25 research outputs found
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<sub>2–<i>x</i></sub>Zr<sub><i>x</i></sub>O<sub>4</sub> 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 Zr<sup>4+</sup> 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/cm<sup>2</sup> 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/cm<sup>2</sup> 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/cm<sup>2</sup>, 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/cm<sup>2</sup> 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<sup>–2</sup> with a specific
capacity of 560 mAh g<sub>Zn</sub><sup>–1</sup> and energy
density of 728 Wh kg<sub>Zn</sub><sup>–1</sup> 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 m<sup>2</sup>/g compared to reduced graphene oxide (RGO,
290 m<sup>2</sup>/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 m<sup>2</sup>/g compared to PANI-PA (0.4 S/cm and 60 m<sup>2</sup>/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/cm<sup>2</sup>, an enhanced areal capacitance of 1.52 F/cm<sup>2</sup>, and a volumetric capacitance of 122 F/cm<sup>3</sup>. 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/cm<sup>2</sup>) the state-of-the-art 20 wt % Pt-supported carbon
catalyst (PtC) (5.4 ± 0.5 mA/cm<sup>2</sup>). 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/cm<sup>2</sup> and a specific capacity of ∼630
mAh g<sup>–1</sup> 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 Co<sub>3</sub>O<sub>4</sub> Nanocubes Supported on Nitrogen-Doped Graphene as an Electrocatalyst for Efficient Water Oxidation
Herein, we report a simple and scalable
synthesis of Co<sub>3</sub>O<sub>4</sub> nanocubes possessing exposed
low surface energy planes supported on nitrogen-doped graphene (Co<sub>3</sub>O<sub>4</sub>-NC/NGr) by a hydrothermal method as an efficient
electrocatalyst for water oxidation. Three different types of morphologies
of Co<sub>3</sub>O<sub>4</sub> (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 Co<sub>3</sub>O<sub>4</sub>-NC/NGr-12h) has shown the highest OER activity. The catalyst displayed
an overpotential (η) of ∼280 mV at 10 mA/cm<sup>2</sup> in 1 M KOH solution, which is lower than that of the other prepared
samples such as Co<sub>3</sub>O<sub>4</sub>-NC/NGr-3h (∼348
mV), Co<sub>3</sub>O<sub>4</sub>-NC/NGr-9h (∼356 mV), Co<sub>3</sub>O<sub>4</sub>-NC/NGr-24h (∼320 mV), Co<sub>3</sub>O<sub>4</sub>-NC/Gr-12h (∼300 mV) and Co<sub>3</sub>O<sub>4</sub> (∼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 Co<sub>3</sub>O<sub>4</sub> nanocubes (Co<sub>3</sub>O<sub>4</sub>-NC) and the importance of the doped nitrogen
in the carbon framework for the uniform dispersion and direct coupling
with Co<sub>3</sub>O<sub>4</sub>-NC have been examined. The controlled
interplay of the exposed crystal planes of Co<sub>3</sub>O<sub>4</sub> 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 Co<sub>3</sub>O<sub>4</sub> 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 (Co<sub>3</sub>O<sub>4</sub>) 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 Co<sub>3</sub>O<sub>4</sub> nanoparticles, viz., cubic, blunt edged cubic, and spherical,
by using a simple hydrothermal method. We found that the spherical
Co<sub>3</sub>O<sub>4</sub> nanoparticle supported NGr catalyst (Co<sub>3</sub>O<sub>4</sub>–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 Co<sub>3</sub>O<sub>4</sub>–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. Co<sub>3</sub>O<sub>4</sub>–SP/NGr-24h catalyst
has shown several hours of flat discharge profile at the discharge
rates of 10, 20, and 50 mA/cm<sup>2</sup> with a specific capacity
and energy density of ∼590 mAh/g<sub>–Zn</sub> and ∼840
Wh/kg<sub>–Zn</sub>, respectively, in the primary Zn–air
battery system. In conjunction, Co<sub>3</sub>O<sub>4</sub>–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/cm<sup>2</sup>. The present results, thus demonstrate that the proper combination
of the tuned morphology of Co<sub>3</sub>O<sub>4</sub> 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<sup>–1</sup>. 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<sup>–1</sup> as well as a high areal capacitance of 1.3
F cm<sup>–2</sup> at a current density of 10 A g<sup>–1</sup>. The device also exhibits high rate capability, delivering 1217
F g<sup>–1</sup> at a current density of 50 A g<sup>–1</sup> and a high energy density of 30 W h kg<sup>–1</sup> at a
power density of 2 kW kg<sup>–1</sup>
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
H<sub>2</sub> 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 N<sub>2</sub> adsorption isotherms
and charge–discharge curves. Among the carbons synthesized,
C-MOF-2 shows maximal surface area of 1378 m<sup>2</sup>/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 oligophenyleneÂvinylene
as donor moieties (P<sub>1</sub> and P<sub>2</sub>, respectively)
were synthesized using the direct (hetero) arylation (DHAP) polymerization
route. Nitrile groups were introduced at the vinylene linkage in one
copolymer (P<sub>2</sub>) 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.
P<sub>2</sub> 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 (P<sub>1</sub>). The two polymers
were evaluated as type III supercapacitor materials by preparing composite
electrodes with carbon nanotubes (CNTs) and employing 0.5 M H<sub>2</sub>SO<sub>4</sub> as the electrolyte. Their performance was compared
with that of PÂ(NDI2OD-T2) as a reference polymer. The polymer P<sub>2</sub> 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 P<sub>2</sub>, P<sub>1</sub> exhibits a specific
capacitance of 84 F/g, while the corresponding value for the reference
polymer PÂ(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 oligophenyleneÂvinylene
as donor moieties (P<sub>1</sub> and P<sub>2</sub>, respectively)
were synthesized using the direct (hetero) arylation (DHAP) polymerization
route. Nitrile groups were introduced at the vinylene linkage in one
copolymer (P<sub>2</sub>) 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.
P<sub>2</sub> 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 (P<sub>1</sub>). The two polymers
were evaluated as type III supercapacitor materials by preparing composite
electrodes with carbon nanotubes (CNTs) and employing 0.5 M H<sub>2</sub>SO<sub>4</sub> as the electrolyte. Their performance was compared
with that of PÂ(NDI2OD-T2) as a reference polymer. The polymer P<sub>2</sub> 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 P<sub>2</sub>, P<sub>1</sub> exhibits a specific
capacitance of 84 F/g, while the corresponding value for the reference
polymer PÂ(NDI2OD-T2) is 61 F/g under identical conditions