reductin reactin takes place at very high verptentials, i.e., at very negative ptentials with respect t E eq,it is reasnable t assume that transprt f O 2 tward the electrde interface might becme a limiting factr. Assuming that a linear cncentratin gradient is established acrss the diffusin layer thickness, the recmbinatin rate can be described by nfa Cd D ā l a l I dif = 28 d where D is the diffusin cefficient fr O 2 in the electrlyte, ā l is the activity f xygen in the bulk f the electrlyte, is the activity cefficient fr disslved xygen, and d is the average diffusin layer thickness, thrugh which O 2 must be transprted t the interface. In the extreme case, when the recmbinatin rate is cmpletely dminated by mass transprt, a l at the electrde surface equals zer and the diffusin-cntrlled recmbinatin current I dif can be reduced t I dif = nfa CdD p, 29 d in which the partial xygen pressure Eq. 6 is again intrduced. Frm Eq. 29 it is clear that a linear dependence between the recmbinatin current and p is t be expected under purely diffusincntrlled cnditins. Experimental All experiments have been perfrmed with cmmercial AA-size NiCd batteries f the type 80AAS Matsushita Battery Industries. It is well knwn that, due t the initiated xygen recmbinatin reactin, a lt f heat will be prduced inside the batteries, leading t a significant temperature rise. 0 In rder t perfrm prper kinetic studies at cnstant temperatures, water jackets made f well-heat cnducting, cpper were tightly adjusted arund the batteries. T further imprve the heat exchange between the batteries and the water jackets, the batteries were lubricated with a heat-cnducting resin heat sink cmpund 340 frm Dw Crning, Belgium befre attachment. The temperature was cntrlled by means f a thermstat Neslab within the temperature range f 0-60 C accuracy is 0.5 C. The temperature f the batteries was measured by means f Pt-00 thermcuples within an accuracy f 0.5 C. T measure the gas pressure build-up during the experiments, small hles with a diameter f mm were drilled in the flat bttms f the batteries. T reduce the risk f electrlyte lss, this was dne just befre the experiments were started. Subsequently, the batteries tgether with the water jackets were placed int hlders t which pressure transducers Transamerica Instruments type n were attached. Duble O-ring cnstructins ensured gas-tight sealing between the batteries and transducers. The pressure was measured within an accuracy f 0.0 bar. In rder nt t disturb the battery behavir t much, the dead vlume f the pressure transducer, including the supply tube, was kept as small as pssible and was calibrated t be.7 ml. This is less than the battery vid vlume, which was analyzed t be.8 ml. Separate electrical cntacts were used fr current flw and vltage measurements. Befre starting the kinetic measurements all batteries were activated fr at least 20 cycles under thermstatic cnditins 20 C until cnstant charge/discharge behavir was attained. The cndi-

5 A426 tins f such an activatin cycle cnsisted f a 3-h charging perid, using a 400-mA current 0.5 C-rate fllwed by an pen-circuit perid f 60 min. Subsequently, the batteries were discharged with 400 ma until the cutff vltage f 0.9 V was reached. After a resting perid f 5 min, the batteries were additinally deepdischarged, using a lwer current f 80 ma 0. C, again until the same cutff vltage was reached. The activatin cycle was cmpleted by a resting perid f 5 min. The kinetics f the xygen evlutin and recmbinatin reactins was investigated during cntinuusly vercharging by stepwise changing the vercharging current. Fully discharged batteries were therefre initially charged under mderate current 80 ma cnditins fr 20 h 200% depth-f-charge t ensure a steady-state situatin in the vercharge regin t be attained, as culd be recgnized n the cnstant values f the three measured parameters V, P, and T. After this perid the current was lwered stepwise dwn t 5 ma. The actual kinetic measurement was perfrmed by increasing the current stepwise up t 2 A. The stabilizatin time needed t attain a new steady state was bviusly dependent n the applied current, and the vercharging perid therefre ranged between 50 and 400 min fr the high and lw current regin, respectively. This prcedure was repeated at every temperature. T get the batteries used t a new ambient temperature, they were reactivated fr three cycles befre the kinetic measurements were perfrmed. The reactivatin cycling regime was the same as that used fr the activatin cycles described abve. Flexible, in-huse-designed, autmatic cycling equipment was used t perfrm bth the cycling experiments and the custm-type vercharging experiments. V, P, and T values were cntinuusly recrded. Impedance measurements were perfrmed with cnventinal equipment Autlab frm Ecchemie under galvanstatic cntrl during cntinuus vercharging with a cnstant dc-current I dc in the range f 80 I dc 20 ma and using an alternating current f 5 ma. The investigated frequency range was frm 0 t 0 4 Hz. The equilibrium ptential measurements f the Ni and Cd electrdes were perfrmed under the same mechanical cnditins as thse in sealed batteries. After the batteries were cmpletely activated and, subsequently, fully discharged, the caps accmmdating the vents were remved and extra electrlyte with exactly the same cmpsitin as that used in the cmmercial batteries was added. 5 An Hg/HgO reference electrde was clsely psitined abve the winded electrde package. Als in this experimental setup, the battery was thermstated using the cpper water jacket. The temperature was cntrlled at 20 C. The fully discharged batteries were charged stepwise fr 2.5 min with 0.5 C 400 ma. After current interruptin, the electrdes were allwed t relax t their equilibrium ptential fr 5 h befre the next charging step was initiated. T prevent the uncertainty f the state-f-charge SOC at higher values f SOC, resulting frm the cmpetitin with the xygen evlutin reactin at the Ni electrde, a SOC value f 75% was taken as eq maximum, after which the current was reversed. Hwever, sme E Cd measurements were als perfrmed after current interruptin in the vercharging regin where the SOC f the Ni electrde is prly defined. Bth the Ni electrde E Ni and Cd electrde ptential E Cd were mnitred against the reference electrde. T check these measurements, the battery vltage E NiCd was als inspected and was fund t be in perfect agreement with the individual electrde ptentials in all cases, i.e., E NiCd - E Ni -E Cd 2 mv. Results Several activatin cycles measured at 20 C are shwn in Fig.. The first charging cycle curve a reveals that charging ccurs at rather high vltages, especially in the regin where the cmpetitin with the xygen evlutin and recmbinatin reactins ccurs. After 80 min, the charging current was interrupted fr h, which was fllwed by discharging the battery. The vercharging behavir changed drastically during the fllwing charging prcedures as Figure. Example f activatin cycles f an 80AAS NiCd battery during charging 400 ma, pen-circuit, and discharging 400 ma at 20 C. Cycle a, cycle 2 b, cycle 3 c, and cycle 20 d, after which the characteristics d nt change anymre. curves b and c shw fr the secnd and third charging cycle, respectively. After apprximately 20 cycles curve d, the vltage curves had stabilized and did nt change anymre. The vltage dependence during bth the pen-circuit and discharging perid is nt very activatin-dependent, indicating that activatin is mainly related t the xygen recmbinatin cycle. In line with this bservatin, the discharge capacities are mre r less the same fr all cycles. It shuld be emphasized that the appearance f the vltage curves shwn in Fig. may deviate frm thse nrmally reprted, such as the characteristic dv/dt behavir in the vercharge regin. 0 This is due t the special temperature-cntrlled cnditins adpted in the present experiments, which are rather uncmmn in battery research. After the activatin perid was cmpleted, the kinetic investigatins were started. Part f the vercharging prcedure is shwn in Fig. 2. At t =0 arbitrary value, the charging current was increased frm.2 t.28 A. The vltage increases rather abruptly and relaxes tward a new steady-state value within abut 40 min, while the pressure, n the ther hand, slwly increases tward the new steady state. The relaxatin times f the vltage and pressure curves are clearly cupled t ne anther. After stabilizatin, the current Figure 2. Typical example f part f the vltage V, pressure P, and temperature T develpment during cntinuus vercharging at 20 C. This part refers t the high-current regin, where I is increased stepwise frm.2 t.28 A at t = 0 via.44 A t = 50 min t.60 A t = 300 min. After 450 min, the current is switched ff and the battery is allwed t relax t the equilibrium state under pen-circuit cnditins.

6 A427 Figure 3. Overcharging current as a functin f the established steady-state partial xygen pressure inside the NiCd battery at varius temperaturecntrlled cnditins. Figure 4. Ni electrde curves a and b and Cd electrde curves d and e pen-circuit ptential measured in an pened battery against an Hg/HgO reference electrde at 20 C during intermittent charging curves a and d and discharging curves b and e with 400 ma. Current-n perid is 2.5 min and current-ff perid tk 300 min befre each ptential measurement. Curve c represents the theretically expected curve fr the Ni electrde based n the Nernst equatin, cnsidering the attractin r repulsin energies t be negligible. 0 Figure 5. High-frequency regin f a typical impedance measurement perfrmed during cntinuusly vercharging 60 ma a NiCd battery thermstated at 20 C. In this example, a steady-state battery vltage f.477 V was established. was further increased in tw steps via.44 A at 50 min up t.60 A after 300 min see Fig. 2. After the final steady-state situatin was accmplished at the highest current, the current was switched ff and bth the vltage and pressure relaxed t their lw equilibrium state. The temperature was fixed at 20 C thrughut the experiment in this example, as Fig. 2 reveals. The dependence f the established steady-state xygen pressure n the applied vercharging current is shwn in Fig. 3. Althugh sme spread in the results is fund, especially at lw temperatures, very likely due t the severe develpment f xygen bubbles at the Ni electrde, it can be seen that straight lines passing thrugh the rigin are btained at all temperatures. As expected, the partial xygen pressures are significantly reduced at higher temperatures. The influence f the battery SOC n the electrde ptentials f bth the Ni and Cd electrde inside an pened NiCd battery is revealed in Fig. 4. Curves a and b refer t the pen-circuit vltage OCV dependencies f the Ni electrde during charging and discharging, respectively. The ptentials were measured against an Hg/HgO reference electrde. A large hysteresis was fund between charging and discharging, which amunts t abut 00 mv. In cntrast t the Ni electrde, the pen-circuit ptential f the Cd electrde des nt shw any dependence n the SOC within an accuracy f 2 mv. N sign f any hysteresis between the charging curve d and discharging OCV curve e was fund in this case. The differences between measured OCV values fr the Ni and Cd electrde are in perfect agreement within 2 mv with the measured battery vltage at all SOC values, as expected. Similar measurements have been perfrmed in the vercharging regin. Since the SOC f the Ni electrde is rather unreliable in this regin due t the parasitic xygen reactin, the x axis f Fig. 4 becmes highly unreliable. Hwever, the Cd electrde ptential was fund t remain at the same cnstant value, as indicated in Fig. 4, curves d and e. This is indeed t be expected frm the fact that bth slid Cd and Cd OH 2 species are still present in the Cd electrde as the capacity f this electrde is much larger than that f the capacity-determining Ni electrde. In rder t crrect the measured battery vltage fr the hmic lsses ccurring under current flwing cnditins, the sealed batteries were subjected t impedance measurements under steady-state vercharging cnditins at varius currents at 20 C. A typical example f such an impedance measurement, using an vercharging current f 60 ma, is shwn in Fig. 5. Only the mst infrmative part f the impedance spectrum at high frequencies is pltted. The intercept with the x axis reveals the real part f the impedance, crrespnding t an hmic resistance f 23 m. This value was fund t be independent f the applied vercharging current within ±0.5 m and is in agreement with what is usually accepted as hmic resistance fr this type f aqueus battery system.,0 Cnsidering the temperature dependency f the inic cnductivity T f strng alkaline electrlytes, 4 the hmic resistances at different temperatures R T can be calculated, accrding t R T = R T The as-calculated values are used fr the Ohmic lss crrectins at the ther temperatures. An example f a crrected current-vltage curve btained with a sealed rechargeable battery under steady-state vercharging cnditins at 0 C is shwn in Fig. 6. Tw separate regins can clearly be distinguished. These tw regins becme even mre prnunced when the current is pltted n a lgarithmic scale in Fig. 7. It is evident that at bth lw and high currents, the kinetics beys simple Tafel relatinships, which are represented by the straight lines f curves a and b, respectively. In the intermediate current range,

7 A428 Figure 6. Current-vltage relatinship measured during cntinuusly vercharging a NiCd battery thermstated at 0 C. The battery vltage has been crrected fr the hmic vltage drp. The vercharging current was increased stepwise see Fig. 2. a transfer frm ne Tafel regin t anther is fund. Figure 8 shws the results at varius temperatures. As expected frm imprved kinetics, the curves are shifted tward lwer vltages at higher temperatures. Figure 8. Current-vltage relatinships measured at varius cntrlled temperatures during cntinuus vercharging. The present experimental results reveal that fully activated NiCd batteries Fig. can be vercharged t a large extent under temperature-cntrlled cnditins. These cnditins are required in rder t prperly investigate the kinetic prperties f bth the xygen evlutin and recmbinatin reactins. Discussin The chemical and physical prcesses visualizing the xygen evlutin and recmbinatin prcess inside NiCd batteries are schematically depicted in Fig. 9. The xygen frmed at the Ni electrde disslves int the electrlyte O 2 and will, subsequently, be transprted t the gas phase O 2. Since the xygen slubility in alkaline electrlyte is relatively pr, 4 transprtatin via the gas phase by means f gas bubbles will be the mst dminant transprtatin mechanism. Accrding t Eq. 5, the availability f water is essential fr the xygen recmbinatin t ccur. Redisslutin f O 2 int the electrlyte and diffusin tward the Cd-electrde/electrlyte interface is therefre an essential step. The availability f three-phase bundary regins seems t be crucial in this respect and semiwetted electrdes are therefre generally thught t be very favrable fr a high recmbinatin rate, as is schematically indicated in Fig. 9. Figure 7. Same current-vltage relatinship as shwn in Fig. 6, nw pltted n a semilgarithmic scale filled circles. The characteristic lw-current and high-current Tafel regins are represented by curves a and b, respectively. Using the parameter values f these tw Tafel regins, an verall currentvltage dependence curve c can be calculated, accrding t Eq and Eq. 47. Figure 9. Schematic representatin f the xygen recmbinatin cycle. The Ni and Cd electrdes are separated by a separatr, which is immersed in the strng alkaline electrlyte. Oxygen frmatin is initiated at the Ni electrde/ electrlyte interface. Small gas bubbles are frmed and the gas will be transprted t the gas phase. Recmbinatin starts by redisslutin f xygen in the electrlyte and will subsequently be reduced at the Cd-electrde/ electrlyte interface. The recmbinatin rate is strngly dependent n the diffusin layer thickness, thrugh which xygen has t be transprted and is mst favrable at the three-phase bundaries, as is schematically indicated by arrws a - c.

8 A429 Figure 0. Electrchemical representatin f the xygen recmbinatin prcess in a NiCd battery. The diffusin-cntrlled xygen recmbinatin prcess at the Cd electrde under steady-state vercharging cnditins takes place at negative ptentials with respect t the xygen standard redx ptential E eq and is represented by the ptential-independent current-vltage curves a. Oxygen recmbinatin takes place at the pen-circuit ptential f Cd electrde curve b. Under these steady-state cnditins, the rate f xygen recmbinatin is equal t the rate f xygen evlutin curve c, taking place at the pen-circuit ptential f the Ni electrde E eq Ni n dashed curves d and is represented by I. Instantaneusly increasing the vercharging current by an amunt I 2 t I 3 I 3 = I + I 2 immediately induces the ptential at the Cd electrde t decrease and that f the Ni electrde t increase. As a result, the battery vltage increases simultaneusly see Fig. 2. After relaxatin tward the new steady-state situatin, bth xygen evlutin at the Ni electrde curve e and xygen recmbinatin curve a at the Cd electrde take place again at E eq Ni and E eq Cd, respectively. Figure 2 indicates that by changing the vercharge current, a new steady-state cnditin is attained within apprximately 40 min. Characteristic f this steady-state cnditin is that bth the battery vltage and partial xygen pressure have becme cnstant. Cnsequently, accrding t Eq. 9 and 0, the rate f xygen evlutin is equal t that f xygen recmbinatin and the rates f the main electrchemical strage reactins, I Ni and I Cd, are diminished t zer. This implies that Eq. applies. Fr cnvenience, we first discuss the relevant cnsequences fr the xygen recmbinatin kinetics and after that cnsider the xygen evlutin kinetics under these cnditins. Oxygen recmbinatin. Pltting the supplied battery current as a functin f the steady-state partial xygen pressure clearly shws that the recmbinatin reactin is linearly dependent n p at all temperatures Fig. 3. Assuming that the recmbinatin reactin is purely kinetically cntrlled, it is evident that x rec in Eq. 25 must equal. Since fr a purely diffusin-cntrlled recmbinatin a similar dependency applies see Eq. 29 and it can theretically be shwn that this als hlds fr a cmbined, first-rder, kinetic/ diffusin-cntrlled, reactin, it is impssible t cnclude frm this linear dependency what the rate-determining step fr xygen reductin at the Cd electrde is. Hwever, cnsidering that the Cdelectrde ptential E Cd = V vs. a standard hydrgen electrde SHE 0, at which the xygen reductin reactin takes place, is mre than.2 V negative with respect t the equilibrium ptential f the xygen redx cuple E eq = V vs. SHE at bar 0,it is mre than likely that xygen reductin will be fully transprtcntrlled. This is als the mechanism we adpted in ur earlier battery mdeling wrk. 5,9-2 The diffusin-cntrlled, ptential-independent recmbinatin current at the Cd electrde is schematically represented by curves a in Fig. 0. In additin, the electrchemical charge-transfer prcesses f the Cd and Ni electrde are indicated by curves b and Figure. Calculated slpe, as btained frm the current-pressure curves shwn in Fig. 3 see als crrespnding Eq. 29, as a functin f the reciprcal temperature. d, e, respectively. As pinted ut abve, n current is cnsumed by bth the Ni and Cd reactin under steady-state cnditins and, eq cnsequently, xygen evlutin and recmbinatin take place at E Ni and E eq Cd, respectively. Under these cnditins, the xygen evlutin rate at the Ni electrde, schematically indicated by I in curve c, equals the recmbinatin rate at the Cd electrde I in curve a. The meaning f I 2 and I 3 is relevant fr the nn-steady-state cnditins, as discussed later. Accrding t Eq. 29, infrmatin abut the diffusin prcess can be btained frm the slpes f the lines fund in Fig. 3. This equatin shws that mst f the factrs influencing the recmbinatin kinetics can be cnsidered as cnstants, including A Cd, D,, and. Mre difficult t access is the diffusin layer thickness, d.it is clear frm Fig. 9 that nt nly A Ni but als A Cd refers t the wetted part f the Ni and Cd electrde, respectively. Evidently, redisslutin and transprtatin tward the electrde interface will ccur fast near the slid-electrlyte-gas three-phase edges where the diffusin layer thickness is extremely small see psitin a in Fig. 9. The diffusin layer thickness increases quickly farther away frm these three-phase edges, as is indicated by psitins b and c. Cnclusively, there is a cntinuum in values fr d. In rder t deal with such a cntinuum, d is cnsidered t be an average diffusin layer thickness in the present wrk. The as-btained slpes frm the I vs. p relatinship Fig. 3 have been pltted accrding t an Arrhenius relatinship in Fig.. A linear dependence is fund at temperatures up t 40 C. It has been shwn by Davis et al. 6 that the slubility f xygen in the strng alkaline slutins used in rechargeable batteries is independent f the temperature within a temperature range f 0-60 C and hence that remains essentially cnstant. Assuming that bth A Cd and d are als temperature-independent, the bserved Arrhenius behavir can be attributed t the temperature dependency f D. Such dependence can be expressed by D = D exp E a O 2 3 where D a is the preexpnential factr and E is the activatin energy fr the diffusin cefficient f xygen in strng alkaline slutin. 0 The lw value fr the bserved activatin energy f 9.4 kj/ml is mainly related t the viscsity f the aqueus electrlyte and is in line with thse generally reprted fr diffusincntrlled prcesses. 7

9 A430 At higher temperatures, a clear deviatin frm the linear Arrhenius dependence is bserved. It is suggested that this might be due t the fact that the wetting ability f the electrde is changing, resulting in a larger number f three-phase regins and, cnsequently, in a favrable cmbinatin f a larger wetted surface area A Cd and smaller values fr d see Eq. 29. It shuld, hwever, be emphasized that this deviatin tward higher values is in fact very attractive fr the recmbinatin reactin, keeping the internal gas pressure f the battery relatively lw at higher temperatures. It is desirable t keep the internal gas pressure inside rechargeable batteries as lw as pssible. Frm the present results, it becmes evident that this can be achieved by increasing the recmbinatin rate. Accrding t the lines f reasning utlined abve, this can be accmplished by increasing the number f three-phase bundary regins see Fig. 0, i.e., by creating a larger number f bth wetted and dry regins. By making use f specific additives creating bth hydrphilic and hydrphbic regins, this wuld cntribute t an enhanced recmbinatin rate, keeping the internal gas pressure at mre favrable levels. Oxygen evlutin. The individual electrde ptentials inside uncapped NiCd batteries have been studied by means f a reference electrde as a functin f SOC in Fig. 4. Apart frm the hysteresiseffect characteristic fr the Ni electrde, the OCV behavir is in gd agreement with what is expected n the basis f a nrmal slid-slutin dependence, as represented by the Nernst equatin. 5,9 Fr example, the agreement f the theretical curve c with the experimental curve a btained during charging is very clse, especially, up t 50% SOC. A value f V fr E Ni has been used in the calculatin f curve c. A similar fit during discharging curve b reveals a value f V fr E Ni during discharging. Similar values fr the Ni hysteresis have been reprted befre Mre relevant fr the present wrk is that curves d and e f Fig. 4 unequivcally shw that the OCV f the Cd electrde is independent f its SOC. This als hlds after relaxatin frm the vercharging regin. Interestingly, this implies that the Cd electrde can be cnsidered as an internal reference inside sealed NiCd batteries under the assumptin that nne f the current flwing thrugh this electrde is used t drive the main electrchemical chargetransfer reactin Eq. 2 and hence that the verptential fr the Cd reactin is zer. This result prves that the assumptin f a cnstant Cd electrde ptential in the derivatin f the xygen evlutin kinetics Eq. 2 is indeed fulfilled and that Tafel dependencies are t be expected. Figure 7 shws, hwever, that a simple semilgarithmic Tafel behavir is nt fund. Tw Tafel lines are fund at all temperatures Fig. 8. Such tw-step prcesses have been reprted befre fr the xygen evlutin reactin in alkaline slutins Accrding t Eq. 3, fur electrns are invlved in the verall reactin. Mst likely, electrn transfer will take place in successive steps, invlving I =4 F 4 A 4 Ni k a k a 2 k a 3 k a 4 a OH exp F F 3 A 3 Ni k a 2 k a 3 k a 4 exp F E + F 3 A Ni + F 3 A 3 Ni k c k c 2 k a 4 exp F E F 4 A Ni well knwn t be invlved in mst f the prpsed xygen evlutin reactin mechanisms. 2,24 In rder t make the cmplexity f the mathematics nt t high, nly a sequential series f the fur individual charge-transfer steps are cnsidered in the present derivatin. Fr a mre detailed listing f the varius pssible xygen evlutin mechanisms, the reader is referred t Ref The fllwing general reactin scheme can, in the simplest case, be cnsidered S S 2 + e 32 S 2 S 3 + e S 3 S 4 + e S 4 S 5 + e 35 with the verall reactin S S 5 +4e 36 where S =4OH ; S 2, S 3, S 4 are the intermediate species S ; and S 5 =O 2 l. It is again assumed that the partial current-vltage dependencies fr each charge-transfer step Eq can be described by a Butler-Vlmer-type f relatinship, i.e. I j = FA Ni k a j a Sj exp jf E O 2 FA Ni k c j a Sj+ exp j F E 37 where k a j and k c j are the rate cnstants fr the andic and cathdic partial reactin, respectively, a j and a j+ are the activities f the S j and S j+ species, respectively, and j are the individual chargetransfer cefficients fr each charge-transfer step j, where j ranges frm t 4. Evidently, since electrn transfer takes place in individual steps, bth n and all reactins rders are nw unity in all cases. It shuld, furthermre, be nted that it is essential t use the electrde vltage rather than verptential see Ref. 2 fr detailed derivatins and that E = E Ni. In additin, a steady-state situatin is assumed fr each reactin step, i.e., the cncentratins f intermediate species S 2, S 3, and S 4 are cnsidered cnstant. Under these cnditins, the partial currents are equal I = I 2 = I 3 = I 4 38 and the xygen evlutin current is the summatin f the partial currents I = I + I 2 + I 3 + I 4 39 Eliminating the activities f the intermediate species frm Eq. 37 by using Eq. 38 and 39, the fllwing general expressin fr the xygen current is btained 4 k c k c 2 k c 3 k c 4 a exp F 3 k c k a 3 k a 4 exp F E + F 3 A Ni E 3 k c k c 2 k c 3 exp F E E 40 single-electrn transfer. The change frm ne Tafel slpe t anther suggests that the rate-determining step f the xygen evlutin reactin is changing as a functin f the applied current. The mathematics fr such a multistep xygen evlutin reactin will be derived belw. Apart frm charge transfer, adsrbed intermediate species are Similar expressins have been described in the literature befre by, fr example, Vetter and Christiansen et al. 23,27 Hwever, cntrary t the previus derivatins, we have nt assumed that each reactin step has the same equilibrium ptential. Therefre, the current in Eq. 40 is expressed in terms f the xygen ptential rather than verptential.

10 Bth the andic and cathdic branch f the Butler-Vlmer equatin can be recgnized at the tw terms f the numeratr in Eq. 40. The influence f the fur sequential charge-transfer steps can be recgnized at the varius terms f the denminatr. Depending n the values f the rate cnstants k a j, k c j, and E, the denminatr can be dminated by either ne r mre f the fur expnential terms. This is the rigin f the presence f ne r mre Tafel slpes in the standard lg I vs. E plt. Cnsidering that fr single-electrn transfer reactins must be within 0 and, it is bvius that ) Fr very high values f E, it is clear that the first expnential term in the denminatr f Eq. 40 will be dminant. Which expnent will be dminant fr the lwer values f E becmes apparent frm the inspectin f the Tafel plts. In ur particular case, nly tw different Tafel slpes are bserved. Interestingly, the slpe fr mderate values f E is f the rder f /0.054 V in Fig. 7, indicating a unity term in Eq. 4. This is nly pssible in ne specific case: fr mderate values f E, the expnent crrespnding t will be dminant. The remaining expnential factrs crrespnding t and can then be neglected. Assuming that the simplified reactin scheme f Eq applies, this wuld imply that in the first steep part f the semilgarithmic Tafel plts see Fig. 7 and 8, the secnd charge-transfer step is rate-determining, while at higher currents the first charge-transfer step becmes ratedetermining. Tgether with the fact that the xygen evlutin ccurs at relatively high verptentials, the cathdic part in the numeratr can again be neglected, resulting in a significantly simpler expressin fr the xygen evlutin current I = F 3 A 3 Ni k a 2 k a 3 k a 4 exp F 4F 4 A 4 Ni k a k a 2 k a 3 k a 4 a OH exp F E + F 3 A Ni k c K 2 = 4FA Ni k a k a 2 a OH E 3 k c k a 3 k a 4 exp F exp + 2 F E Cd E A43 46 The results presented in Fig. 7 and 8 have been analyzed accrding t Eq The slid lines in these figures represent the fit results. It is clear that the agreement between the measurements symbls and the mathematical expressin lines is excellent in all cases. Frm these fits, the mst relevant parameters have been derived as a functin f temperature. In ur earlier NiCd mdeling wrk, 0 we nly adpted a nebranch xygen evlutin reactin. The present results shw, hwever, that this assumptin is far t simple. In rder t cpe with the mre cmplex xygen reactin sequence, we mre recently mdified these NiCd mdels by adpting the fllwing equatin 5 I = I li h 47 I l + I h where I l and I h refer t lw and high current regin dependency, respectively. A similar equatin is ften adpted fr mixed kinetic/ diffusin-cntrlled reactins. 28 In the present case, Eq. 47 can be interpreted frm the pint f a kinetic/kinetic-cntrlled reactin, since at different vltages, different steps f a multistep reactin scheme are rate-determining. 27 In the Appendix, it is shwn that the general Eq. 44 can be easily transfrmed int expressin 47. Frm a practical pint f view, Eq. 47 is, hwever, mre cnvenient. Evidently, exactly the same simulatin results are btained in bth cases slid lines in Fig. 7 and 8. Using this cnvenient frm Eq. 47 and assuming that the redx ptentials fr all electrchemical charge-transfer steps are the same, extraplating the straight lines f Fig. 7 and 8 t the standard redx ptential f the xygen redx reactin yield values fr K see Eq. 2. It shuld, hwever, be nted in this respect that bth the values fr E and E Cd are temperature-dependent, accrding t Eq. 22. As adpted befre, a value f +649 and 204 J/ml K fr S and S Cd, respectively, has been used fr this temperature crrectin r I = a FA Ni k exp F E O 2 + 4a OH k c FA Ni k a a k exp + 2 F 2 E 43 Since E = E Ni = E NiCd + E Cd and E Cd has been prven t be cnstant under steady-state cnditins, we finally btain I = in which and K exp F NiCd E + K 2 exp + 2 F K = 4FA Ni k a a OH exp F E Cd E NiCd Figure 2 shws the as-btained values fr K as a functin f the reciprcal temperature. Althugh the scatter is substantial, straight lines can be derived fr bth the lw and high current regins. Frm the slpe f the lw-current dependence curve a, an activatin energy f 66.2 kj/ml is calculated. Surprisingly, the high-current results d nt reveal any temperature dependency curve b. Such temperature-independent behavir has been reprted befre fr the hydrgen evlutin reactin. 2 The values fr can be calculated frm the slpe f the Tafel lines in Fig. 7 and 8. Bth the lw-current alpha l and highcurrent alpha h clearly reveal the same temperature dependence in Fig. 3, with l significantly higher than h. Cnway reprted and discussed the temperature dependence f alpha extensively and attributed this t the temperature-dependent free-energy curves. 2 S far, nly the steady-state results have been discussed. Figure 2 revealed that after raising the vercharging current, the battery vltage instantaneusly increases and, subsequently, slwly decreases in time tward a new steady-state value. On the ther hand, the pressure rises slwly and the new steady-state pressure is reached after the same vltage relaxatin time. In rder t explain this behavir, we must discuss Fig. 0 in smewhat mre detail. Referring t steady-state currents, I, in Fig. 0 it is clear that the rates f xygen evlutin and xygen recmbinatin are exactly

11 A432 Figure 2., as btained frm extraplating the lw curve a and high current curve b Tafel lines Eq. 2 shwn in Fig. 7 and 8 t E eq,asa functin f the reciprcal temperature. balanced, as is schematically indicated in the current-vltage curves a and c. Here it is imprtant t nte that the diffusin-cntrlled xygen recmbinatin current is, accrding t Eq. 29, prprtinal t the partial xygen pressure inside the battery and that the evlutin and recmbinatin reactin take place at E eq Ni and E eq Cd, respectively. An instantaneus change f the current t a higher level by an amunt I 2 i.e., I applied = I + I 2, see Fig. 2 cannt induce an immediate pressure increase as the xygen recmbinatin rate remains at the same level I see Fig. 0. Cnsequently, the increased current I 2 must at least partly flw tward the main electrchemical strage reactin f the Cd electrde under the cnditin that I applied = I + I 2 = I 3. This results in the instantaneus, but temprary, change f the Cd electrde ptential tward mre negative vltages see the I 2 vltage psitin in Fig. 0 and a small increase f SOC f the Cd electrde. As the xygen evlutin rate at the Ni electrde is at a higher level, it is evident that the partial xygen pressure will slwly rise as the experiments in Fig. 2 indeed indicate. As a result, the recmbinatin current plateau starts t increase and the Cd reactin rate will simultaneusly diminish slwly t zer. Since the Cd electrde ptential is independent f SOC Fig. 4, the new steady-state situatin is reached when the xygen pressure is adjusted at that level where I equals I rec again =I 3 in Fig. 0. Under that cnditin, the Cd electrde ptential has returned t the same Figure 3. l and h, as btained frm the respective slpes f the lwcurrent and high-current Tafel lines Eq. 2 shwn in Fig. 7 and 8, as a functin f temperature. steady-state vltage as befre E eq Cd and I 2 has becme zer. The situatin fr the Ni electrde is smewhat different as E eq Ni is dependent n SOC Fig. 4, especially at higher levels f SOC. Here, a new steady-state situatin is reached I 3 in curve c f Fig. 0, where the current-ptential curves f the Ni electrde and, cnsequently, the OCV are shifted tward mre psitive values cmpare dashed lines f curves d. A reversal f a vltage shift is, in cntrast t the Cd case, nt t be expected fr the Ni electrde, and hence the battery vltage reductin upn current increase as bserved in Fig. 2 must be attributed t the specific Cd electrde characteristics. In previus wrk, we have described ur mdeling activities fr aqueus battery systems, including NiCd and NiMH, 5,9-2 and rganic Li-in systems. 5,29 The present results shw that the xygen recmbinatin cycle inside NiCd is rather cmplex. It is very likely that the results can als be applied t ther Ni-based battery systems, which are based n a similar xygen recmbinatin mechanism. A requirement is, f curse, that the kinetics f bth xygen evlutin and recmbinatin reactins des nt differ substantially. Since it is well knwn that the kinetics f electrchemical reactins strngly depends n the cncentratins f the electractive species invlved, it is evident that in rder t apply the present kinetic parameter values, the electr chemical cnditins may nt differ t much. Since the NiMH battery system is based n a similar xygen recmbinatin cycle, 2 includes a similar type f Ni electrde, and als perates in a similar strng alkaline electrlyte, it seems reasnable t assume that the kinetics derived in the present paper can als be adpted fr this battery system. It shuld, hwever, be emphasized that minr changes in the chemical cmpsitin between NiCd and NiMH batteries might be imprtant and might induce kinetic and even mechanistic changes. It is wrthwhile t nte that the cmplexity inside NiMH is much higher as hydrgen will certainly cntribute t the ttal gas pressure inside NiMH batteries. It has recently been experimentally cnfirmed by in situ Raman spectrscpic measurements. 30,3 These mre detailed measurements carried ut with NiMH batteries will be reprted separately in the near future. Cnclusins The kinetics f the electrchemical xygen evlutin and xygen recmbinatin reactin has been investigated under thermstated, steady-state cnditins at NiCd batteries within a temperature range f 0-60 C. The electrde ptential f the Cd electrde was shwn t be cnstant under these steady-state vercharging cnditins, allwing us in situ t quantify the xygen evlutin kinetics at the Ni electrde. The xygen evlutin reactin was clearly fund t be a tw-step prcess. Each step was characterized by a specific Tafel slpe and revealed that the rate-determining step is changing frm lw-current vercharging t high-current vercharging. A transfer frm ne rate-determining step t anther takes place in the intermediate regin. The intermediate kinetics culd be perfectly described by a serial reactin sequence. Oxygen recmbinatin kinetics was fund t be first-rder in xygen pressure, mst likely diffusin-cntrlled by xygen redisslutin frm the gas phase int the electrlyte, preferably in the slid/electrlyte/gas, three-phase bundary layers near the Cd electrde. Recmbinatin is enhanced at higher temperatures, resulting in significantly reduced pressures. Due t the extremely high verptential at which the xygen reductin at the Cd electrde ccurs, it is mst likely that the reactin is cntrlled by transprt f xygen thrugh the electrlyte. Apparent diffusin rate cnstants have been derived, which were fund t be dependent n the ambient temperature.