Ni(OH)2 Nanoflakes as Active Material for High

Feb 8, 2016 - Herein, a proof-of-concept of novel hybrid rechargeable battery based on electrochemical reactions of both nickel–zinc and zinc–air ...
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Self-Assembled NiO/Ni(OH)2 Nanoflakes as Active Material for HighPower and High-Energy Hybrid Rechargeable Battery Dong Un Lee, Jing Fu, Moon Gyu Park, Hao Liu, Ali Ghorbani Kashkooli, and Zhongwei Chen* Department of Chemical Engineering, Waterloo Institute for Nanotechnology, Waterloo Institute for Sustainable Energy, University of Waterloo, 200 University Avenue West, Waterloo, Ontario N2L 3G1, Canada

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ABSTRACT: Herein, a proof-of-concept of novel hybrid rechargeable battery based on electrochemical reactions of both nickel−zinc and zinc−air batteries is demonstrated using NiO/Ni(OH)2 nanoflakes self-assembled into mesoporous spheres as the active electrode material. The hybrid battery operates on two sets of fundamentally different battery reactions combined at the cell level, unlike in other hybrid systems where batteries of different reactions are simply connected through an external circuitry. As a result of combining nickel−zinc and zinc−air reactions, the hybrid battery demonstrates both remarkably high power density (volumetric, 14 000 W L−1; gravimetric, 2700 W kg−1) and energy density of 980 W h kg−1, significantly outperforming the performances of a conventional zinc−air battery. Furthermore, the hybrid battery demonstrates excellent charge rate capability up to 10 times faster than the rate of discharge without any capacity and voltage degradations, which makes it highly suited for large-scale applications such as electric vehicle propulsion and smart-grid energy storage. KEYWORDS: Nickel−zinc batteries, zinc−air batteries, hybrid batteries, high power, high energy

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electrochemical reactions to achieve a hybrid system.15−17 For instance, automotive industry has attempted to create a high performance hybrid energy storage system.18 However, the reported hybrid technology was based on a combination of two separate devices electronically, as opposed to the proposed method of hybridizing fundamental electrochemical reactions at the cell level. Having said this, to the best of our knowledge, a truly hybridized energy storage system that benefits from multiple reactions exhibited by a single active material has not been reported so far in the literature. In this present work, a proof-of-concept of a hybrid rechargeable battery is demonstrated at the cell level, where the electrochemical reactions of nickel−zinc and zinc−air batteries drive the battery to achieve both high power and high energy densities without increasing system complexity, physical dimensions, and the number of active materials used. Zinc−air batteries have recently attracted much attention from both academia and industry due to its advantages such as safe operation, cost effectiveness, and high energy density (1312 W h kg−1),13,14 while nickel−zinc batteries have long been utilized as affordable and environmentally benign high power energy device (above 3000 W kg−1).19−21 The unique hybrid rechargeable battery introduced here is a single electrochemical cell possessing characteristics of both nickel−zinc and zinc−air batteries, complementing the advantages of one another. The unique hybrid zinc−air/nickel rechargeable battery is a single electrochemical cell possessing characteristics of two very

ith continuous increase in fuel prices and rising concerns of environmental issues, much effort has been focused on technological advancement of various forms of energy conversion and storage systems such as fuel-cells, batteries and supercapacitors.1−4 Among them, lithium-based technologies such as lithium-ion batteries are considered one of the most investigated and used rechargeable systems for a wide range of applications due to its relatively high discharge voltage, high Coulombic efficiency, and good cycle stability.5−8 However, the use of lithium-ion batteries, particularly for plug-in hybrid electric vehicles (PEV) and all electric vehicles (EV), is limited by its relatively low energy density resulting in an insufficient driving range to fulfill the current market demands.9,10 To address this, metal−air rechargeable batteries, such as lithium−air and zinc−air batteries, have become the focal points of research due to their extremely high energy density and flat discharge profile.1,11,12 Electrically rechargeable zinc−air batteries in particular are considered highly promising due to their additional advantages of low cost, safe operation, and environmental benignity.13,14 However, the development of metal−air batteries in general for large-scale applications has been impeded by a number of technical challenges, including limited power density stemming from its relatively low discharge voltage and poor rate capability. To address these challenges, recent efforts for metal−air developments have been focused on the generation of both high power and high energy densities by combining multiple electrochemical reactions. Until now, however, reports on hybrid energy systems have been primarily based on either simply connecting two or more separate devices through an external circuitry into a larger pack or employing multiple active materials that exhibit different © 2016 American Chemical Society

Received: November 27, 2015 Revised: January 27, 2016 Published: February 8, 2016 1794

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Figure 1. Proof-of-concept of hybrid zinc−air/nickel rechargeable battery. (a) Schematic illustration of electrochemical processes in hybrid zinc−air/ nickel battery (top) and conventional zinc−air battery (bottom). (b) Solid-state hybrid rechargeable battery prototype. (c) Flexible hybrid battery demonstration.

In terms of the electrochemical reactions, NiO/Ni(OH)2 nanoflake mesoporous spheres undergo Faradaic redox reactions (eqs 1 and 2) and reversible electrocatalytic oxygen reduction and evolution reactions (eq 3). For both Faradaic and electrocatalytic reactions, the opposite electrode reaction is zinc reduction and oxidation (eq 4)

different types of battery, complementing the advantages of one another. For instance, when utilized in an electric vehicle, high current demands during acceleration can be provided by high power density resulting from the nickel−zinc reaction, while low current demands during cruising can be fulfilled by high energy density generated by the zinc−air reaction. To realize a true hybrid energy storage system, it is imperative to develop an active electrode material, which typically constitutes the largest portion of the total device cost, that is affordable, simple to fabricate, and environmentally benign, particularly for large-scale commercialization purposes. In fact, one of the long goals of the research community has been to develop a single active compound, which has multifunctional electrochemical characteristics for utilization as a unified electrode material in hybrid systems. The present work taps into fulfilling this goal by using nickel oxide/nickel hydroxide (NiO/Ni(OH)2) nanoflakes self-assembled into mesoporous spheres as a single active electrode material for the hybrid zinc−air/nickel rechargeable battery. This nonprecious transition metal-based active material is not only costeffective but is also capable of exhibiting both Faradaic redox reactions, and reversible oxygen electrochemical reactions, which correspond to the fundamental driving forces of nickel− zinc and zinc−air batteries, respectively. The active surface area of NiO/Ni(OH)2 has been significantly increased to further enhance its electrochemical properties by employing a surfactant-assisted solvothermal synthetic route followed by a controlled calcination step. NiO plays an integral role in providing electrical conductivity for the composite so that the charge transfer occurs more readily and the material utilization is maximized.22 The synthesis procedure is relatively very simple yet allows the formation of high surface area nanoscale materials in high yields, which is interesting for large-scale production and utilization in energy storage systems such as this one.

Discharge

←⎯⎯⎯⎯⎯⎯⎯⎯ NiO + OH− NiOOH + e− ⎯⎯⎯⎯⎯⎯→

(1)

Charge

Discharge

←⎯⎯⎯⎯⎯⎯⎯⎯ Ni(OH)2 + OH− NiOOH + H 2O + e− ⎯⎯⎯⎯⎯⎯→ Charge

2OH

(2)

Discharge −←⎯⎯⎯⎯⎯⎯⎯⎯

1 O + H 2O + 2e− ⎯⎯⎯⎯⎯⎯→ 2 2 Charge

(3)

Discharge

←⎯⎯⎯⎯⎯⎯⎯⎯ Zn(OH)24 − + 2e− Zn + 4OH− ⎯⎯⎯⎯⎯⎯→ Charge

(4)

The rapid kinetics of nickel−zinc Faradaic redox reactions (eqs 1 and 2) generates reversible pseudocapacitive currents within narrower voltage window compared to the electrocatalytic oxygen reduction and evolution reactions. This in turn generates higher power density compared to the zinc−air battery reaction due to higher working voltages. Despite the capabilities to harness higher power, nickel−zinc reactions are limited in terms of generating a high energy density due to the limited amount of active material that can be stored inside the battery. To complement this, zinc−air battery’s electrocatalytic oxygen reactions (eq 3) can provide extremely high energy densities due to its advantage of utilizing oxygen as the primary source of fuel, which is available virtually in unlimited quantities 1795

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Figure 2. Morphological characterization of NiO/Ni(OH)2 nanoflake-derived mesoporous spheres. (a) Schematic illustration of mechanism of formation of NiO/Ni(OH)2 mesoporous spheres. (b,c) Scanning electron microscopy (SEM), (d,e) transmission electron microscopy (TEM), and (f,g) high-resolution TEM images of NiO/Ni(OH)2 mesoporous spheres. (f, inset) SAED pattern of an individual NiO/Ni(OH)2 nanoflakes.

in the atmospheric air. To further build on the advantages of the hybrid rechargeable battery, compact and lightweight solidstate cell design has been used to evaluate the performance, which resulted in attainment of remarkably high power and energy densities. In the present work, a proof-of-concept of novel hybrid rechargeable battery operating under the combined electrochemical reactions of nickel−zinc and zinc−air batteries at the cell level using a single active material is demonstrated. The operation of this unique hybrid rechargeable zinc−air/nickel battery is governed by two distinct pairs of electrochemical processes, namely reduction and oxidation reactions of active nickel species, and oxygen reduction and evolution reactions, whereas the operation of a conventional rechargeable zinc−air battery is solely governed by the oxygen reactions (Figure 1a). Hence, the discharge process of the hybrid battery is a combination of oxygen reduction reaction (ORR) and reduction of active nickel species, while the charge process is a combination of oxygen evolution reaction (OER) and oxidation of active nickel species. The most significant benefit of having both nickel−zinc and zinc−air reactions in the hybrid battery is the capability to harness both high power and high energy densities, complementing the weaknesses of each type of battery. The high power and high energy generated by the hybrid battery is thus highly attractive for a wide range of applications, including the ones that have not been possible due to the lack of either power or energy density of the current

battery technologies. Electric vehicle (EV) and hybrid electric vehicle (HEV) applications are particularly interesting for the implementation of hybrid battery because high power can facilitate acceleration of the vehicle, while high energy can extend driving range. To fully realize the advantages of the hybrid battery, the present study used a solid-state thin cell prototype to evaluate the performance, which consists of active material loaded gas diffusion layer, PVA gel membrane, and zinc film (Figure 1b). The use of compact and lightweight design of the solid-state thin battery design having a total thickness of only 0.5 mm, as opposed to physically larger and heavier liquid electrolyte based cells, significantly improves power and energy densities in terms of both gravimetrically and volumetrically. In addition, the thin battery opens possibilities for fabrication of flexible hybrid cells, which is very interesting for emerging wearable electronic applications (Figure 1c). To realize practically viable and cost-effective hybrid battery, a nonprecious transition metal based NiO/Ni(OH)2 composite is used as a single active component in the battery electrode without further combination or modification with other electrochemically active species. NiO/Ni(OH)2 is known to have excellent electrochemical properties including pseudocapacitance23,24 and bifunctional activity toward ORR and OER,25,26 which makes it highly suitable for use in this novel hybrid rechargeable battery. To obtain the active material having a specific nanoscale morphology for enhanced electrocatalytic properties, a facile surfactant-assisted solvothermal 1796

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Figure 3. Crystal structure, elemental composition, and surface area analyses of NiO/Ni(OH)2 mesoporous spheres. (a) XRD patterns of Ni(OH)2 and NiO/Ni(OH)2 mesoporous spheres before and after calcination, respectively. (b) Full XPS survey and (c) deconvoluted high-resolution Ni 2p3/2 XPS peak of NiO/Ni(OH)2 mesoporous spheres. (d) Nitrogen adsorption−desorption isotherms of Ni(OH)2 and NiO/Ni(OH)2.

hybrid battery concept because each pore acts as a reservoir for the electrolyte, readily providing reactants during the rapid ion exchange of the Faradaic redox of nickel−zinc battery reactions. Even though the TEM analysis of precalcined Ni(OH)2 shows relatively loosely packed nanoflakes on the outer shell of the spheres (Figure S2a), the core is observed to be solid, which is indicative of the lack of porosity like in NiO/Ni(OH)2 (Figure S2a, inset). High-resolution TEM image of an individual NiO/ Ni(OH)2 nanoflake shows fringes (Figure 2f), and selected area electron diffraction (SAED) shows a clear pattern that match to (111), (200), and (220) crystal orientations of NiO (Figure 2f, inset), both of which are indicative of crystalline nature of the mesoporous spheres. The crystal structure near the edge of an individual nanoflake shows fringes that correspond to the dspacing of 0.25 nm, which again matches that of (111) crystal orientation of NiO (Figure 2g). Similarly, Ni(OH)2 exhibits crystalline nature as observed by fringes in the high-resolution TEM image (Figure S2b), and a clear SAED pattern (Figure S2b, inset) even prior to calcination. These morphological features and crystalline natures of NiO/Ni(OH)2 nanoflakes self-assembled into mesoporous spheres are crucial for obtaining high porosity with as much active sites exposed as possible, which makes it an ideal candidate for the unique hybrid rechargeable battery presented in this study. To further elucidate the crystalline nature of NiO/Ni(OH)2, X-ray diffraction (XRD) has been conducted before and after calcination (Figure 3a). Prior to calcination, the peaks obtained by the XRD pattern matches those of alpha-phase Ni(OH)2 (αNi(OH)2) (Database: JCPDS 22-0444). After calcination in air at 250 °C for 2 h, which has resulted in 30% weight reduction as confirmed by thermogravimetric analysis (TGA) (Figure

technique has been carried out followed by a simple calcination, resulting in NiO/Ni(OH)2 self-assembled into mesoporous spheres (Figure 2a).27 Even though the exact mechanism of the formation is still not clear, this type of nanoscale morphology has been reported to enhance the overall electrochemical properties by increasing the active site exposure and facilitating rapid movement of active ions during the electrochemical reactions.28 Scanning electron microscopy (SEM) analysis of NiO/Ni(OH)2 indeed reveals nanoflake structures that have self-assembled into spherical structures (Figure 2b). A highmagnification SEM image shows that individual nanoflakes exhibit highly rippled structure, which helps to prevent it from completely stacking with neighboring nanoflakes, resulting in the formation of interparticle spacing between them, leading to a significantly increased active surface area (Figure 2c). Prior to the calcination step during the synthesis, the material consists of only Ni(OH)2, which also has similar spherical morphology made up of rippled nanoflakes (Figure S1a). However, the spheres are observed to be much more densely packed, resulting in significantly reduced interparticle spacing between the nanoflakes (Figure S1b), which likely leads to the loss of active surface area. The importance of the calcination step is also clearly visible in the transmission electron microscopy (TEM) image of NiO/Ni(OH)2 which shows relatively brighter localized spots smaller than 100 nm, indicative of mesoporous nature of the spheres (Figure 2d). Highmagnification TEM image again reveals rippled nanoflake structures, which have retained their morphology even after the calcination process, creating interparticle spacing between neighboring nanoflakes (Figure 2e). The observed mesoporosity of the spheres are extremely important particularly for this 1797

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Figure 4. Solid-state single-cell hybrid battery performance. (a) Galvanodynamic charge and discharge voltage behaviors. (b) Full discharge profiles obtained at current densities of 1, 5, and 10 mA cm−2. (c) Galvanostatic charge−discharge cycling obtained at 1 mA cm−2 with each cycle consisting of 35 min charge followed by 35 min discharge. (d) Galvanostatic charge−discharge voltage profiles obtained at 40th, 50th, 60th, and 70th cycles. All tests have been performed using NiO/Ni(OH)2 mesoporous spheres.

Ni0 and Ni2+, while the second pair (Ba/Bc) is due to the conversion between Ni2+ and Ni3+. These Faradaic reactions are characterized by very rapid ion exchange kinetics, which give rise the observed high pseudocapacitive currents. In addition, the redox reactions, which occur within a narrower voltage window compared to that of the catalytic oxygen reactions, result in higher working discharge voltage which in turn leads to a high power density. Similarly, uncalcined Ni(OH)2 shows two pairs of peaks A and B due to the same reactions undergone by nickel (Figure S5a). However, the pseudocapacitive current generated by Ni(OH)2 is relatively much smaller compared to that of NiO/Ni(OH)2. This superior redox reactions of NiO/Ni(OH)2 is attributed to the improved electrical conductivity over the overall material through partially converting Ni(OH)2 to NiO through the controlled calcination step, while the activity of the pure Ni(OH)2 is likely limited by its intrinsically poor electrical conductivity. To compare with NiO/Ni(OH)2, a commonly used and investigated bifunctionally active oxygen electrocatalyst Co3O4 has been evaluated under the same conditions (Figure S5b). Co3O4 also shows a pair of small peaks (Ca/Cc), which corresponds to the Faradaic redox conversions between Co2+ and Co3+. The pseudocapacitive currents from these peaks are relatively much smaller than those of NiO/Ni(OH)2 due to high stability of cobalt ions in its spinel crystal structure, preventing them from readily undergoing prolific Faradaic reduction and oxidation. This indicates that even though Co3O4 is an excellent candidate as an efficient oxygen electrocatalyst for conventional rechargeable zinc−air batteries as widely reported in the literature, its application is limited for hybrid rechargeable battery applications due to insufficient currents generated by Faradaic redox reactions. In the oxygen-saturated electrolyte, the CV profile obtained with NiO/Ni(OH)2 shows current densities generated at the negative and positive ends of the voltage window, which correspond to ORR and OER, respectively,

S3), the peaks obtained by XRD matches primarily those of NiO with low intensity peaks corresponding to α-Ni(OH)2, which indicates that the active material is indeed a composite material consisting of NiO and α-Ni(OH)2. This particular composition of the active material has advantages over pure NiO due to the capability of α-Ni(OH)2 to transfer more than one electron per nickel ion during Faradaic redox reaction.29−31 The surface elemental composition of NiO/Ni(OH)2 has been confirmed by XPS, which has shown peaks present at binding energies that correspond to nickel and oxygen (Figure 3b). Deconvoluting the high-resolution Ni 2p 3/2 spectrum reveals peaks that correspond to both NiO and Ni(OH)2 (Figure 3c) consistent with the results obtained by XRD and EDXS above. The mesoporosity of NiO/Ni(OH)2 observed by electron microscopy is confirmed by Brunauer−Emmett−Teller (BET) analysis, where Type IV isotherm of nitrogen absorption− desorption profile is observed with a distinct hysteresis loop in the high range of relative pressure (0.5−1.0) (Figure 3d). The specific surface area of NiO/Ni(OH)2 from the BET analysis is found to be 220 m2 g−1, which is significantly higher than 85 m2 g−1 obtained with Ni(OH)2, again corroborating the electron microscopic results. By applying a simple calcination process, a composite of NiO/Ni(OH)2 having high surface area and mesoporosity has been synthesized, which very well suits as the active electrode material for the hybrid rechargeable battery as demonstrated in the following sections. To confirm hybridized nickel−zinc and zinc−air electrochemical reactions of NiO/Ni(OH)2 mesoporous spheres, the half-cell evaluation using three electrode set up has been carried out in 0.1 M KOH electrolyte. In the nitrogen-saturated electrolyte, the cyclic voltammetry (CV) profile of NiO/ Ni(OH)2 is found to show two distinct pairs of peaks (Aa/Ac and Ba/Bc), corresponding to Faradaic redox reactions of the active nickel species (Figure S4). The first pair (Aa/Ac) corresponds to the conversions between nickel oxidation states 1798

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mitigated by introducing 3+ cobalt, manganese, and aluminum ions as dopants to stabilize the crystal structure of αNi(OH)2.37,38 The cycle stability of the hybrid battery has been studied by disassembling the battery after the cycling test and characterizing the zinc foil electrode by SEM and XRD. Prior to cycling, the zinc foil is observed to be uniformly distributed of zinc particles within the carbon nanofiber matrix, which reinforces the overall electrode structure and increases the electrical conductivity of the zinc electrode (Figure S8a). Additionally, the zinc foil is observed to be highly porous in nature, which significantly enhances the diffusion of the electrolyte, thereby increasing the active zinc material utilization. After cycling the battery, the zinc foil is observed to consist of small rice grain-like zinc crystals uniformly distributed throughout the carbon nanofiber matrix (Figure S8b). This observation is contrary to the well-known shape change of zinc electrodes associated with the formation of zinc dendrites, where growths of relatively much large sized branches made of sharp needle-like structures are typically observed.39−41 In addition to the SEM analysis, the XRD pattern of the zinc foil after cycling is observed to consist of both zinc and zinc oxide (Figure S8c), indicative of good rechargeability of the zinc electrode. These promising results of the zinc foil are attributed to the additives used such as carbon nanofibers during the zinc electrode fabrication, which significantly enhances the electrical conductivity, thereby promotes a uniform growth of small grains of zinc crystals. Further analysis of the zinc foil after higher depths of charge and discharge is crucial for potential commercialization of the hybrid battery, hence it is the topic of our continuous research. To emphasize practical efficacy of the hybrid battery, its performance has been evaluated in ambient conditions using atmospheric air instead of pressurized pure oxygen. The galvanodynamic charge and discharge profiles show a similar two step reaction, consistent with the above results (Figure S9a). Interestingly, even after exhausting active nickel redox species upon first charge or discharge, the battery still operated as a conventional zinc−air battery, utilizing nickel material exclusively as an oxygen electrocatalyst (Figure S10). The pure Ni(OH)2 electrode, when galvanodynamically charged and discharged, reveals similar two-step voltage behaviors, however, the capacity arising from the Faradaic reactions is relatively much smaller (Figure S9c). This is consistent with the amount of current observed from the half-cell testing, hence the lack of capacity exhibited by Ni(OH)2 is also likely due to intrinsically low electrical conductivity of Ni(OH)2, preventing high material utilization. The conventional zinc−air battery (rechargeable, but non-hybrid) tested using Co3O4 electrode is observed to be completely absent of the two-step charge and discharge voltage behaviors (Figure S9e). Instead, linear voltage curves are demonstrated, consistent with previously reported results of rechargeable zinc−air batteries.14,42 The performance advantage of the hybrid battery over the conventional zinc−air battery is highlighted by a significantly increased peak power density (1,000 W kg−1 and 5,400 W L−1 in terms of mass and volume, respectively) obtained at a relatively low current density of 25 mA cm−2 (Figure S9b). This is directly due to higher discharge voltage obtained at the lower current density region due to the rapid kinetics of nickel redox compared to relatively slower rate of ORR, resulting in lower discharge voltage. Compared to the hybrid battery, pure Ni(OH)2 electrode also demonstrates a peak power density, however much smaller in magnitude (860 W kg−1 and 4,600 W

while the pseudocapacitive currents arising from the redox reactions are still observed. The CV profiles of Ni(OH)2 and Co 3 O 4 also show ORR and OER currents, but the pseudocapacitive currents arising from pure Ni(OH)2 is significantly inferior due to its poor electrical conductivity, also impeding the kinetics of the electrocatalytic oxygen reactions.22 To address this, Ni(OH)2 is commonly thermally dehydrated via a simple heat treatment process to form NiO which greatly increases the electrical conductivity and in turn improves the charge transportation and material utilization.22,32,33 Likewise, the thermal treatment has been carried out in this study in terms of temperature (Figure S6) and duration (Figure S7) to optimize the activities of Ni(OH)2. On the basis of the evaluation of the crystal structure and electrochemical activity by XRD and CV, respectively, the heat treatment condition of 250 °C for 2 h has been found to result in the highest pseudocapacitive, oxygen reduction and evolution reaction currents for Ni(OH) 2 mesospheres presented in this study. Building on the fundamental electrochemical activities observed above, the performance of NiO/Ni(OH)2 mesoporous spheres has been evaluated using a single-cell hybrid battery. To evaluate the galvanodynamic response, the hybrid battery has been charged and discharged from the current density of zero to 30 mA cm−2 (Figure 4a). The charge process reveals two voltage steps where the lower voltage near 1.7 V corresponds to the oxidation of active nickel species and the higher voltage near 2.1 V corresponds to the OER. Similarly, the discharge process reveals two distinct voltage steps where higher step near 1.7 V corresponds to the reduction of active nickel species, and the lower step near 1.1 V corresponds to the ORR. These observed charge and discharge voltage behaviors are consistent with previous reports on rechargeable zinc−air and nickel−zinc batteries.13,19,34,35 In addition, the stepwise voltage profiles observed during both charging and discharging are clear indications of two distinct electrochemical reactions of nickel−zinc and zinc−air batteries facilitated by NiO/Ni(OH)2 mesoporous spheres. Galvanostatically discharging the hybrid battery at current densities of 1, 5, and 10 mA cm−2 results in similar voltage profiles with slightly decreasing working voltages and specific capacities with increasing current density. This trend is commonly observed with zinc−air batteries due to slower kinetics of ORR at higher applied current densities resulting in larger overpotentials. The discharge profile obtained at 10 mA cm−2 with the hybrid battery still demonstrates highly competitive discharge voltage of 1.3−1.2 V versus Zn and even superior specific capacity of 800 mA h g−1 compared to currently the best published result in the literature (Figure 4b).13 This specific capacity normalized by the combined mass of active nickel species and the zinc utilized is equivalent to the gravimetric energy density of 980 W h kg−1. To the best of our knowledge, energy density of this magnitude has never been reported in the literature, which highlights high energy density advantage from the zinc−air battery reaction of the hybrid battery. In addition to the charge and discharge performance, the hybrid battery demonstrates good electrochemical durability with stable capacity and charge/discharge voltages over 70 cycles with each cycle consisting of 35 min charge followed by 35 min discharge even in 6.0 M KOH (Figure 4c,d). A slight decrease in the capacity of the nickel− zinc battery reaction over repeated cycling is ascribed to the phase conversion from α-Ni(OH)2 to thermodynamically more stable β-Ni(OH)2 in alkaline electrolytes,36 which can be 1799

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Figure 5. Single-cell demonstration of the hybrid battery for electrical vehicle applications. (a,b) Schematic illustration of high power usage during regenerative breaking and acceleration, respectively. (c) Reversed current rate galvanodynamic charge and discharge profiles and (d) power density profile of NiO/Ni(OH)2 mesoporous spheres.

L−1 in terms of mass and volume, respectively) obtained at lower current density of 21 mA cm−2 (Figure S9d). As expected, the Co3O4 electrode exhibits a power density profile of a typical conventional zinc−air battery, which is absent of a peak power density due to the lack of Faradaic nickel redox reactions (Figure S9f). In fact, the power density of the conventional zinc−air battery obtained with Co3O4 electrode is the lowest compared to the other two nickel based electrodes (570 W kg−1 and 3,000 W L−1 in terms of mass and volume, respectively). In fact, it is only half of the power density obtained with the hybrid battery at the same current density of 25 mA cm−2. Similar to the results of galvanodynamic results, charging and discharging the hybrid battery galvanostatically at a fixed current density results in similar stepwise voltage profiles (Figure S11). Having demonstrated excellent charge and discharge performance along with good electrochemical durability, the viability of the hybrid battery for practical applications such as EVs and HEVs, which require both high power and energy densities, is demonstrated. This is done by reversing the range of applied current density during the galvanodynamic test so that the battery is charged and discharged from high to low current density (instead of low to high). As illustrated, the reversed applied current density during charge and discharge simulate regenerative breaking (Figure 5a) and high power acceleration (Figure 5b), respectively. A video demonstration using a fan mimics vehicle acceleration and cruising based on high power and high energy densities generated by the hybrid battery, respectively. (Movie S1). During high to low current galvanodynamic charging, the hybrid battery is charged from 60

mA cm−2, resulting in a voltage plateau near 1.9 V with a small overpotential, which is most likely due to the rapid oxidation of the active nickel species (Figure 5c). The energy harnessed from regenerative breaking can be used to quickly charge the battery in this high current region, which can be discharged during the next high power acceleration. Upon continued charging to lower current densities, the battery voltage slightly increases due to the depletion of active nickel species and the onset of OER is associated with higher overpotential. As the applied current density reaches zero, the battery voltage also gradually decreases as the overpotential for OER decreases as well. Surprisingly, when the battery is charged from a lower starting current density of 40 mA cm−2, the battery voltage shows a single plateau near 1.9 V, most likely due to relatively smaller overpotential for OER overlapping with the fast kinetics of nickel reactions. This charge behavior is highly favorable because the lowered charge voltage can significantly reduce air electrode degradation associated with carbon corrosion. During high power discharge of the hybrid battery starting from 60 mA cm−2, a relatively higher voltage plateau near 1.6 V is observed due to the low overpotential of the reduction of the active nickel species. The high power density generated in this region can facilitate high power acceleration of a vehicle. Upon depletion of active nickel, discharging of the battery is sustained by high energy density resulting from ORR as observed by a broad dip in the voltage profile near the low current density region due to higher overpotential for ORR. The power density profile obtained by reversed current density discharge highlights the performance advantage of the hybrid battery, demonstrating significantly higher peak power density (gravi1800

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Nano Letters metric, 2700 W kg−1; volumetric, 14000 W L−1) obtained at a very high current density of 60 mA cm−2 (Figure 5d). This high power density obtained is most likely due to the complementary effect of rapid Faradaic redox reaction and ORR occurring simultaneously until the active nickel species is exhausted at which point the active material acts as an efficient oxygen electrocatalyst to continue to generate current, resulting in high energy density. As an extension of the EV application, the rate charge capability of nickel−zinc reaction of the hybrid battery has been evaluated, which aims to address relatively slow charging times of lithium-ion batteries currently used in EVs. By taking advantage of the hybrid battery’s pseudocapacitance stemming from the rapid nickel redox reactions, significantly reduced charging time without sacrificing the discharge capacity is demonstrated. The hybrid battery charged at different current rates of 1, 3, and 9 mA cm−2 (Figure S12a,c,e, respectively) are shown to retain virtually 100% capacity of the nickel−zinc reaction when discharged at the current density of 1 mA cm−2 (Figure S12b,d,f, respectively). This remarkable charge rate capability allows charging of the hybrid battery at 9 mA cm−2 in 2.4 min, resulting in 22 min of discharge, the same discharge capacity as the battery charged at much lower rate of 1 mA cm−2. Upon cycling the hybrid battery using charging rate of 10 mA cm−2 and discharging rate of 1 mA cm−2 over 20 cycles, the capacity due to zinc redox reactions has remained the same, while charge voltage has improved from 2.17 to 2.08 V (Figure S13). This signifies the advantage of the hybrid battery over the conventional zinc−air battery in terms of not only both high power and high energy densities but also extremely fast charging due to rapid reaction kinetics of nickel redox. To summarize, a single active material, NiO/Ni(OH)2, has been utilized to demonstrate proof-of-concept of novel hybrid rechargeable battery, which exhibits electrochemical properties of both nickel−zinc and zinc−air batteries at the cell level. The fast kinetics of Faradaic redox reactions of the active nickel species, and high specific capacity with a flat voltage plateau of the oxygen reduction reaction result in high power density (gravimetric, 2700 W kg−1; volumetric, 14000 W L−1) and high energy density of 980 W h kg−1. Having both high power and energy density, the practical viability of the hybrid battery for electric and hybrid electric vehicle applications has been demonstrated by conducting galvanodynamic charge and discharge from high to low current. Initially at high current, Faradaic redox and oxygen reactions discharged simultaneously has resulted in synergistically enhanced power density, while at low current, the battery’s voltage has been maintained by oxygen reduction reaction resulting in high energy density. Lastly, the hybrid battery demonstrated up to 10 times faster charge rate compared to that of discharge without sacrificing any capacity loss. The novel concept of hybrid rechargeable presented in this study is interesting for a wide range of applications including electric vehicles, grid-energy storage, consumer electronics, and wearable technologies and opens the door for further development of advanced energy systems having both high power and energy density.

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Experimental procedure for the synthesis of NiO/ Ni(OH)2 nanoflake mesospheres and further physicochemical and electrochemical analyses. (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions

D.U.L. and J.F. contributed equally. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the Natural Sciences and Engineering Research Council of Canada (NSERC) through grants to Z.C. and the University of Waterloo.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.5b04788. 1801

DOI: 10.1021/acs.nanolett.5b04788 Nano Lett. 2016, 16, 1794−1802

Letter

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DOI: 10.1021/acs.nanolett.5b04788 Nano Lett. 2016, 16, 1794−1802