Trash to Treasure: Waste Eggshells as Chemical Reactors for the

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Trash to Treasure: Waste Eggshells as Chemical Reactors for the Synthesis of Amorphous Co(OH)2 Nanorod Arrays on Various Substrates for Applications in Rechargeable Alkaline Batteries and Electrocatalysis Xinghua Meng and Da Deng* Department of Chemical Engineering and Materials Science, Wayne State University, 5050 Anthony Wayne Drive, Detroit, Michigan 48202, United States S Supporting Information *

ABSTRACT: Bioinspired synthesis has been attracting much attention. Here, we demonstrate a novel approach to directly use waste eggshells as a reactor system for controlled synthesis of nanostructures formed on different substrates. This approach can recycle and transform the “trash” of waste eggshells into “treasure” of unique reactor systems for nanofabrication. The eggshell reactor system can provide unique conditions for the formation of nanostructures on various substrates. Using Co(OH)2 as a model, amorphous Co(OH)2 nanorod arrays, which cannot be synthesized conventionally by direct mixing of precursors, have been successfully formed on various substrates, including Ni foam, metal foil, and glass. To illustrate their potential applications, we use the as-fabricated amorphous Co(OH)2 nanorod arrays on Ni foam as (1) binder-free electrodes for rechargeable alkaline batteries, demonstrating impressively good electrochemical performances, and (2) electrocatalyst for oxygen evolution reaction, demonstrating improved electrocatalytic performances as compared to their crystalline counterpart. We believe the idea outlined here, using eggshell reactor system, can be further expanded to synthesize many different functional materials and precursors which can find additional applications, including self-cleaning, catalysis, sensor, electrochromic devices, etc. KEYWORDS: eggshell, nanofabrication, nanoarrays, battery, electrocatalysis

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INTRODUCTION Sophisticated biological systems have long been rich sources of inspirations for engineers and scientists. For example, the eggshell formation in nature has been attracting much attention for bioinspired synthesis of functional materials recently.1−3 A eggshell is formed by reaction between carbonate ions from the metabolism of an embryo and calcium ions from the uterus on the eggshell membrane. The as-formed interstitial calcium carbonate structures on the eggshell membrane provide a robust and protective system that can protect the embryo from infection, control mass exchange, prevent water loss, and provide a source of calcium for the embryonic skeleton.4 Eggshell-inspired and diaphragm-assisted synthesis of nanostructures is emerging as a promising method for nanofabrication. Inspired by formation of eggshells, diaphragm-assisted reactor systems can provide unique reaction conditions that could be dramatically different from that of directly mixed reactor systems. To illustrate, we assume a model reaction where reactant A reacts with reactant B to form a material AB in a solution. Taking concentrations of reactants (CA and CB) for example, in conventional direct mixing and precipitation, the concentrations of two reactants decrease proportionally at © XXXX American Chemical Society

the same time along with amount of reactants consumed and reaction time (Figure 1a). The crystallization of AB can hardly be controlled under ambient conditions, but harsh conditions,

Figure 1. Illustrations to compare the difference in changes of concentrations for reactant A and B along with time inside reactors: (a) directly mixed conventional reaction system vs (b) diaphragmassisted regulated reactor system. Assuming reactant A is regulated and maintained by the diaphragm. Note: the changes in normalized concentration are plotted for illustration purpose only. Received: November 2, 2016 Accepted: January 24, 2017

A

DOI: 10.1021/acsami.6b14053 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 2. Schematic to outline the idea of using emptied eggshell as a chemical reactor system: (a) Eggshell is directly used to separate two solutions containing the reactants inside and outside of the eggshell. (b) Cross section view to show that a piece of substrate, Ni foam here for illustration, is inserted inside the eggshell. ESM stands for natural eggshell membrane. The model solutions selected here are simple couple of NaOH and CoSO4 solutions outside and inside of the eggshell, respectively. (c) Amorphous Co(OH)2 nanorod arrays are formed on the surface of Ni foam after reaction.

expensive RuO2 is typically studied for supercapacitors or “pseudocapacitors”. Recently, Co(OH)2 is being intensively investigated as a potential alternative to replace RuO2 as highperformance electrodes in “pseudocapacitors”.5−7 However, Co(OH)2 electrodes demonstrate very different electrochemical profiles as compared to that of RuO2 or MnO2. Rectangle shaped CV curves and triangle shaped charge− discharge curves, as electrochemical signatures for supercapacitors, are typically not observed for Co(OH)2 electrodes. The Faradaic reactions observed on the Co(OH)2 electrodes suggest battery behaviors rather than capacitor behaviors. We would prefer to refer Co(OH)2 materials as battery electrode instead of “pseudocapacitor” electrodes as well.5−8 On the other hand, Co(OH)2, as battery electrodes, can be coupled with activated carbon, as supercapacitor electrodes, to make hybrid supercapacitors to offer both high energy and high power. There are typically two types of Co(OH)2, namely, αCo(OH)2 and β-Co(OH)2. α-Co(OH)2 is expected to be more electrochemically active than that of crystalline β-Co(OH)2. The good electrochemical performance of α-Co(OH)2 is attributed to its poor crystallinity and turbostratically disordered structure.9 The hydrotolcite-like α-Co(OH)2 is formed by stacked Co(OH)2−x layers intercalated with anions to balance charge.10 The expanded interlayers spacing of αCo(OH)2 leading to higher electrochemical activity that of βCo(OH)2. In contrast to that of α-Co(OH)2 and β-Co(OH)2, amorphous Co(OH)2 is relatively less studied.11 In fact, it is believed that amorphous Co(OH)2 with dominantly disordered structures may be highly attractive for application as electrodes for electrochemical energy storage. However, it is still a challenging task to synthesize amorphous Co(OH)2 under mild conditions by a simple method. Electrochemical deposition was employed to prepare amorphous Co(OH)2 recently.11,12 Experimental parameters, including concentration of reactants, pH, temperature, additives, and the presence of salts could determine the phases and structures of Co(OH)2 formed.9,10 The direct mixing of Co2+ salt and OH− salt typically lead to the formation of crystalline β-Co(OH)2, not α-Co(OH)2 or amorphous Co(OH)2.2 Therefore, it is required to carefully regulate the reaction conditions to achieve the formation of amorphous Co(OH)2 and minimize the formation of crystalline Co(OH)2. The eggshell provides a barrier to control the

such as high temperature and pressure, extremely basic or acidic conditions, microwave radiation and ultrasonic treatments, are typically used. It is known that material formation is highly sensitive to a number of parameters, particularly concentration. Therefore, it will be interesting to maintain the concentration of the reactants at constant in the entire period of material synthesis to offer additional parameters for manipulation and crystallization control. The use of diaphragm to separate the two reactants will help to achieve constant concentration for at least one of the reactants. Therefore, the system could offer a unique nucleation environment not available under conventional conditions (Figure 1b), achieving possible materials synthesis at neutral conditions. Using the concept and idea outlined above, Nafion was used to mimic that of eggshell membrane for synthesis of nanoarrays of bristles1 and flakes.3 On the other hand, the emptied eggs or waste eggshells, with mechanical robustness and selective permeability, could be directly employed as a natural container or reactor for nanofabrication,2 which still required further systematically investigation. The eggshell with eggshell membrane could be considered as a natural separator that can control the mass transfer inward and/or outward of the eggshell. At the same time, the formation of nanostructures is known to be determined by experimental conditions, particularly concentration of reactants and temperature. In other words, the eggshell system can create a unique reaction and nucleation conditions for the formation of functional nanostructures. On the other hand, waste eggshell generated by food consumption in a large amount daily is always difficult to dispose. The approach to use waste eggshell as reactor systems could recycle and transform the “trash” of waste eggshells into “treasure” of unique reaction systems for nanofabrication. At the same time, nanofabrication with desired structures formed on substrates under mild conditions is still a challenging task. Therefore, it will be scientifically interesting and environmentally beneficial to employ waste eggshells for the synthesis of nanostructures (e.g., Co(OH)2 nanorods) on substrates for advanced applications, including electrochemical energy storage and electrocatalysis. Electrochemical applications of functional materials are always attracting much attention. Electrochemical energy storage devices, including batteries and supercapacitors, play increasing roles in modern society. However, their performances need to be dramatically improved. Traditionally, B

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Figure 3. (a) Optical images of substrate of Ni foam before and after formation of amorphous Co(OH)2 nanorod arrays via eggshell-assisted synthesis. (b) XRD pattern showing dominant peaks from the Ni foam substrate indicating amorphous nature of the Co(OH)2. (c) EDS analysis of the Co(OH)2 nanorods, where the detected elements of S and C are associated with intercalated [SO4]2− and [CO3]2− and elements of Ni and Au are associated with Ni foam and sputtered Au coating, respectively. (d) FTIR analysis revealed the presence of [SO4]2− and [CO3]2− in the amorphous Co(OH)2. was washed by a plenty of water to remove the remained albumen. The emptied eggshell was put inside a beaker and the hole was positioned on the top (Figure 2a). To the eggshell reactor, 30 mL water solution of 1 M CoSO4 and 10 mM cetrimonium bromide (CTAB) was filled. A water solution of 1 M NaOH was filled outside of eggshell to the same level as the solution inside of the eggshell to minimize pressure difference between the two sides. A piece of substrate (e.g., Ni foam, glass, Cu or Ti foils) was placed inside the eggshell. The beaker was covered with para film and maintained at 50 °C for a certain time. After reaction, the substrates covered with amorphous Co(OH)2 nanorod arrays was washed by water and dried in an oven overnight. In contrast, direct mixing of the same two solutions was carried out as the control where pink color crystalline βCo(OH)2 was formed. Materials Characterization. X-ray diffraction analysis was done on a Rigaku Smart Lab X-ray Diffractometer with Cu Kα radiation. The nanostructures were characterized by FESEM (JEOL JSM-7600 coupled with EDX), TEM (JEOL 2010 TEM instrument, with accelerating voltage of 200 kV). Electrochemical Tests. (1) For electrochemical energy storage, the as-prepared Co(OH)2 nanorod arrays on Ni foam as the working electrode, a Pt mesh as counter electrode, and Ag/AgCl reference electrode in a 1 M KOH aqueous solution were put into a standard electrochemical cell. Cyclic voltammetry (CV) and galvanostatic charge−discharge cycling tests were done on a CHI660D electrochemical workstation. (2) For OER electrocatalysis, the same system was used. Before measurement, 1 M KOH was saturated with O2 by purging with O2 gas for at least 1 h and kept O2-saturated during the measurement. Linear sweep voltammetry (LSV) between 1.0 and 1.75 V (vs RHE) was carried out at scan rate of 5 mV s−1. Stability of amorphous Co(OH)2 as electrocatalyst was measured at a current of 10 mA cm−2 for 12 h. All potentials were converted to vs reversible hydrogen electrode (RHE) for convience (ERHE = EAg/AgCl + 0.197 + 0.059pH, 95% iR compensation considered).13,14 In comparison, crystalline Co(OH)2 prepared by direct mixing of the same two reactants were tested similarly.

concentration of reactants and pH, creating a suitable environment for the formation of amorphous Co(OH)2. Herein, we demonstrate the direct use of emptied eggshells as a chemical reactor system for the controlled synthesis of amorphous Co(OH)2 on various substrates. The idea is illustrated in Figure 2. We select the simple couple of NaOH and CoSO4 solutions for the formation of amorphous Co(OH)2 as the model. Emptied eggshell is employed to separate the two solutions where NaOH solution is put outside and CoSO4 solution is put inside the eggshell (Figure 2a). A piece of substrate (e.g., Ni foam, Ti foil or glass) is inserted into CoSO4 solution inside the eggshell (Figure 2b). After reaction, amorphous Co(OH)2 nanorod arrays are formed on the substrates which can be easily collected (Figure 2c). To demonstrate the potential applications of the as-synthesized amorphous Co(OH)2 nanorod arrays on substrates, the amorphous Co(OH)2 nanorod arrays formed on Ni foam have been preliminarily investigated for batteries and electrocatalysis. Impressively, the as-prepared amorphous Co(OH)2 nanorod arrays, as binder-free electrodes, demonstrate highly stable specific capacity of ∼171 mAh g−1 at testing current of 2 A g−1 with reasonably good cycling performance. Additionally, amorphous Co(OH)2 nanorod arrays on Ni foam can be used directly as OER electrocatalyst, demonstrating impressive electrocatalytic performances, achieving smaller over potential and smaller Tafel slop as compared to that of crystalline Co(OH)2.

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EXPERIMENTAL SECTION

Materials Synthesis. In a typical procedure, hen eggs obtained from a local supermarket were carefully broken and the egg white and egg yolk were emptied from the broken hole. The emptied eggshell C

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Figure 4. (a) Low-magnification FESEM image showing the extensive coverage and (b) high-magnification FESEM image showing the detailed nanostructures of the amorphous Co(OH)2 nanorod arrays on the Ni foam. (c) TEM image showing the detailed structure of the amorphous Co(OH)2 nanorods peeled off from the substrate. (d) SAED showing the amorphous nature of the Co(OH)2 nanorods with diffused and broad halo rings.

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RESULTS AND DISCUSSION Figure 3a shows the optical images of the original bare Ni foam and the Ni foam after reaction inside a eggshell reaction system. The uniform color change suggests that the as-formed amorphous Co(OH)2 nanorod arrays on the Ni foam is well distributed. The brown color on the surface of the Ni foam is different from that of pink color of β-Co(OH)2 or blue color of α-Co(OH)2.15,16 The brown color is very uniformly distributed thorough the Ni foam indicating good coating thorough the foam. The control experiment using the same two reactant solutions but by directly mixing inside a covered beak under the same reaction conditions only formed crystalline β-Co(OH)2 power which was pink color (Figure S1 in the Supporting Information). XRD analysis did not detect any distinguishable peaks from crystalline α-Co(OH)2 or β-Co(OH)2 but was dominated by the substrate of Ni foam (Figure 3b). The hump observed at around 25 degree was from the glass sample holder. SAED analysis also evidenced that the Co(OH)2 nanorods had no distinguishable diffraction spots but a broad and diffused halo ring (Figure 4d). When the amorphous Co(OH)2 was annealed in argon and air, crystalline CoO and Co3O4 could be obtained, respectively (Figure S2 in the SI). The simple couple of NaOH and CoSO4, and the annealed products of cobalt oxides (CoO and Co3O4) all evidenced the formation of Co(OH)2. Therefore, the as formed Co(OH)2 should be amorphous. EDS analysis showed the presence of elements S and C (Figure 3c), which could be attributed to the intercalated or absorbed [SO4]2− and [CO3]2− ions to balance the charge of

Co(OH)2−x. FTIR analysis also confirmed the presence of [SO4]2− and [CO3]2− ions in amorphous Co(OH)2 (Figure 3d). The broad peak at ∼3400 cm−1 is attributed to the O−H stretching vibration of intercalated water molecules and hydrogen-bound O−H groups.17 The peak at ∼1602 cm−1 is associated with the bending mode of water molecules.18 The observed peak at 1386 cm−1 is associated with stretching vibration υ (CO3) and the sharp peak at around 832 cm−1 is attributed to δ(CO3).18 The peak around ∼1160 cm−1 with a shoulder at 1072 cm−1 is assigned to the vibration of intercalated sulfate anions.19 The peaks at 609 cm−1 and below could be assigned to Co−OH bending vibrations mode of Co(OH)2.17 The [SO4]2− ions were from the precursor CoSO4 salt and the [CO3]2− ions could be from the CaCO3 in eggshell. The presentence of [SO4]2− and [CO3]2− ions could affect the morphology and crystallinity of the amorphous Co(OH)2 formed.18,20−23 We hypothesize that the presence of both [SO4]2− and [CO3]2− ions also could enhance the stability of the amorphous Co(OH)2 or delay its degradation into crystalline Co(OH)2. In order words, the eggshell reaction system provides the right conditions for the formation of stable Co(OH)2 in the amorphous phase. Interestingly, when Co(NO3)2 and CoCl2 were used as precursors, crystalline αCo(OH)2 and highly crystalline Co2(OH)3Cl were formed, respectively in our eggshell reactor system. Low-magnification FESEM image shows the extensive coverage and uniform distribution of the amorphous Co(OH)2 nanorod arrays on the Ni foam (Figure 4a). Zoom-in analysis D

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Figure 5. Proposal mechanism of the diaphragm-regulated synthesis of amorphous Co(OH)2 on Ni foam and the observed changes in surface morphology on the inserted Ni foam substrate and changes in pH inside the eggshell along with time.

pH inside the eggshell reactor was immediately measured to be around ∼4.8 maintaining its original acidic nature of CoSO4 aqueous solution. In other words, eggshell was the diaphragm to physically separate the two solutions temporarily. This separation is important. In contrast, direct mixing of the two solution would lead to the formation of crystalline β-Co(OH)2 microparticles (Figure S1 in the SI). The eggshell is porous, and the lined protein membrane is permeable allowing ions to be transferred.30,31 Therefore, ions could slowly diffuse and cross the eggshell diaphragm. In daily life, the processed food of salted eggs suggests the eggshell with membrane is permeable to salt ions. The transfer of OH− ions into the eggshell reactor is evidenced by the increase of pH from 4.8 to about 6.2 after 1 day. At the same time, the air pores on the eggshell could be filled by Co(OH)2 microflake aggregates (Figure S4 in the SI). The experimental observation suggests that Co2+ ions diffused outward could be immediately consumed by the high concentration of OH− ions presented in the air pores in eggshell forming crystalline β-Co(OH)2 aggregates. The deposited crystalline β-Co(OH)2 microflake aggregates could block the air pores or reduced the rates of ion diffusion. In other words, the amount of OH− ions entered the inside of the eggshell reactors and the amount of Co2+ ions moved outward could be controlled. Experimentally, the pH would be a constant at around 7 after four days and stabilized (Figure S3 in the SI). In other words, the pH inside the eggshell reactor was well regulated by the eggshell system. The regulated of pH close to neutral condition is critical, because it is known that the phase of Co(OH)2 formed is highly sensitive to pH environment. The eggshell reactor provided the right conditions with mild pH for the formation of amorphous Co(OH)2 instead of β-Co(OH)2 inside the eggshell reactor. The inserted Ni form with large surface area could promote heterogeneous nucleation on the surface. The nearly neutral pH conditions and low temperature will not promote the fast formation of Co(OH)2. The slow reaction was evidenced by reaction time requires is typically 7 days. In other words, the formed Co(OH)2 may lack of long-range order, in contrast to that of directly mixed reactants, forming amorphous Co(OH)2 instead of crystalline β-Co(OH)2. At the same time, the

reveals the detailed 1D structure of the Co(OH)2 nanorod arrays (Figure 4b). The nanorods are not cylindrical in shape. The free end is typically narrow in needlelike morphology, and the end attached on the substrate is typically broad in sheetlike morphology. The aggregation of the free tips of many nanorod forming bundles is also observed (highlighted in red in Figure 4b), which suggests the nanorods can be bent and are mechanically robust. The aggregation of the tips could be attributed to the positively charged surface of the Co(OH)2−x layers that are attracted by negatively charge anions and/or van der Waals force between the tips.1 TEM was used to further reveal the detailed structures of the nanorods (Figure 4c). Interestingly, the nanorods have thin branches which are typically sheetlike. The aggregation of the tips is observed as well (highlighted in red in Figure 4c). The mixing of sheet- and needlelike structures is different from those 1D structured Co(OH)2 prepared by other methods.18,20−22,24−27 Highmagnification TEM image of a typical Co(OH)2 nanorod shows that it is porous with dotlike light contrast (Figure S5 in SI). The observation clearly indicates that the nanorod is poorly crystalline or amorphous.28,29 It has been reported that [CO3]2− ions can limit lateral growth of Co(OH)2 crystals.18,21 At the same time, the large anionic radii of [CO3]2− and [SO4]2− ions intercalated in Co(OH)2 could have reduce the crystallinity of the host.23 In other words, the intercalation of [SO 4 ] 2− and [CO 3 ] 2− ions could have distorted the organization of Co(OH)2−x layers significantly leading to the formation of amorphous Co(OH)2. Therefore, based on XRD, SAED, and TEM analysis, we conclude that the as-formed Co(OH)2 is amorphous. It is particularly interesting to note that our method could be employed for the synthesis of amorphous Co(OH)2 nanorod arrays on various other substrates similarly, including glass, Ti, and Cu foil as substrates (Figure S8 and 9 in the SI). In order to understand the possible formation mechanism involved, additional experiments were carried out. The pH environment inside the eggshell reactor was monitored, and the products formed on the nickel foam were characterized along with time. After the eggshell reactor was filled with the CoSO4 aqueous solution and immersed into the solution of NaOH, the E

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Figure 6. Electrochemical performances of electrodes of amorphous Co(OH)2 nanorod arrays on Ni foam prepared by eggshell reactor system: (a) CV curves at various scanning rates from 5 to 50 mV s−1. (b) Galvanostatic charge−discharge voltage profiles at different current densities from 2 to 20 A g−1. Bare Ni foam as control was measured and plotted in parts a and b. (c) Effects of testing current on the specific capacity obtained. (d) Specific capacity vs cycle plots over 3000 cycles.

small crystals.34,35 Besides, CTAB could also be present between the interlayers of Co(OH)2−x layers enlarging the expansion along c-axis direction.36 Third, the superstation in solution is controlled to be in pH around 7 or neutral conditions. The pH neutral conditions could promote the dissolution−renucleation of Co(OH)2.37 Experimentally, we also observed that the morphology of the deposited solid on Ni foam changed along with the reaction time, clearly evidenced the dissolution-renucleation process. The amorphous phase formed in the initial stage of kinetically driven crystallization could be stabilized by the intercalation of various anions and even CTAB as discussed previously. In other words, our eggshell supported reaction system could enable the formation of stable amorphous Co(OH)2. However, further study is still required to understand the mechanism which is our ongoing effort. The electrochemical performances of the as-synthesized amorphous Co(OH)2 nanorod arrays on Ni foam were preliminarily evaluated (Figure 6). The cyclic voltammetry (CV) profiles obtained a various scanning rates show two pairs of broad redox peaks (Figure 6a), which could be associated with Faradaic reactions of the Co(OH)2.6,38 The reversible reactions involved could be6,38

[SO4]2− ions from CoSO4 salt and [CO3]2− ions from the CaCO3 in eggshell could affect the structures of Co(OH)2 formed.18,20−23 We hypothesize that the presence of both [SO4]2− and [CO3]2− ions could enhance the stability of the asformed amorphous Co(OH)2. In order words, the eggshell reaction system provides the right conditions for the formation of stable Co(OH)2 in the amorphous phase. Figure 5 illustrates the eggshell regulated transfer of ions and the measured pH changes inside the eggshell reactor. We also observed that the structures formed on the surface of Ni foam also change with time, indicating the dissolution−renucleation process involved (Figure S6 in the SI). We hypothesized the plausible mechanism of forming amorphous Co(OH)2 in the unique eggshell reactor system instead of a crystalline one in conventional reactor systems as follows. First, the types and amount of anions could have played important roles in the formation of amorphous Co(OH)2. Without strong alkali medium, the positively charged Co(OH)2−x layers are easily intercalated by negatively charged anions and water molecules.32 The eggshell reactor could stabilize pH at around 7. Therefore, the abundant [SO4]2− inside the eggshell reactor could be intercalated into the Co(OH)2−x layers during formation of Co(OH)2. The presence of intercalated [SO4]2 was confirmed by FTIR. The large ionic radii of [SO4]2− could decrease the crystallinity of the host.23 The experimental evidence also suggested the enhancement effect of [SO4]2− in stabilization of amorphous Co(OH)2. In contrast, [NO3]− anions could stabilize crystalline α-Co(OH)2 (Figure S7 in SI). Similarly, [CO3]2− ion could have been adsorbed and intercalated.33 Second, the presence of cation surfactant CTAB (10 mM) could combine with [OH]− and hinder the crystallization by accelerating the dissolution of

Co(OH)2 + OH− ↔ CoOOH + H 2O + e−

(1)

CoOOH + OH− ↔ CoO2 + H 2O + e−

(2)

The pair of redox peaks marked with p1 and p4 could be attributed to reaction 1, and the pair of p2 and p3 peaks could be attributed to the reaction 2.39 The shifts in potentials were observed for the peaks when the scan rate increased, where the cathodic peaks shifted in more negative potential while the F

DOI: 10.1021/acsami.6b14053 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces anodic peaks shifted in more positive potential. The potential shifts could be attributed to the internal resistance of the electrodes.40,41 When the scanning rates were dramatically changed, the two pairs of redox peaks were still observed and the overall CV shapes were the same, indicating relatively good electron conduction.11,40 The CV analysis also provided electrochemical evidence to indicate the formation of Co(OH)2 on the substrates. Figure 6b illustrates the galvanostatic charge−discharge profiles at various current densities ranging from 2 to 20 A g−1. The charge−discharge profiles are typical for Co(OH)2 reported in the literature,38,42,43 which provided more electrochemical evidence for the formation of Co(OH)2. The nearly symmetrical charging and discharging profiles suggest good reversibility. The CV curves with obvious redox peaks and charge− discharge profiles with plateaus clearly indicate that the electrochemical performances of the amorphous Co(OH)2 nanorod arrays coated Ni foam shuld be considered as battery instead of commonly defined capacitor behaviors. For supercapacitor electrodes of activated carbon or pseudocapacitor electrodes of MnO2 or RuO2, their CV curves are typically rectangle in shapes and their charge−discharge profiles are typically triangle in shape. In literature, it is still a common practice to generally consider those Faradaic reactions on and near the surface of the hydroxides and oxides as pseudocapacitor behaviors.11,44−47 To incorporate Faradaic reactions in those electrodes, including Co(OH)2 electrodes, the capacitance is calculated as C=

nanorods. The decrease in specific capacity with the increase of testing current densities could be attributed less utilization of the active materials and the internal resistance of the electrodes.48 Impressively, even when the current density was set at as high as 20 A g−1, a reasonably good specific capacity of 83 mAh g−1 was still achieved, even no binder or conductivity enhancer were used.11 We also noted that the mass loading of Co(OH)2 on current Ni foram collectors reported in the literature is typically low, for example, at 0.124 mg cm−2 and up to 0.62 mg cm−2.11,49 In contrast, we could grow about 1 mg cm−2 of active amorphous Co(OH)2 on Ni foam using our eggshell reactor system. Figure 6d shows the excellent cycling performance of the as-prepared amorphous Co(OH)2 based binder-free electrodes over at least 3000 cycles at testing current of 10 A g−1. The observed increase in specific capacity over first 1000 cycles could be attributed to electrode activation.11,50−54 The electrode demonstrated good stability for over 6000 cycles (Figure S10 in the SI). We also evaluated the electrochemical performances of crystalline β-Co(OH)2 prepared by direct mixing of the same two reactant solutions (Figure S11 in the SI). Obviously, crystalline β-Co(OH)2 demonstrated much lower specific capacity and poorer cyclability as compared to amorphous Co(OH)2 nanorod arrays on Ni foam. The specific capacity of bare Ni foam (∼0.02 mAh g−1) was too small to be counted. In another proof-of-concept demonstration, our amorphous Co(OH)2 nanorod arrays on Ni foams were used as electocatalyst for oxygen revolution reaction (OER). Amorphous phase Co(OH)2 are expected to be better electrocatalyst for the oxygen evolution reaction (OER) as compared to their crystalline counterparts.55,56 Experimentally, the as-synthesized amorphous Co(OH)2 nanorod arrays demonstrated significantly different OER properties from that of crystalline βCo(OH)2 (Figure 7). The dramatic current increase at around ∼1.49 V for amorphous Co(OH)2 is associated with water oxidization (Figure 7a). In comparison, the dramatic current increase occurred at round ∼1.5 V for crystalline β-Co(OH)2 and 1.53 V for bare Ni foam substrate (Figure 7a). The earlier onset and higher OER current density suggest that the amorphous Co(OH)2 is better catalyst as compared to the crystalline one. We also estimated the Tafel slop of the amorphous Co(OH)2 nanorod arrays which is ∼79 mV dec−1 (Figure 7b). In comparison, the Tafel slop for crystalline βCo(OH)2 is about ∼132 and ∼228 mV dec−1 for bare Ni foam substrate. The chronopotentiometry analysis at a high current density of 10 mA cm−2 was carried out (Figure 7c). Both amorphous and crystalline β-Co(OH)2 demonstrated good stability over at least 12 h. The potential required to maintain the constant current for amorphous Co(OH)2 is significantly lower than that of crystalline Co(OH)2. The results suggest that electrocatalytic activity of the amorphous Co(OH)2 was improved as compared to the crystalline one. The improved electrocatalytic performances of amorphous Co(OH)2 nanorod arrays could be attributed to the large exposed surface area, lack of long-range order and presence of large number of defects associated with the amorphous phase.56 In other words, the amorphous Co(OH)2 could provide significantly more active sites for OER reactions, as compared to that of crystalline Co(OH)2. The formation of amorphous Co(OH)2 nanoarrays directly on 3D Ni foam also could be beneficial from the perspectives of enhanced electrode integration and facilitated ion diffusion, achieving high degree of utilization of active materials.57 Therefore, our preliminary

∫ I dt /mV

where C is the average specific capacitance (F/g), I is the testing current, dt is the time differential, m is the mass of the active electrode materials, and V is the voltage range of one seep segment. One of the limitations of this calculation is that the specific capacitance will be significantly affected by the voltage ranges selected. Whether this basic equation is suitable or not for estimation the capacitance of Co(OH)2 or other metal hydroxides and oxides, except MnO2 and RuO2, is still debatable giving its wide acceptance.11,44−47 However, we would prefer to use specific capacity (mAh/g) instead specific capacitance (F/g) in order to more actually interpret our results observed in Figure 6. The specific capacity could be calculated based on the following equation: SC =

∫ I dt / m

where SC is the specific capacity (mAh g−1), I is the testing current, dt is the time differential, and m is the mass of the active electrode materials. Specific capacity is an important electrochemical parameter to determine the performance of the materials in electrochemical charge storage. Based on Figure 6b, the specific capacity (SC) of the amorphous Co(OH)2 electrodes was estimated to be 172, 152, 101, and 83 mAh g−1 when testing at current densities of 2, 4, 10, and 20 A g−1, respectively (Figure 6c). The theoretical specific capacity of Co(OH)2 based on two electrons transfer is 577 mAh g−1. The significantly lower specific capacity measured as compared to that of theoretical value suggests that low degree of utilization of the active materials. One should note that the testing currents used here are very high for battery electrodes. Under such high currents, the redox reactions mainly occurred on or near the surface of the Co(OH)2 G

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types of precursors used in our eggshell reactor system, particularly anions, played critical roles in determine the phase, composition and morphology of the products obtained. Our ongoing efforts are to systematically explore a series of other cations and anions systems and to gain better fundamental understanding. The idea outlined here using waste eggshells as chemical reactor systems to regulate reactants can, in principle, be expanded to synthesize numerous other functional materials or precursors. We also believe that it should be possible to scale up the production by using a large number of waste eggshells fixed into parallel reactors or using artificial eggshells, which is our ongoing effort.

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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/acsami.6b14053. Additional results, including XRD and SEM of the crystalline β-Co(OH)2 control, XRD of the annealed samples, pH vs time plot, additional TEM images, timecourse experiments, results with different cobalt salts, FESEM images of rod arrays formed on various substrates, optical images, cyclability plot, and electrochemical performances of the crystalline control (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (D.D.). ORCID

Da Deng: 0000-0002-8855-5347 Author Contributions

D.D. conceived the idea and wrote the paper. X.M. carried out all the experiments and collected the experimental results. Notes

Figure 7. Direct comparison amorphous Co(OH)2 nanorod arrays formed on Ni foam and crystalline β-Co(OH)2 coated on Ni foam in OER: (a) Polarization curves obtained by linear sweep voltammetry at a scan rate of 5 mV/s. (b) Tafel plots of based on part a. Bare Ni foam was measured as a control in parts a and b. (c) Chronopotentiometry plots recorded over 12 h at a constant currents density of 10 mA cm−2.

The authors declare no competing financial interest.

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REFERENCES

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CONCLUSION In summary, amorphous Co(OH)2 nanorod arrays have been successfully fabricated on various substrates in simple and unique eggshell reactor systems. The as-prepared amorphous Co(OH)2 nanorod arrays on Ni foam as binder-free electrodes demonstrated improved electrochemical performances. Good rate performance and high specific capacity, as well as good cycling performance over at least 6000 cycles, were achieved. The results suggested that the amorphous Co(OH)2 nanorod arrays can be used in alkaline rechargeable batteries and as well as battery electrodes in hybrid supercapacitors if coupled with capacitor electrodes. The amorphous Co(OH)2 nanoarrays could be used as better electrocatalyst for OER as compared to that of crystalline Co(OH)2. Additionally, the formation of amorphous Co(OH)2 nanorod arrays on various substrates was demonstrated which could find applications in self-cleaning surface, batteries, catalysis, electrochromic devices, etc. The H

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