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Hybrid nickel-metal hydride/hydrogen battery Hiroki Uesato a, Hiroki Miyaoka a,b, Takayuki Ichikawa c, Yoshitsugu Kojima a,b,* a
Graduate School of Advanced Sciences of Matter, Hiroshima University, 1-3-1, Kagamiyama, Higashi-Hiroshima, Hiroshima, 739-8530, Japan b Natural Science Center for Basic Research and Development, Hiroshima University, 1-3-1, Kagamiyama, Higashi-Hiroshima, Hiroshima, 739-8530, Japan c Graduate School of Engineering, Hiroshima University, 1-4-1, Kagamiyama, Higashi-Hiroshima, Hiroshima, 739-8527, Japan
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abstract
Article history:
High capacity, high efficiency and resource-rich energy storage systems are required to
Received 25 October 2018
store large scale excess electrical energy from renewable energy. We proposed “Hybrid
Received in revised form
Nickel-Metal Hydride/Hydrogen (Ni-MH/H2) Battery” using high capacity AB5-type
13 December 2018
hydrogen storage alloy and high-pressure H2 gas as negative electrode active materials. It
Accepted 17 December 2018
was experimentally confirmed that hydrogen gas can be utilized as an active material of
Available online xxx
negative electrode by the presence of the AB5-type hydrogen storage alloy. The experimental average cell voltage suggested that H2 gas passed through the alloy in the form of
Keywords:
atoms. The calculated gravimetric energy density of this hybrid battery increased up to 1.5
Nickel metal hydride battery
times of the conventional Ni-MH battery with low content of rare-earth element which is
Metal hydride
32 wt% of the Ni-MH battery.
High dissociation pressure
© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
High-pressure hydrogen Energy storage
Introduction Energy storage technology is required for efficient utilization of renewable energy in the world [1,2]. Developments of super capacitors, superconducting magnetic energy storage systems, fly wheels, hydrogen-based energy storage systems, secondary batteries, pumped storage hydroelectric systems, and compressed air energy storage systems have been carried out [1e4]. Among them, many researchers are conducting study on hydrogen-based energy storage systems and secondary batteries [5e7], such as lithium-ion batteries, nickel-
metal hydride (Ni-MH) batteries, sodium-sulfur batteries, redox-flow batteries, and lead-acid batteries for local leveling of renewable energy because of their large energy density [1e4]. Hydrogen is a resource-rich element and generated by water electrolysis reaction. A typical hydrogen-based energy storage system consists of a water electrolyser, a hydrogen storage tank and a fuel cell. This system can store large amount of electrical energy in the form of hydrogen, and then the energy efficiency is about 40% [3]. Ni-MH battery is also famous secondary battery using hydrogen for large scale stationary energy storage [4,8]. This
* Corresponding author. Natural Science Center for Basic Research and Development, Hiroshima University, 1-3-1, Kagamiyama, Higashi-Hiroshima, Hiroshima, 739-8530, Japan E-mail address: [email protected] (Y. Kojima). https://doi.org/10.1016/j.ijhydene.2018.12.114 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Uesato H et al., Hybrid nickel-metal hydride/hydrogen battery, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.12.114
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battery has a good safety, cycle properties, cost performance, compared with lithium-ion battery [9,10]. This battery stores electric energy as the solid hydride phase of alloys. The energy efficiency of this battery is 70e90% [11]. At the positive electrode, the electrochemical charge/ discharge reactions of Ni-MH batteries are expressed by the following equation: NiðOHÞ2 þ OH %NiOOH þ H2 O þ e
(1)
where, facing right and left show charge and discharge steps, respectively. At the negative electrode, the reactions are expressed by the equation: M þ H2 O þ e %MH þ OH
(2)
where M and MH are a hydrogen storage alloy and the hydrogen storage alloy absorbed hydrogen (metal hydride). Then, the total reactions are described as follows: NiðOHÞ2 þ M%NiOOH þ MH
La0.24Ce0.54Pr0.06Nd0.16 used for MmNi4.12Co0.79 and atomic ratio of Mm is La0.26Ce0.53Pr0.05Nd0.16 used for MmNi4.16Co0.6Mn0.23Al0.05. The alloys were crushed to obtain powder below 50 mm (median diameter: 30 mm). The MmNi4.12Co0.79 alloy was mixed with acrylic resin and carboxymethyl cellulose (CMC) water solution until it was a paste. The paste was coated and dried on a punched Ni current collector. The negative electrode has 3.75 wt% acrylic resin and 1.25 wt% CMC per alloy. The coating amount is 40 mg/cm2 and circular negative electrode with the diameter of 10 mm was punched (estimated capacity: 12.5 mAh). The paste including MmNi4.16Co0.6Mn0.23Al0.05 was coated on the Ni current corrector using the same method of MmNi4.12Co0.79. We also used the negative electrode from commercial battery (NP2, Primearth EV Energy Co., Ltd.). The positive electrode with the dimeter of 10 mm was punched from the commercial battery. The charge capacities of these cells determined by the amount of b-Ni(OH)2 in the positive electrode are about 20 mAh.
(3)
At both electrodes, oxidation/reduction reactions take place in an alkaline electrolyte such as 30 wt% KOH aqueous solution. AB5-type hydrogen storage alloys (AB5-type alloy) having gravimetric capacity below 300 mAh/g (290 mAh/g, density 8.0 g/cm3) are commercially used for negative electrode active materials [12e14]. These alloys with low plateau dissociation pressure of 0.01e0.02 MPa at 293 K have been developed to prevent increasing inside pressure above 0.1 MPa at maximum working temperature of 333 K [13,15,16]. Hydrogen storage tank filled a hydrogen storage alloy below hydrogen gas pressure of 1 MPa (MH tank) was developed for fuel cell vehicles [17]. After that, high pressure gas technology has been advanced in the field of hydrogen [18,19]. It has been reported that “Hybrid Hydrogen Tank (High-pressure MH Tank)” using hydrogen storage alloys with high dissociation pressure and high-pressure hydrogen gas at 35 MPa can improve hydrogen storage performances such as storage capacity and low temperature performance of the MH tank [20,21]. The hydrogen densities of the hybrid hydrogen tank are 1.4 times of the MH tank [20]. In this paper, we propose “Hybrid Nickel-Metal Hydride/ Hydrogen Battery” using AB5-type metal hydride with high dissociation pressure and high-pressure hydrogen gas (H2) to improve the energy density and decrease the amount of rareearth elements. The electrochemical properties were investigated by the specially designed high-pressure electrochemical cell.
High-pressure electrochemical cell The high-pressure electrochemical cell (HP-cell) is shown in Fig. 1. The cell was specially designed and assembled to investigate the charge-discharge properties under highpressure hydrogen (H2, 99.99999%, Taiyo Nippon Sanso Co.) and argon (Ar, 99.99999%, Taiyo Nippon Sanso Co.) atmospheres. The inner volume of the cell was 23.8 cm3. The pressures were measured by a pressure sensor (520.9K0503L401W, Huba Control), and monitored by digital indicator (KSM801, Krone Co.). Potassium hydroxide KOH aqueous solution of 30 wt% (6 mol/L) was used with polyolefin separator (thickness: 0.15 mm, Japan Vilene Co., Ltd.). The punched positive electrode with diameter of 10 mm was completely soaked in the KOH aqueous solution to prevent the self-discharge reaction with hydrogen gas directly [22]. The negative electrode with diameter of 10 mm was partially soaked in the KOH aqueous solution so that the surface of the alloy contact with H2 gas in the same way of the negative electrode of “NickeleHydrogen (NieH2) battery” [23] using Ptbased catalyst. The electrochemical properties of the cell were measured by charge-discharge test apparatus (HJ1001 SD-8, Hokuto Denko Co.) in the voltage between 1.0 and 1.6 V with current density of 2.54 mA/cm2 (0.1C) at 293 K and 253 K by the mixture of sodium chloride (NaCl) and ice. The cell capacities
Experimental Materials The negative electrode active materials used in this study are AB5-type alloys with high dissociation pressure such as MmNi4.12Co0.79 and MmNi4.16Co0.6Mn0.23Al0.05 (Japan Metals & Chemicals Co., Ltd.). The mischmetal (Mm) consists of 23.98e25.67 wt% La, 53.75e53.26 wt% Ce, 5.77e5.10 wt% Pr, and 16.50e15.97 wt% Nd. Atomic ratio of Mm is
Fig. 1 e Schematic diagram of specially designed highpressure electrochemical cell (HP-cell).
Please cite this article as: Uesato H et al., Hybrid nickel-metal hydride/hydrogen battery, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.12.114
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of these cells determined by the amount of b-Ni(OH)2 in the positive electrodes are about 20 mAh. Before the electrochemical measurements, positive electrode and negative electrode were activated by a charging and discharging under 3.0 MPa H2 atmosphere.
Characterization Pressure-composition-temperature (PCT) profiles were measured under H2 pressure of 0.001e5.0 MPa and at 273e293 K by the Sievert type apparatus (Suzuki Shokan Co., Ltd.). Powder X-ray diffraction (XRD) measurements were carried out to characterize the structure of electrode active materials after charge and discharge. The XRD patterns were measured by RINT-2500V (Rigaku Co.) in which the X-ray source is Cu-Ka with 40 kV/200 mA of output power. All the samples were placed on the glass plates and covered by polyimide sheets (Kapton®, Du Pont-Toray Co., Ltd.) to prevent the exposure to air. Hydrogen and oxygen released from electrodes during charge was detected by gas chromatography (GC-14B, Shimadzu Co., Ltd.) at 0.10 MPa Ar atmosphere.
Results Concept Conceptive picture of the “Hybrid Nickel-Metal Hydride/ Hydrogen (Ni-MH/H2) Battery” is shown in Fig. 2. This battery consists of positive electrode using Ni(OH)2 as a active material and hybrid negative electrode, which is composed of hydrogen storage alloy with high dissociation pressure (M) and high-pressure H2 as active materials. The plateau dissociation pressure of the MH is above 0.1 MPa and the gravimetric hydrogen density is 1.4e1.5 wt% [24]. The negative electrode reaction is divided into two reactions, although the positive electrode reaction is identical with the commercial Ni-MH battery. One reaction represents hydrogen insertion and deinsertion in the alloy during change and discharge. The other reaction represents hydrogen gas is desorbed and absorbed during the charging and discharging in the vessel, respectively. Previously, “Nickel-Metal Hydride/Hydrogen Hybrid Battery using Alkali Ion Conducting Separator” which is a combination of hydrogen storage alloy and hydrogen dissociation catalyst such as platinum, was patented [25]. In this patent,
Fig. 2 e Conceptive picture of “Hybrid Nickel-Metal Hydride/Hydrogen (Ni-MH/H2) battery”.
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the current carrying species in electrolyte were exclusively alkali metal ion such as sodium ion (Naþ). However, in our concept, the current carrying species in electrolyte is hydroxide ion (OH). Additionally, hydrogen storage alloy with high dissociation pressure is utilized as both of a negative electrode active material and a hydrogen dissociation catalyst in our concept. AB5-type alloy with high dissociation pressure leads large volumetric capacity of 3300e3400 mAh/cm3 assuming that the density is 8.6 g/cm3 [24], while the calculated gravimetric capacity is 380e400 mAh/g according to a general method in reference [26]. However, high-pressure H2 gas has large gravimetric capacity of 26,600 mAh/g, while the volumetric capacities at 293 K are 629 mAh/cm3 (0.0237 g/cm3), 475 mAh/ cm3 (0.0179 g/cm3), and 207 mAh/cm3 (0.00780 g/cm3) under 35 MPa, 25 MPa, and 10 MPa, respectively [27]. Thus, this hybrid battery is expected to improve the gravimetric energy densities of Ni-MH by changing the molar ratio of the alloy and compressed hydrogen gas under various pressure.
Metal hydride Fig. 3 shows pressure composition isotherm (PCI) for hydrogen desorption process of MmNi4.12Co0.79 at 293 K. The PCI of the hydrogen storage alloy used in commercial Ni-MH batteries is also shown as a reference. The plateau dissociation pressures were obtained by an equilibrium pressure at desorbing hydrogen which is 50% of the maximum absorbed hydrogen amount [24]. The dissociation pressure of MmNi4-12Co0.79 is 2.0 MPa at 293 K and has a higher than that of the AB5-type alloys in the commercial batteries (0.012 MPa at 293 K). The dissociation pressure of MmNi4.16Co0.6Mn0.23Al0.05 is 0.35 MPa at 293 K (Supplementary Fig. 1). XRD indicates that MmNi4.12Co0.79 and MmNi4.16Co0.6Mn0.23Al0.05 have hexagonal CaCu5 type crystal structure [28,29], which is similar to AB5type alloy in the commercial batteries (Supplementary Fig. 2). The lattice constants of these alloys are smaller than the value
Fig. 3 e Pressure composition isotherms (PCIs) of MmNi4.12Co0.79 and AB5-type alloy in the commercial batteries at 293 K.
Please cite this article as: Uesato H et al., Hybrid nickel-metal hydride/hydrogen battery, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.12.114
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of AB5-type alloy in the commercial batteries. It has been reported that the dissociation pressure of AB5-type alloys increases with decreasing of the cell volume [26,30,31]. MmNi4.12Co0.79 and MmNi4.16Co0.6Mn0.23Al0.05 also follow the trend (cell volume of MmNi4.12Co0.79: 0.0082 nm3/a0 ¼ 0.484 nm/c0 ¼ 0.406 nm, cell volume of MmNi4.16Co0.6Mn0.23Al0.05: 0.0084 nm3/a0 ¼ 0.497 nm/c0 ¼ 0.394 nm, cell volume of AB5etype alloy in the commercial batteries: 0.0087 nm3/ a0 ¼ 0.504 nm/c0 ¼ 0.397 nm). The reversible hydrogen capacities between 5 and 103 MPa is 1.4e1.5 wt%. Theoretically calculated electrochemical capacity was 380e400 mAh/g. This capacity was higher than a value of the AB5-type alloy in commercial battery (below 300 mAh/g). The standard enthalpy change (DH) and the standard entropy change (DS) of hydrogen desorption reactions were estimated by van't Hoff equation shown in Eq. (4) [21,32]. ln
PeqH2 P0
¼
DH DS RT R
(4)
where PeqH is the dissociation pressure of hydrogen storage 2 alloy (MPa), P0 is standard pressure (0.101 MPa), R is gas constant (8.314 J/K$mol), T is temperature (K). The standard enthalpy change (heat of formation) DH and the standard entropy change DS were 25.4 kJ/mol-H2 and 108 J/K$mol-H2, respectively. The DH and the DS of MmNi4.16Co0.6Mn0.23Al0.05 were 30.5 kJ/mol-H2 and 115 J/ K$mol-H2, respectively. The absolute values of the DH are small compared with the conventional AB5-type alloy used for the Ni-MH batteries (DH:34.8 kJ/mol-H2 DS:101 J/K$mol-H2). Those values of DS are smaller than entropy of H2 (131 J/ K$mol-H2) [27].
Fig. 4 e Charge (upper) and discharge (lower) curves of HPcell using MmNi4.12Co0.79 as a negative electrode material with current density of 2.54 mA/cm2 (0.1C) at 293 K, (C) gas pressure in HP-cell under 0.1 MPa Ar during charge.
Electrochemical properties Upper figure of Fig. 4 shows charge curves of the high-pressure electrochemical cell (HP-cell) composed of b-Ni(OH)2 and MmNi4.12Co0.79 under H2 of 1.0 and 3.0 MPa which is below and above the dissociation pressure of the alloy, and Ar of 0.10 MPa. The charge experiment was carried out up to the capacities of 20 mAh which is determined by theoretical capacity of positive electrode in the cell. The charge curves under H2 and Ar have similar shape, suggesting that the same electrochemical reaction would proceed independent of the H2 pressure in the cell. Under Ar atmosphere of 0.10 MPa, gas pressure increases with the charging. The released gas during the charging process was analyzed by a gas chromatography. As a result, the released gas is H2 (Supplementary Fig. 3). The partial pressure of H2 in the HP-cell was increased from 0 to 0.035 MPa during the charge measurement under 0.10 MPa Ar. This H2 amount estimated from the pressure gain approximately corresponds to the formation of NiOOH from Ni(OH)2 in the positive electrode. However, the molar ratio of H/Ni(OH)2 is estimated to be 0.82. It is presumed that the small amount of hydrogen (molar ratio: 0.18) is electrochemically absorbed in the alloy even if below the dissociation pressure, or it was remained in electrochemically inactive Ni(OH)2. “Fuel Cell/Battery (FCB)” which combined the Ni-MH battery and fuel cell system, has been proposed to realize high capacity energy storage [33,34]. At the FCB system, hydrogen and oxygen are released by over charge. At the Ni-MH/H2 battery proposed in this work, only hydrogen is released by full charge and released gas is different from FCB system. Lower figure of Fig. 4 also shows discharge curves of the HP-cell composed of NiOOH and the hydrogenated MmNi4.12Co0.79 with high-pressure H2 at 3.0 MPa which is above the dissociation pressure of the alloy. The discharge capacity of the cell is 17.4 mAh, although the theoretical capacity of MH was 12.5 mAh without gaseous H2. The discharge capacity was changed from 17.4 to 18.3 mAh by repeated measurements (Supplementary Fig. 4). Namely, H2 can be utilized in the discharging process. The capacity was down to 12.5 mAh by soaking of MH in KOH aqueous solution (Supplementary Fig. 4). This indicates that the surface of the alloy contacted with H2 is a key to utilize the H2. The discharge capacities are drastically decreased at 1.0 MPa of H2 and 0.10 MPa of Ar, which is below the dissociation pressure. The small amount of discharge capacity below the dissociation pressure also suggests that the alloy includes hydrogen atom because hydrogen release rate is slow. Noble metal catalysts such as Pt is required as catalysts for H2 dissociation in the Nie H2 battery [23,35]. On the other hand, the above results suggest that AB5-type alloy plays a role as a hydrogen storage material and catalyst for H2 dissociation at above the dissociation pressure in the proposed Ni-MH/H2 battery. Thus, the highcost catalysts are not necessary. Fig. 5 shows XRD patterns of the positive electrodes after the charging and discharging processes. The peaks corresponding to Ni current corrector was observed for all samples as a background. XRD pattern of as-purchased positive electrode shows peaks at 2q of 19.03, 33.09, 38.51, 59.07, and 62.67 . These peaks originate in (001), (100), (101), (110), and (111)
Please cite this article as: Uesato H et al., Hybrid nickel-metal hydride/hydrogen battery, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.12.114
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E ¼ E0
RT PeqH2 ln 0 2F P
(5)
where PeqH is the dissociation pressure of hydrogen storage 2 alloy (MPa), P0 is standard pressure (0.101 MPa), R is gas constant (8.314 J/K$mol), T is temperature (K), F is Faraday constant (96485C/mol). E of hydrogen storage alloy is 0.828 V vs SHE (Standard Hydrogen Electrode: þ0.000 V) [4]. The electromotive force of the cell (EEMF) is determined by the theoretical potential difference of the electrodes using the hydrogen storage alloys and Ni(OH)2 (þ0.52 V vs SHE) [37]. Eemf ¼ 0:52 E
Fig. 5 e XRD patterns of the positive electrodes as purchased and after charge/discharge conditions under H2 and Ar gas together with the data of b-Ni(OH)2 (JCPDS file No. 00-014-0117), b-NiOOH (JCPDS file No. 00-006-0141), gNiOOH (JCPDS file No. 00-006-0075), Ni (JCPDS file No. 00004-0850).
planes of b-Ni(OH)2 having Cd(OH)2 type hexagonal structure [36]. The interlayer distance of b-Ni(OH)2 is 0.472 nm. After charging under 3.0 MPa of H2 and 0.10 MPa of Ar, XRD patterns of positive electrode show peaks at 2q of 18.97e18.95, 38.61, and 66.19e66.07 . Those 3 peaks corresponds to diffraction from (001), (002), and (110) planes of b-NiOOH [37,38]. The peaks at 19.03 of b-Ni(OH)2 shifts a little to lower angle of 18.97e18.95 after the charging, and then the interlayer distance of b-NiOOH is 0.474 nm. It is considered that the expansion of (001) plane of NiOOH is suppressed in the positive electrode by additives [38]. The peaks at 2q of 12.81e12.83 and 25.81e25.67 originate in (003) and (006) planes of gNiOOH, respectively. The interlayer distance of g-NiOOH is 0.694e0.695 nm. b-Ni(OH)2 was changed to b- and small amount of g-NiOOH after the charging process under 3.0 MPa H2 and 0.10 MPa Ar. The formation of g structure can be explained by the overcharge [36]. Small amount of Ni(OH)2 is remained after the charging. After discharging, b- and gNiOOH is changed to b-Ni(OH)2 at H2 of 3.0 MPa. For the negative electrodes, the XRD patterns of the negative electrode active materials after the charging and discharging processes show substantially same patterns. AB5type metal hydride with high dissociation pressure is unstable under XRD measurement condition at atmospheric pressure (Supplementary Fig. 5). Thermodynamic parameters such as hydrogen dissociation pressure P of the hydrogen storage alloys are correlated with theoretical potential (E) [32,39]. The E of MmNi4.12Co0.79 is calculated by the following equation.
(6)
The average cell voltage which is assumed to be proportional to the Eemf (open circuit voltage), was defined as the arithmetic mean of the charge and discharge voltages at 50% of each capacity. Fig. 6 shows the dependences of the average cell voltage on dissociation pressure of the negative electrode material at 293 K and 253 K. The EEMF of the cell is also shown in the same Fig. 6 as a function of the dissociation pressure of AB5-type alloy. The experimental average cell voltages are determined by the thermodynamic parameters of H2, whether the electrochemical reaction proceeds at the interface between the KOH aqueous solution and H2 via the surface of AB5-type alloy. However, Fig. 6 shows that the experimental average cell voltages linearly increase with the logarithm of the dissociation pressure. The slope is similar to the dependence of EEMF on logarithm of dissociation pressure. This result suggests that the electrochemical reaction proceeds at the interface between the KOH aqueous solution and the AB5-type alloy.
Fig. 6 e The dependences of the average cell voltage and EEMF on dissociation pressure of the AB5-type alloys.
Please cite this article as: Uesato H et al., Hybrid nickel-metal hydride/hydrogen battery, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.12.114
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1x H2 þ xMH þ ð1 xÞNiOOH ð1 xÞNiðOHÞ2 þ xMH% 2 Number of H atom in MH x: Number of H atom in MH and gas
(7)
where x is the ratio of the number of hydrogen atoms in MH to the total number of hydrogen atoms in MH and compressed hydrogen. x is in the regions of 0 < x < 1. Hydrogen molecules dissociate into atoms and pass through MH during discharging. At the initial stage of charge and at the later stage of discharge, the conventional reaction according to the equation below occurs [4]: xNiðOHÞ2 þ xM%xNiOOH þ xMH
(8)
The total reaction is expressed by the following equation. NiðOHÞ2 þ xM%NiOOH þ
Fig. 7 e The dependence of the cell voltage on H2 pressure in the HP-cell at 293 K, using MmNi4.12Co0.79 (red) and AB5type alloy in the commercial batteries (black) as negative electrode materials. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
The potential of negative electrode can be approximately determined by thermodynamic parameters of AB5-type alloy such as the dissociation pressure. Additionally, the experimental average cell voltage was measured under several H2 pressure. Fig. 7 shows the dependences of the average cell voltage on H2 pressure. The experimental average cell voltages used MmNi4.12Co0.79 are approximately constant despite of internal pressures in the cell (Average: 1.41 V). In the case of using the commercial negative electrode, it is also indicated the similar trend exists (Average: 1.36 V). The difference in the average of the cell voltage by the negative electrode materials corresponds to the results in Fig. 6. It is indicated that the electrochemical reaction doesn't proceed at the interface between KOH aqueous solution and H2. This result is different from NieH2 battery using Pt-based catalyst [4].
1x H2 þ xMH ð0 < x < 1Þ 2
(9)
We have calculated theoretical capacities of the hybrid battery combining the AB5-type alloy with high dissociation pressure and compressed hydrogen, together with the data of Ni-MH battery. At an operating pressure of 35 MPa at 293K, the volume of a tank (10 L) for gaseous hydrogen allows storage capacity of 0.23 kg-H2. The assumed density of alloys with high dissociation pressure is 8.6 gcm3. When the tank (10 L) is filled with the alloy of 0e43 kg (packing ratio: 0e50%), the gravimetric capacities Cng and volumetric capacities, Cnv are shown in Fig. 8. Cng decreases with the packing ratio and has 471 mAh/g at 50%. Cnv increase linearly with the packing ratio of the alloy and has 2030 mAh/cm3 at 50%. Cng and Cnv of negative electrode active materials such as AB5-type alloy with high dissociation pressure including compressed hydrogen at 35 MPa and conventional AB5-type alloy used for Ni-MH battery are 801 mAh/g (1049 mAh/cm3) and 290 mAh/g (1160 mAh/cm3), respectively. Here, the volume ratio of the alloy to the compressed hydrogen is 15%.
Discussion The above results about experimental average voltage of the cell suggested that hydrogen gas is released after passing through the alloy in the form of atoms and the gas is dissociated into atoms and pass through the alloy during the charging and discharging processes, respectively. Therefore, at the later stage of charge and initial stage of discharge, the overall reaction can be expressed by the following equation:
Fig. 8 e The dependences of gravimetric/volumetric densities on packing ratio.
Please cite this article as: Uesato H et al., Hybrid nickel-metal hydride/hydrogen battery, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.12.114
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Table 1 e Calculated energy densities of hybrid and NiMH batteries without binder, electrolyte, current collector, separator and vessel. Energy density Eg/Wh/kg Ev/Wh/L
Hybrid battery
Ni-MH battery
275 497
189 516
Assuming that the capacity of the negative electrode active materials are equal to positive electrode active materials [40], then, gravimetric and volumetric energy densities Eg (Wh/kg), Ev (Wh/L) of the battery are expressed by the following equation,
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gas is released after passing through the alloy in the form of atoms, and the gas is dissociated into atoms and pass through the alloy during the charging and discharging processes, respectively. It is expected that this battery will provide high capacity and high efficiency energy storage system reduced rare-earth element.
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2018.12.114.
references
1:3Cng Eg ¼ 92:7Cng þ1 26800
(10)
1:3Cnv Ev ¼ 92:7Cnv 1 þ1 26800 2:08
(11)
Here, we used battery voltage of 1.3 V. Molecular weight of Ni(OH)2 is 92.7 and the quantity of electricity per electron is 26800 mAh/mol. The packing density of Ni(OH)2 is 2.08 g/cm3, by assuming that the packing ratio is 50%. We have calculated the gravimetric and volumetric energy densities of the hybrid and conventional Ni-MH batteries according to Eqs. 10 and 11. Table 1 shows energy densities of the hybrid and the Ni-MH batteries without binder, electrolyte, current collector, separator and vessel. The gravimetric density increases up to 1.5 times, although the volumetric density has similar value. The required amount of AB5-type alloys in the hybrid battery decreases down to 32 wt% compared with the Ni-MH battery. As a result, the amount of rare-earth element can be decreased. The hydrogen-based energy storage system is composed of electrolyser, hydrogen storage tank, and fuel cell. The total efficiency is about 40%. The energy storage and generation efficiency of Ni-MH battery is 70e90% [11]. In this work, coulombic efficiency is 87e92% and energy efficiency is 74e76% (see Fig. 4). The energy efficiency is similar to Ni-MH battery and two times larger than that of the hydrogenbased energy storage system.
Conclusion We proposed “Hybrid Nickel-Metal Hydride/Hydrogen (NiMH/H2) Battery” using AB5-type metal hydride (MH) with high dissociation pressure (>0.1 MPa) and high-pressure hydrogen gas (H2) as negative electrode materials. The electrochemical properties under high-pressure H2 atmosphere were investigated by the high-pressure electrochemical cell. As a result, hydrogen gas was reversibly utilized for charge and discharge reaction at above the dissociation pressure of the AB5-type alloy. It is experimentally demonstrated that the concept of the hybrid battery can be realized. The cell voltage obtained by the experiments was consistent with the theoretical voltage estimated by dissociation pressure of the AB5-type alloys. This indicates that hydrogen
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[18] Mori D, Hirose K. Recent challenges of hydrogen storage technologies for fuel cell vehicles. Int J Hydrogen Energy 2009;34:4569e74. [19] Zheng J, Liu X, Xu P, Liu P, Zhao Y, Yang J. Development of high pressure gaseous hydrogen storage technologies. Int J Hydrogen Energy 2012;37:1048e57. [20] Mori D, Kobayashi N, Matsunaga T, Toh K, Kojima Y. Highpressure MH tank. Materia Jpn 2005;44:257e9. [21] Kojima Y, Kawai Y, Towata SI, Matsunaga T, Shinozawa T, Kimbara M. Development of metal hydride with high dissociation pressure. J Alloy Compd 2006;419:256e61. [22] Mao Z, Visintin A, Srinivasan S, Appleby AJ, Lim HS. Microcalorimetric study of the self-discharge of the NiOOH/ Ni(OH)2 electrode in a hydrogen environment. J Appl Electrochem 1992;22:409e14. [23] Thaller LH, Zimmerman AH. Overview of the design, development, and application of nickel-hydrogen batteries. 2003. [24] Sandrock G. A panoramic overview of hydrogen storage alloys from a gas reaction point of view. J Alloy Compd 1999;293e295:877e88. [25] Joshi AV, Gordon JH, Bhavaraja S. Nickel-metal hydride/ hydrogen hybrid battery using alkali ion conducting separator. United States Patent, No. 9209445 B2; 2015. [26] Cuevas F, Joubert JM. Intermetallic compounds as negative electrodes of Ni/MH batteries. Appl Phys A 2001;238:225e38. [27] Lemmon EW, McLinden MO, Friend DG. In: Linstrom PJ, Mallard WG, editors. "Thermophysical Properties of Fluid Systems" in NIST Chemistry WebBook, NIST Standard Reference Database Number 69. Gaithersburg, MD: National Institute of Standards and Technology, https://webbook.nist. gov/chemistry/.. gan A, Chabre Y, Bouet J, [28] Latroche M, Percheron-Gue Pannetier J, Ressouche E. Intrinsic behaviour analysis of substituted LaNi5-type electrodes by means of in-situ neutron diffraction. J Alloy Compd 1995;231:537e45. [29] Vogt T, Reilly JJ, Johnson JR, Adzic GD, McBreen J. Site preference of cobalt and deuterium in the structure of a
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Hybrid nickel-metal hydride/hydrogen battery Hiroki Uesato a, Hiroki Miyaoka a,b, Takayuki Ichikawa c, Yoshitsugu Kojima a,b,* a
Graduate School of Advanced Sciences of Matter, Hiroshima University, 1-3-1, Kagamiyama, Higashi-Hiroshima, Hiroshima, 739-8530, Japan b Natural Science Center for Basic Research and Development, Hiroshima University, 1-3-1, Kagamiyama, Higashi-Hiroshima, Hiroshima, 739-8530, Japan c Graduate School of Engineering, Hiroshima University, 1-4-1, Kagamiyama, Higashi-Hiroshima, Hiroshima, 739-8527, Japan
article info
abstract
Article history:
High capacity, high efficiency and resource-rich energy storage systems are required to
Received 25 October 2018
store large scale excess electrical energy from renewable energy. We proposed “Hybrid
Received in revised form
Nickel-Metal Hydride/Hydrogen (Ni-MH/H2) Battery” using high capacity AB5-type
13 December 2018
hydrogen storage alloy and high-pressure H2 gas as negative electrode active materials. It
Accepted 17 December 2018
was experimentally confirmed that hydrogen gas can be utilized as an active material of
Available online xxx
negative electrode by the presence of the AB5-type hydrogen storage alloy. The experimental average cell voltage suggested that H2 gas passed through the alloy in the form of
Keywords:
atoms. The calculated gravimetric energy density of this hybrid battery increased up to 1.5
Nickel metal hydride battery
times of the conventional Ni-MH battery with low content of rare-earth element which is
Metal hydride
32 wt% of the Ni-MH battery.
High dissociation pressure
© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
High-pressure hydrogen Energy storage
Introduction Energy storage technology is required for efficient utilization of renewable energy in the world [1,2]. Developments of super capacitors, superconducting magnetic energy storage systems, fly wheels, hydrogen-based energy storage systems, secondary batteries, pumped storage hydroelectric systems, and compressed air energy storage systems have been carried out [1e4]. Among them, many researchers are conducting study on hydrogen-based energy storage systems and secondary batteries [5e7], such as lithium-ion batteries, nickel-
metal hydride (Ni-MH) batteries, sodium-sulfur batteries, redox-flow batteries, and lead-acid batteries for local leveling of renewable energy because of their large energy density [1e4]. Hydrogen is a resource-rich element and generated by water electrolysis reaction. A typical hydrogen-based energy storage system consists of a water electrolyser, a hydrogen storage tank and a fuel cell. This system can store large amount of electrical energy in the form of hydrogen, and then the energy efficiency is about 40% [3]. Ni-MH battery is also famous secondary battery using hydrogen for large scale stationary energy storage [4,8]. This
* Corresponding author. Natural Science Center for Basic Research and Development, Hiroshima University, 1-3-1, Kagamiyama, Higashi-Hiroshima, Hiroshima, 739-8530, Japan E-mail address: [email protected] (Y. Kojima). https://doi.org/10.1016/j.ijhydene.2018.12.114 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Uesato H et al., Hybrid nickel-metal hydride/hydrogen battery, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.12.114
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battery has a good safety, cycle properties, cost performance, compared with lithium-ion battery [9,10]. This battery stores electric energy as the solid hydride phase of alloys. The energy efficiency of this battery is 70e90% [11]. At the positive electrode, the electrochemical charge/ discharge reactions of Ni-MH batteries are expressed by the following equation: NiðOHÞ2 þ OH %NiOOH þ H2 O þ e
(1)
where, facing right and left show charge and discharge steps, respectively. At the negative electrode, the reactions are expressed by the equation: M þ H2 O þ e %MH þ OH
(2)
where M and MH are a hydrogen storage alloy and the hydrogen storage alloy absorbed hydrogen (metal hydride). Then, the total reactions are described as follows: NiðOHÞ2 þ M%NiOOH þ MH
La0.24Ce0.54Pr0.06Nd0.16 used for MmNi4.12Co0.79 and atomic ratio of Mm is La0.26Ce0.53Pr0.05Nd0.16 used for MmNi4.16Co0.6Mn0.23Al0.05. The alloys were crushed to obtain powder below 50 mm (median diameter: 30 mm). The MmNi4.12Co0.79 alloy was mixed with acrylic resin and carboxymethyl cellulose (CMC) water solution until it was a paste. The paste was coated and dried on a punched Ni current collector. The negative electrode has 3.75 wt% acrylic resin and 1.25 wt% CMC per alloy. The coating amount is 40 mg/cm2 and circular negative electrode with the diameter of 10 mm was punched (estimated capacity: 12.5 mAh). The paste including MmNi4.16Co0.6Mn0.23Al0.05 was coated on the Ni current corrector using the same method of MmNi4.12Co0.79. We also used the negative electrode from commercial battery (NP2, Primearth EV Energy Co., Ltd.). The positive electrode with the dimeter of 10 mm was punched from the commercial battery. The charge capacities of these cells determined by the amount of b-Ni(OH)2 in the positive electrode are about 20 mAh.
(3)
At both electrodes, oxidation/reduction reactions take place in an alkaline electrolyte such as 30 wt% KOH aqueous solution. AB5-type hydrogen storage alloys (AB5-type alloy) having gravimetric capacity below 300 mAh/g (290 mAh/g, density 8.0 g/cm3) are commercially used for negative electrode active materials [12e14]. These alloys with low plateau dissociation pressure of 0.01e0.02 MPa at 293 K have been developed to prevent increasing inside pressure above 0.1 MPa at maximum working temperature of 333 K [13,15,16]. Hydrogen storage tank filled a hydrogen storage alloy below hydrogen gas pressure of 1 MPa (MH tank) was developed for fuel cell vehicles [17]. After that, high pressure gas technology has been advanced in the field of hydrogen [18,19]. It has been reported that “Hybrid Hydrogen Tank (High-pressure MH Tank)” using hydrogen storage alloys with high dissociation pressure and high-pressure hydrogen gas at 35 MPa can improve hydrogen storage performances such as storage capacity and low temperature performance of the MH tank [20,21]. The hydrogen densities of the hybrid hydrogen tank are 1.4 times of the MH tank [20]. In this paper, we propose “Hybrid Nickel-Metal Hydride/ Hydrogen Battery” using AB5-type metal hydride with high dissociation pressure and high-pressure hydrogen gas (H2) to improve the energy density and decrease the amount of rareearth elements. The electrochemical properties were investigated by the specially designed high-pressure electrochemical cell.
High-pressure electrochemical cell The high-pressure electrochemical cell (HP-cell) is shown in Fig. 1. The cell was specially designed and assembled to investigate the charge-discharge properties under highpressure hydrogen (H2, 99.99999%, Taiyo Nippon Sanso Co.) and argon (Ar, 99.99999%, Taiyo Nippon Sanso Co.) atmospheres. The inner volume of the cell was 23.8 cm3. The pressures were measured by a pressure sensor (520.9K0503L401W, Huba Control), and monitored by digital indicator (KSM801, Krone Co.). Potassium hydroxide KOH aqueous solution of 30 wt% (6 mol/L) was used with polyolefin separator (thickness: 0.15 mm, Japan Vilene Co., Ltd.). The punched positive electrode with diameter of 10 mm was completely soaked in the KOH aqueous solution to prevent the self-discharge reaction with hydrogen gas directly [22]. The negative electrode with diameter of 10 mm was partially soaked in the KOH aqueous solution so that the surface of the alloy contact with H2 gas in the same way of the negative electrode of “NickeleHydrogen (NieH2) battery” [23] using Ptbased catalyst. The electrochemical properties of the cell were measured by charge-discharge test apparatus (HJ1001 SD-8, Hokuto Denko Co.) in the voltage between 1.0 and 1.6 V with current density of 2.54 mA/cm2 (0.1C) at 293 K and 253 K by the mixture of sodium chloride (NaCl) and ice. The cell capacities
Experimental Materials The negative electrode active materials used in this study are AB5-type alloys with high dissociation pressure such as MmNi4.12Co0.79 and MmNi4.16Co0.6Mn0.23Al0.05 (Japan Metals & Chemicals Co., Ltd.). The mischmetal (Mm) consists of 23.98e25.67 wt% La, 53.75e53.26 wt% Ce, 5.77e5.10 wt% Pr, and 16.50e15.97 wt% Nd. Atomic ratio of Mm is
Fig. 1 e Schematic diagram of specially designed highpressure electrochemical cell (HP-cell).
Please cite this article as: Uesato H et al., Hybrid nickel-metal hydride/hydrogen battery, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.12.114
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of these cells determined by the amount of b-Ni(OH)2 in the positive electrodes are about 20 mAh. Before the electrochemical measurements, positive electrode and negative electrode were activated by a charging and discharging under 3.0 MPa H2 atmosphere.
Characterization Pressure-composition-temperature (PCT) profiles were measured under H2 pressure of 0.001e5.0 MPa and at 273e293 K by the Sievert type apparatus (Suzuki Shokan Co., Ltd.). Powder X-ray diffraction (XRD) measurements were carried out to characterize the structure of electrode active materials after charge and discharge. The XRD patterns were measured by RINT-2500V (Rigaku Co.) in which the X-ray source is Cu-Ka with 40 kV/200 mA of output power. All the samples were placed on the glass plates and covered by polyimide sheets (Kapton®, Du Pont-Toray Co., Ltd.) to prevent the exposure to air. Hydrogen and oxygen released from electrodes during charge was detected by gas chromatography (GC-14B, Shimadzu Co., Ltd.) at 0.10 MPa Ar atmosphere.
Results Concept Conceptive picture of the “Hybrid Nickel-Metal Hydride/ Hydrogen (Ni-MH/H2) Battery” is shown in Fig. 2. This battery consists of positive electrode using Ni(OH)2 as a active material and hybrid negative electrode, which is composed of hydrogen storage alloy with high dissociation pressure (M) and high-pressure H2 as active materials. The plateau dissociation pressure of the MH is above 0.1 MPa and the gravimetric hydrogen density is 1.4e1.5 wt% [24]. The negative electrode reaction is divided into two reactions, although the positive electrode reaction is identical with the commercial Ni-MH battery. One reaction represents hydrogen insertion and deinsertion in the alloy during change and discharge. The other reaction represents hydrogen gas is desorbed and absorbed during the charging and discharging in the vessel, respectively. Previously, “Nickel-Metal Hydride/Hydrogen Hybrid Battery using Alkali Ion Conducting Separator” which is a combination of hydrogen storage alloy and hydrogen dissociation catalyst such as platinum, was patented [25]. In this patent,
Fig. 2 e Conceptive picture of “Hybrid Nickel-Metal Hydride/Hydrogen (Ni-MH/H2) battery”.
3
the current carrying species in electrolyte were exclusively alkali metal ion such as sodium ion (Naþ). However, in our concept, the current carrying species in electrolyte is hydroxide ion (OH). Additionally, hydrogen storage alloy with high dissociation pressure is utilized as both of a negative electrode active material and a hydrogen dissociation catalyst in our concept. AB5-type alloy with high dissociation pressure leads large volumetric capacity of 3300e3400 mAh/cm3 assuming that the density is 8.6 g/cm3 [24], while the calculated gravimetric capacity is 380e400 mAh/g according to a general method in reference [26]. However, high-pressure H2 gas has large gravimetric capacity of 26,600 mAh/g, while the volumetric capacities at 293 K are 629 mAh/cm3 (0.0237 g/cm3), 475 mAh/ cm3 (0.0179 g/cm3), and 207 mAh/cm3 (0.00780 g/cm3) under 35 MPa, 25 MPa, and 10 MPa, respectively [27]. Thus, this hybrid battery is expected to improve the gravimetric energy densities of Ni-MH by changing the molar ratio of the alloy and compressed hydrogen gas under various pressure.
Metal hydride Fig. 3 shows pressure composition isotherm (PCI) for hydrogen desorption process of MmNi4.12Co0.79 at 293 K. The PCI of the hydrogen storage alloy used in commercial Ni-MH batteries is also shown as a reference. The plateau dissociation pressures were obtained by an equilibrium pressure at desorbing hydrogen which is 50% of the maximum absorbed hydrogen amount [24]. The dissociation pressure of MmNi4-12Co0.79 is 2.0 MPa at 293 K and has a higher than that of the AB5-type alloys in the commercial batteries (0.012 MPa at 293 K). The dissociation pressure of MmNi4.16Co0.6Mn0.23Al0.05 is 0.35 MPa at 293 K (Supplementary Fig. 1). XRD indicates that MmNi4.12Co0.79 and MmNi4.16Co0.6Mn0.23Al0.05 have hexagonal CaCu5 type crystal structure [28,29], which is similar to AB5type alloy in the commercial batteries (Supplementary Fig. 2). The lattice constants of these alloys are smaller than the value
Fig. 3 e Pressure composition isotherms (PCIs) of MmNi4.12Co0.79 and AB5-type alloy in the commercial batteries at 293 K.
Please cite this article as: Uesato H et al., Hybrid nickel-metal hydride/hydrogen battery, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.12.114
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of AB5-type alloy in the commercial batteries. It has been reported that the dissociation pressure of AB5-type alloys increases with decreasing of the cell volume [26,30,31]. MmNi4.12Co0.79 and MmNi4.16Co0.6Mn0.23Al0.05 also follow the trend (cell volume of MmNi4.12Co0.79: 0.0082 nm3/a0 ¼ 0.484 nm/c0 ¼ 0.406 nm, cell volume of MmNi4.16Co0.6Mn0.23Al0.05: 0.0084 nm3/a0 ¼ 0.497 nm/c0 ¼ 0.394 nm, cell volume of AB5etype alloy in the commercial batteries: 0.0087 nm3/ a0 ¼ 0.504 nm/c0 ¼ 0.397 nm). The reversible hydrogen capacities between 5 and 103 MPa is 1.4e1.5 wt%. Theoretically calculated electrochemical capacity was 380e400 mAh/g. This capacity was higher than a value of the AB5-type alloy in commercial battery (below 300 mAh/g). The standard enthalpy change (DH) and the standard entropy change (DS) of hydrogen desorption reactions were estimated by van't Hoff equation shown in Eq. (4) [21,32]. ln
PeqH2 P0
¼
DH DS RT R
(4)
where PeqH is the dissociation pressure of hydrogen storage 2 alloy (MPa), P0 is standard pressure (0.101 MPa), R is gas constant (8.314 J/K$mol), T is temperature (K). The standard enthalpy change (heat of formation) DH and the standard entropy change DS were 25.4 kJ/mol-H2 and 108 J/K$mol-H2, respectively. The DH and the DS of MmNi4.16Co0.6Mn0.23Al0.05 were 30.5 kJ/mol-H2 and 115 J/ K$mol-H2, respectively. The absolute values of the DH are small compared with the conventional AB5-type alloy used for the Ni-MH batteries (DH:34.8 kJ/mol-H2 DS:101 J/K$mol-H2). Those values of DS are smaller than entropy of H2 (131 J/ K$mol-H2) [27].
Fig. 4 e Charge (upper) and discharge (lower) curves of HPcell using MmNi4.12Co0.79 as a negative electrode material with current density of 2.54 mA/cm2 (0.1C) at 293 K, (C) gas pressure in HP-cell under 0.1 MPa Ar during charge.
Electrochemical properties Upper figure of Fig. 4 shows charge curves of the high-pressure electrochemical cell (HP-cell) composed of b-Ni(OH)2 and MmNi4.12Co0.79 under H2 of 1.0 and 3.0 MPa which is below and above the dissociation pressure of the alloy, and Ar of 0.10 MPa. The charge experiment was carried out up to the capacities of 20 mAh which is determined by theoretical capacity of positive electrode in the cell. The charge curves under H2 and Ar have similar shape, suggesting that the same electrochemical reaction would proceed independent of the H2 pressure in the cell. Under Ar atmosphere of 0.10 MPa, gas pressure increases with the charging. The released gas during the charging process was analyzed by a gas chromatography. As a result, the released gas is H2 (Supplementary Fig. 3). The partial pressure of H2 in the HP-cell was increased from 0 to 0.035 MPa during the charge measurement under 0.10 MPa Ar. This H2 amount estimated from the pressure gain approximately corresponds to the formation of NiOOH from Ni(OH)2 in the positive electrode. However, the molar ratio of H/Ni(OH)2 is estimated to be 0.82. It is presumed that the small amount of hydrogen (molar ratio: 0.18) is electrochemically absorbed in the alloy even if below the dissociation pressure, or it was remained in electrochemically inactive Ni(OH)2. “Fuel Cell/Battery (FCB)” which combined the Ni-MH battery and fuel cell system, has been proposed to realize high capacity energy storage [33,34]. At the FCB system, hydrogen and oxygen are released by over charge. At the Ni-MH/H2 battery proposed in this work, only hydrogen is released by full charge and released gas is different from FCB system. Lower figure of Fig. 4 also shows discharge curves of the HP-cell composed of NiOOH and the hydrogenated MmNi4.12Co0.79 with high-pressure H2 at 3.0 MPa which is above the dissociation pressure of the alloy. The discharge capacity of the cell is 17.4 mAh, although the theoretical capacity of MH was 12.5 mAh without gaseous H2. The discharge capacity was changed from 17.4 to 18.3 mAh by repeated measurements (Supplementary Fig. 4). Namely, H2 can be utilized in the discharging process. The capacity was down to 12.5 mAh by soaking of MH in KOH aqueous solution (Supplementary Fig. 4). This indicates that the surface of the alloy contacted with H2 is a key to utilize the H2. The discharge capacities are drastically decreased at 1.0 MPa of H2 and 0.10 MPa of Ar, which is below the dissociation pressure. The small amount of discharge capacity below the dissociation pressure also suggests that the alloy includes hydrogen atom because hydrogen release rate is slow. Noble metal catalysts such as Pt is required as catalysts for H2 dissociation in the Nie H2 battery [23,35]. On the other hand, the above results suggest that AB5-type alloy plays a role as a hydrogen storage material and catalyst for H2 dissociation at above the dissociation pressure in the proposed Ni-MH/H2 battery. Thus, the highcost catalysts are not necessary. Fig. 5 shows XRD patterns of the positive electrodes after the charging and discharging processes. The peaks corresponding to Ni current corrector was observed for all samples as a background. XRD pattern of as-purchased positive electrode shows peaks at 2q of 19.03, 33.09, 38.51, 59.07, and 62.67 . These peaks originate in (001), (100), (101), (110), and (111)
Please cite this article as: Uesato H et al., Hybrid nickel-metal hydride/hydrogen battery, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.12.114
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E ¼ E0
RT PeqH2 ln 0 2F P
(5)
where PeqH is the dissociation pressure of hydrogen storage 2 alloy (MPa), P0 is standard pressure (0.101 MPa), R is gas constant (8.314 J/K$mol), T is temperature (K), F is Faraday constant (96485C/mol). E of hydrogen storage alloy is 0.828 V vs SHE (Standard Hydrogen Electrode: þ0.000 V) [4]. The electromotive force of the cell (EEMF) is determined by the theoretical potential difference of the electrodes using the hydrogen storage alloys and Ni(OH)2 (þ0.52 V vs SHE) [37]. Eemf ¼ 0:52 E
Fig. 5 e XRD patterns of the positive electrodes as purchased and after charge/discharge conditions under H2 and Ar gas together with the data of b-Ni(OH)2 (JCPDS file No. 00-014-0117), b-NiOOH (JCPDS file No. 00-006-0141), gNiOOH (JCPDS file No. 00-006-0075), Ni (JCPDS file No. 00004-0850).
planes of b-Ni(OH)2 having Cd(OH)2 type hexagonal structure [36]. The interlayer distance of b-Ni(OH)2 is 0.472 nm. After charging under 3.0 MPa of H2 and 0.10 MPa of Ar, XRD patterns of positive electrode show peaks at 2q of 18.97e18.95, 38.61, and 66.19e66.07 . Those 3 peaks corresponds to diffraction from (001), (002), and (110) planes of b-NiOOH [37,38]. The peaks at 19.03 of b-Ni(OH)2 shifts a little to lower angle of 18.97e18.95 after the charging, and then the interlayer distance of b-NiOOH is 0.474 nm. It is considered that the expansion of (001) plane of NiOOH is suppressed in the positive electrode by additives [38]. The peaks at 2q of 12.81e12.83 and 25.81e25.67 originate in (003) and (006) planes of gNiOOH, respectively. The interlayer distance of g-NiOOH is 0.694e0.695 nm. b-Ni(OH)2 was changed to b- and small amount of g-NiOOH after the charging process under 3.0 MPa H2 and 0.10 MPa Ar. The formation of g structure can be explained by the overcharge [36]. Small amount of Ni(OH)2 is remained after the charging. After discharging, b- and gNiOOH is changed to b-Ni(OH)2 at H2 of 3.0 MPa. For the negative electrodes, the XRD patterns of the negative electrode active materials after the charging and discharging processes show substantially same patterns. AB5type metal hydride with high dissociation pressure is unstable under XRD measurement condition at atmospheric pressure (Supplementary Fig. 5). Thermodynamic parameters such as hydrogen dissociation pressure P of the hydrogen storage alloys are correlated with theoretical potential (E) [32,39]. The E of MmNi4.12Co0.79 is calculated by the following equation.
(6)
The average cell voltage which is assumed to be proportional to the Eemf (open circuit voltage), was defined as the arithmetic mean of the charge and discharge voltages at 50% of each capacity. Fig. 6 shows the dependences of the average cell voltage on dissociation pressure of the negative electrode material at 293 K and 253 K. The EEMF of the cell is also shown in the same Fig. 6 as a function of the dissociation pressure of AB5-type alloy. The experimental average cell voltages are determined by the thermodynamic parameters of H2, whether the electrochemical reaction proceeds at the interface between the KOH aqueous solution and H2 via the surface of AB5-type alloy. However, Fig. 6 shows that the experimental average cell voltages linearly increase with the logarithm of the dissociation pressure. The slope is similar to the dependence of EEMF on logarithm of dissociation pressure. This result suggests that the electrochemical reaction proceeds at the interface between the KOH aqueous solution and the AB5-type alloy.
Fig. 6 e The dependences of the average cell voltage and EEMF on dissociation pressure of the AB5-type alloys.
Please cite this article as: Uesato H et al., Hybrid nickel-metal hydride/hydrogen battery, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.12.114
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1x H2 þ xMH þ ð1 xÞNiOOH ð1 xÞNiðOHÞ2 þ xMH% 2 Number of H atom in MH x: Number of H atom in MH and gas
(7)
where x is the ratio of the number of hydrogen atoms in MH to the total number of hydrogen atoms in MH and compressed hydrogen. x is in the regions of 0 < x < 1. Hydrogen molecules dissociate into atoms and pass through MH during discharging. At the initial stage of charge and at the later stage of discharge, the conventional reaction according to the equation below occurs [4]: xNiðOHÞ2 þ xM%xNiOOH þ xMH
(8)
The total reaction is expressed by the following equation. NiðOHÞ2 þ xM%NiOOH þ
Fig. 7 e The dependence of the cell voltage on H2 pressure in the HP-cell at 293 K, using MmNi4.12Co0.79 (red) and AB5type alloy in the commercial batteries (black) as negative electrode materials. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
The potential of negative electrode can be approximately determined by thermodynamic parameters of AB5-type alloy such as the dissociation pressure. Additionally, the experimental average cell voltage was measured under several H2 pressure. Fig. 7 shows the dependences of the average cell voltage on H2 pressure. The experimental average cell voltages used MmNi4.12Co0.79 are approximately constant despite of internal pressures in the cell (Average: 1.41 V). In the case of using the commercial negative electrode, it is also indicated the similar trend exists (Average: 1.36 V). The difference in the average of the cell voltage by the negative electrode materials corresponds to the results in Fig. 6. It is indicated that the electrochemical reaction doesn't proceed at the interface between KOH aqueous solution and H2. This result is different from NieH2 battery using Pt-based catalyst [4].
1x H2 þ xMH ð0 < x < 1Þ 2
(9)
We have calculated theoretical capacities of the hybrid battery combining the AB5-type alloy with high dissociation pressure and compressed hydrogen, together with the data of Ni-MH battery. At an operating pressure of 35 MPa at 293K, the volume of a tank (10 L) for gaseous hydrogen allows storage capacity of 0.23 kg-H2. The assumed density of alloys with high dissociation pressure is 8.6 gcm3. When the tank (10 L) is filled with the alloy of 0e43 kg (packing ratio: 0e50%), the gravimetric capacities Cng and volumetric capacities, Cnv are shown in Fig. 8. Cng decreases with the packing ratio and has 471 mAh/g at 50%. Cnv increase linearly with the packing ratio of the alloy and has 2030 mAh/cm3 at 50%. Cng and Cnv of negative electrode active materials such as AB5-type alloy with high dissociation pressure including compressed hydrogen at 35 MPa and conventional AB5-type alloy used for Ni-MH battery are 801 mAh/g (1049 mAh/cm3) and 290 mAh/g (1160 mAh/cm3), respectively. Here, the volume ratio of the alloy to the compressed hydrogen is 15%.
Discussion The above results about experimental average voltage of the cell suggested that hydrogen gas is released after passing through the alloy in the form of atoms and the gas is dissociated into atoms and pass through the alloy during the charging and discharging processes, respectively. Therefore, at the later stage of charge and initial stage of discharge, the overall reaction can be expressed by the following equation:
Fig. 8 e The dependences of gravimetric/volumetric densities on packing ratio.
Please cite this article as: Uesato H et al., Hybrid nickel-metal hydride/hydrogen battery, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.12.114
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Table 1 e Calculated energy densities of hybrid and NiMH batteries without binder, electrolyte, current collector, separator and vessel. Energy density Eg/Wh/kg Ev/Wh/L
Hybrid battery
Ni-MH battery
275 497
189 516
Assuming that the capacity of the negative electrode active materials are equal to positive electrode active materials [40], then, gravimetric and volumetric energy densities Eg (Wh/kg), Ev (Wh/L) of the battery are expressed by the following equation,
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gas is released after passing through the alloy in the form of atoms, and the gas is dissociated into atoms and pass through the alloy during the charging and discharging processes, respectively. It is expected that this battery will provide high capacity and high efficiency energy storage system reduced rare-earth element.
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2018.12.114.
references
1:3Cng Eg ¼ 92:7Cng þ1 26800
(10)
1:3Cnv Ev ¼ 92:7Cnv 1 þ1 26800 2:08
(11)
Here, we used battery voltage of 1.3 V. Molecular weight of Ni(OH)2 is 92.7 and the quantity of electricity per electron is 26800 mAh/mol. The packing density of Ni(OH)2 is 2.08 g/cm3, by assuming that the packing ratio is 50%. We have calculated the gravimetric and volumetric energy densities of the hybrid and conventional Ni-MH batteries according to Eqs. 10 and 11. Table 1 shows energy densities of the hybrid and the Ni-MH batteries without binder, electrolyte, current collector, separator and vessel. The gravimetric density increases up to 1.5 times, although the volumetric density has similar value. The required amount of AB5-type alloys in the hybrid battery decreases down to 32 wt% compared with the Ni-MH battery. As a result, the amount of rare-earth element can be decreased. The hydrogen-based energy storage system is composed of electrolyser, hydrogen storage tank, and fuel cell. The total efficiency is about 40%. The energy storage and generation efficiency of Ni-MH battery is 70e90% [11]. In this work, coulombic efficiency is 87e92% and energy efficiency is 74e76% (see Fig. 4). The energy efficiency is similar to Ni-MH battery and two times larger than that of the hydrogenbased energy storage system.
Conclusion We proposed “Hybrid Nickel-Metal Hydride/Hydrogen (NiMH/H2) Battery” using AB5-type metal hydride (MH) with high dissociation pressure (>0.1 MPa) and high-pressure hydrogen gas (H2) as negative electrode materials. The electrochemical properties under high-pressure H2 atmosphere were investigated by the high-pressure electrochemical cell. As a result, hydrogen gas was reversibly utilized for charge and discharge reaction at above the dissociation pressure of the AB5-type alloy. It is experimentally demonstrated that the concept of the hybrid battery can be realized. The cell voltage obtained by the experiments was consistent with the theoretical voltage estimated by dissociation pressure of the AB5-type alloys. This indicates that hydrogen
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Please cite this article as: Uesato H et al., Hybrid nickel-metal hydride/hydrogen battery, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.12.114