Electrochemical properties of layered double ...

J Solid State Electrochem DOI 10.1007/s10008-014-2671-0

FEATURE ARTICLE

Electrochemical properties of layered double hydroxides containing 3d metal cations Pierre Vialat & Fabrice Leroux & Christine Mousty

Received: 27 August 2014 / Revised: 13 October 2014 / Accepted: 23 October 2014 # Springer-Verlag Berlin Heidelberg 2014

Abstract The electrochemical behavior of layered double hydroxides (LDHs) containing 3d metal is presented here by analyzing the recent research progress described in the literature. Electron transfer within these inorganic lamellar materials can be promoted by redox-active cation sites (nickel, cobalt, iron, or manganese) located within the layer structure, with a potential window varying between −0.1 and 0.8 V/SCE in aqueous electrolyte, depending on the nature of metal cations and on the LDH composition. Additives, such as metal complexes, metal oxides, or nanocarbons to form hybrid or composite LDH-based materials, can further improve these redox properties. The electrochemical behavior of those LDH materials and their hybrid derivatives and composites, investigated mainly by cyclic voltammetry and electrochemical impedance spectroscopy, is described. A special attention is paid on NiAl-LDH and CoAl-LDH, as illustrative examples of LDH electrochemistry.

Keywords Layered double hydroxides . Transition metal . Electrode . Faradic versus pseudo-capacitive electrochemical reaction

Electronic supplementary material The online version of this article (doi:10.1007/s10008-014-2671-0) contains supplementary material, which is available to authorized users. P. Vialat : F. Leroux : C. Mousty Clermont Université, Université Blaise Pascal, Institut de Chimie de Clermont-Ferrand, BP 10448, 63000 Clermont-Ferrand, France P. Vialat : F. Leroux : C. Mousty (*) CNRS, UMR 6296, ICCF, 63171 Aubiere, France e-mail: [email protected]

Introduction Layered double hydroxides (LDHs) are synthetic lamellar solids with positively charged brucite-like layers of mixed metal hydroxides separated by interlayer hydrated anions, defined by the general formula [M 2+ 1 − x M 3+ x (OH) 2 ] x+ [(An−)x/n, yH2O] (abbreviated as M2+M3+-A, where M2+ and M3+ are respectively divalent and trivalent metals and An− is the interlayer anion compensating the positive charge of the metal hydroxide layers). The tunable chemical composition, the layered structure, and the anion exchange capacity of LDH make them highly attractive materials as electrode modifiers to be used for both electrochemical detection (chemical sensors and biosensors) and energy-storage devices (alkaline or lithium batteries and supercapacitors) [1]. Additionally, the LDH family, fabricated by soft chemistry route, is of interest regarding energy sustainability and most of them are not concerned by Registration, Evaluation, Authorization and Restriction of Chemicals (REACH). Indeed, during the last 10 years, one can notice a pronounced increase of interest from the electrochemist community for these LDH modified electrodes with a number of publications multiplied by five in the last 10 years. For instance, trace analysis can be realized after accumulation of electroactive molecules, i.e., methyl parathion, phenol, or bisphenol into the LDH modified electrodes [1–4]. These 2D-layered materials are also reported as enzyme host matrices in the development of biosensors [5]. However, most of the LDH materials used in these applications are not electronically conductive. To meet efficient and reproducible electrochemical performance, LDH modified electrodes are thus mainly prepared as thin films coated on a working electrode surface (glassy carbon, platinum, indium tin oxide) by solvent casting, layer by layer assembly, or electrodeposition [1, 5, 6]. Electron transfer within these inorganic lamellar materials can further be promoted by different strategies: (1) the

J Solid State Electrochem

intercalation of redox active anions between the LDH layers and (2) the presence of transition metal cations within the LDH intralayer domain, itself. For instance, electroactive molecules and metal complexes bearing anionic groups such as anthraquinone mono- and disulfonate (AQS) [7, 8], 2,2′-azino-bis-3-ethylbenzothiazoline-6-sulfonate [9, 10], nitroxide [11] and ferrocene derivatives [11–14], hexacyanoferrate [15], or porphyrins [16, 17] can be intercalated into LDH interlayer domains, rendering the LDH hybrid structure electroactive. These intercalated redox mediators are used as electron relays in electrocatalytic detection processes or to regenerate the enzyme active sites in biosensors or biofuel cells [in ref. 1, 5]. However, only a small fraction of these electroactive species intercalated in LDH (≤10 % mole) participates in the electrochemical reactions which are controlled by a slow diffusional process; the apparent diffusion coefficients range between 10 −11 and 10−10 cm2 s−1 [9, 15]. A loss or gain of ions from the electrolyte appears to be the general charge-balancing mechanism occurring during the electron transfer, as depicted by electrochemical quartz crystal microbalance (EQCM) measurements [18]. Diffusion of anions within LDH film, which depends on the LDH affinity towards these anions, is a limiting process for the electrochemical reactions [11, 12, 19]. However, incorporation of surfactant molecules onto LDH structure can increase the permeability within the film and plays a role in charge-balancing process, as well [2, 14, 20]. Pioneered in the late 1990s by Villemure et al., an electronhopping mechanism using electron relay within the LDH layer was proposed adopting redox-active cation sites (nickel, cobalt, or manganese) of LDH-type lattice [21]. At the same time, Demourgues-Guerlou and Delmas have tested NiFeLDH and NiMn-LDH as positive electrode materials in Ni/ Cd cells [22, 23]. Indeed, all these metal cations which are characterized according to the following standard potentials (E°) (NiII/NiIII 2.56, CoII/CoIII 1.92, MnII/MnIII 1.5, FeII/FeIII 0.77 V/NHE) [24, 25] can endow redox properties to the LDH layer structure. More recently, the development of hierarchical nanocomposites or mixed composites based on the combination of LDHs and nanocarbons (i.e., carbon nanotubes CnT, exfoliated graphene GnS) has been reported, improving the electronic conductivity of the composites in comparison to the pristine LDHs [26]. In particular, graphene has attracted a great attention due to its high surface area, electrical conductivity, high flexibility, and mechanical strength [27]. Therefore, GnS@LDH composites are mainly studied in so-called electrochemical supercapacitors requiring capacitance and/or pseudocapacitance properties under high power density [1]. Evidently, the combination of these strategies may open new potentialities in regard to applications in the area of energy storage or electrochemical analysis. This feature article highlights the recent research progress, developed in our own

group and worldwide, to investigate and hence improve electron transfer reaction using 3d metal containing LDH (Ni, Co, Fe, and Mn). A special attention is paid on NiAl-LDH, CoAlLDH, and mixed CoNiAl-LTH, as illustrative examples of LDH electrochemistry. The synthesis and the physical characterization of the LDH prepared in our laboratory are summarized in the experimental part.

Experimental section Reactants Al(NO3)3·9H2O (Acros, 99 %), Ni(NO3)2 (Acros, 99 %), Co(NO3)2·6H2O (Acros, 99 %), Fe(NO3)3·9H2O (Acros, 99 %), Mg2Al(NO3)2·6H2O (Acros, 98 %), graphite powder, and KOH (Sigma-Aldrich) were used as received without further purification. Deionized water was employed for all the experiments. Synthesis All the LDH compounds, except CoCo-LDH [28], were prepared by coprecipitation route [29]. Typically, 25 mL of the nitrate salts (Ni(NO3)2, Co(NO3)2, Mg(NO3)2, and Fe(NO3)3 or Al(NO3)3) solution was prepared with a total cationic concentration of 0.1 M. In order to keep a constant MII/MIII ratio of 2, the amount of nitrate salts was adjusted for each synthesis. The nitrate salt solution was then added dropwise into a reactor with a constant flow of 0.275 mL/min. Throughout this addition, the pH was maintained constant at a value of 9.5 by the simultaneous addition of a 0.2 M NaOH solution. The reaction was carried out under N2 atmosphere to avoid carbonate contamination. The addition of the nitrate salt solution was complete within 3 h, and the suspensions were immediately centrifuged at 4,500 rpm without any aging in order to quench the crystal growth to yield small platelets. The solids recovered by centrifugation were washed three times with deionized water and dried in air at 30 °C overnight. CoCo-LDH was prepared through a topochemical oxidative reaction (TOR) under air [28]. Physical characterization Elemental compositions of the LDH were determined by inductively coupled plasma atomic emission spectrometry (ICP-AES) with a PerkinElmer Optima 3000XL spectrometer. LDH samples were solubilized using concentrated hydrochloric acid, and the solutions were diluted to be in a 0–100-ppm analysis range for each cation. Standard solutions were prepared using nitrate salts of Co, Al, Mg, Fe, and Ni. The values reported in Table SI1 are the average values of three successive analyses.


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Powder X-ray diffraction (PXRD) measurements were performed using a Siemens D501 diffractometer in BraggBrentano geometry with Cu-Kα1/α2 radiation. Data were collected between 4.0 and 70.0 2θ (degrees), with a step size of 0.08 2θ (degrees) and a counting time of 3 s/step. PXRD patterns of the samples correspond to the LDH structure (Fig. SI1). The position of 00l diffraction lines are consistent with the presence of nitrate intercalated anions (dspacing ≈ 0.86 nm), except in the case of iron-containing phases (Mg2Fe, Ni2Fe, Co2Fe with dspacing ≈0.78 nm) corresponding to the intercalation of carbonate. A reference product (Co2AlCO3) was also synthesized presenting a basal spacing of 0.75 nm.

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Electrochemical behavior of NiAl-LDH Electrochemical response of Ni2Al-NO3 recorded in cyclic voltammetry is shown in Fig. 1a. A progressive increase of current peak occurs upon cycling, reaching a maximum value after ≈10 cycles. A noticeable change of color at the electrode surface shows that the oxidation process starts from the edge of the coating and propagates to the center of the NiAl-LDH thin film. The typical signals observed in the potential window between 0.3 and 0.6 V correspond to reversible oxidation of NiII sites in LDH lattice. Redox process within 3d metal LDH may be depicted by the following reaction: MII −LDH þ OH − sol

M 1−x II M x III ðOH Þ−LDH þ xe−

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LDH modified electrodes were prepared as thin LDH films coated on Pt electrodes (A=0.07 cm2). Before deposition, the Pt electrode surface was polished with 1-μm diamond paste and washed with acetone and then polished again with 0.04-μm alumina slurry to be finally rinsed with water and ethanol. Then, 10 μL of a 2 mg/mL LDH aqueous suspension (20 μg) was deposited on the Pt electrode and allowed to dry in air. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) measurements were made using BioLogic Science Instruments SP-150. The experiments were carried out in 0.1 M KOH solutions using a three-electrode cell, including a saturated calomel electrode (SCE) as reference electrode, a platinum counter electrode, and the platinum electrode coated with LDH thin films as working electrode. EIS measurements were recorded at the open circuit potentials after oxidation cycling (10 cycles, OCPf). A sinusoidal potential modulation with an amplitude of 10 mV (peak to peak) was applied with a frequency from 25 mHz to 100 kHz. The Nyquist plots obtained were fitted with ZView software to determine equivalent circuits.

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Fig. 1 a Evolution of cyclic voltammograms of Ni2Al-NO3 as a function of cycling (n) and b cyclic voltammogram of GnS@Ni2Al-NO3 in 0.1 KOH (v=5 mV s−1). (Adapted from [29], copyright 2013, with permission from Elsevier Ltd.)

Charge transportation occurs through the LDH layers via electron hopping between localized redox centers (3d metal cations) and ion motion across the pores and channels of the material to preserve the electroneutrality of the LDH structure [30–33]. Indeed, the effect of scan rate on the electrochemical response of NiAl-LDH shows that the electrochemical reaction is governed by a diffusional process, with an apparent diffusion coefficient (Dapp) in the range of 5–50×10−10 cm2 s-1, depending on the electrolyte [34, 35]. Obviously, two regions can be distinguished in these porous samples, the external LDH coating/electrolyte solution interface and the internal LDH coating/electrolyte interface located inside the pores, channels, and interlayer domains, the latter being less accessible for OH− ions. A total charge (Qtot), which denotes the charge related to the whole active layer, can be estimated from the extrapolation of the electrical charge Q at v→0 (measured from the area under the voltammetric peak) on a plot Q−1 versus v½, whereas


the outer charge Qout at v→∞, which is proportional to the outer active surface, can be extrapolated from the Q vs v−1/2 plot [36]. For NiAl-LDH in 0.1 M KOH, these values are respectively 7 and 0.1 C g−1 [29], leading to a contribution of the external surface corresponding only to 1.4 % of Qtot. Peak position and intensity depend also on the pH of the electrolyte solution and on its composition [29, 37, 38]. Surprisingly, alkaline cations seem to play a key role in the charge compensation following the oxidation process. Indeed, EQCM measurements [38] and chemical analysis [29] confirm a progressive incorporation of alkali ions within the LDH film during oxidation. Hydrated cations, such as Na+, can modify the hydrated state of the LDH coating, facilitating the mobility of charge-balancing species (OH− or H+/Na+) associated to the electron transfer. The crystallinity of the material affects also the electrochemical behavior; hence, the presence of the 2H1 stacking motifs in the 3R1 NiAl-LDH matrix results in an increased electrochemical signal [35]. Phase modification occurring during the oxidation process can be followed by X-ray diffraction. Two reactions may occur: (1) an anion exchange of intercalated anions by OH− from the electrolyte and (2) the formation of βNi(OH)2 phase [35, 39]. Consequently, the structural stability of NiAl-LDH upon electrochemical cycling depends on the electrolyte concentration, the LDH crystallinity, and the temperature. Oxidation of NiAl-LDH films is accompanied by a color change from green clear to black. However, this electrochromism-type process is not totally reversible since reduction of oxidized nickel sites back to NiII-LDH is limited and requires a long reduction time [40]. Thus, the reduction peak observed in cyclic voltammetry corresponds to the reduction of Ni sites next to the electrode surface. Interestingly, Villemure et al. have shown that electroactive anions with redox potentials below that of NiII sites, i.e., Fe(CN)64−, Ru(CN)64−, Mo(CN)84−, and IrCl2− added to the electrolyte solution or intercalated in the LDH interlayer spaces, are effective at mediating the full reduction of NiAl-LDHox films [41]. Similar effect is also reported when electroactive cations, Ru (bpy)32+ and Co (bpy)32+, are added in the electrolyte [40]. All these data suggest that electron transfer is the limiting step in NiAl-LDH. The combination of exfoliated graphene (GnS) and LDH into hierarchical nanocomposites can improve the electrical properties by building up electron percolation pathways. These composite materials can be prepared by in situ precipitation of LDH on reduced graphite oxide [29, 42, 43]. Figure 1b shows cyclic voltammogram of GnS@NiAl-NO3 composite in 0.1 M KOH. In comparison to the pristine LDH, a significant increase of the current peak intensity is observed and the difference between anodic and cathodic peak potentials (ΔEp) decreases. For instance, in 0.1 M KOH, the percentage of electroactive Ni sites (calculated from CV peak

integration) is ten times higher for GnS@NiAl-NO3 than for NiAl-NO3. A smaller peak separation (ΔEp) confirms a faster electron transfer rate with a calculated rate constant (k) 2.5fold that of the pristine NiAl-LDH [43]. EIS of NiAl-LDH shows characteristic semicircles which can be fitted using an equivalent circuit adapted from pseudocapacitance models, confirming that the electrochemical behavior of NiAl-LDH is governed by both electronic and ionic contributions [44] (Fig. SI2). The amplitude of semicircular arcs (in relation with electron transfer resistance Ret) depends on the applied potential and hence on the oxidation state of Ni atoms, showing an increase of the electronic conductivity in the oxidized material [35, 45, 46]. Scavetta et al. have confirmed from EIS data analysis that the kineticlimiting step of the overall electrochemical process in NiAlLDH is the one related to the electron hopping [45]. This fact explains the decrease of Ret from 7,000 Ω for NiAl-NO3 to 2,000 Ω for GnS@NiAl-NO3 composite, confirming the role of GnS as electron percolant [29, 42]. However, the capacitance characteristics of these composites remain quite low in the range of 300–700 F g−1 [29, 42]. These materials are mainly used as electrocatalyst in the development of sensors [1, 47], as illustrated by Li et al. for the detection of dopamine [48].

Electrochemical behavior of CoAl-LDH As shown in Fig. 2(a), the voltammogram of Co2Al-NO3 is different than that of Ni2Al-NO3. The peaks, which are broader and more intense, are situated at lower potentials (E1/2 =0.460 V/SCE), and the peak separation is smaller (ΔEp =82 mV in 0.1 M KOH). Moreover, the nature of intercalated anion (nitrate or carbonate) does not modify significantly the electrochemical signal, with only a small shift of oxidation peak towards lower potential from carbonate to nitrate in CV (Fig. 2). Even though the electron process is also under diffusion control (ip increases linearly with v1/2), the role of alkaline cation seems to be less significant than with NiAl-LDH and no activation of the electrochemical response is observed upon potential cycling [29]. The apparent diffusion coefficient (Dapp) is 2×10−8 cm2 s−1, and Qtot and Qout are 172 and 18 C g−1, respectively. All these values are higher than those found for NiAl-LDH, suggesting that the overall electrochemical process in CoAl-LDH is more efficient and faster than in NiAl-LDH. Moreover, the percentage of external surface involves in the electrochemical process is significantly enhanced (Qout/Qtot =10 %). Nyquist plot of EIS data of CoAl-LDH displays a quasistraight line and was not much affected by the potential cycling or the nature of electrolyte (Fig. 2b) [29, 49]. Consequently, only two constant phase components (CPE) are used in the equivalent circuit refinement (Fig. SI2),


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Fig. 2 a Cyclic voltammogramm (v=5 mV s−1) and b Nyquist representation of electrochemical impendance spectroscopy data at open circuit potential after ten CV cycles (25 mHz–100 kHz, 10 mV) of Co2Al-NO3 (a) and Co2Al-CO3 (b). (Adapted from [29], copyright 2013, with permission from Elsevier Ltd.)

suggesting that the two resistant components (Ret and Rd) vanished within the investigated frequency range. According to diffusion models proposed by Scholz et al. for redox reaction through microcrystals [31–33], if the electrons move quickly along the LDH layers towards the electrode and the ion diffusion is slower, the film of the oxidized product is formed also at the outer surface with an accumulation of ions mainly at the solid/electrolyte interface. This means that the faradic process regarding the surface component may be defined more like a pseudocapacitive behavior. The electrochemical stability of CoAl-LDH in alkaline solution (7 M KOH) was studied by Hu et al. [50]. The transformation from LDH to β-Co(OH)2 degrades the electrochemical performance because β-Co(OH) 2 has poorer electrochemical activity. However, lower KOH concentration or addition of Al(OH)3 in the electrolyte can retard this transformation. On the other hand,

Zhang’s group suggests the possible insertion of lithium cations and hydroxyls as ion pairs into the lattice of CoAl-LDH during electrochemical cycling, which differs from the mechanism in NaOH and KOH [51]. Moreover, the same group has shown that Fe(CN)63−/4− ions dissolved in the electrolyte solution act as electron relay with CoAl-LDH, decreasing charge transfer resistance and increasing exchange current density [52]. Similarly, ordered ultrathin films based on CoAl-LDH and Fe porphyrins (FeTSPP) [17] or naphthol green B (NGB) [53] have been fabricated by the layer-by-layer method, where those iron complexes serve as mediator facilitating the electron transfer on CoII/CoIII. Among various LDH materials, CoAl-LDH is known for its relatively high specific capacitance (≈600 F g−1) depending on its synthesis method [1]. In attempts to improve the electrochemical performance of these CoAl-LDH-based electrodes, nanocomposites have been studied. For instance, several papers have, in the last 2 years, reported on the capacitance of GnS@CoAl-LDH [26, 54–57]. However, in these cases, the capacitances are generally similar to those reported for the pristine CoAl-LDH. More recently, other strategies have been proposed. Duan’s group has described the preparation of poly(3,4-ethylenedioxythiophene) (PEDOT)@CoAlLDH core/shell nanoplatelet array on a flexible Ni foil substrate as a high-performance pseudocapacitor [58]. The largely enhanced pseudocapacitor behavior of the PEDOT@LDH electrode is related to the synergistic effect of its individual components: the CoAl-LDH nanoplatelet core provides abundant energy storage capacity, while both highly conductive PEDOT shell and porous architecture facilitate the electron/ mass transport in the redox reaction. Hierarchical MnO2@CoAl-LDH nanocomposites were also prepared, displaying a high specific capacitance (1,088 F g−1 at 1 A g−1), high rate capability, and excellent long-term cycling life due to a synergetic effect between CoAl-LDH and the pseudocapacitive manganese oxide [59]. Layered three component hydroxides CoxNiyAlz-LTH with varying compositions have been prepared by different methods, i.e., coprecipitation, hydrothermal synthesis, and electrodeposition [29, 60–64]. Interestingly, the voltammograms obtained with these materials show shifts in the redox peak towards less negative values by an increasing amount of Co (x) in the layer (Fig. 3a) [29, 62, 64]. Moreover, a correlation between the charge transfer resistance (Ret, determined by EIS) and the intercation distance inside the layer (a cell parameter determined by XRD) is observed, underlining the formation of solid solution (Fig. 3b) [29]. Hence, Co content has a significant effect to reduce the charge transfer resistance [29, 64], to improve the reversibility of the electrochemical process [63], and to enhance the specific capacitance which can reach a value of 1,300 F g−1 [62, 63]. The improved electrochemical characteristics can be attributed to the


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Fig. 4 Cyclic voltammogramms of (a) Mg2Fe-CO3 and (b) Ni2Fe-CO3 (in 0.1 M KOH, v=5 mV s−1)

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Fig. 3 a Evolution of the cyclic voltammogramms as a function of x in CoxNi2−2xAl-NO3 in 0.1 M KOH (v=5 mV s−1) and b evolution of charge transfer resistance (Ret) as a function of cell parameter a. (Adapted from [29], copyright 2013, with permission from Elsevier Ltd.)

synergistic effects of Co and Ni, by affecting both the ionic and the electric conductivity of the materials.

LDH containing other 3d metal cations: FeIII and MnII/III Iron is also a potential candidate to improve electrochemical properties of LDH; nevertheless, it is the magnetic properties which are mainly investigated or exploited so far, for instance, for NiFe and CoFe LDH phases [65–67]. In cyclic voltammetry, Mg2Fe-CO3 does not display electrochemical signal within the potential window investigated in 0.1 M KOH (Fig. 4(a)). However, one can notice a shift of the rising edge of the oxygen evolution reaction (OER) which starts at 0.5 V/ SCE, as expected since iron oxyhydroxide is known to be an efficient electrocatalyst for OER in alkaline medium [68]. Interestingly, Shao et al. have recently shown that the redox current is significantly enhanced for hierarchical MgFe-LDH

microspheres, with a reversible redox signal clearly observed between 0 and 0.6 V/Hg/HgO (−0.1 to 0.5 V/SCE) in 1 M NaOH [69]. Moreover, this material exhibits an excellent electrocatalytic activity for ethanol electrochemical oxidation. These authors demonstrate clearly that 3D architecture with enhanced surface area and a suitable mesopore distribution is beneficial to the mass transport of electrolyte and thus improves the faradic redox global reaction. For Ni2Fe-CO3, a significant increase in oxidation current is observed while the reduction current remains similar to that observed with the Ni2Al-LDH (Fig. 4(b)). In a previous study, Qui and Villemure have shown that the position of the peak potential depends on the upper limit in potential used to record the voltammograms in phosphate buffer solution (pH 7.9) [70]. NiFe-LDH shaped in nanoplates is found to be highly active for OER in alkaline medium (1 M KOH) [71]. In the same vein, the corresponding CnT@NiFe-LDH composite exhibits higher electrocatalytic activity and stability for OER than commercial Ir catalysts. The substitution of AlIII by FeIII in cobalt-based LDH causes a negative shift of both anodic and cathodic peak potentials (E1/2 =0.145 V/SCE) (Fig. 5). However, the current peak remains similar to those obtained for CoAl-LDH. The electrochemical behavior of iron-containing LDH was further studied by EIS. Figure 6 displays the Nyquist plots of the IES spectra for Mg2Fe, Ni2Fe, and Co2Fe LDH. A semicircle-like shape was observed with the following sequence of the electron transfer resistance: MgFe > NiFe > CoFe. This underlines the synergistic effect between iron and cobalt on the electron transfer mechanism. Abellan et al. have recently investigated the electrochemical behavior of CoFe-LDH in 6 M KOH [65]. This sample exhibits high specific capacitance of 500 F g−1 at 2 A g−1 associated to a catalytic activity towards OER, as well. Manganese is also an interesting element due to its large number of oxidation states. However, the main problem is the control of its oxidation state during its incorporation within the


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Fig. 5 Cyclic voltammogramm of (a) Co2Al-CO3, (b) Co2Fe-CO3, and (c) CoII5,7CoIII-CO3 (in 0.1 M KOH, v=5 mV s−1)

LDH layer to prevent the formation of impurity. Qiu and Villemure found an average oxidation state of +3.52 in MgMn-CO3 sample, suggesting the presence MnIV in the LDH structure [21]. The voltammetric peaks observed between 0 and 0.4 V/SCE were attributed to MnIII/MnIV redox couples. Reduction below 3+ did not occur even after several hours of electrolysis at negative potentials. Recently, other MnIII-containing LDH or their carbon composites have been tested as supercapacitors, namely CoMn [72] and NiMn [73, 74], with a particularly good performance for the nanostructurated CNT@NiMn composite (2,960 F g−1 at 1.5 A g−1) [26]. Other papers report on the synthesis and platelet exfoliation combining GnS and MnIIAl or MnIIFe LDH for electrode materials [75–77]. However, further study will be required to fully understand the behavior of Mncontaining LDH as well as to better stabilize their performance upon cycling.

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Fig. 6 Nyquist representation of EIS data of (a) Mg2Fe-CO3, (b) Ni2FeCO3, and (c) Co2Fe-CO3 at open circuit potential after ten CV cycles (in 0.1 M KOH, 25 mHz–100 kHz, 10 mV)

The last type of LDH which will be presented here is LDH materials composed by the same metal cation but with two different oxidation states. The most relevant compounds corresponding to this definition are green rusts (GR) which are Fe II/III oxyhydroxycarbonate (FeII6(1 − x)FeIII6xO12H2(7 − 3x)CO3) with in situ redox flexibility (1/3≤×≤2/3) [78]. The electrochemical behavior of GR-CO3 and GR-SO4, prepared by electrodeposition, has been studied by cyclic voltammetry coupled with EQCM [79]. A complex formation and transformation pathway is described, showing that GR can act as both electron donor and acceptor with the possibility for a global one electron per Fe atom exchange, from the fully reduced product exGR-FeII to the ferrous exGR-FeIII. Similarly, cobalt hydroxide β-Co(OH)2 can be converted by a TOR under air to a CoIICoIII-CO3 LDH phase with a CoIII/Cotot of 0.13 [28]. This compound, characterized by CV in 0.1 M KOH aqueous electrolyte, exhibits well-defined reversible peaks at E1/2 =0.160 V/SCE with higher current than those observed for Co2Al-CO3 or Co2Fe-CO3 (Fig. 5). According to the composition of these compounds and the total amount of LDH coated on the electrode surface, the percentage of electroactive CoII involved in the electrochemical process can be estimated from the oxidation peak area. The obtained percentages are 28 % mole for CoIICoIII-CO3 and only 10 % mole for Co2Al-CO3 or Co2Fe-CO3, underlining the electrochemical oxidation of CoII to be more efficient in the CoIICoIII-CO3 structure. The resulting electrochemical behavior was scrutinized along with the structural evolution occurring upon electrochemical treatment by coupling analysis techniques. It appeared that, after the electrochemical oxidation, the mean amount of CoIII present in the LDH layer increased from 13 to 60 %. The net balance between half cycles, characteristic of the electrochemical efficiency, deviates from 1, resulting in an average value stabilized at 30 % of CoIII after reduction. This results in the usual ratio of 2 to 1 between divalent and trivalent cations in the LDH composition. Finally, the C o I I C o I I I -C O 3 s a m p l e d i s p l a y s a c h a r a c t e r i s t i c pseudocapacitance behavior with a high charge capacity of 1,490 F g−1 at 0.5 A g−1, determined by galvanostatic cycling. Similarly, TOR oxidation of CoII was also reported in the preparation of ZnCo-LDH, using hydrogen peroxide as oxidant [80, 81]. The existence of a mixed oxidation state of CoII/ CoIII is also found in these compounds with a ratio CoII/CoIII close to 1. ZnCo-LDH films exhibit large specific capacitance [81] and an efficient electrocatalytic activity for OER [80]. The overpotential for water oxidation is 100 mV lower than for monometallic Co materials (Co(OH)2 and Co3O4).


Conclusion This overview of recent results highlights the growing importance of LDH containing 3d transition metal cations as electrode modifiers with potential applications as electrocatalytic materials (alcohol oxidation [47], OER, (bio)sensors [5], and supercapacitors [1]. Redox processes into and onto the primary (layers stacking) and secondary (stacked aggregates) layered structures result in a limitation due to the slower process between electron hopping on localized redox centers (3d metal cations) coupled with the ion motion across pores, channels, and interlayer domains. Further work is still needed to fully understand and improve the charge transport within these materials. Some insights have been featured in this article, for instance, the 3D nanostructuration of LDH deposits on the surface electrode using the layer-by-layer method with delaminated nanoparticles of LDH or using hierarchical LDH microspheres. These 3D architectures with enhanced surface area and suitable mesopore distribution are beneficial to the mass transport of electrolyte and improve the faradic redox reaction. In order to enhance the electrochemical exposure, mixing composites from LDH with conductive materials (CnT, GnS, metal oxides, or conductive polymers) seems to be a promising route to reach high performance. The challenge is still to control the directional growth of LDH into these composite materials. Finally, the combination of transition metal-based LDH with intercalated redox molecules to form hybrid materials has been little exploited until now and it should open up promising applications in biosensors and biofuel cell development with the electrochemical regeneration of active sites of the enzymes [5] as well as in photovoltaic domain [82].

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