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Enhanced kinetics of hydrogen electrosorption in AB5 hydrogen storage alloy decorated with Pd nanoparticles
Electrochemistry Communications 100 (2019) 100–103
Contents lists available at ScienceDirect
Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom
Short communication
Enhanced kinetics of hydrogen electrosorption in AB5 hydrogen storage alloy decorated with Pd nanoparticles
T
Katarzyna Hubkowskaa, , Michał Soszkob, Michał Krajewskia, Andrzej Czerwińskia,b,c ⁎
a
Faculty of Chemistry, University of Warsaw, Pasteura 1, 02-093 Warsaw, Poland Industrial Chemistry Research Institute, Rydygiera 8, 01-793 Warsaw, Poland c Biological and Chemical Research Centre, University of Warsaw, Żwirki i Wigury 101, 02-089 Warszawa, Poland b
ARTICLE INFO
ABSTRACT
Keywords: AB5 alloy Hydrogen storage alloys Ni-MH battery Palladium nanoparticles Hydrogen sorption
The surface of an AB5 alloy (LaMmNi4.1Al0.3Mn0.4Co0.45) was modified with Pd nanoparticles using a microwave-assisted polyol method. Cyclic voltammetry (CV) and chronoamperometry (CA) were used to characterize the electrochemical behavior of the resulting composite in 6 M KOH electrolyte. The microwave-assisted polyol method makes it possible to obtain Pd nanoparticles of irregular shapes and sizes, unevenly distributed on the AB5 surface. CV and CA results showed that the kinetics of hydrogen absorption/desorption processes were enhanced after modification of the AB5 surface with Pd nanoparticles. Furthermore, no electrochemical activation is required in the case of the Pd-modified AB5 alloy.
1. Introduction The global demand for energy, especially in the automotive industry, has grown exponentially over recent decades. There has therefore been a significant effort to develop new, efficient and green sources of electricity and energy storage options as an alternative to existing systems based on fossil fuels. Among these, nickel–metal hydride (NiMH) batteries are considered to be particularly environmentally friendly, economical and highly efficient energy sources for use in electric vehicles. Even though more attention has been focused on Li ion systems (which are characterized by a higher power density), the most common hybrid electric vehicles, including Toyota's latest hydrogen fuel cell car, the Mirai, are equipped with a NiMH battery stack as an auxiliary power source [1,2]. AB5 type alloys, which are based on a LaNi5 matrix, are used in NiMH batteries for hydrogen storage [1]. Powders based on pure lanthanum–nickel binary alloys do not exhibit the hydrogen absorption kinetics and durability required to achieve high battery performance, so it is necessary to modify them. The most common step is to mix the binary alloy with small amounts of transition group metals such as Co, Mn, Fe etc., resulting in a greater hydrogen capacity and improved electrochemical stability [3]. It has also been found that addition of noble metals to AB5 alloys can significantly increase their hydrogen absorption kinetics. The gaseous hydrogen absorption properties of AB5-type alloys are reflected by their electrochemical behavior in
⁎
aqueous solutions and (especially) in closed NiMH battery systems. The literature gives great insight into the electrochemical characteristics of various AB5-type electrode materials [4–8] and Pd [9–11], however, there are only a few reports (described below) which provide data on the hydrogen sorption kinetics and electrochemical profiles of lanthanum–nickel alloys modified with noble metals, especially nanoparticles. Zaluski et al. demonstrated the catalytic effect of Pd on hydrogen sorption in a LaNi5 alloy, among others [12]. It was found that after modification with Pd nanoparticles, the rate of gaseous hydrogen sorption increased, there was no need for activation and the composite was much more resistant to air impurities. The influence of Pd surface modification on the enhancement of discharge capacity and rate capability of alloy–hydride electrodes was studied by Barsellini et al. [13]. The researchers postulate that the improved performance of the Pdmodified electrode is connected with the catalytic effect of Pd. Ambrosio and Ticianelli [14] performed electroless deposition of Pd on a LaNi4.7Sn0.3 surface by Cu replacement and as a result they achieved complete coverage of the alloy surface. They found that a Pd/Cu coating results in lower charge–discharge overpotentials, lower charge transfer resistance and increases the number of cycles that can be performed. Visintin et al. [15] studied the effect of adding Ni, Pd, Pt polycrystalline powders to AB5-type alloy. They noticed that metal additives enhance the rate capability and facilitate the activation process. The enhancement mechanism was studied by Shan et al., who mixed palladium black with AB5 powder at various ratios and examined
Corresponding author. E-mail address: [email protected] (K. Hubkowska).
https://doi.org/10.1016/j.elecom.2019.02.007 Received 7 December 2018; Received in revised form 6 February 2019; Accepted 6 February 2019 Available online 07 February 2019 1388-2481/ © 2019 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).
Electrochemistry Communications 100 (2019) 100–103
K. Hubkowska et al.
its response to gaseous hydrogen exposure under 99.0 kPa [16]. Freshly prepared Pd-modified AB5 powder obtained by mechanical grinding showed a significant increase in hydrogen absorption rate and stability, even after two years exposure to air. Williams et al., using a surface nano-engineering approach (functionalization of the AB5 and electroless deposition of Pd), obtained both continuous Pd layers and Pd nanoparticles with a high surface coverage factor [17,18]. The authors found that the presence of both forms of Pd on a LaNi5-type mischmetal alloy has a great influence on its hydrogen sorption properties; however, modification of the chemical state of pristine AB5 alloy by treating its surface with fluorine ions can have a significant effect as well. The influence of co-milling AB5 with Pd black on the hydrogen sorption properties was examined by Modibane et al. [19]. The composite obtained by co-milling (≤5 wt% Pd) does not exhibit any improvement in hydrogen sorption performance or poisoning tolerance. In this short communication, for the first time, we examined the electrochemical behavior of an AB5 alloy powder covered with Pd nanoparticles (ca. 3.5 wt%) obtained by microwave-assisted polyol synthesis. The presented data show an extreme difference in cyclic voltammetry and chronoamperometry profiles between the modified and unmodified powders, providing an insight into hydrogen sorption enhancement in the Pd-modified composite. Since the decoration of AB5 with Pd NPs causes a significant increase in the maximum discharging current, the specific power of the cell will increase, becoming comparable with the performance of lithium-ion batteries. However, the specific energy (which depends on the range of oxidation potentials, hydrogen oxidation charge and composite mass [20]) remains almost the same after modification. We emphasize the role of CV and CA measurements in studying the catalytic properties of noble metals (Pd) when added to commercially used hydrogen-absorbing materials.
SEM images of the Pd-modified AB5 powders were obtained using a Zeiss Merlin FE-SEM spectrometer with a Bruker Quantax 400 EDX analyzer (127 eV resolution). The electrochemical measurements were carried out using a threeelectrode configuration, with a limited volume electrode (LVE, where the thickness of the electrode was close to the alloy particle diameter, ca. 50 μm) made from hydrogen-absorbing powder as the working electrode, a gold plate as the counter electrode and HgO/Hg as the reference electrode. The electrolyte was 6 M solution of KOH in distilled water. The LVE electrode was prepared by pressing the hydrogen-absorbing powder without any binding additives (which can distort the results of electrochemical experiments) into two gold meshes (9 mm diameter) on a hydraulic press under 6 t pressure for 3 min [21,22]. Afterwards, the resulting mesh was placed in Teflon® casing, connected to the potentiostat/galvanostat through gold wire and placed in KOH solution. The electrochemical measurements were conducted on an AUTOLAB® PGSTAT30 potentiostat/galvanostat and took the form of cyclic voltammetry and chronoamperometry. The electrodes were first activated by CV in the potential range (−1.1)–(−0.4) V (vs. HgO/Hg) at a scan rate of 2 mV s−1 for 50 consecutive cycles. Afterwards, absorption/desorption CA experiments were conducted by polarizing the electrodes at −1.0 and − 0.6 V (vs. HgO/Hg) for 2·104 and 2.5·104 s, respectively. The stability of the current at the end of the CA charging/ discharging experiment ensures that the electrode is fully charged/ discharged. The current values presented in the CV and CA curves are normalized by the active mass of the electrode. 3. Results and discussion Fig. 1 presents FE-SEM images of the AB5 alloy decorated with Pd. The flat surface of the AB5 particles (of ca. 50 μm diameter) is covered with spherical/irregular-shaped Pd crystallites of diameter between 20 and 80 nm. The palladium crystallites are distributed unevenly with regions of higher Pd surface concentration. EDX mapping (Fig. 1D) confirms that even for Pd agglomerates which exceed the size of 1 μm, nano-sized particles can be distinguished. The qualitatively estimated surface coverage with palladium remains low due to the small amount of PdCl2 used in the synthesis. This is an important observation and should be taken into account when discussing the electrochemical properties of the Pd-modified AB5 alloy. Fig. 2 shows the CV curves of pristine and palladium-modified AB5 alloy before activation (Fig. 2A) and after 50 CV cycles (Fig. 2B). On the pristine AB5 curve, one can see signals originating from hydrogen absorption into the alloy and H2 evolution on the surface of the LVE electrode (below −0.9 V vs. HgO/Hg) and hydrogen desorption from the AB5 alloy (peak at ca. −0.67 V vs. HgO/Hg). After deposition of palladium nanoparticles on the surface of the AB5 alloy, the electrochemical properties of the composite changed drastically. Two significant peaks appeared, at −0.94 and −0.81 V (vs. HgO/Hg) (Fig. 2A), originating from hydrogen absorption/desorption into/out of the palladium nanoparticles, respectively. Moreover, the normalized current associated with the desorption of hydrogen from the AB5 alloy (peak at ca. −0.67 V vs. HgO/Hg) also increased. It is evident that deposition of palladium nanoparticles on the surface of the AB5 alloy enhanced its electrochemical properties by accelerating the absorption/desorption reaction kinetics, which, as a result, increased the current associated with these processes during CV experiments. Furthermore, it can be seen that in the case of Pd-modified AB5 powder, activation cycles are not required, as the characteristic signals of pure AB5 alloy are present even in the first CV scan. The fact that there is no significant change in the CV curve between the first and the fiftieth cycle (Fig. 2A and B – red curve) shows that the Pd-modified alloy is stable in the 6 M KOH electrolyte. This indicates that the composite is electrochemically stable under the measurement conditions. Slight changes in the voltammetry curve can be caused by the effect of the strongly alkaline solution on the CV behavior of Pd. This is connected with the fact that the
2. Experimental LaMmNi4.1Al0.3Mn0.4Co0.45 powder (VARTA Microbattery, referred to as AB5 in text) was used as the base material for the synthesis of the AB5 misch-metal alloy modified with Pd nanoparticles (ca. 3.5 wt%). The modified alloy containing 3.5 wt% Pd was selected for further examination, since it had the lowest concentration of electrochemically stable Pd particles under the electrochemical measurement conditions (strongly alkaline media, applied hydrogen sorption potentials). Detailed characteristics of the unmodified AB5 used in this study are given in the literature [21,22]. Palladium(II) chloride (Alfa Aesar, analytical grade) suspended in ethylene glycol (Avantor Performance Materials, analytical grade) was used as a precursor for the Pd nanoparticles. The suspension was prepared by mixing PdCl2 with ethylene glycol at a 1 mg:1 ml ratio followed by homogenizing the mixture in a sonication bath for 10 min at room temperature. 25 ml of freshly prepared PdCl2/EG suspension was poured into a 50 ml flask and mixed with AB5 powder (ca. 440 mg). The mixture was intensively stirred for 30 min, after which the flask was placed in a CEM Discover microwave reactor and heated to 185 °C. Decomposition of PdCl2 was performed for 30 min at constant temperature (185 °C, measured with a CEM-integrated optical sensor) controlled by microwave heating. The power was automatically adjusted between 1 and 150 W by the CEM software. After a given reaction time the microwave reactor was turned off and the mixture was allowed to cool. The reaction was carried out under reflux and the mixture was intensively stirred during both heating and cooling stages. The cooled mixture was centrifuged and the powder was separated from the ethylene glycol during decanting. The collected powder was washed several times with purified water (double distilled and filtered in a Millipore system) and dried at 250 °C for 2 h. The ethylene glycol recovered after the reaction was clear and colorless which confirmed that 100% of the PdCl2 in the mixture had been decomposed. A similar procedure for the synthesis of Pt nanoparticles has been described in the literature as the microwaveassisted polyol method [23,24]. 101
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Fig. 1. Surface morphology of AB5 alloy modified with Pd nanoparticles (3.5 wt%). (A–B): FE-SEM images of the same surface region at different magnifications; (C–D): SEM image and PdLα-sensitive EDX mapping.
‘electrochemical response’ of Pd is very sensitive even to low concentrations of impurities – this is a well-known problem in alkaline media [11,25]. It should be emphasised that in case of thin-film Pd electrodes (ca. 0.5 μm), the Pd electrode must be subjected to a hydrogen pretreatment procedure (many successive CV cycles and CA steps over the potential range, ensuring full hydrogen insertion and removal) to obtain reproducible results [11]. Thus, it is very significant that this procedure is not required for Pd-modified AB5 electrodes and the CV response of the composite is stable after many CV cycles and CA charging/discharging steps. Fig. 3 presents the chronoamperometry desorption curves of pristine and Pd-modified AB5 alloy. After deposition of Pd nanoparticles on the surface of the AB5 crystals, the initial values of the current densities associated with hydrogen desorption are greatly enhanced. Moreover, it can be seen that the rate at which the current density decreases with time is higher for Pd-modified powder than for the pristine material,
which suggests enhanced kinetics for hydrogen desorption reactions after the deposition of Pd nanoparticles. The enhanced kinetics of hydrogen absorption/desorption into/out of the AB5 alloy are related to the influence of the palladium nanoparticles on the absorbed hydrogen transport, in which hydrogen first adsorbs and then absorbs onto the surface/into the palladium nanoparticles (indicated by the presence of characteristic Pd hydrogen absorption/desorption peaks) and then, in an absorbed state, diffuses through the palladium nanoparticles and becomes incorporated into the AB5 alloy (the increased current values for hydrogen absorption/desorption are in the potential ranges characteristic of AB5 alloy). In case of the pristine AB5 alloy LaMm-Ni4.1Al0.3Mn0.4Co0.45, the theoretical hydrogen sorption capacity does not exceed 290 mAh/g. This value was calculated from the hydrogen gas sorption experiment [26] and is the maximum value that could be obtained from electrochemical measurements in the case of pristine AB5. The addition of Pd to the composite does not
Fig. 2. (A) 1st and (B) 50th CV curves of pristine and Pd-modified AB5 alloy. The insets show more accurately the CV curves of pristine AB5. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.) 102
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Acknowledgments This research was funded by the National Science Centre (Poland), grant number: 2015/17/B/ST8/03377 (ID 289956). References [1] L. Ouyang, J. Huang, H. Wang, J. Liu, M. Zhu, Progress of hydrogen storage alloys for Ni-MH rechargeable power batteries in electric vehicles: a review, Mater. Chem. Phys. 200 (2017) 164–178. [2] Toyota Mirai Technical Specifications, http://media.toyota.co.uk/wp-content/files_ mf/1444919532151015MToyotaMiraiTechSpecFinal.pdf. [3] J. Kleperis, G. Wójcik, A. Czerwiński, J. Skowroński, M. Kopczyk, M.J. BełtowskaBrzezińska, Electrochemical behavior of metal hydrides, J. Solid State Electrochem. 5 (2001) 229–249. [4] H. Cunmao, Z. Yufen, W. Jian, Second phase and electrode characteristics of rareearth-based AB5+x alloys, J. Alloys Compd. 231 (1995) 546–549. [5] F.C. Ruiz, P.S. Martinez, E.B. Castro, R. Humana, H. Peretti, A. Visintin, Effect of electrolyte concentration on the electrochemical properties of an AB5-type alloy for Ni/MH batteries, Int. J. Hydrog. Energy 38 (2013) 240–245. [6] Z. Ma, W. Zhou, C. Wu, D. Zhu, L. Huang, Q. Wang, Z. Tang, Y. Chen, Effects of size of nickel powder additive on the low-temperature electrochemical performances and kinetics parameters of AB5-type hydrogen storage alloy for negative electrode in Ni/MH battery, J. Alloys Compd. 660 (2016) 289–296. [7] E. Teliz, S. Cammardella, F. Zinola, V. Diaz, Temperature performance of AB5 hydrogen storage alloy for Ni-MH batteries, Int. J. Hydrog. Energy 41 (2016) 19684–19690. [8] W. Zhou, D. Zhu, Z. Tang, C. Wu L. Huang, Z. Ma, Y. Chen, Improvement in lowtemperature and instantaneous high-rate output performance of Al-free AB5-type hydrogen storage alloy for negative electrode in Ni/MH battery: effect of thermodynamic and kinetic regulation via partial Mn substituting, J. Power Sources 343 (2017) 11–21. [9] T. Graham, On the absorption and dialytic separation of gases by colloid septa, Philos. Trans. R. Soc. Lond. 156 (1866) 399–439. [10] S. Szpak, P.A. Mosier-Boss, S.R. Scharber, Charging of the Pd/nH system: role of the interphase, J. Electroanal. Chem. 337 (1992) 147–163. [11] K. Hubkowska, M. Soszko, M. Symonowicz, M. Łukaszewski, A. Czerwiński, Electrochemical behavior of a Pd thin film electrode in concentrated alkaline media, Electrocatalysis 8 (2017) 295–300. [12] L. Zaluski, A. Zaluska, P. Tessier, J.O. Ström-Olsen, R. Schulz, Catalytic effect of Pd on hydrogen absorption in mechanically alloyed Mg2Ni, LaNi5 and FeTi, J. Alloys Compd. 217 (1995) 295–300. [13] D. Barsellini, A. Visintin, W.E. Triaca, M.P. Soriaga, Electrochemical characterization of a hydride-forming metal alloy surface-modified with palladium, J. Power Sources 124 (2003) 309–313. [14] R.C. Ambrosio, E.A. Ticianelli, Studies on the influence of palladium coatings on the electrochemical and structural properties of a metal hydride alloy, Surf. Coat. Technol. 197 (2005) 215–222. [15] A. Visintin, E.B. Castro, S.G. Real, W.E. Triaca, C. Wang, M.P. Soriaga, Electrochemical activation and electrocatalytic enhancement of a hydride-forming metal alloy modified with palladium, platinum and nickel, Electrochim. Acta 51 (2006) 3658–3667. [16] X. Shan, J.H. Payer, W.D. Jennings, Mechanism of increased performance and durability of Pd-treated metal hydriding alloys, Int. J. Hydrog. Energy 34 (2009) 363–369. [17] M. Williams, A.N. Nechaev, M.N. Lototsky, V.A. Yartys, J.K. Solberg, R.V. Denys, C. Pineda, Q. Li, V.M. Linkov, Influence of aminosilane surface functionalization of rare earth hydride-forming alloys on palladium treatment by electroless deposition and hydrogen sorption kinetics of composite materials, Mater. Chem. Phys. 115 (2009) 136–141. [18] M. Williams, M.V. Lototsky, V.M. Linkov, A.N. Nechaev, J.K. Solberg, V.A. Yartys, Nanostructured surface coatings for the improvement of AB5-type hydrogen storage intermetallics, Int. J. Energy Res. 33 (2009) 1171–1179. [19] K.D. Modibane, M. Lototskyy, M.W. Davids, M. Williams, M.J. Hato, K.M. Molapo, Influence of co-milling with palladium black on hydrogen sorption performance and poisoning tolerance of surface modified AB5-type hydrogen storage alloy, J. Alloys Compd. 750 (2018) 523–529. [20] M. Łukaszewski, K. Hubkowska, U. Koss, A. Czerwiński, Characteristic of hydrogensaturated Pd-based alloys for the application in electrochemical capacitors, J. Solid State Electrochem. 16 (2012) 2533–2539. [21] Z. Rogulski, J. Dłubak, M. Karwowska, M. Krebs, E. Pytlik, M. Schmalz, A. Gumkowska, A. Czerwiński, Studies on metal hydride electrodes containing no binder additives, J. Power Sources 195 (2010) 7517–7523. [22] M. Karwowska, T. Jaroń, K.J. Fijałkowski, P.J. Leszczyński, Z. Rogulski, A. Czerwiński, Influence of electrolyte composition and temperature on behaviour of AB5 hydrogen storage alloy used as negative electrode in Ni–MH batteries, J. Power Sources 263 (2014) 304–309. [23] M. Tsuji, M. Hashimoto, Y. Nishizawa, M. Kubokawa, T. Tsuji, Microwave-assisted synthesis of metallic nanostructures in solution, Chem. Eur. J. 11 (2005) 440–452. [24] M. Nogami, R. Koike, R. Jalem, G. Kawamura, Y. Yang, Y. Sasaki, Synthesis of porous single-crystalline platinum nanocubes composed of nanoparticles, J. Phys. Chem. Lett. 1 (2010) 568–571. [25] M.H. Martin, A. Lasia, Hydrogen sorption in Pd monolayers in alkaline solution, Electrochim. Acta 54 (2009) 5292–5299. [26] M. Karwowska, K.J. Fijałkowski, A. Czerwiński, Comparative study of hydrogen electrosorption from alkali metals electrolytes and hydrogen sorption from gas phase in AB5 alloy, Electrochim. Acta 252 (2017) 381–386.
Fig. 3. Chronoamperometry desorption curves of pristine and Pd-modified AB5 alloy. The inset (a) shows the initial 300 s of discharging; the inset (b) shows an enlarged area of the CA discharging curves.
significantly affect the capacity (there might in fact be a slight decrease). To conclude, the addition of Pd nanoparticles has no significant impact on the maximum hydrogen sorption capacity of the composite, but strongly facilitates the permeation of hydrogen through the Pd/AB5 phase boundary. This is the result of a different mechanism of hydrogen transport, to a minor extent involving the corrosive oxide and hydroxide layers forming on the surface of the less noble alloy components, which hinder diffusion of hydrogen ions from the electrolyte to the surface of the AB5 grains. This results in higher discharge currents, and leads directly to an increase in the cell power. The electrochemical results that we obtained confirmed our predictions regarding the influence of Pd modification on the kinetics of hydrogen sorption in AB5 alloys. However, further studies are required for a more thorough examination of the observed phenomena. 4. Conclusions In summary, we successfully modified an AB5 alloy by surface deposition of palladium nanoparticles using a microwave-assisted polyol method for the first time. The SEM analysis showed an uneven deposition of Pd particles in the AB5 matrix. Examination of the prepared composite using cyclic voltammetry and chronoamperometry showed better electrochemical properties towards hydrogen absorption/desorption reactions than were obtained using the pristine AB5 alloy. Furthermore, activation of the electrode is not required after modification with Pd nanoparticles. The improved electrochemical performance is due to incorporation of palladium nanoparticles into the composite, which led to an enhancement of the absorption/desorption reaction kinetics. This paper reports, for the first time, the enhanced kinetics of the hydrogen sorption process in an AB5 alloy decorated with Pd nanoparticles using basic electrochemical methods (CA – an increase in the discharge current; CV – an increase in the hydrogen absorption/desorption signals). Modification of the alloy surface with palladium increases the value of the hydrogen oxidation current by several times in the first few dozen seconds, which in the case of application of this anode material in metal hydride batteries (MH–NiOOH) would significantly increase their initial power. Our research group is currently working towards reducing the Pd content of the composite and further investigating the mechanism of the absorption/desorption reactions.
103
Contents lists available at ScienceDirect
Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom
Short communication
Enhanced kinetics of hydrogen electrosorption in AB5 hydrogen storage alloy decorated with Pd nanoparticles
T
Katarzyna Hubkowskaa, , Michał Soszkob, Michał Krajewskia, Andrzej Czerwińskia,b,c ⁎
a
Faculty of Chemistry, University of Warsaw, Pasteura 1, 02-093 Warsaw, Poland Industrial Chemistry Research Institute, Rydygiera 8, 01-793 Warsaw, Poland c Biological and Chemical Research Centre, University of Warsaw, Żwirki i Wigury 101, 02-089 Warszawa, Poland b
ARTICLE INFO
ABSTRACT
Keywords: AB5 alloy Hydrogen storage alloys Ni-MH battery Palladium nanoparticles Hydrogen sorption
The surface of an AB5 alloy (LaMmNi4.1Al0.3Mn0.4Co0.45) was modified with Pd nanoparticles using a microwave-assisted polyol method. Cyclic voltammetry (CV) and chronoamperometry (CA) were used to characterize the electrochemical behavior of the resulting composite in 6 M KOH electrolyte. The microwave-assisted polyol method makes it possible to obtain Pd nanoparticles of irregular shapes and sizes, unevenly distributed on the AB5 surface. CV and CA results showed that the kinetics of hydrogen absorption/desorption processes were enhanced after modification of the AB5 surface with Pd nanoparticles. Furthermore, no electrochemical activation is required in the case of the Pd-modified AB5 alloy.
1. Introduction The global demand for energy, especially in the automotive industry, has grown exponentially over recent decades. There has therefore been a significant effort to develop new, efficient and green sources of electricity and energy storage options as an alternative to existing systems based on fossil fuels. Among these, nickel–metal hydride (NiMH) batteries are considered to be particularly environmentally friendly, economical and highly efficient energy sources for use in electric vehicles. Even though more attention has been focused on Li ion systems (which are characterized by a higher power density), the most common hybrid electric vehicles, including Toyota's latest hydrogen fuel cell car, the Mirai, are equipped with a NiMH battery stack as an auxiliary power source [1,2]. AB5 type alloys, which are based on a LaNi5 matrix, are used in NiMH batteries for hydrogen storage [1]. Powders based on pure lanthanum–nickel binary alloys do not exhibit the hydrogen absorption kinetics and durability required to achieve high battery performance, so it is necessary to modify them. The most common step is to mix the binary alloy with small amounts of transition group metals such as Co, Mn, Fe etc., resulting in a greater hydrogen capacity and improved electrochemical stability [3]. It has also been found that addition of noble metals to AB5 alloys can significantly increase their hydrogen absorption kinetics. The gaseous hydrogen absorption properties of AB5-type alloys are reflected by their electrochemical behavior in
⁎
aqueous solutions and (especially) in closed NiMH battery systems. The literature gives great insight into the electrochemical characteristics of various AB5-type electrode materials [4–8] and Pd [9–11], however, there are only a few reports (described below) which provide data on the hydrogen sorption kinetics and electrochemical profiles of lanthanum–nickel alloys modified with noble metals, especially nanoparticles. Zaluski et al. demonstrated the catalytic effect of Pd on hydrogen sorption in a LaNi5 alloy, among others [12]. It was found that after modification with Pd nanoparticles, the rate of gaseous hydrogen sorption increased, there was no need for activation and the composite was much more resistant to air impurities. The influence of Pd surface modification on the enhancement of discharge capacity and rate capability of alloy–hydride electrodes was studied by Barsellini et al. [13]. The researchers postulate that the improved performance of the Pdmodified electrode is connected with the catalytic effect of Pd. Ambrosio and Ticianelli [14] performed electroless deposition of Pd on a LaNi4.7Sn0.3 surface by Cu replacement and as a result they achieved complete coverage of the alloy surface. They found that a Pd/Cu coating results in lower charge–discharge overpotentials, lower charge transfer resistance and increases the number of cycles that can be performed. Visintin et al. [15] studied the effect of adding Ni, Pd, Pt polycrystalline powders to AB5-type alloy. They noticed that metal additives enhance the rate capability and facilitate the activation process. The enhancement mechanism was studied by Shan et al., who mixed palladium black with AB5 powder at various ratios and examined
Corresponding author. E-mail address: [email protected] (K. Hubkowska).
https://doi.org/10.1016/j.elecom.2019.02.007 Received 7 December 2018; Received in revised form 6 February 2019; Accepted 6 February 2019 Available online 07 February 2019 1388-2481/ © 2019 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).
Electrochemistry Communications 100 (2019) 100–103
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its response to gaseous hydrogen exposure under 99.0 kPa [16]. Freshly prepared Pd-modified AB5 powder obtained by mechanical grinding showed a significant increase in hydrogen absorption rate and stability, even after two years exposure to air. Williams et al., using a surface nano-engineering approach (functionalization of the AB5 and electroless deposition of Pd), obtained both continuous Pd layers and Pd nanoparticles with a high surface coverage factor [17,18]. The authors found that the presence of both forms of Pd on a LaNi5-type mischmetal alloy has a great influence on its hydrogen sorption properties; however, modification of the chemical state of pristine AB5 alloy by treating its surface with fluorine ions can have a significant effect as well. The influence of co-milling AB5 with Pd black on the hydrogen sorption properties was examined by Modibane et al. [19]. The composite obtained by co-milling (≤5 wt% Pd) does not exhibit any improvement in hydrogen sorption performance or poisoning tolerance. In this short communication, for the first time, we examined the electrochemical behavior of an AB5 alloy powder covered with Pd nanoparticles (ca. 3.5 wt%) obtained by microwave-assisted polyol synthesis. The presented data show an extreme difference in cyclic voltammetry and chronoamperometry profiles between the modified and unmodified powders, providing an insight into hydrogen sorption enhancement in the Pd-modified composite. Since the decoration of AB5 with Pd NPs causes a significant increase in the maximum discharging current, the specific power of the cell will increase, becoming comparable with the performance of lithium-ion batteries. However, the specific energy (which depends on the range of oxidation potentials, hydrogen oxidation charge and composite mass [20]) remains almost the same after modification. We emphasize the role of CV and CA measurements in studying the catalytic properties of noble metals (Pd) when added to commercially used hydrogen-absorbing materials.
SEM images of the Pd-modified AB5 powders were obtained using a Zeiss Merlin FE-SEM spectrometer with a Bruker Quantax 400 EDX analyzer (127 eV resolution). The electrochemical measurements were carried out using a threeelectrode configuration, with a limited volume electrode (LVE, where the thickness of the electrode was close to the alloy particle diameter, ca. 50 μm) made from hydrogen-absorbing powder as the working electrode, a gold plate as the counter electrode and HgO/Hg as the reference electrode. The electrolyte was 6 M solution of KOH in distilled water. The LVE electrode was prepared by pressing the hydrogen-absorbing powder without any binding additives (which can distort the results of electrochemical experiments) into two gold meshes (9 mm diameter) on a hydraulic press under 6 t pressure for 3 min [21,22]. Afterwards, the resulting mesh was placed in Teflon® casing, connected to the potentiostat/galvanostat through gold wire and placed in KOH solution. The electrochemical measurements were conducted on an AUTOLAB® PGSTAT30 potentiostat/galvanostat and took the form of cyclic voltammetry and chronoamperometry. The electrodes were first activated by CV in the potential range (−1.1)–(−0.4) V (vs. HgO/Hg) at a scan rate of 2 mV s−1 for 50 consecutive cycles. Afterwards, absorption/desorption CA experiments were conducted by polarizing the electrodes at −1.0 and − 0.6 V (vs. HgO/Hg) for 2·104 and 2.5·104 s, respectively. The stability of the current at the end of the CA charging/ discharging experiment ensures that the electrode is fully charged/ discharged. The current values presented in the CV and CA curves are normalized by the active mass of the electrode. 3. Results and discussion Fig. 1 presents FE-SEM images of the AB5 alloy decorated with Pd. The flat surface of the AB5 particles (of ca. 50 μm diameter) is covered with spherical/irregular-shaped Pd crystallites of diameter between 20 and 80 nm. The palladium crystallites are distributed unevenly with regions of higher Pd surface concentration. EDX mapping (Fig. 1D) confirms that even for Pd agglomerates which exceed the size of 1 μm, nano-sized particles can be distinguished. The qualitatively estimated surface coverage with palladium remains low due to the small amount of PdCl2 used in the synthesis. This is an important observation and should be taken into account when discussing the electrochemical properties of the Pd-modified AB5 alloy. Fig. 2 shows the CV curves of pristine and palladium-modified AB5 alloy before activation (Fig. 2A) and after 50 CV cycles (Fig. 2B). On the pristine AB5 curve, one can see signals originating from hydrogen absorption into the alloy and H2 evolution on the surface of the LVE electrode (below −0.9 V vs. HgO/Hg) and hydrogen desorption from the AB5 alloy (peak at ca. −0.67 V vs. HgO/Hg). After deposition of palladium nanoparticles on the surface of the AB5 alloy, the electrochemical properties of the composite changed drastically. Two significant peaks appeared, at −0.94 and −0.81 V (vs. HgO/Hg) (Fig. 2A), originating from hydrogen absorption/desorption into/out of the palladium nanoparticles, respectively. Moreover, the normalized current associated with the desorption of hydrogen from the AB5 alloy (peak at ca. −0.67 V vs. HgO/Hg) also increased. It is evident that deposition of palladium nanoparticles on the surface of the AB5 alloy enhanced its electrochemical properties by accelerating the absorption/desorption reaction kinetics, which, as a result, increased the current associated with these processes during CV experiments. Furthermore, it can be seen that in the case of Pd-modified AB5 powder, activation cycles are not required, as the characteristic signals of pure AB5 alloy are present even in the first CV scan. The fact that there is no significant change in the CV curve between the first and the fiftieth cycle (Fig. 2A and B – red curve) shows that the Pd-modified alloy is stable in the 6 M KOH electrolyte. This indicates that the composite is electrochemically stable under the measurement conditions. Slight changes in the voltammetry curve can be caused by the effect of the strongly alkaline solution on the CV behavior of Pd. This is connected with the fact that the
2. Experimental LaMmNi4.1Al0.3Mn0.4Co0.45 powder (VARTA Microbattery, referred to as AB5 in text) was used as the base material for the synthesis of the AB5 misch-metal alloy modified with Pd nanoparticles (ca. 3.5 wt%). The modified alloy containing 3.5 wt% Pd was selected for further examination, since it had the lowest concentration of electrochemically stable Pd particles under the electrochemical measurement conditions (strongly alkaline media, applied hydrogen sorption potentials). Detailed characteristics of the unmodified AB5 used in this study are given in the literature [21,22]. Palladium(II) chloride (Alfa Aesar, analytical grade) suspended in ethylene glycol (Avantor Performance Materials, analytical grade) was used as a precursor for the Pd nanoparticles. The suspension was prepared by mixing PdCl2 with ethylene glycol at a 1 mg:1 ml ratio followed by homogenizing the mixture in a sonication bath for 10 min at room temperature. 25 ml of freshly prepared PdCl2/EG suspension was poured into a 50 ml flask and mixed with AB5 powder (ca. 440 mg). The mixture was intensively stirred for 30 min, after which the flask was placed in a CEM Discover microwave reactor and heated to 185 °C. Decomposition of PdCl2 was performed for 30 min at constant temperature (185 °C, measured with a CEM-integrated optical sensor) controlled by microwave heating. The power was automatically adjusted between 1 and 150 W by the CEM software. After a given reaction time the microwave reactor was turned off and the mixture was allowed to cool. The reaction was carried out under reflux and the mixture was intensively stirred during both heating and cooling stages. The cooled mixture was centrifuged and the powder was separated from the ethylene glycol during decanting. The collected powder was washed several times with purified water (double distilled and filtered in a Millipore system) and dried at 250 °C for 2 h. The ethylene glycol recovered after the reaction was clear and colorless which confirmed that 100% of the PdCl2 in the mixture had been decomposed. A similar procedure for the synthesis of Pt nanoparticles has been described in the literature as the microwaveassisted polyol method [23,24]. 101
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Fig. 1. Surface morphology of AB5 alloy modified with Pd nanoparticles (3.5 wt%). (A–B): FE-SEM images of the same surface region at different magnifications; (C–D): SEM image and PdLα-sensitive EDX mapping.
‘electrochemical response’ of Pd is very sensitive even to low concentrations of impurities – this is a well-known problem in alkaline media [11,25]. It should be emphasised that in case of thin-film Pd electrodes (ca. 0.5 μm), the Pd electrode must be subjected to a hydrogen pretreatment procedure (many successive CV cycles and CA steps over the potential range, ensuring full hydrogen insertion and removal) to obtain reproducible results [11]. Thus, it is very significant that this procedure is not required for Pd-modified AB5 electrodes and the CV response of the composite is stable after many CV cycles and CA charging/discharging steps. Fig. 3 presents the chronoamperometry desorption curves of pristine and Pd-modified AB5 alloy. After deposition of Pd nanoparticles on the surface of the AB5 crystals, the initial values of the current densities associated with hydrogen desorption are greatly enhanced. Moreover, it can be seen that the rate at which the current density decreases with time is higher for Pd-modified powder than for the pristine material,
which suggests enhanced kinetics for hydrogen desorption reactions after the deposition of Pd nanoparticles. The enhanced kinetics of hydrogen absorption/desorption into/out of the AB5 alloy are related to the influence of the palladium nanoparticles on the absorbed hydrogen transport, in which hydrogen first adsorbs and then absorbs onto the surface/into the palladium nanoparticles (indicated by the presence of characteristic Pd hydrogen absorption/desorption peaks) and then, in an absorbed state, diffuses through the palladium nanoparticles and becomes incorporated into the AB5 alloy (the increased current values for hydrogen absorption/desorption are in the potential ranges characteristic of AB5 alloy). In case of the pristine AB5 alloy LaMm-Ni4.1Al0.3Mn0.4Co0.45, the theoretical hydrogen sorption capacity does not exceed 290 mAh/g. This value was calculated from the hydrogen gas sorption experiment [26] and is the maximum value that could be obtained from electrochemical measurements in the case of pristine AB5. The addition of Pd to the composite does not
Fig. 2. (A) 1st and (B) 50th CV curves of pristine and Pd-modified AB5 alloy. The insets show more accurately the CV curves of pristine AB5. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.) 102
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Acknowledgments This research was funded by the National Science Centre (Poland), grant number: 2015/17/B/ST8/03377 (ID 289956). References [1] L. Ouyang, J. Huang, H. Wang, J. Liu, M. Zhu, Progress of hydrogen storage alloys for Ni-MH rechargeable power batteries in electric vehicles: a review, Mater. Chem. Phys. 200 (2017) 164–178. [2] Toyota Mirai Technical Specifications, http://media.toyota.co.uk/wp-content/files_ mf/1444919532151015MToyotaMiraiTechSpecFinal.pdf. [3] J. Kleperis, G. Wójcik, A. Czerwiński, J. Skowroński, M. Kopczyk, M.J. BełtowskaBrzezińska, Electrochemical behavior of metal hydrides, J. Solid State Electrochem. 5 (2001) 229–249. [4] H. Cunmao, Z. Yufen, W. Jian, Second phase and electrode characteristics of rareearth-based AB5+x alloys, J. Alloys Compd. 231 (1995) 546–549. [5] F.C. Ruiz, P.S. Martinez, E.B. Castro, R. Humana, H. Peretti, A. 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Fig. 3. Chronoamperometry desorption curves of pristine and Pd-modified AB5 alloy. The inset (a) shows the initial 300 s of discharging; the inset (b) shows an enlarged area of the CA discharging curves.
significantly affect the capacity (there might in fact be a slight decrease). To conclude, the addition of Pd nanoparticles has no significant impact on the maximum hydrogen sorption capacity of the composite, but strongly facilitates the permeation of hydrogen through the Pd/AB5 phase boundary. This is the result of a different mechanism of hydrogen transport, to a minor extent involving the corrosive oxide and hydroxide layers forming on the surface of the less noble alloy components, which hinder diffusion of hydrogen ions from the electrolyte to the surface of the AB5 grains. This results in higher discharge currents, and leads directly to an increase in the cell power. The electrochemical results that we obtained confirmed our predictions regarding the influence of Pd modification on the kinetics of hydrogen sorption in AB5 alloys. However, further studies are required for a more thorough examination of the observed phenomena. 4. Conclusions In summary, we successfully modified an AB5 alloy by surface deposition of palladium nanoparticles using a microwave-assisted polyol method for the first time. The SEM analysis showed an uneven deposition of Pd particles in the AB5 matrix. Examination of the prepared composite using cyclic voltammetry and chronoamperometry showed better electrochemical properties towards hydrogen absorption/desorption reactions than were obtained using the pristine AB5 alloy. Furthermore, activation of the electrode is not required after modification with Pd nanoparticles. The improved electrochemical performance is due to incorporation of palladium nanoparticles into the composite, which led to an enhancement of the absorption/desorption reaction kinetics. This paper reports, for the first time, the enhanced kinetics of the hydrogen sorption process in an AB5 alloy decorated with Pd nanoparticles using basic electrochemical methods (CA – an increase in the discharge current; CV – an increase in the hydrogen absorption/desorption signals). Modification of the alloy surface with palladium increases the value of the hydrogen oxidation current by several times in the first few dozen seconds, which in the case of application of this anode material in metal hydride batteries (MH–NiOOH) would significantly increase their initial power. Our research group is currently working towards reducing the Pd content of the composite and further investigating the mechanism of the absorption/desorption reactions.
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