Improvement of electrochemical properties of NdMgNi-based hydrogen storage alloy electrodes by evaporation-polymerization coating of polyaniline
Graphical abstract
Introduction
LaMgNi-based hydrogen storage alloys have been widely studied owing to their high capacity as alternative for the recently-used negative electrode materials, AB5-type rare earth-based hydrogen storage alloys [1], [2], [3], [4], [5]. However, this type of alloy is subject to poor cycling stability and La is pointed to be responsible for the poor cycling stability. To improve the cycling stability, elements suitable for occupying the La-site in the super-stacking structure has been tried, such as Sm, Ca, Pr, Nd [6], [7], [8], [9], [10]. Nd complete or partial substitution for La becomes a possible choice to obtain a more stable electrode material upon charging/discharging since Nd tends to be more stable in alkaline electrolyte [2], [11]. Nd element is with smaller atomic radius according to lanthanide contraction and consequently varies super-stacking structure, as well as electrochemical properties.
In order to remain excellent discharge characteristics for the NdMgNi-based alloy electrodes, all steps in the alloy electrode reaction should be taken into consideration. It is generally accepted that the electrode reaction during charging/discharging includes mainly two steps, electrochemical reaction on the surface and hydrogen diffusion in the bulk [12], [13], [14]. As components and preparation parameters decide bulk structure, the bulk structure would not be altered when we choose a certain composition. What we are able to do to ameliorate the overall electrochemical properties is to modifying interface conditions of the electrodes. And some reports have shown that it is possible to compensate for loss in discharge capacity and other electrochemical properties by modifying the electrode surface [15], [16], [17].
Among different surface modification methods such as acidic etching, basic boiling, microencapsulation, surface coating which brings an electro-active layer on the surface has been considered reasonable. As for modification layers, different types have been reported. Metallic modification layers have been reported earliest and most often for their high conductivity and high catalytic effect on hydrogen anodic reaction. Ni, Co, Cu and their compounds were usually used as modification materials [18], [19], [20], [21]. Sun et al. [22] performed Ni-coating on the Ti1.4V0.6 Ni alloy surface, inhibiting V dissolution from the electrode and improving discharge capability at high discharge current density. Li et al. [23] enhanced electrochemical properties of metal hydride electrodes by in-situ grown Co3O4. Co3O4 was a conductive additive, increasing catalytic activity and current utilization for the electrodes and therefore promoted the discharge capacity and capability at high current density. Kuang et al. [24] got a Cu/Cu2O composite coating on LaNi5 surface and increased the alloy electrode discharge capacity. In recent years carbon materials, such as graphene has been found effective on promoting electrochemical reaction on the alloy electrode surface. Cui et al. [25] modified AB5-type MmNi3.55Co0.75Mn0.4Al0.3 hydrogen storage alloy with graphene nano-platelets by ball milling to improve the high rate dischargeability. Lan et al. [26] succeed in getting a nanocomposite modification layer on the La0.7Mg0.3(Ni0.85Co0.15)3.5 alloy surface and improved the reversible hydrogen storage property. Conductive polymers were also excellent modifiers. Polypyrrole (PPY) has been used in metal hydride electrode modification and increased high rate dischargeability [27]. Ni-PTFE composite layer was encapsulated on the AB5-type alloy surface, improving cycling stability [28]. Polyaniline (PANI) has been modified to the alloy electrode surface by different methods, milling, electroless deposition, as well as electroplating. The PANI modification layer promoted electron transferring process and therefore enhanced electrochemical reaction on the alloy surface [29], [30], [31]. Most reported PANI modification was performed between solid-solid phase or solid-liquid phase process. In the solid-solid process it is difficult to cover the alloy particle uniformly. However, in the solid-liquid phase process the aqueous solution exerted unavoidable influence to the alloy when the particles are dropped into aqueous solution, especially for a long time bath.
In order to eliminate any property deterioration we combine a quick surface cleaning and a subsequent evaporation-polymerization (solid-gas process) coating PANI on the NdMgNi-based alloy surface and then morphology and electrochemical properties for the PANI coating alloy electrodes are studied.
Section snippets
Experimental
Nd0.7Mg0.3Ni3.0 alloy was prepared by induction melting components Nd, Mg and Ni. The alloy ingots were then annealed at 900 °C for 8 h. After heat treatment, the ingots were crushed and milled into small particles with a size of 37–74 μm. The alloy powders were washed with HF for about 1 min, filtered and laid on a porous supporting material. Then the alloy supporter was put into aniline vapor and kept shaking for a certain period, 10, 20, 30, 45 and 60 min to get polyaniline (PANI)
Effects of PANI coating on microstructure
Fig. 1 shows Morphology and infrared (IR) results for alloy powders. The as milled alloy powders have smooth surface with some small particles adhering to the alloy surface (Fig. 1a). As PANI evaporation-polymerization coating is performed for 10 min, thin honeycomb-like species appear on the surface (Fig. 1b). As the coating time increases to 20 min or longer, the coating PANI layer becomes thicker, and gradually transform into a coral-like appearance (Fig. 1c–e). To reveal distribution of the
Conclusions
Polyaniline (PANI) was modified onto surface of Nd0.7Mg0.3Ni3.0 hydrogen storage alloy by an evaporation-polymerization method. Main results are summarized as follows.
- 1.
SEM mapping and infrared results show that PANI covers the alloy surface uniformly. The PANI coating layer suffers from a reversible redox reaction and remains stable upon charging/discharging cycles.
- 2.
Electrochemical kinetics shows that exchange current density and limiting current density for the electrodes increases remarkably
Acknowledgements
This work was financially supported by the National Natural Science Foundation of China (NOs. 21303157, 51771164 and 51571173), and Scientific Research Projects in Colleges and Universities in Hebei Province (QN2016002).
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