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New fabrication method for nickel metal hydride electrodes by sintering alloy onto a foam substrate
\
Pergamon PII]
Int[ J[ Hydro`en Ener`y\ Vol[ 12\ No[ 01\ pp[ 0068Ð0072\ 0887 Þ 0887 International Association for Hydrogen Energy Elsevier Science Ltd All rights reserved[ Printed in Great Britain S9259Ð2088"86#99045Ð9 9259Ð2088:87 ,08[99¦9[99
NEW FABRICATION METHOD FOR NICKEL METAL HYDRIDE ELECTRODES BY SINTERING ALLOY ONTO A FOAM SUBSTRATE W[ METZGER\ R[ WESTFALL and A[ HERMANN Department of Physics\ University of Colorado\ Boulder\ Colorado 79298\ U[S[A[
Abstract*A new method for fabricating nickel metal hydride electrodes has been investigated[ A slurry consisting of hydriding alloy mixed with nickel powder was pasted onto nickel foam substrates[ These were then roller pressed to 0[1 mm thickness and sintered in an inert atmosphere for 09 min at 799>C[ The role of the di}erent processing steps as well as the performance of di}erent materials has been characterized[ Initial cycles give an energy density comparable to that obtained with conventional methods and a much higher fast rate discharge capacity[ Þ 0887 International Association for Hydrogen Energy
INTRODUCTION
EXPERIMENTAL
Throughout the past 09 years\ the need for rechargeable nickel cadmium cells has increased dramatically due to the popularization of a number of cordless appliances such as cellular phones and laptop computers[ Since cad! mium is both toxic and scarce\ there is a strong need for a substitute[ Nickel metal hydride batteries have a voltage\ rate capability\ overcharge protection\ and charge retention comparable to the nickel cadmium battery\ and a higher energy density[ Traditionally\ a short cycle life has impeded the nickel metal hydride battery from penetrating the market place^ however\ recent advances have overcome this shortcoming ð0Ł[ Researchers thus far have used primarily one method for fabricating the metal hydride electrode[ After pre! paring the hydriding alloy\ the alloy is mixed with an organic binder and then hot pressed onto the substrate typically at around 299>C and 399 kgf cm−1 ð0Ð4Ł[ The binder causes a decrease in energy density and high rate discharge capacity while reducing activation time ð5Ł[ In this work an alternative method was investigated[ Hydriding alloy was mixed with INCO "International Nickel Company# T!109 nickel powder\ pasted onto a nickel foam substrate\ roller pressed to 0[1 mm thickness\ and sintered at 799>C in an inert atmosphere for 09 min[ This eliminates the need for a binder and increases the fast!rate discharge capacity[
The nickel metal hydride alloy used in these experi! ments was La9[40Ce9[22Nd9[00Pr9[4Ni2[4Co9[6Al9[3Mn9[2 pur! chased from the Rhone Poulenc company[ This composition is very similar to a number of alloys which have been used with the traditional method of electrode fabrication ð0Ł[ An adsorption and desorption PCT curve was measured by the Rhone Poulenc company using a Sieverts type gas!solid reactor apparatus\ their results are given in Fig[ 0[
Author to whom correspondence should be addressed[
Fig[ 0[ Absorption "A# and desorption "B# PCT curves of the La9[40Ce9[22Nd9[00Pr9[94Ni2[4Co9[6Al9[3Mn9[2 alloy prepared by Rhone Poulenc "with permission from Rhone Poulenc#[ 0068
0079
W[ METZGER et al[
This alloy powder was mixed with _ne INCO nickel T!109 powder "particle size 9[4Ð0[9 mm# in a slurry\ and then pasted onto INCO nickel foam with dimensions 4[9×1[4×9[08 cm2[ To maximize energy density and
cycle life\ it is important that the active material uni! formly _ll the void space in the nickel foam[ Despite the small particle size\ the backscattering electron images in Fig[ 1 "a# and "b# indicate that the slurry did not penetrate
(a)
(b)
Fig[ 1[ Backscattering electron images of the metal hydride electrode cross section[ Average pore size of the nickel foam substrate is "a# 499 mm^ "b# 699 mm[
NEW FABRICATION METHOD FOR NICKEL METAL HYDRIDE ELECTRODES
0070
Fig[ 2[ Backscattering electron images of the cross!section of a negative electrode prepared without roller pressing[
nickel foam with an average pore size of 499 mm\ but did penetrate nickel foam with a pore size of 699 mm[ The electrode was allowed to dry and then roller pre! ssed to a thickness of 0[1 mm[ It can be seen by comparing the scanning electron microscope photographs in Figs 1 "b# and 2 that this step further pulverizes the material and helps to achieve a more uniform mixture and distribution throughout the substrate[ After roller pressing\ the electrodes were placed in a tube furnace and sintered at 799>C for 09 min in a 89:09 argonÐhydrogen atmosphere[ Electrochemical capacity and short!term stability properties were measured by charging the electrodes in an open cell with a current density of 049 mA:g for 1[4 h and discharging them with an average current density of 099 mA:g to 9[59 V on the negative electrode with respect to Hg:HgO reference electrodes[ Sintered nickel plaque and 5 M KOH were used as the counter electrodes and electrolyte respectively[ Scanning electron microscopy and backscattering electron images were performed using an ISI SX29 scanning electron microprobe and a JOEL 7599 microprobe respectively[ For cross!sectional back! scattering images\ cross!sections were cut out of the electrodes prior to placing them in the electrochemical cell\ placed in epoxy and polished\ and then carbon coated[
RESULTS AND DISCUSSION The physical characteristics of the INCO nickel 109 powder are best illustrated by the SEM photograph in Fig[ 3[ The high aspect ratio serves to form a three dimen! sional network upon sintering which binds the hydriding alloy to the substrate as shown in Fig[ 4[ Discharge capacity during the initial cycles prior to full activation was 129 mAh:g of metal hydride alloy\ and 045 mAh:g of the entire negative electrode[ The current capacity at a discharge rate of 3[4 C was 89) of the capacity at a discharge rate of 9[5 C[ Typical values for electrodes containing polymeric binder and a similar hydriding alloy are 39Ð69) ð0\ 5Ł[ This con_rms the supposition by Sakai et al[ ð5Ł that capability for high rates is lowered by the use of a polymeric binder because the electrical contact between alloy particles becomes worse[ By replacing polymeric binder with sintered nickel 109 powder\ the high rate discharge capability is dras! tically improved\ as expected[ Voltage characteristics throughout charging and dis! charging cycles were quite standard[ The equilibrium voltage on the negative was −9[803 V with respect to a Hg:HgO reference electrode[ The cathode internal resist! ance was 9[4 V[ Throughout 04 cycles\ no pulverization or signi_cant weight loss were observed on the negative electrode if the
0071
W[ METZGER et al[
Fig[ 3[ SEM photograph of INCO nickel 109 powder "with permission from INCO#[
electrode was initially free of large cracks and _ssures[ We did _nd that if the electrode initially had cracks or _ssures\ these were quickly aggravated by charge and discharge cycles[ Discharge capacity was climbing steadily with each cycle^ however\ full activation was not attained[ A long term cycle stability analysis was not performed[ CONCLUSION A metal hydride electrode was made by pasting a hyd! riding alloy mixed with INCO nickel 109 powder onto a nickel foam substrate and sintering at 799>C for 09 min[
It was found that the electrodes produced using this new method gave respectable discharge capacities comparable to electrodes made from hot pressing hydriding alloy mixed with organic binders[ By removing the need for an organic binder and implementing a network of nickel powder throughout the electrode\ the electrical contact between alloy particles*and consequently the fast rate discharge capacity*were drastically improved[ Short term stability characteristics were good\ pro! vided no cracks or _ssures were present in the original electrode[ Sintering characteristics depend on the chemical com! position of the alloy[ We have found that this new method
NEW FABRICATION METHOD FOR NICKEL METAL HYDRIDE ELECTRODES
0072
Fig[ 4[ Top!view SEM photograph of a metal hydride electrode[
works well for LaNi4 and other alloys containing di}erent proportions of La\ Ce\ Nd\ Pr\ Ni\ Co\ Al and Mn than the alloy studied here[ However\ it is not known how each individual metal or other compositions a}ect the sintering process[ Acknowled`ements*The authors are grateful to Phil Lyman "Boundless Corp[# for his cooperation and assistance throughout this project[ The paper is supported by the Entrepreneur Technology Assistance Program administered by the Department of Energy\ the Colorado Advanced Materials Institute and the Rocky Flats Local Impact Initiative[
REFERENCES 0[ Sakai\ T[\ Yoshinaga\ H[\ Miyamura\ H[\ Kuriyama\ N[ and Ishikawa\ H[\ J[ Alloys Comp[\ 0881\ 079\ 26[
1[ Geng\ M[\ J[ Alloys Comp[\ 0884\ 106\ 89[ 2[ Geng\ M[ J[ of Alloys Comp[\ 0883\ 104\ 040[ 3[ Sakai\ T[\ Hazama\ T[\ Miyamura\ H[\ Kuriyama\ N[\ Kato\ A[ and Ishikawa\ H[\ J[ Less!Common Met[\ 0880\ 061\ 0064[ 4[ Sakai\ T[\ Miyamura\ H[\ Kuriyama\ N[\ Kato\ A[\ Oguro\ K[\ Ishikawa\ H[ and Iwakura\ C[\ J[ Less!Common Met[\ 0889\ 048\ 016[ 5[ Sakai\ T[\ Takagi\ A[\ Kinoshita\ K[\ Kuriyama\ N[\ Miya! mura\ H[ and Ishikawa\ H[\ J[ Less!Common Met[\ 0880\ 061\ 0074[ 6[ Tang\ W[\ Gai\ Y[ and Zheng\ H[\ J[ of Alloys Comp[\ 0884\ 113\ 181[ 7[ Sakai\ T[\ Ishikawa\ H[\ Oguro\ K[\ Iwakura\ C[ and Yone! yama\ H[\ J[ Electrochem[ Soc[\ 0876\ 023\ 447[ 8[ Williams\ J[ J[ G[ and Buschow\ K[ H[ J[\ J[ Less!Common Met[\ 0876\ 018\ 02[
Pergamon PII]
Int[ J[ Hydro`en Ener`y\ Vol[ 12\ No[ 01\ pp[ 0068Ð0072\ 0887 Þ 0887 International Association for Hydrogen Energy Elsevier Science Ltd All rights reserved[ Printed in Great Britain S9259Ð2088"86#99045Ð9 9259Ð2088:87 ,08[99¦9[99
NEW FABRICATION METHOD FOR NICKEL METAL HYDRIDE ELECTRODES BY SINTERING ALLOY ONTO A FOAM SUBSTRATE W[ METZGER\ R[ WESTFALL and A[ HERMANN Department of Physics\ University of Colorado\ Boulder\ Colorado 79298\ U[S[A[
Abstract*A new method for fabricating nickel metal hydride electrodes has been investigated[ A slurry consisting of hydriding alloy mixed with nickel powder was pasted onto nickel foam substrates[ These were then roller pressed to 0[1 mm thickness and sintered in an inert atmosphere for 09 min at 799>C[ The role of the di}erent processing steps as well as the performance of di}erent materials has been characterized[ Initial cycles give an energy density comparable to that obtained with conventional methods and a much higher fast rate discharge capacity[ Þ 0887 International Association for Hydrogen Energy
INTRODUCTION
EXPERIMENTAL
Throughout the past 09 years\ the need for rechargeable nickel cadmium cells has increased dramatically due to the popularization of a number of cordless appliances such as cellular phones and laptop computers[ Since cad! mium is both toxic and scarce\ there is a strong need for a substitute[ Nickel metal hydride batteries have a voltage\ rate capability\ overcharge protection\ and charge retention comparable to the nickel cadmium battery\ and a higher energy density[ Traditionally\ a short cycle life has impeded the nickel metal hydride battery from penetrating the market place^ however\ recent advances have overcome this shortcoming ð0Ł[ Researchers thus far have used primarily one method for fabricating the metal hydride electrode[ After pre! paring the hydriding alloy\ the alloy is mixed with an organic binder and then hot pressed onto the substrate typically at around 299>C and 399 kgf cm−1 ð0Ð4Ł[ The binder causes a decrease in energy density and high rate discharge capacity while reducing activation time ð5Ł[ In this work an alternative method was investigated[ Hydriding alloy was mixed with INCO "International Nickel Company# T!109 nickel powder\ pasted onto a nickel foam substrate\ roller pressed to 0[1 mm thickness\ and sintered at 799>C in an inert atmosphere for 09 min[ This eliminates the need for a binder and increases the fast!rate discharge capacity[
The nickel metal hydride alloy used in these experi! ments was La9[40Ce9[22Nd9[00Pr9[4Ni2[4Co9[6Al9[3Mn9[2 pur! chased from the Rhone Poulenc company[ This composition is very similar to a number of alloys which have been used with the traditional method of electrode fabrication ð0Ł[ An adsorption and desorption PCT curve was measured by the Rhone Poulenc company using a Sieverts type gas!solid reactor apparatus\ their results are given in Fig[ 0[
Author to whom correspondence should be addressed[
Fig[ 0[ Absorption "A# and desorption "B# PCT curves of the La9[40Ce9[22Nd9[00Pr9[94Ni2[4Co9[6Al9[3Mn9[2 alloy prepared by Rhone Poulenc "with permission from Rhone Poulenc#[ 0068
0079
W[ METZGER et al[
This alloy powder was mixed with _ne INCO nickel T!109 powder "particle size 9[4Ð0[9 mm# in a slurry\ and then pasted onto INCO nickel foam with dimensions 4[9×1[4×9[08 cm2[ To maximize energy density and
cycle life\ it is important that the active material uni! formly _ll the void space in the nickel foam[ Despite the small particle size\ the backscattering electron images in Fig[ 1 "a# and "b# indicate that the slurry did not penetrate
(a)
(b)
Fig[ 1[ Backscattering electron images of the metal hydride electrode cross section[ Average pore size of the nickel foam substrate is "a# 499 mm^ "b# 699 mm[
NEW FABRICATION METHOD FOR NICKEL METAL HYDRIDE ELECTRODES
0070
Fig[ 2[ Backscattering electron images of the cross!section of a negative electrode prepared without roller pressing[
nickel foam with an average pore size of 499 mm\ but did penetrate nickel foam with a pore size of 699 mm[ The electrode was allowed to dry and then roller pre! ssed to a thickness of 0[1 mm[ It can be seen by comparing the scanning electron microscope photographs in Figs 1 "b# and 2 that this step further pulverizes the material and helps to achieve a more uniform mixture and distribution throughout the substrate[ After roller pressing\ the electrodes were placed in a tube furnace and sintered at 799>C for 09 min in a 89:09 argonÐhydrogen atmosphere[ Electrochemical capacity and short!term stability properties were measured by charging the electrodes in an open cell with a current density of 049 mA:g for 1[4 h and discharging them with an average current density of 099 mA:g to 9[59 V on the negative electrode with respect to Hg:HgO reference electrodes[ Sintered nickel plaque and 5 M KOH were used as the counter electrodes and electrolyte respectively[ Scanning electron microscopy and backscattering electron images were performed using an ISI SX29 scanning electron microprobe and a JOEL 7599 microprobe respectively[ For cross!sectional back! scattering images\ cross!sections were cut out of the electrodes prior to placing them in the electrochemical cell\ placed in epoxy and polished\ and then carbon coated[
RESULTS AND DISCUSSION The physical characteristics of the INCO nickel 109 powder are best illustrated by the SEM photograph in Fig[ 3[ The high aspect ratio serves to form a three dimen! sional network upon sintering which binds the hydriding alloy to the substrate as shown in Fig[ 4[ Discharge capacity during the initial cycles prior to full activation was 129 mAh:g of metal hydride alloy\ and 045 mAh:g of the entire negative electrode[ The current capacity at a discharge rate of 3[4 C was 89) of the capacity at a discharge rate of 9[5 C[ Typical values for electrodes containing polymeric binder and a similar hydriding alloy are 39Ð69) ð0\ 5Ł[ This con_rms the supposition by Sakai et al[ ð5Ł that capability for high rates is lowered by the use of a polymeric binder because the electrical contact between alloy particles becomes worse[ By replacing polymeric binder with sintered nickel 109 powder\ the high rate discharge capability is dras! tically improved\ as expected[ Voltage characteristics throughout charging and dis! charging cycles were quite standard[ The equilibrium voltage on the negative was −9[803 V with respect to a Hg:HgO reference electrode[ The cathode internal resist! ance was 9[4 V[ Throughout 04 cycles\ no pulverization or signi_cant weight loss were observed on the negative electrode if the
0071
W[ METZGER et al[
Fig[ 3[ SEM photograph of INCO nickel 109 powder "with permission from INCO#[
electrode was initially free of large cracks and _ssures[ We did _nd that if the electrode initially had cracks or _ssures\ these were quickly aggravated by charge and discharge cycles[ Discharge capacity was climbing steadily with each cycle^ however\ full activation was not attained[ A long term cycle stability analysis was not performed[ CONCLUSION A metal hydride electrode was made by pasting a hyd! riding alloy mixed with INCO nickel 109 powder onto a nickel foam substrate and sintering at 799>C for 09 min[
It was found that the electrodes produced using this new method gave respectable discharge capacities comparable to electrodes made from hot pressing hydriding alloy mixed with organic binders[ By removing the need for an organic binder and implementing a network of nickel powder throughout the electrode\ the electrical contact between alloy particles*and consequently the fast rate discharge capacity*were drastically improved[ Short term stability characteristics were good\ pro! vided no cracks or _ssures were present in the original electrode[ Sintering characteristics depend on the chemical com! position of the alloy[ We have found that this new method
NEW FABRICATION METHOD FOR NICKEL METAL HYDRIDE ELECTRODES
0072
Fig[ 4[ Top!view SEM photograph of a metal hydride electrode[
works well for LaNi4 and other alloys containing di}erent proportions of La\ Ce\ Nd\ Pr\ Ni\ Co\ Al and Mn than the alloy studied here[ However\ it is not known how each individual metal or other compositions a}ect the sintering process[ Acknowled`ements*The authors are grateful to Phil Lyman "Boundless Corp[# for his cooperation and assistance throughout this project[ The paper is supported by the Entrepreneur Technology Assistance Program administered by the Department of Energy\ the Colorado Advanced Materials Institute and the Rocky Flats Local Impact Initiative[
REFERENCES 0[ Sakai\ T[\ Yoshinaga\ H[\ Miyamura\ H[\ Kuriyama\ N[ and Ishikawa\ H[\ J[ Alloys Comp[\ 0881\ 079\ 26[
1[ Geng\ M[\ J[ Alloys Comp[\ 0884\ 106\ 89[ 2[ Geng\ M[ J[ of Alloys Comp[\ 0883\ 104\ 040[ 3[ Sakai\ T[\ Hazama\ T[\ Miyamura\ H[\ Kuriyama\ N[\ Kato\ A[ and Ishikawa\ H[\ J[ Less!Common Met[\ 0880\ 061\ 0064[ 4[ Sakai\ T[\ Miyamura\ H[\ Kuriyama\ N[\ Kato\ A[\ Oguro\ K[\ Ishikawa\ H[ and Iwakura\ C[\ J[ Less!Common Met[\ 0889\ 048\ 016[ 5[ Sakai\ T[\ Takagi\ A[\ Kinoshita\ K[\ Kuriyama\ N[\ Miya! mura\ H[ and Ishikawa\ H[\ J[ Less!Common Met[\ 0880\ 061\ 0074[ 6[ Tang\ W[\ Gai\ Y[ and Zheng\ H[\ J[ of Alloys Comp[\ 0884\ 113\ 181[ 7[ Sakai\ T[\ Ishikawa\ H[\ Oguro\ K[\ Iwakura\ C[ and Yone! yama\ H[\ J[ Electrochem[ Soc[\ 0876\ 023\ 447[ 8[ Williams\ J[ J[ G[ and Buschow\ K[ H[ J[\ J[ Less!Common Met[\ 0876\ 018\ 02[