In Situ X-Ray Absorption Spectroscopy Studies of Metal Hydride Electrodes

In situ x-ray absorption spectroscopy (XAS) studies were done on three metal hydride electrodes, LaNi~, LaNi~.sSns.2, La0.sCe02Ni48Sn0.2, in 6M KOH. Ex situ measurements were also made on dry uncycled electrodes and on material from an La0.sCe0.2Ni48Sn0.2 electrode that had been cycled 25 times. Comparison of the in situ XAS at the Ni K and at the La L3 edge of charged and discharged electrodes indicates large changes in the electronic and structural characteristics on introduction of hydrogen. Results at the Ce L2 edge in La~.sCe~.2Ni4.~Sn0.~ show a transition from a mixed valent c~ to a -/-like Ce state as the lattice expands during charge. Ex situ x-ray absorption near-edge structures (XANES) at the Ni K edge indicate that the additions of either Ce or Sn fill empty Ni 3d states. The Ni K edge extended x-ray absorption fine structures (EXAFS) for all three alloys in the dry uncharged state were similar, indicating that minor substitutions for either the A or B component do not substantially change the structure. The Sn substitution causes an increase both in a and c axis as evidenced from increase in the Ni-Ni and the Ni-La distances. Partial substitution of La by Ce causes a slight contraction in the Ni-La distance. The Ni XANES and EXAFS indicate that about 6 % of the Ni in the La0.sCe0.2Ni~.sSn02 corroded after 25 cycles. Ce XANES on the cycled electrode indicates some corrosion of Ce and the formation of Ce (III) state. The results indicate that XAS is a very useful technique for the study of alloy hydrides, particularly the role of electronic structure, the environment around minor constituents, and the corrosion of individual components. Recent advances in the development of stable metal hydride alloy electrodes have led to their use as a replacement for cadmium anodes in rechargeable alkaline batteries. I Present battery electrodes are either AB2 or AB5 type alloys. The performance and life of these alloys greatly depend on their composition. In the case the AB5 type alloys, substitution of either component in the prototype alloy, LaNi~, with small amounts of other alloying elements can have major effects in the performance and stability of the alloy. Previous results with Ce, Sn, and Co substitution have demonstrated promising results in battery electrodes. 2 Recent results 3 have shown that Sn and Co substitution for some of the Ni causes a lowering of the hydrogen plateau pressure. Partial substitution of La with Ce results in improved corrosion resistance and cycle life. The Ce substitution also causes an increase in the hydrogen plateau pressure. An understanding of the mechanism of these effects would help in optimizing metal hydrides for various hydrogen storage and battery applications. Most previous studies for understanding the role of various substituents in metal hydride alloys have focused on the structural aspects of such substitutions (via application of x-ray and neutron diffraction techniques). However, electronic effects, such as the role of empty d states are important for hydrogen storage. ~ X-ray absorption spectroscopy (XAS) has the ability to probe in situ, both electronic (from the x-ray absorption near-edge structure, XANES) and geometric parameters (from the extended x-ray absorption fine structure, EXAFS) with element specificity. Most previous XAS studies on hydrides have been done in the gas phase. Tanaka et al. ~ and Lengeler and Zeller 6 did early XANES work on several metal hydrides. In the case of NiH0.a~, changes in the XANES on hydriding were attr ibuted to changes in the density of empty p states. ~ Similar conclusions were made for VH0.n. 5 Later Garcia et al. used both XANES and EXAFS to study the alloy hydrides CeRu2H3.7~ and CeFe2H375 at the Ce L~ edge and the Fe K edge] More recently Suenobu et al. have done XAS studies on amorphous LaNia thin films, involving gas-phase hydriding. 8' 9 Recently, an in situ XAS study on LaNi5 electrode has been reported. ~~ * Electrochemical Society Active Member. 2278 The present study focuses on the application of in situ XAS to elucidate the electronic and geometric parameters as a result of (i) substitution of both A and B components of the prototype AB~ alloy, LaNis, by Ce and Sn and (it) the effect of charging and discharging the metal hydride electrode in 6M KOH. Experimental Alloy preparation and characterization.--All the alloys were prepared from high purity (>99.9%) starting components with the exception of Ce, which was of commercial purity. The alloys were prepared by an arc melting technique under He atmosphere. The melting procedure involved several remelting steps, each time followed by turning over the ingot. This was followed by an annealing step at a temperature of I173K for 48 h, after which x-ray diffraction patterns were obtained for each alloy, and its lattice parameters determined. The alloys were first characterized by obtaining the hydrogen pressure composition isotherms (PCT) in a modified Sievert's type apparatus according to the usual procedure, n This yielded the hydrogen plateau pressure and the hydrogen equilibrium storage capacity in the gas phase. The molar volume of hydrogen in the hydride phase was determined by comparing the lattice parameters of the hydrided and dehydrided phases using x-ray diffraction. The procedure for determining the lattice parameter and molar volume of hydrogen is given elsewhere. 12 Electrochemical and cycling studies.--Electrochemical characterization measurements and cycling tests were made on electrodes prepared by pressing a mix, comprised of the alloy, carbon black (Vulcan XC-72), and 33 weight percent (w/o) PTFE binder (Teflon T-30), onto an 80 mesh nickel screen. The electrodes were tested for their initial capacity, charge-discharge characteristics, and cycle life in 6M KOH. The electrode potential was monitored using an Hg/HgO reference electrode and the capacities were measured to a cut of potential of 0.7 V. Cycle life was measured using a Bitrode battery cycler by charging at 0.5 C rate for 3 h and discharging at the same rate to a cutoff cell potential of 1.0 V vs. a nickel hydroxide counterelectrode. After cycling, the electrodes were washed to remove KOH. The active material was removed from the current collector, mixed with boron nitride (BN), and pressed into a pellet for XAS studies. d. Electrochem. Soc., Vol. 142, No. 7, July 1995 9 The Electrochemical Society, Inc. ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 129.10.247.223 Downloaded on 2017-02-26 to IP J. Electrochem. Soc., Vol. 142, No. 7, July 1995 9 The Electrochemical Society, Inc. Table I. Physicochemical properties of modified AB5 type alloys. 2279 Cell Plateau Lattice Parameters volume pressure ~ VH Composition a (s c (/k) (s arm Hmax/FU ( s LaNi~ 5.009 3.970 86.31 1.80 6.90 3.5 b LaNi4.sSn02 5.056 4.019 88.96 1.01 6.26 3.2 La0.sCe02Ni4.sSn02 5.033 4.038 88.6 1.08 5.73 All reported values are at a temperature of 299 K. b Obtained from Ref. 35. In situ X A S studies.--X-ray absorption spectroscopic (XAS) measurements were conducted at the beam lines X23A2 and X11 at the National Synchrotron Light Source (NSLS). Details of the monochromator design and energy resolution of the respective beam lines are given in detail elsewhere, z3-~5 To eliminate second harmonics the beam was detuned by 15 % at the Ni K edge and by 50 % at the La and Ce L edges. In situ XAS measurements were made, in the transmission mode, at the La L3, Ni K and Ce L3 edges, at the end of charge and discharge. The electrodes for the in situ XAS measurements and for measurements on dry samples were thin disks (0.25 mm thick and 19 mm in diam) that comprised the alloy, carbon black (Vulcan XC-72), vitreous carbon fibers, and a polyvinylidine fluoride binder. They were prepared using a standard vacuum table paper making technique. 13 Prior to electrode fabrication, all the alloys were activated by subjecting each to several hydriding-dehydriding cycles in the Sievert's apparatus. This produces a fine powder with a Brunauer, Emmett, and Teller method (BET) surface area of ~0.5 m2/gm, or a particle size of ~5 p~m that eliminates thickness effects in the EXAFS and avoids the need for an electrochemical activation procedure. In the last dehydriding cycle the H content of the solids was measured by venting aliquots of desorbed H2 gas into a measured volume; these values are listed in Table I. For detailed accounts of the activation procedure see Ref. 16 and 17. The separator was a single layer of a polyamide felt (0.125 mm thick) combined with a single layer of a 0.025 mm thick radiation grafted polyethylene separator. The counterelectrode was a 0.125 mm thick Grafo'il disk. All components were soaked in 6M KOH and incorporated into a spectroelectrochemical cell that is described elsewhere. ~3 The data acquisition for XAS comprised of three 12 in. ionization detectors (incidence Io, transmittance It, reference Ir~). The reference channel was primarily for internal calibration of the edge positions and was used in conjunction with pure foils or oxide samples of the respective elements. For the Ni K edge pure N~ was used in all the chambers, while for the La/Ce L3 edges a mixture of 80% He and 20% N2 was used in the incidence chamber while passing pure N2 in the transmittance and reference chambers. The metal hydride electrodes for the in situ studies were charged at a constant current of 1 mA for 16 h and discharged at a current of 2 mA to a potential of -0.5 V. The charging rate and' time allowed for considerable overcharge because the electrodes contained only =50 mg of the alloy. XAS scans were run within 30 rain of the termination of charge. Data analysis.-The data analysis package used for the XANES analysis was the University of Washington analysis program. TM The data analysis was done according to procedures described in detail elsewhere. 13-~5'~8-2~ The EXAFS data analysis used computer algorithms developed by Koningsberger and co-workers. ~8-2~ The most comprehensive analysis of EXAFS was carried out on the data for dry uncycled electrodes at the Ni K edge. The Fourier filtering and analysis of the EXAFS spectra were conducted according to procedure described in detail elsewhere. ~3-~'22 The windows for the forward and inverse Fourier transforms were chosen on the basis of nodes in the EXAFS in order to avoid excessive termination errors. 2a The slight variations in the Ak and Ar ranges, however, do not effect the outcome of the results. 23 The analysis was confined to the first major peak in the Fourier transform at 2.2 A. Analysis of the EXAFS spectra for the alloys was done using one, two, three, and four shell fits using an iterative least square technique. ~ For fitting the sample data, phase and amplitude parameters derived from standard materials as well as those calculated using the University of Washington FEFF program (version 4.08) were employed. 2~ The phase and amplitude parameters for Ni-Ni coordination shells were obtained from liquid N~ data for a pure Ni foil (6 ~m thick). The Ni-La and Ni-Sn phase and amplitude parameters were calculated theoretically using the FEFF program based on Cartesian coordinates input for the Wurtzite structure. The cell constants were varied to yield first shell bond distances that were equivalent to the sum of the atomic radii. In these theoretical calculations an So 2 value of 0.7 was used throughout. Results and Discussion X-ray diffraction, physicochemical, and electrochemical characterization.--Table I shows the results of x-ray diffraction analysis for the alloys. All the alloys were single phase and possessed the hexagonal CaCu5 type (space group P6/mmm) structure, typical of the AB~ type alloys. As indicated by the lattice parameters in Table I, substitution of B component (Ni) by Sn in the AB~ metal hydride lattice (LaNis) results in an increase in the lattice parameters and, hence, the cell volume. However, substitution of the A component (La) with Ce causes a minor shrinkage along the a axis. The structural change due to partial substitution of Ni by Sn is reflected in the lowering of the hydrogen plateau pressure which improves the prospects for charging efficiency and charge retention of the hydride electrode. This structural change due to substitution is also reflected in a decrease in the gas-phase hydrogen storage capacity per formula unit (FU) and a reduced molar volume of hydrogen (V~) The electrochemical performance characteristics of the metal hydride electrodes are given in Table II. The results indicate that substitution of Sn for Ni in the LaN% lattice significantly improves the hydride stability. This is consistent with the reduction in the gas-phase hydrogen plateau pressure, and is in agreement with the recent results of Rathnakumar et. al. 26 The results in Table II indicate that these materials containing Sn or Ce give a very significant improvement in the cycle life relative to the LaN%. From these results it is clear that the substitution of the AB~ type alloys with small amounts of Table II. Performance characteristics of hydride electrodes with modified ABs-type alloys. Rate Initial Discharge capability Decay rate capacity potential 1C/3C a (mAh/g Composition (mAh/gh) [mV(Hg/HgO)] (%) cycle) LaNi~ 350 b 4 5 % b'c LaNi~.sSn0.~ 296 900 93 0.93 La08Ce0.2Ni4.~Sn02 315 935 83 0.77 Ratio of capacity based on electrodes charged and discharged at C and C/3 rate, respectively. b Obtained from Ref. 35. ~ After 100 cycles. ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 129.10.247.223 Downloaded on 2017-02-26 to IP