Abstract
In order to improve the hydrogen storage capacity of conventional La2Mg17 electrode alloys, a nanocrystalline/amorphous-structured La2Mg17-Ni composite material was produced by high energy ball milling in the presence of TiF3. Subsequent analysis of the discharge/charge cycle performances of this electrode material revealed that its cycle stability and electrochemical capacity were greatly improved, with the latter reaching a maximum value of 787.07 mAh/g with optimisation of the TiF3 addition. Moreover, a remarkable enhancement in the reversibility of electrochemical reactions on the material’s surface was also observed. Hydrogen diffusion coefficients for the material were calculated by means of a potential step method, confirming that TiF3 markedly improves the long-range diffusion of hydrogen within the material.
1. Introduction
Of the various energy conversion and storage materials that have been developed over recent years, amorphous composites based on a La-Mg system alloy that is combined with Ni powder by mechanical milling are considered amongst the most promising, by virtue of their high electrochemical discharge capacity [1–3]. Indeed, Y. Wang and X. Wang [4] have already demonstrated that the ball milling of Pr5Mg41 with 200 wt.% Ni can achieve an excellent electrochemical hydrogen storage capacity, reaching as high as 1016.2 mAh/g. Rousselot et al. [5] have also obtained an improved discharge capacity of 475 mAh/g through the high energy ball milling of an Mg-Ti alloy and noted a microscale structural transformation of the Mg-Ti phase during charge/discharge cycles.
There has, however, been little consideration given to the use of TiF3 as a catalyst in electrochemical applications; its use instead has been largely confined to the adsorption and release of gaseous hydrogen. Notable exceptions include the work of Liu et al. [6, 7], in which TiF3 doping of LiAlH4 during ball milling was used to achieve rapid hydride decomposition. Ma et al. [7, 8] have also demonstrated that using 4 mol.% TiF3 as a catalyst significantly improves the hydrogen absorption/desorption performance of Mg alloys. This is made possible by the inherently high corrosion resistance of Ti, and the resulting relative instability of its fluoride. The use of TiF3 as a catalyst for electrochemical hydrogen storage therefore certainly warrants further research. To this end, this paper explores the effect of TiF3 addition on the formation of a La2Mg17 alloy-Ni powder composite by mechanical grinding and how this influences its room-temperature electrochemical properties.
2. Experimental Materials and Methods
2.1. Alloy Preparation and Mechanical Ball Milling
Ingots of La2Mg17 alloy were prepared under a 0.04 MPa helium atmosphere using a vacuum induction furnace and once cooled they were mechanically pulverized to a +200 mesh powder. Composite material samples, each with a weight of 10 g, were obtained through further mechanical grinding of the La2Mg17 alloy powder in conjunction with 150 wt.% Ni and wt.% TiF3 (where , hereafter denoted as TiF3-0, TiF3-3, and TiF3-5). The purity of both the Ni powder and TiF3 was greater than 99.7%, and Cr-Ni stainless steel balls were charged at a weight ratio of 40 : 1. All milling was performed under an argon atmosphere in a laboratory planetary mill at a speed of 350 rpm for 60 hours. The resulting composite samples were handled in an argon-filled glove box and sealed in vessels under an argon atmosphere until required for testing.
2.2. Microstructural Analysis
The phase of structure of the composite samples was analysed by X-ray diffraction (XRD, X’Pert Pro MPD), with Jade6.0 software that is used to determine the material composition. The morphology of the as-cast alloy and ball-milled composite powders was examined scanning electron microscopy (SEM, JSM 6400, and FEI Quanta 400) and optical microscopy (Neophot 21).
2.3. Measurement of Electrochemical Properties
Round electrode pellets measuring 15 mm in diameter were prepared by cold pressing a 1 : 4 mixture (by weight) of the composite powder and a carbonyl nickel powder with 35 MPa of pressure. After being allowed to dry for 4 hours, the electrode pellets were then immersed for 24 hours in a 6 mol/L KOH electrolyte solution to fully wet the electrodes prior to electrochemical measurement. These electrochemical measurements were performed at 30°C using a trielectrode open cell, consisting of a working electrode and a sintered Ni(OH)2/NiOOH counter electrode. The charge/discharge current density used was 40 mAh/g.
Tafel polarization curves were obtained for each sample within a potential range of −1.2 to +1.0 V (versus Hg/HgO) using an electrochemical workstation (PARSTAT 2273) and a scan rate of 5 mV/s at 50% DOD. To determine the potentiostatic discharge, fully charged electrodes were discharged at 500 mV potential steps for 5000 s, and the results were interpreted using CorrWare electrochemistry corrosion software.
3. Results and Discussion
3.1. Microstructure of the Composite
The morphologies of the composite samples shown in Figure 1 indicate that ball milling results in homogeneous spherical particles (with radius being about 10 μm), with some localised fusion. Moreover, the broad and diffuse peaks evident in the accompanying XRD spectra indicate the formation of an amorphous/nanocrystalline structure, though there is also evidence of a residual Ni phase. In the case of TiF3-5, this Ni peak is greatly reduced and the remaining peaks become broader and shifted to the right. However, this suggests that the use of a TiF3 catalyst may be conducive to the formation of more amorphous phases. This effect is only slight and therefore it warrants further investigation to confirm such a connection.
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3.2. Discharge Capacity and Cycle Stability
The evolution of the discharge capacity of the composite over 20 cycles that is shown in Figure 2 was used to characterise its activation performance. That is, given that all samples were clearly fully activated after the first cycle, they reached their maximum discharge capacity and activation performance at room temperature. This can be attributed to the modification effect of the Ni powder, a material which itself has a high electrocatalytic activity. However, it was also found that the composite electrode material is more readily oxidised in the electrolyte, leading to a greater attenuation of its capacity; the decline in capacity was primarily ascribed to corrosion of the Mg. Nevertheless, the maximum discharge capacity of TiF3-3 is clearly and greatly improved, reaching a maximum of 787.07 mAh/g. Furthermore, TiF3-5 exhibits a reduced attenuation at the fourth cycle that would greatly improve the capacity of the material and it is considered to be most likely due to a recombination of Ni and Mg in response to the corrosive environment.
3.3. Charge and Discharge Voltage Characteristics
From the first circulation charge/discharge curves shown in Figure 3, we can see that the use of TiF3 as a catalyst increases the material’s charge resistance, which directly affects the activation charge of the material. This is explained by the fact that Ti helps to increase the transfer of charge between occupied active sites, with the overall rate of charge transfer determining the charging resistance [9]. Figure 3 shows the diffusion of hydrogen atoms, and ions are strong in TiF3-3 under these same conditions and, as mentioned above, are attributable to the amorphous/nanocrystalline structure generated by ball milling. It is claimed by Zhu et al. [10] that the hydrogenated reaction mechanism of a hydrogen storage nanocomposite alloy is self-catalysing due to the presence of different nanoscale phases, whereas a single-phase alloy is regulated by nucleation and growth. This would mean that a nanocomposite alloy has an inherently superior hydrogen storage performance.
3.4. High Rate Discharge Ability (HRD)
The electrochemical hydrogen storage kinetics of the composite samples can be defined by their high rate discharge ability, as calculated by [11]where and are the discharge capacities of an electrode charged-discharged at current densities of and 40 mA/g, respectively. The HRD data obtained is listed in Table 1, which demonstrates that a certain improvement is seen with the addition of the catalyst. This can be explained by the TiF3-assisted reunion of Ni that creates the amorphous structure observed, which is more conducive to the transfer of hydrogen atoms and ions [9].
3.5. Dynamic Properties of Electrode Materials
Figure 4(a) shows the exchange current density () of the electrode materials, as derived from the Tafel anodic and cathodic polarization curves by slope extrapolation. This value is a key parameter in determining the hydrogen desorption reaction kinetics, as it defines the reversibility of the electrochemical reaction and surface electrochemical activity for the hydride electrode [12, 13]. From the values given in Table 2, we can see that there is a clear increase in with increasing TiF3 addition. This indicates that the specific surface area of reaction is increased, thereby increasing the electrocatalytic activity of the electrode. In the Tafel polarization curves of all the samples (Figure 4(a)), there is a clear increase in the anodic current density to a limiting current density, , beyond which the current decreases. This is indicative of the oxidation of the electrode surface, with the resulting oxidation product inhibiting the penetration of hydrogen atoms [14, 15]. The decrease in anodic charge current density with repeated cycling also suggests that charging becomes more difficult, and, hence, may be regarded as the critical passivation current density, which is determined by the diffusion of hydrogen in the bulk electrode during anodic polarization [16]. The numerical values of (Table 2) also show that an increase in TiF3 creates a similar trend to that identified in the aforementioned electrochemical test results, confirming that TiF3 induces excellent electrochemical performance.
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The hydrogen diffusion coefficient of each sample was measured using the potential step method, with the semilogarithmic curves of anodic current versus working duration depicted in Figure 4(b). If it is assumed that the composite particles are perfectly spherical, then the hydrogen diffusion coefficient can be calculated by [17] where is the diffusion current density (A/g), is the bulk hydrogen diffusion coefficient (cm2/s), is the initial hydrogen concentration in the composite (mol/cm3), is the hydrogen concentration on the surface of the composite (mol/cm3), is the alloy particle radius (cm), is the density of the hydrogen storage composite (g/cm3), and is the discharge time (s). It is assumed here that μm, as this is the aperture size of the sieve over which it was screened. The in (3) refers to the slope of the linear portion of the semilogarithmic curve of anode current versus working time (Table 2). The values calculated in this way, assuming a slope of 55, are shown in Figure 4(b). This demonstrates that TiF3 greatly improves the hydrogen diffusion coefficient, varying from cm2/s for TiF3-0 to cm2/s and cm2/s for TiF3-3 and TiF3-5, respectively. It is therefore easy to see that a more uniform distribution of TiF3 in the composite provides more favourable electrochemical reaction activation, thereby facilitating long-range diffusion of hydrogen atoms and improving the electrochemical performance of the composite material.
4. Conclusion
Ball-milled composites of La2Mg17, Ni, and TiF3 were found to have a uniform amorphous/nanocrystalline structure, albeit a small amount of residual Ni phase. The addition of TiF3 was shown to improve both the corrosion resistance and maximum discharge capacity of the composite, with the latter reaching a maximum of 787.07 mAh/g. Furthermore, the TiF3 catalyst improved the high rate discharge capacity and charge/discharge reversibility, especially that TiF3 extended discharge platform of material by a wide margin. The electrochemical kinetics revealed that both Ni and TiF3 improve the exchange current density and limiting current density, with subsequent measurement of the hydrogen diffusion coefficient, verifying that TiF3 improves the electrochemical dynamics of the composite.
Conflict of Interests
The authors declare that there is no conflict of interests regarding the publication of this paper.