表面包覆La-Mg-Ni系A_2B_7型贮氢电极合金的电化学性能研究
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摘要
La-Mg-Ni系A_2B_7型贮氢合金是近来发现的综合电化学性能较好的电极合金，具有重要的开发应用前景，但电极合金循环稳定性较差，因而如何提高该类合金电极循环稳定性是该类合金目前应用研究的关键问题。据此本文以A_2B_7型贮氢合金La_(1.5)Mg_(0.5)Ni_7为研究对象，通过对合金表面包覆Ni-P、Co-P和Cu—P及退火处理，系统的研究了表面包覆对A_2B_7型贮氢合金电极电化学性能的影响。
首先本文对A_2B_7型贮氢合金La_(1.5)Mg_(0.5)Ni_7进行化学镀不同厚度Ni-P处理，包覆处理后对合金粉末表面进行SEM观察。研究表明，包覆镍颗粒均为球形，且均匀弥散于合金表面，合金粉末刨面光镜照片显示合金粉末包覆的镍层厚度较为均匀，呈连续包覆状态。电化学测试结果表明，表面包覆镍处理的La_(1.5)Mg_(0.5)Ni_7贮氢合金电极放电容量有所降低，而循环稳定性有所提高，但改善并不明显。线性极化扫描和电化学阻抗谱等分析结果表明，包覆后合金电极的交换电流密度(I_0)以及电化学阻抗均有较好的改善。
对于A_2B_7型贮氢合金La_(1.5)Mg_(0.5)Ni_7，本文还通过对其化学镀不同厚度Co-P处理及6.78％Co-P镀态合金进行不同温度退火处理，系统地研究了包覆及退火对贮氢合金电极电化学性能的影响。SEM形貌观察显示镀钴晶粒较为细小，退火处理使镀层表面更为平整且镀层颗粒增大，同时合金表面有孔隙出现。XRD分析表明，化学镀Co-P镀层为晶态，是磷在钴中的固熔体；经退火处理，晶粒长大，应力减小；随着温度的升高，合金镀层结构及内部合金均发生变化，镀层结构出现CoP_3相及β—Co相，且基体合金La Ni_5逐渐增多。773K退火1小时是合金镀层结构和电化学性能变化的转折温度。电化学测试结果表明表面包覆钴放电容量明显提高，包覆10％Co-P后放电容量可高达433.1mAh／g，比未包覆合金放电容量提高50mAh／g以上，但循环稳定性均有所降低。包覆钴后，放电曲线出现两个平台，合金电极出现放电双平台及容量提高与镀层钴的氧化而导致吸放氢有关。低温退火处理使合金的循环稳定性较镀态合金有所改善，但对合金电极其它电化学性能影响不大；高温退火处理，随着退火温度升高，合金电极的放电容量逐渐减小，活化性能和循环稳定性均呈恶化趋势。合金电极动力学分析表明，表面包覆钴改善了合金电极的高倍率放电性能，合金电极的交换电流密度I_0。低温退火处理对合金电极动力学性能影响较小，高温退火使合金电极动力学性能变差。
最后本文对贮氢合金La_(1.5)Mg_(0.5)Ni_7表面包覆铜进行了系统研究，使用次磷酸钠作还原剂及用硫酸溶液浸蚀两种方法对合金粉末进行化学镀铜，同时对比研究了在铜镀层表面再覆镍和钴处理对合金电极电化学性能的影响。SEM形貌观察结果表明，用次磷酸钠作还原剂所获铜镀层晶粒非常细密；用硫酸溶液浸蚀镀铜，镀层的形态与硫酸的浓度有很大关系。电化学测试结果表明包覆铜、铜-镍处理均改善了合金电极的循环稳定性能，包覆10％Cu处理的合金电极循环寿命增加最为明显，140次循环容
量保持率从包覆前的66.1％提高到73.2％。经不同浓度硫酸浸蚀镀铜后，合金电极的循环寿命均有较好改善，但合金的放电容量下降明显。合金电极动力学分析表明表面包覆铜、铜-镍和铜-钴均改善了合金电极的动力学性能。
A_2B_7-type hydrogen storage alloy of La-Mg-Ni system is a kind of new type high capacity electrode alloy with better overall electrochemical properties which was found in recent years, this kind of alloy possesses an important prospect of development and utilization. However, the cyclic life of alloy electrodes is poor, then it is the key question of unilization researches that how to improve the cyclic life of alloy electrodes. So in this paper, the influence of Ni-P、 Co-P and Cu coatings on the electrochemical properties of A_2B_7-type hydrogen storage alloy electrode was studied.
Firstly, in this paper the Ni-P coating by chemical plating was systematically studied. The results of SEM showed that some global grain was covered on the surface of alloys. The section optic microscope picture of powder alloy showed that the Ni-P coatings was very symmetrical.The electrochemical results showed that, after Ni-P coating, the cyclic stability of the alloy electrodes was some improved, while the maximum discharge capacity was reduced. The kinetic properties of surface microencapsulation alloy electrode were improved by testing kinetic parameter i.e., a higher exchange density I_0 and lager high rate dischargeability.
For further improving the performances of the A_2B_7-type alloy electrodes, The effect of Co-P coatings and annealing on the electrochemical properties of La_(1.5)Mg_(0.5)Ni_7 hydrogen storage alloy electrode was systematically studied. The results of SEM showed that many small cobalt global grains were covered on the surface of alloys. Heat treatment make cobalt grains grow and some pore appear on the surface. XRD show that electroless plated Co-P alloys coating is crystalline,being a solid solution of phosphors in cobalt matrix with finer grains as plated .when heated, grains growth stress relaxation occur in the coating.With the heating temperature increasing, coating crystal structure was changed,metallic compounds CoP_3 and β— Co phases deposition and allotropic transformation from α—Co, at the same time, LaNi_5 phases increase. The metamorphic temperature is 773K. The electrochemical results showed: material has a higher capacity and has two discharge voltage compared to the bare alloy due to the faradaic reaction of cobalt during discharge, the discharge capacity of coating electrodes could be up to 433.1mAh/g, but the cycle life of coated material was not improved compared to the bare alloy, comparing to coating alloys,When the coating alloys was treated at the low heating temperature ,whose capacity approximated to coating alloys and cycling stability was improved ,with the heating temperature increasing, the capacity and cycling stability were all deteriorated.The Co-P coated powder electrode is better than bare relectrode in exchange current density I_0 limiting current density I_0. The kinetic properties of alloy electrode change rarely when the coating alloys were annealed at the low temperature, but
with the heating temperature increasing, the kinetic properties were deteriorated.
Finally, in order to improve the cyclic stability of A_2B_7-type alloy electrodes, the Cu coating by chemical plating was systematically investigated.We deal with alloy make use of two different methods, at the same time, the cover Ni-P and Co-P on the Cu coating alloy electrodes were also studied.The picture of SEM showed the copper aggradation surface with hypophosphite reducer was covered with very close-grained grains,the surface layer forms of alloys by erode plating were associated with the chroma of vitriol .The electrochemical results showed the cycle life of the materials coated by Cu and Cu-Co were improved .Cu coating could obviously improve overall electrochemical properties of alloy electrodes. After 10% Cu coating the capacity retention rate after 140 charge/discharge cycles could reach73.2%, whereas the discharge capacity retaining rate of bare electrode was 66.1%. The cycle life of Cu coating alloys by erode plating all were improved obviously, but their discharge capacity minished evidently. The kinetic properties testing show: The Cu、 Cu-Co、 Cu-Ni surface microencapsulation all obviously improved the kinetic properties of alloy electrodes.
首先本文对A_2B_7型贮氢合金La_(1.5)Mg_(0.5)Ni_7进行化学镀不同厚度Ni-P处理，包覆处理后对合金粉末表面进行SEM观察。研究表明，包覆镍颗粒均为球形，且均匀弥散于合金表面，合金粉末刨面光镜照片显示合金粉末包覆的镍层厚度较为均匀，呈连续包覆状态。电化学测试结果表明，表面包覆镍处理的La_(1.5)Mg_(0.5)Ni_7贮氢合金电极放电容量有所降低，而循环稳定性有所提高，但改善并不明显。线性极化扫描和电化学阻抗谱等分析结果表明，包覆后合金电极的交换电流密度(I_0)以及电化学阻抗均有较好的改善。
对于A_2B_7型贮氢合金La_(1.5)Mg_(0.5)Ni_7，本文还通过对其化学镀不同厚度Co-P处理及6.78％Co-P镀态合金进行不同温度退火处理，系统地研究了包覆及退火对贮氢合金电极电化学性能的影响。SEM形貌观察显示镀钴晶粒较为细小，退火处理使镀层表面更为平整且镀层颗粒增大，同时合金表面有孔隙出现。XRD分析表明，化学镀Co-P镀层为晶态，是磷在钴中的固熔体；经退火处理，晶粒长大，应力减小；随着温度的升高，合金镀层结构及内部合金均发生变化，镀层结构出现CoP_3相及β—Co相，且基体合金La Ni_5逐渐增多。773K退火1小时是合金镀层结构和电化学性能变化的转折温度。电化学测试结果表明表面包覆钴放电容量明显提高，包覆10％Co-P后放电容量可高达433.1mAh／g，比未包覆合金放电容量提高50mAh／g以上，但循环稳定性均有所降低。包覆钴后，放电曲线出现两个平台，合金电极出现放电双平台及容量提高与镀层钴的氧化而导致吸放氢有关。低温退火处理使合金的循环稳定性较镀态合金有所改善，但对合金电极其它电化学性能影响不大；高温退火处理，随着退火温度升高，合金电极的放电容量逐渐减小，活化性能和循环稳定性均呈恶化趋势。合金电极动力学分析表明，表面包覆钴改善了合金电极的高倍率放电性能，合金电极的交换电流密度I_0。低温退火处理对合金电极动力学性能影响较小，高温退火使合金电极动力学性能变差。
最后本文对贮氢合金La_(1.5)Mg_(0.5)Ni_7表面包覆铜进行了系统研究，使用次磷酸钠作还原剂及用硫酸溶液浸蚀两种方法对合金粉末进行化学镀铜，同时对比研究了在铜镀层表面再覆镍和钴处理对合金电极电化学性能的影响。SEM形貌观察结果表明，用次磷酸钠作还原剂所获铜镀层晶粒非常细密；用硫酸溶液浸蚀镀铜，镀层的形态与硫酸的浓度有很大关系。电化学测试结果表明包覆铜、铜-镍处理均改善了合金电极的循环稳定性能，包覆10％Cu处理的合金电极循环寿命增加最为明显，140次循环容
量保持率从包覆前的66.1％提高到73.2％。经不同浓度硫酸浸蚀镀铜后，合金电极的循环寿命均有较好改善，但合金的放电容量下降明显。合金电极动力学分析表明表面包覆铜、铜-镍和铜-钴均改善了合金电极的动力学性能。
A_2B_7-type hydrogen storage alloy of La-Mg-Ni system is a kind of new type high capacity electrode alloy with better overall electrochemical properties which was found in recent years, this kind of alloy possesses an important prospect of development and utilization. However, the cyclic life of alloy electrodes is poor, then it is the key question of unilization researches that how to improve the cyclic life of alloy electrodes. So in this paper, the influence of Ni-P、 Co-P and Cu coatings on the electrochemical properties of A_2B_7-type hydrogen storage alloy electrode was studied.
Firstly, in this paper the Ni-P coating by chemical plating was systematically studied. The results of SEM showed that some global grain was covered on the surface of alloys. The section optic microscope picture of powder alloy showed that the Ni-P coatings was very symmetrical.The electrochemical results showed that, after Ni-P coating, the cyclic stability of the alloy electrodes was some improved, while the maximum discharge capacity was reduced. The kinetic properties of surface microencapsulation alloy electrode were improved by testing kinetic parameter i.e., a higher exchange density I_0 and lager high rate dischargeability.
For further improving the performances of the A_2B_7-type alloy electrodes, The effect of Co-P coatings and annealing on the electrochemical properties of La_(1.5)Mg_(0.5)Ni_7 hydrogen storage alloy electrode was systematically studied. The results of SEM showed that many small cobalt global grains were covered on the surface of alloys. Heat treatment make cobalt grains grow and some pore appear on the surface. XRD show that electroless plated Co-P alloys coating is crystalline,being a solid solution of phosphors in cobalt matrix with finer grains as plated .when heated, grains growth stress relaxation occur in the coating.With the heating temperature increasing, coating crystal structure was changed,metallic compounds CoP_3 and β— Co phases deposition and allotropic transformation from α—Co, at the same time, LaNi_5 phases increase. The metamorphic temperature is 773K. The electrochemical results showed: material has a higher capacity and has two discharge voltage compared to the bare alloy due to the faradaic reaction of cobalt during discharge, the discharge capacity of coating electrodes could be up to 433.1mAh/g, but the cycle life of coated material was not improved compared to the bare alloy, comparing to coating alloys,When the coating alloys was treated at the low heating temperature ,whose capacity approximated to coating alloys and cycling stability was improved ,with the heating temperature increasing, the capacity and cycling stability were all deteriorated.The Co-P coated powder electrode is better than bare relectrode in exchange current density I_0 limiting current density I_0. The kinetic properties of alloy electrode change rarely when the coating alloys were annealed at the low temperature, but
with the heating temperature increasing, the kinetic properties were deteriorated.
Finally, in order to improve the cyclic stability of A_2B_7-type alloy electrodes, the Cu coating by chemical plating was systematically investigated.We deal with alloy make use of two different methods, at the same time, the cover Ni-P and Co-P on the Cu coating alloy electrodes were also studied.The picture of SEM showed the copper aggradation surface with hypophosphite reducer was covered with very close-grained grains,the surface layer forms of alloys by erode plating were associated with the chroma of vitriol .The electrochemical results showed the cycle life of the materials coated by Cu and Cu-Co were improved .Cu coating could obviously improve overall electrochemical properties of alloy electrodes. After 10% Cu coating the capacity retention rate after 140 charge/discharge cycles could reach73.2%, whereas the discharge capacity retaining rate of bare electrode was 66.1%. The cycle life of Cu coating alloys by erode plating all were improved obviously, but their discharge capacity minished evidently. The kinetic properties testing show: The Cu、 Cu-Co、 Cu-Ni surface microencapsulation all obviously improved the kinetic properties of alloy electrodes.
引文
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