Synthesis of La₂(Ni_1-xCo_x)₇(x = 0.05, 0.1, 0.2) Hydrogen Storage Alloys by the Electro-Deoxidation Technique

Abstract

La 2 (Ni 1-x Co x ) 7 (x = 0.05, 0.1, 0.2) alloys were synthesized directly from sintered mixture of La 2 O 3 + NiO + CoO in the molten CaCl 2 electrolyte by the electro-deoxidation method at 850 • C and the electrochemical hydrogen storage characteristics of the synthesized alloys were observed. Sintering (at 1200 • C for 3 h) converted the hygroscopic La 2 O 3 (by the reaction with NiO) into the non-hygroscopic LaNiO 3 , La 3 Ni 2 O 6.5 and La 4 Ni 3 O 9 depending on the Co content of the oxide mixture. The X-ray diffraction peaks indicated that La 2 NiO 4 was the main La-Ni-O phase formed as a result of partial reduction or dissociation reaction to initiate the LaNi 5 phase formation. The target La 2 Ni 7 phase formed much later than LaNi 5 phase probably due to the sluggish reduction of LaOCl. It was observed that La 2 (Ni 1-x Co x ) 7 (x = 0.05, 0.1, 0.2) alloys had promising discharge capacities changed between 227 mA h g −1 (La 2 (Ni 0.95 Co 0.05 ) 7 ) and 332 mA h g −1 (La 2 (Ni 0.8 Co 0.2 ) 7 ) depending on the alloy Co content. This work clearly indicated that the electro-deoxidation was very effective method in the synthesis of the hydrogen storage materials.The extensive developments in the portable electronic devices and the requirement for the environmental friendly transportation industry withdraw the attention of many researchers on the nickel metal hydride batteries. 1-3 The big portion of the works is conducted on the development of the high performance electrode alloy with the proper synthesis methods for these secondary batteries. 4-9 Recently a novel technique called electro-deoxidation, which is also known as Fray-Farthing-Chen (FFC) Cambridge process, 10,11 is reported as very promising for the economical synthesis of the hydrogen storage alloys. [12][13][14] One of the well-known commercial alloy groups for the nickel metal hydride batteries is lanthanum-nickel based alloys and they are generally synthesized by the melting and casting under the protective atmosphere. [15][16][17][18][19] Of course the individual elements (La, Ni and other additive elements) must be already extracted and refined for the melting and casting processes. The cast products need annealing for several hours to get the structural homogeneity. 20 Obviously this production pathway for the La-Ni based hydrogen storage alloys is not very encouraging especially for the large scale applications like battery systems of the electrical vehicles. The electro-deoxidation method looks more cost-effective since it provides direct synthesis of the alloy with the final stoichiometry from the raw materials (oxides). 21 In this work, as the continuation of our previous work, 22 La 2 (Ni 1-x Co x ) 7 (x = 0.05, 0.1, 0.2) hydrogen storage alloy was synthesized by the molten salt electro-deoxidation method for the first time in the literature. In our previous work we presented the alloy development stages of La 2 Ni 7 during electro-deoxidation. 22 In this work part of Ni was replaced by Co systematically and then the alloy developmen...

Direct synthesis of La-Mg-Ni-Co type hydrogen storage alloys from oxide mixtures

“…fied the alloy development mechanism, which was reported in our previous publications [10][11][12] , by taking into account the role of Mg additive element. The charge/discharge experiments were conducted with the synthesized alloys to observe the effect of Mg, which is indispensable hydride forming element [19][20][21] , as an additive material on the hydrogen storage characteristics of A 2 B 7 type La-Ni based alloys.…”

“…The extension of the electro-deoxidation up to 4 h yields the accumulation of the chemically formed LaOCl phase as in Reaction ( 6 ) [10,11] :…”

“…The effectiveness of the electrodeoxidation becomes much more apparent especially if the elements/alloys have very complex production steps in their ordinary synthesis pathways. Utilization of the electro-deoxidation was extended to the synthesis of the energy storage materials since the economics is also very important consideration in the production of these materials due to the potential large scale applications of them like in the battery systems of the electric vehicles [7][8][9][10][11][12] .…”

“…Lanthanum-nickel based hydride forming alloys have tedious production process that after the melting and casting under the protective atmosphere the long term annealing is needed to get the structural homogeneity [13][14][15][16][17][18] . Therefore we proposed the usage of the electro-deoxidation method as more cost-effective synthesis pathway for these alloy groups and we reported the alloy development mechanisms for AB 5 and A 2 B 7 alloys during the electro-deoxidation [10][11][12] . In this work, we synthesized A 2 B 7 type alloys by replacing part of La with Mg ((La 1 −x Mg x ) 2 (Ni 0.8 Co 0.2 ) 7 ( x = 0.125, 0.25, 0.5) alloys and we modi- * Corresponding author.…”

Aspects of the Application of Electrochemistry to the Extraction of Titanium and Its Applications

“…This gives the opportunity for creation of alloys that are dif cult to make using co-melting, especially in cases where the boiling point of one element in the alloy is higher than the melting point of the other element. A further advantage is that the alloys are synthesised at temperatures around typically 800 to 900 C and thus do not undergo signi cant phase changes on cooling, unlike alloys prepared in the molten state [30][31][32][33][34][35][36][37][38][39][40][41][42][43][44][45][46] . Rather surprisingly, low oxygen Ti alloys have been produced directly from synthetic rutile and processed by conventional and novel consolidation techniques such as isostatic pressing and 3D printing.…”