Zirconium-Substituted Cobalt Ferrite Nanoparticle Supported N‑doped Reduced Graphene Oxide as an Efficient Bifunctional Electrocatalyst for Rechargeable Zn–Air Battery

Solvothermal synthesis of zirconium-substituted cobalt ferrite nanoparticles was accomplished by the introduction of zirconium (Zr) in the spinel matrix to obtain a cost-effective and robust electrocatalyst that does not use noble metals. A variation in the cobalt ferrite structure CoFe_2–<i>x</i>Zr_<i>x</i>O₄ with Zr (0.1–0.4) substitution has significantly altered the overpotential for the electrocatalytic oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), leading to an optimum composition of CFZr(0.3). The incorporation of the foreign Zr⁴⁺ ion in the cobalt ferrite spinel lattices has effectively enhanced the oxygen evolution reaction (OER) activity in comparison to the parent cobalt ferrite (CF) nanocrystals. However, a nominal change in the ORR current density has been observed due to Zr incorporation. For the OER, the Zr-substituted catalyst has shown a 40 mV negative shift in the overpotential in comparison with the CF nanoparticles at 10 mA/cm² current density. Interestingly, the in situ grafting of Zr-substituted cobalt ferrite nanoparticles over N-doped reduced graphene oxide (CFZr(0.3)/N-rGO) results in remarkably enhanced performance during the ORR and moderately favored the OER with an overall potential difference (Δ<i><i>E</i></i>) of 0.840 V. The enhanced bifunctional electrocatalytic activity of the material is crucial for the fabrication of high-performance rechargeable Zn–air batteries (ZABs). The prepared catalyst exhibited an overpotential of 80 mV for the ORR in comparison with the state-of-the-art (Pt/C) catalyst and an overpotential of 340 mV at 10 mA/cm² current density for the OER from the standard value (1.23 V vs RHE). This potential bifunctional electrocatalyst has been employed as an electrode material for the fabrication of a primary ZAB, where it exhibited discharge capacities of 727 and 730 mAh/g at current densities of 20 and 30 mA/cm², respectively, under ambient conditions. The notable performance of the catalyst as a bifunctional material is observed during the cycling of the rechargeable ZAB. The prepared catalyst showed an increase of 200 mV in the overall operating overpotential after cycling for 10 cycles at 15 mA/cm² in comparison to the 350 mV increase shown by the Pt/C catalyst

Cu–Pt Nanocage with 3‑D Electrocatalytic Surface as an Efficient Oxygen Reduction Electrocatalyst for a Primary Zn–Air Battery

Cu–Pt nanocage (CuPt-NC) intermetallic structures have been prepared by an in situ galvanic displacement reaction. The structures are found to be well organized within the framework demarcated with distinguishing arms, having clear edges and corners with a size of ∼20 nm. The unique nanocage structure possessing large specific surface area and better structural integrity helps to achieve improved electrochemical oxygen reduction reaction activity and stability in alkaline solution in comparison to the commercially available 20 wt % Pt/C. CuPt-NC shows 50 mV positive onset potential shift with significantly higher limiting current in comparison to Pt/C. Interestingly, CuPt-NC has shown 2.9- and 2.5-fold improved mass activity and specific activity, respectively, for ORR at 0.9 V vs RHE in comparison to Pt/C. Moreover, the stability of CuPt-NC has been tested by an accelerated durability test under alkaline conditions. CuPt-NC has been subsequently utilized as the air electrode in a primary Zn–air battery and is found to possess 1.30- and 1.34-fold improved power density and current density at 1 V, respectively, in comparison to the state-of-the-art Pt/C catalyst. In addition, CuPt-NC has shown several hours of constant discharge stability at 20 mA cm^–2 with a specific capacity of 560 mAh g_Zn^–1 and energy density of 728 Wh kg_Zn^–1 in the primary Zn–air battery system

3D Polyaniline Porous Layer Anchored Pillared Graphene Sheets: Enhanced Interface Joined with High Conductivity for Better Charge Storage Applications

Here, we report synthesis of a 3-dimensional (3D) porous polyaniline (PANI) anchored on pillared graphene (G-PANI-PA) as an efficient charge storage material for supercapacitor applications. Benzoic acid (BA) anchored graphene, having spatially separated graphene layers (G-Bz-COOH), was used as a structure controlling support whereas 3D PANI growth has been achieved by a simple chemical oxidation of aniline in the presence of phytic acid (PA). The BA groups on G-Bz-COOH play a critical role in preventing the restacking of graphene to achieve a high surface area of 472 m²/g compared to reduced graphene oxide (RGO, 290 m²/g). The carboxylic acid (−COOH) group controls the rate of polymerization to achieve a compact polymer structure with micropores whereas the chelating nature of PA plays a crucial role to achieve the 3D growth pattern of PANI. This type of controlled interplay helps G-PANI-PA to achieve a high conductivity of 3.74 S/cm all the while maintaining a high surface area of 330 m²/g compared to PANI-PA (0.4 S/cm and 60 m²/g). G-PANI-PA thus conceives the characteristics required for facile charge mobility during fast charge–discharge cycles, which results in a high specific capacitance of 652 F/g for the composite. Owing to the high surface area along with high conductivity, G-PANI-PA displays a stable specific capacitance of 547 F/g even with a high mass loading of 3 mg/cm², an enhanced areal capacitance of 1.52 F/cm², and a volumetric capacitance of 122 F/cm³. The reduced charge-transfer resistance (RCT) of 0.67 Ω displayed by G-PANI-PA compared to pure PANI (0.79 Ω) stands out as valid evidence of the improved charge mobility achieved by the system by growing the 3D PANI layer along the spatially separated layers of the graphene sheets. The low RCT helps the system to display capacitance retention as high as 65% even under a high current dragging condition of 10 A/g. High charge/discharge rates and good cycling stability are the other highlights of the supercapacitor system derived from this composite material

Cobalt Ferrite Bearing Nitrogen-Doped Reduced Graphene Oxide Layers Spatially Separated with Microporous Carbon as Efficient Oxygen Reduction Electrocatalyst

The present work discloses how high-quality dispersion of fine particles of cobalt ferrite (CF) could be attained on nitrogen-doped reduced graphene oxide (CF/N-rGO) and how this material in association with a microporous carbon phase could deliver significantly enhanced activity toward electrochemical oxygen reduction reaction (ORR). Our study indicates that the microporous carbon phase plays a critical role in spatially separating the layers of CF/N-rGO and in creating a favorable atmosphere to ensure the seamless distribution of the reactants to the active sites located on CF/N-rGO. In terms of the ORR current density, the heat-treated hybrid catalyst at 150 °C (CF/N-rGO-150) is found to be clearly outperforming (7.4 ± 0.5 mA/cm²) the state-of-the-art 20 wt % Pt-supported carbon catalyst (PtC) (5.4 ± 0.5 mA/cm²). The mass activity and stability of CF-N-rGO-150 are distinctly superior to PtC even after 5000 electrochemical cycles. As a realistic system level exploration of the catalyst, testing of a primary zinc–air battery could be demonstrated using CF/N-rGO-150 as the cathode catalyst. The battery is giving a galvanostatic discharge time of 15 h at a discharge current density of 20 mA/cm² and a specific capacity of ∼630 mAh g^–1 in 6 M KOH by using a Zn foil as the anode. Distinctly, the battery performance of this system is found to be superior to that of PtC in less concentrated KOH solution as the electrolyte

Low Surface Energy Plane Exposed Co₃O₄ Nanocubes Supported on Nitrogen-Doped Graphene as an Electrocatalyst for Efficient Water Oxidation

Herein, we report a simple and scalable synthesis of Co₃O₄ nanocubes possessing exposed low surface energy planes supported on nitrogen-doped graphene (Co₃O₄-NC/NGr) by a hydrothermal method as an efficient electrocatalyst for water oxidation. Three different types of morphologies of Co₃O₄ (i.e., nanocubes, blunt edge nanocubes and spherical particles) have been synthesized by systematically varying the reaction time. Subsequently, their catalytic activity toward oxygen evolution reaction (OER) has been screened in alkaline medium. Among the three different morphologies, the intermediate architecture (i.e., the blunt edged nanocubes designated as Co₃O₄-NC/NGr-12h) has shown the highest OER activity. The catalyst displayed an overpotential (η) of ∼280 mV at 10 mA/cm² in 1 M KOH solution, which is lower than that of the other prepared samples such as Co₃O₄-NC/NGr-3h (∼348 mV), Co₃O₄-NC/NGr-9h (∼356 mV), Co₃O₄-NC/NGr-24h (∼320 mV), Co₃O₄-NC/Gr-12h (∼300 mV) and Co₃O₄ (∼310 mV). Along with that, the electrochemical stability of the catalyst is also found to be remarkably good. The role of the low index planes of Co₃O₄ nanocubes (Co₃O₄-NC) and the importance of the doped nitrogen in the carbon framework for the uniform dispersion and direct coupling with Co₃O₄-NC have been examined. The controlled interplay of the exposed crystal planes of Co₃O₄ and its dispersion and synergistic interaction with the nitrogen-doped graphene are found to be the decisive factors in bringing in the modulated OER activity of the system

Surface-Tuned Co₃O₄ Nanoparticles Dispersed on Nitrogen-Doped Graphene as an Efficient Cathode Electrocatalyst for Mechanical Rechargeable Zinc–Air Battery Application

The most vital component of the fuel cells and metal–air batteries is the electrocatalyst, which can facilitate the oxygen reduction reaction (ORR) at a significantly reduced overpotential. The present work deals with the development of surface-tuned cobalt oxide (Co₃O₄) nanoparticles dispersed on nitrogen-doped graphene as a potential ORR electrocatalyst possessing some unique advantages. The thermally reduced nitrogen-doped graphene (NGr) was decorated with three different morphologies of Co₃O₄ nanoparticles, viz., cubic, blunt edged cubic, and spherical, by using a simple hydrothermal method. We found that the spherical Co₃O₄ nanoparticle supported NGr catalyst (Co₃O₄–SP/NGr-24h) has acquired a significant activity makeover to display the ORR activity closely matching with the state-of-the-art Pt supported carbon (PtC) catalyst in alkaline medium. Subsequently, the Co₃O₄–SP/NGr-24h catalyst has been utilized as the air electrode in a Zn–air battery, which was found to show comparable performance to the system derived from PtC. Co₃O₄–SP/NGr-24h catalyst has shown several hours of flat discharge profile at the discharge rates of 10, 20, and 50 mA/cm² with a specific capacity and energy density of ∼590 mAh/g_–Zn and ∼840 Wh/kg_–Zn, respectively, in the primary Zn–air battery system. In conjunction, Co₃O₄–SP/NGr-24h has outperformed as an air electrode in mechanical rechargeable Zn–air battery as well, which has shown consistent flat discharge profile with minimal voltage loss at a discharge rate of 50 mA/cm². The present results, thus demonstrate that the proper combination of the tuned morphology of Co₃O₄ with NGr will be a promising and inexpensive material for efficient and ecofriendly cathodes for Zn–air batteries

Realizing High Capacitance and Rate Capability in Polyaniline by Enhancing the Electrochemical Surface Area through Induction of Superhydrophilicity

Polyaniline (PANI) as a pseudocapacitive material has very high theoretical capacitance of 2000 F g^–1. However, its practical capacitance has been limited by low electrochemical surface area (ESA) and unfavorable wettability toward aqueous electrolytes. This work deals with a strategy wherein the high ESA of PANI has been achieved by the induction of superhydrophilicity together with the alignment of PANI exclusively on the surface of carbon fibers as a thin layer to form a hybrid assembly. Superhydrophilicity is induced by electrochemical functionalization of the Toray carbon paper, which further induces superhydrophilicity to the electrodeposited PANI layer on the paper, thereby ensuring a high electrode–electrolyte interface. The Toray paper is electrochemically functionalized by the anodization method, which generates a highly active electrochemical surface as well as greater wettability (superhydrophilic) of the carbon fibers. Because of the strong interaction of anilinium chloride with the hydrophilic carbon surface, PANI is polymerized exclusively over the surface of the fibers without any appreciable aggregation or agglomeration of the polymer. The PANI–Toray paper assembly in the solid-state prototype supercapacitor can provide a high gravimetric capacitance of 1335 F g^–1 as well as a high areal capacitance of 1.3 F cm^–2 at a current density of 10 A g^–1. The device also exhibits high rate capability, delivering 1217 F g^–1 at a current density of 50 A g^–1 and a high energy density of 30 W h kg^–1 at a power density of 2 kW kg^–1

Porous Carbons from Nonporous MOFs: Influence of Ligand Characteristics on Intrinsic Properties of End Carbon

Synthesis of porous carbons on direct carbonization of nonporous Zn-based MOFs has been achieved without using any additional carbon precursor. The effect of ligand nature on the resulting carbon porosity has been studied systematically using the MOFs synthesized from rigid and flexible ligands. The linear relations between Zn/C ratio of the MOF versus surface area of the resulting carbon, microporosity versus H₂ uptake achieved in these carbons, and surface area versus specific capacitance of the end carbons have been verified from the gas adsorption, molecular composition, and electrochemical studies, respectively. Cyclic voltammetry and charge–discharge cycling have been carried out to study the capacitive behavior of the carbons. The interdependence of capacitive behavior on the surface area has been analyzed using data derived from N₂ adsorption isotherms and charge–discharge curves. Among the carbons synthesized, C-MOF-2 shows maximal surface area of 1378 m²/g with a specific capacitance of 170 F/g at 1 A/g

Naphthalene Diimide Copolymers by Direct Arylation Polycondensation as Highly Stable Supercapacitor Electrode Materials

Conjugated donor–acceptor copolymers based on naphthalene diimide (NDI) as acceptor and thiophene-terminated oligophenylene­vinylene as donor moieties (P₁ and P₂, respectively) were synthesized using the direct (hetero) arylation (DHAP) polymerization route. Nitrile groups were introduced at the vinylene linkage in one copolymer (P₂) to fine-tune its electrochemical properties. Both polymers show π–π* transition in the 300–480 nm region and intramolecular charge transfer (ICT) from thiophene to NDI in the 500–800 nm region in the absorption spectra. P₂ exhibits a blue-shifted intramolecular charge transfer (ICT) band in the absorption spectrum as well as a lower reduction potential in the cyclic voltammogram compared to the analogous polymer without the nitrile substitution (P₁). The two polymers were evaluated as type III supercapacitor materials by preparing composite electrodes with carbon nanotubes (CNTs) and employing 0.5 M H₂SO₄ as the electrolyte. Their performance was compared with that of P­(NDI2OD-T2) as a reference polymer. The polymer P₂ based supercapacitor exhibits a specific capacitance of 124 F/g with excellent stability up to 5000 cycles with almost 100% retention of the initial capacitance in the potential window of −0.7 to 0.5 V. Compared to P₂, P₁ exhibits a specific capacitance of 84 F/g, while the corresponding value for the reference polymer P­(NDI2OD-T2) is 61 F/g under identical conditions

Naphthalene Diimide Copolymers by Direct Arylation Polycondensation as Highly Stable Supercapacitor Electrode Materials

Conjugated donor–acceptor copolymers based on naphthalene diimide (NDI) as acceptor and thiophene-terminated oligophenylene­vinylene as donor moieties (P₁ and P₂, respectively) were synthesized using the direct (hetero) arylation (DHAP) polymerization route. Nitrile groups were introduced at the vinylene linkage in one copolymer (P₂) to fine-tune its electrochemical properties. Both polymers show π–π* transition in the 300–480 nm region and intramolecular charge transfer (ICT) from thiophene to NDI in the 500–800 nm region in the absorption spectra. P₂ exhibits a blue-shifted intramolecular charge transfer (ICT) band in the absorption spectrum as well as a lower reduction potential in the cyclic voltammogram compared to the analogous polymer without the nitrile substitution (P₁). The two polymers were evaluated as type III supercapacitor materials by preparing composite electrodes with carbon nanotubes (CNTs) and employing 0.5 M H₂SO₄ as the electrolyte. Their performance was compared with that of P­(NDI2OD-T2) as a reference polymer. The polymer P₂ based supercapacitor exhibits a specific capacitance of 124 F/g with excellent stability up to 5000 cycles with almost 100% retention of the initial capacitance in the potential window of −0.7 to 0.5 V. Compared to P₂, P₁ exhibits a specific capacitance of 84 F/g, while the corresponding value for the reference polymer P­(NDI2OD-T2) is 61 F/g under identical conditions