Vishal M. Dhavale

Porous‐Organic‐Framework‐Templated Nitrogen‐Rich Porous Carbon as a More Proficient Electrocatalyst than Pt/C for the Electrochemical Reduction of Oxygen

Porous nitrogen-rich carbon (POF-C-1000) that was synthesized by using a porous organic framework (POF) as a self-sacrificing host template in a nanocasting process possessed a high degree of graphitization in an ordered structural arrangement with large domains and well-ordered arrays of carbon sheets. POF-C-1000 exhibits favorable electrocatalytic activity for the oxygen-reduction reaction (ORR) with a clear positive shift of about 40 mV in the onset potential compared to that of a traditional, commercially available Pt/C catalyst. In addition, irrespective of its moderate surface area (785 m(2)  g(-1)), POF-C-1000 showed a reasonable H(2) adsorption of 1.6 wt % (77 K) and a CO(2) uptake of 3.5 mmol g(-1) (273 K).

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/cm(2) 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/cm(2) 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/cm(2). 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.

Activated nitrogen doped graphene shell towards electrochemical oxygen reduction reaction by its encapsulation on Au nanoparticle (Au@N-Gr) in water-in-oil “nanoreactors”

Encapsulation of nitrogen doped graphene on Au nanoparticle (Au@N-Gr) could be accomplished through a water-in-oil emulsion technique, where the emulsion droplets act as ‘nanoreactors’ and the redox reaction inside the droplets results in the formation of core–shell nanoparticles. The encapsulation of N-Gr on a small quantity of Au (N-Gr:Au wt ratio of 90:10) made the N-Gr layer more conductive and active towards electrochemical oxygen reduction reaction (ORR). The enhanced conductivity helped the system narrow down the ohmic overpotential, and direct electronic interactions between the Au and Gr layers brought in a favourable positive shift to the onset potential for ORR. Encapsulation has helped N-Gr reduce the overpotential by ∼121 mV as compared to N-Gr alone. Apart from this, the oxygen reduction kinetics of Au@N-Gr also appeared to be superior to N-Gr and Au nanoparticles as separate entities due to greater involvement of the preferred 4-electron reduction pathway. At −0.3 V (vs. Hg/HgO), the percentage of hydrogen peroxide (H2O2) (a product formed from the undesirable 2-electron reduction pathway) was found to be 16.5% for Au@Gr, where Au was covered with undoped Gr, which gets reduced to a significantly low level of 6.5% for Au@N-Gr. Au and N-Gr as separate entities give yield of H2O2 as 52.2 and 47.7%, respectively. From these, it can be concluded that the coverage of N-Gr on Au helps decrease the yield of H2O2 drastically apart from the benefits of synergistic interactions in reducing both ohmic and activation overpotentials.

Can enantiomer ligands produce structurally distinct homochiral MOFs

Here, we report a self-assembled homochiral metal–organic framework [Cu1.5(H2LL-leu)(Ac)H2O]n·3H2O (1) obtained from an L-leucine-derived ligand (H4LL-leu) and Cu(Ac)2·H2O in a 1:1 ratio. Coordination-induced conformational change in the ligand has been monitored by circular dichroism which has been further attested by synthesizing a D-leucine-containing enantiomer H4LD-leu and its Cu(II) complex [Cu1.5(H2LD-leu)H2O]n·10H2O (2). Structure determination revealed entirely different structures for the homochiral MOFs (1 and 2) obtained from the L/D-leucine-derived enantiomer ligands under analogous reaction conditions. Further, structural dissimilarity in these MOFs has been judicially supported by proton conductance studies. MOF 1 shows higher proton (10−5 S cm−1) conductance in comparison to 2 (10−6 S cm−1) due to dissimilar alignment of the hydrogen-bonded water molecules in the hydrophilic pocket as well as crystal packing.

Structure and dynamics of benzyl-NX3 (X = Me, Et) trifluoromethanesulfonate ionic liquids.

Ammonium-based benzyl-NX3 (X = methyl, ethyl) trifluoromethanesulfonate (TFA) ionic liquids (ILs) are low cost, nontoxic, thermally stable ion-conducting electrolytes in fuel cells and batteries. In the present study, we have characterized the structure and dynamics of these ILs using molecular dynamics (MD) simulations and ionic conductivity using electro-chemical impedance spectroscopy (EIS) at varying temperature and relative humidity (RH). Results from MD simulations predict that cation-cation and cation-anion interactions are stronger in benzyltrimethylammonium (BzTMA) compared to benzyltriethylammonium (BzTEA) that diminish with increase in RH. Further, the BzTMA cations show both C-H/Ph (center of mass of phenyl ring) and cation-Ph interactions whereas BzTEA cations show only strong cation-Ph interactions. The C-H/Ph interactions (ψ ≥ 90°, d(H-Ph) ≤ 4 Å, θ < 50° and d(C-Ph) ≤ 4.3 Å) in BzTMA cations increase with RH and are highest at RH = 90%. The cumulative impact of electrostatic, cation/Ph, and C-H/Ph interactions results in lower conductivity of BzTMA-TFA IL compared to BzTEA-TFA IL. The EIS measurements show that the trends in ionic conductivity of ILs at RH = 30 and 90% are qualitatively similar to the Nernst-Einstein conductivity from MD simulations. The ionic conductivity of BzTEA-TFA IL is ~3 times higher than BzTMA-TFA IL at 353 K and RH = 90%.

1000-fold enhancement in proton conductivity of a MOF using post-synthetically anchored proton transporters.

Pyridinol, a coordinating zwitter-ionic species serves as stoichiometrically loadable and non-leachable proton carrier. The partial replacement of the pyridinol by stronger hydrogen bonding, coordinating guest, ethylene glycol (EG), offers 1000-fold enhancement in conductivity (10−6 to 10−3 Scm−1) with record low activation energy (0.11 eV). Atomic modeling coupled with 13C-SSNMR provides insights into the potential proton conduction pathway functionalized with post-synthetically anchored dynamic proton transporting EG moieties.