Inhibition of LaNi5 electrode decay in alkaline medium by electroless encapsulation of active powder particles

Abstract

Surface of powdered LaNi 5 intermetallic compound has been modified by active particle coverage with electroless nickel (Ni-P). The electrode degradation process in 6 M KOH solution has been tested across 70 charge/ discharge cycles at −0.5 C/+0.5 C rates. It has been established that after approx. 25-35 initial cycles, the electrode degradation process fulfills first order chemical reaction kinetics law: logarithm of discharge capacity linearly decreases with cycle number. The rate constant for the Ni-P protected material is over 20 % lower than that of as received one. The surface modification also improves the alloy hydrogenation kinetics: exchange current densities of H 2 O/H 2 system are generally greater for modified material and, contrary to uncovered material, do not practically decrease with long-lasting cycling.

Modification of Hydrogenation and Corrosion Properties of Hydrogen Storage Material by Amorphous TiCrFeCoNi HEA Layer

“…After N = 8–15 cycles, full activation of the based electrode material takes place, and then nearly linear capacity fade is visible in Q disch = f( N ) coordinate system with a slope of −0.98 mAh∙g −1 ∙cycle −1 . For HEA modified material, capacities are generally lower by 15–20%, and the range of practically constant capacity is distinctly wider (it occurs between N = 8–35 cycles) in case of lack of capacity fade, i.e., when electrode corrosion rate ( r corr ) is close to zero [ 37 , 38 ]. This means that HSM particles modification by HEA sputtering strongly inhibits corrosion degradation up to 35 cycles.…”

“…For zero-order kinetics, the reaction rate is constant within the whole rectilinear cycling range and equal to the rate constant ( r corr, 0 = k 0 ). Contrary to zero-order behaviour, capacity fade may also obey rectilinearity in the log Q disch − N coordinate system (first-order kinetics) [ 37 , 38 ]. For the first-order kinetics, corrosion rate depends on the actual concentration of the HSM alloy undergoing corrosion ( r corr,I = k I ∙ c M ), so the r corr,I decreases with cycling.…”

“…The electrode pellets ( Φ = 5 mm, h ≈ 0.4 mm, the mass of active material ≈30 mg, determined with an accuracy of 0.1 mg for each individual pellet) have been prepared by careful mixing of the HEA-sputtered powder with 5 mass % of C-graphite (conducting powder), 10 mass % of poly(vinylidene fluoride) (PVDF) as a particle binder, and a small amount of acetone to ensure thick paste consistency. After acetone evaporation, the homogenised mass was pressed with 50 bar force, analogously to previously published papers [ 34 , 35 , 36 , 37 , 38 ].…”

“…In charge/discharge cycling, the electrodes were cathodically charged with the current density of −186 mA∙g −1 (−0.5 C rate) during 9000 s for each cycle. The discharge process continued with a rate of +186 mA∙g −1 (+0.5 C rate) until electrode potential reached E = −0.50 V vs. HgO/Hg (cutoff potential), analogously as described in [ 34 , 35 , 36 , 37 , 38 ]. The electrode charging/discharging has been carried out up to N = 60 cycles (for cycle numbers greater than 65–70, the worsening of electrode coherence occurred which was accompanied by partial loss of particles compactness with binder).…”

Electrochemical Hydrogenation and Corrosion Behaviour of LaNi5-xGex (x = 0.3 and 0.6) Alloys

“…Otherwise, some of the cathodically produced hydrogen may evolve on the M surface in the form of hydrogen gas bubbles (H 2 ). One of the disadvantages of using LaNi 5 as an anode in an Ni-MH cell is its efficiency limited to only 30-50% of its theoretical capacity [16,17]. Moreover, during subsequent discharge cycles, part of the anode undergoes irreversible degradation, which reduces the amount of available energy.…”