Indrajit M. Patil

Three dimensional nanocomposite of reduced graphene oxide and hexagonal boron nitride as an efficient metal-free catalyst for oxygen electroreduction

The present work demonstrates a simple and inexpensive method for the synthesis of a reduced graphene oxide/boron nitride (rGO/BN) nanocomposite using a one step hydrothermal method followed by annealing at high temperature. The structural analysis confirms the formation of a homogeneous composite with coalescence of the graphitic layers of reduced graphene oxide and hexagonal boron nitride (h-BN), making an ideal situation for better oxygen adsorption followed by electroreduction. The morphology study also clearly indicates a uniform distribution of boron nitride particles at both sides of the stratified graphene oxide. Interestingly, the electrochemical study implies that the rGO/BN nanocomposite shows a substantially higher oxygen reduction reaction (ORR) activity with a single step nearly 4-electron transfer pathway and an improved onset potential of ∼0.8 V versus RHE in alkaline conditions. Though the onset potential is inferior to Pt based catalysts, it is much superior to previously reported carbon or h-BN based electrocatalysts. However, the present rGO/BN nanocomposite catalysts show higher stability than commercial Pt/C catalysts even after 10 000 cycles, and hence it could be a first report of ORR by metal-free h-BN based materials. Additionally, this composite catalyst does not have any methanol oxidation reactions that nullify the issues due to the fuel cross-over effect in direct methanol fuel cells.

Enhanced methanol electrooxidation at Pt skin@PdPt nanocrystals

Pt skin growth over PdPt alloy nanocrystals has been described using a simple wet chemical method, where a layer-by-layer epitaxial deposition of Pt on PdPt could be understood by the Stranski–Krastanov mechanism. Initial PdPt alloy nanocrystals grown in a simple wet-chemical method, in the presence of a reducing solvent like N-methyl pyrrolidone (NMP) and a stabilizer like polyvinyl pyrrolidone (PVP), have been used as the substrate for secondary growth of a Pt thin layer. Surface changes have been observed during step-by-step growth of polyhedral Pt skin@PdPt nanocrystals originating from nearly octahedral geometries of PdPt. The methanol electrooxidation activities of two different Pt skin@PdPt nanostructures have been compared with PdPt nanocrystals with similar compositions but without skin structures and commercial RuPt catalysts. A gain factor of 8 towards electrooxidation of methanol in acidic media with activities of 1950 mA mgPt−1 and 3.1 mA cmPt−2 (with lower onset potential compared to the RuPt commercial catalyst), which is believed to be much higher compared to that of previous reports and state-of-art RuPt/C catalysts, indicating better surface properties and core-alloy formation along with improved intraparticle active interfacial sites. Additionally, exciting results of electrooxidation of ethanol and ethylene glycol with 70% and 58% activity retention respectively, after 5000 cycles are also found, demonstrating a facile C–C breaking in such C2 type alcohols.

Carbon Nanotube/Boron Nitride Nanocomposite as a Significant Bifunctional Electrocatalyst for Oxygen Reduction and Oxygen Evolution Reactions

It is an immense challenge to develop bifunctional electrocatalysts for oxygen reduction reactions (ORR) and oxygen evolution reactions (OER) in low temperature fuel cells and rechargeable metal-air batteries. Herein, a simple and cost-effective approach is developed to prepare novel materials based on carbon nanotubes (CNTs) and a hexagonal boron nitride (h-BN) nanocomposite (CNT/BN) through a one-step hydrothermal method. The structural analysis and morphology study confirms the formation of a homogeneous composite and merging of few exfoliated graphene layers of CNTs on the graphitic planes of h-BN, respectively. Moreover, the electrochemical study implies that CNT/BN nanocomposite shows a significantly higher ORR activity with a single step 4-electron transfer pathway and an improved onset potential of +0.86 V versus RHE and a current density of 5.78 mA cm-2 in alkaline conditions. Interestingly, it exhibits appreciably better catalytic activity towards OER at low overpotential (η=0.38 V) under similar conditions. Moreover, this bifunctional catalyst shows substantially higher stability than a commercial Pt/C catalyst even after 5000 cycles. Additionally, this composite catalyst does not show any methanol oxidation reactions that nullify the issues due to fuel cross-over effects in direct methanol fuel cell applications.

Synergistically Enhanced Electrocatalytic Performance of an N-Doped Graphene Quantum Dot-Decorated 3D MoS2–Graphene Nanohybrid for Oxygen Reduction Reaction

Nitrogen-doped graphene quantum dots (N-GQDs) were decorated on a three-dimensional (3D) MoS2–reduced graphene oxide (rGO) framework via a facile hydrothermal method. The distribution of N-GQDs on the 3D MoS2–rGO framework was confirmed using X-ray photoelectron spectroscopy, energy dispersive X-ray elemental mapping, and high-resolution transmission electron microscopy techniques. The resultant 3D nanohybrid was successfully demonstrated as an efficient electrocatalyst toward the oxygen reduction reaction (ORR) under alkaline conditions. The chemical interaction between the electroactive N-GQDs and MoS2–rGO and the increased surface area and pore size of the N-GQDs/MoS2–rGO nanohybrid synergistically improved the ORR onset potential to +0.81 V vs reversible hydrogen electrode (RHE). Moreover, the N-GQDs/MoS2–rGO nanohybrid showed better ORR stability for up to 3000 cycles with negligible deviation in the half-wave potential (E1/2). Most importantly, the N-GQDs/MoS2–rGO nanohybrid exhibited a superior methanol tolerance ability even under a high concentration of methanol (3.0 M) in alkaline medium. Hence, the development of a low-cost metal-free graphene quantum dot-based 3D nanohybrid with high methanol tolerance may open up a novel strategy to design selective cathode electrocatalysts for direct methanol fuel cell applications.

Unusual enhancement in the electroreduction of oxygen by NiCoPt by surface tunability through potential cycling

Chemically ordered interconnected nanostructures of NiCoPt alloy have been prepared using a simple solvothermal process and studied for oxygen reduction reaction (ORR) kinetics. NiCoPt/C catalyst has demonstrated an interesting trend of enhancement in the ORR activity along with long-term durability. The specific activity of 0.744 mA cm−2 for NCP10/C (NiCoPt/C prepared at reaction time of 10 h) is ∼3.7 times higher than that of Pt/C (0.2 mA cm−2). The durability of the catalyst was evaluated over 30k potential cycles in the lifetime regime. More significantly, a novel trend in the enhancement in the ORR activity during stability cycles has been observed for the first time, where a remarkable enhancement of 82% in the specific activity has been observed after 30k potential cycles. Thus, ∼7-fold higher activity of NCP10/C@30k over initial activity of commercial Pt/C would make a tremendous impact on fuel cell technology. Systematic X-ray diffraction studies were performed to supplement subsequent improvement in the ORR activity during potential cycling, where structural changes due to alloying and de-alloying taking place with formation of tetrahexahedron-like surfaces after 15k cycles. Furthermore, transmission electron microscopy (TEM) analysis after 30k durability cycles reveals better stability of NCP10/C nanostructure signifying the retention of Ni and Co due to the chemically ordered structures of NiCoPt alloy catalyst. The observed enhancement in durability might be due to the ordered arrangement of Pt and Ni/Co within the alloy.

Mechanical activation in reduced graphite oxide/boron nitride nanocomposite electrocatalysts for significant improvement in dioxygen reduction

A nanocomposite of reduced graphite oxide (rGO) with hexagonal boron nitride (h-BN) is prepared using a simple hydrothermal method. Significant enhancement in the surface area of the composite is mainly due to the pre-mechanical activation of pristine GO. The structural and morphological study reveals the formation of a homogeneous nanocomposite and masking of rGO sheets over micron sized h-BN particles respectively. Interestingly, the as-synthesized GOBN2–BM composite (nanocomposite of 2 wt% h-BN with mechanically activated GO) catalyst exhibits significant oxygen electroreduction kinetics in terms of onset potential (Eonset = 0.89 V), half-wave potential (E1/2 = 0.74 V) and limiting current density (JL = 4.4 mA cm−2) with a single step ∼4-electron transfer pathway in alkaline medium. Though the composite catalysts exhibit higher overpotential (110 mV) than state-of-the-art Pt/C catalysts, they are much superior to previously reported carbon or h-BN based nanocomposite electrocatalysts. Importantly, the GOBN2–BM nanocomposite shows excellent tolerance towards both methanol oxidation and CO poisoning. Moreover, the nanocomposite catalysts show substantially higher stability than Pt/C catalysts even after 5000 cycles under similar conditions. Additionally, they show a better relative current stability (95% retention) than that of a Pt/C catalyst, signifying immense selectivity and durability towards oxygen electroreduction kinetics. The electrocatalytic oxygen reduction activity of the nanocomposite is mainly attributed to the high surface area (thanks to mechanical activation of GO, leading to increased pore distribution) as well as the synergistic mechanism between the h-BN and carbon network of rGO. Hence, it could be a potential cathode catalyst to replace precious-metal based catalysts in alkaline fuel cells.

B,N,S tri-doped reduced graphite oxide–cobalt oxide composite: a bifunctional electrocatalyst for enhanced oxygen reduction and oxygen evolution reactions

In the present study, we followed a unique approach to synthesize a nanocomposite of B,N,S tri-doped graphite oxide and cobalt oxide. Initially, B,N,S tri-doped carbon quantum dots were prepared by a hydrothermal method using boric acid and L-cysteine as precursors, and were further immobilized on graphite oxide in the presence of a cobalt precursor to synthesise a nanocomposite of cobalt oxide and B,N,S tri-doped graphite oxide. The crystal structure and morphology of the BNS/rGO–Co nanocomposite were studied by X-ray diffraction (XRD) and scanning electron microscopy (SEM) imaging, respectively. Electrochemical studies indicated a substantially higher electrocatalytic activity of the catalyst with an onset potential (Eonset) of 0.87 V vs. RHE and a current density (JL) of 4.4 mA cm−2 at 1600 rpm in alkaline conditions. Additionally, rotating ring disc electrode (RRDE) measurements confirmed a single step ∼4 electron transfer pathway, similar to that of Pt/C catalyst. Interestingly, the BNS/rGO–Co nanocomposite shows enhanced stability (up to 5000 cycles under similar conditions) and a high tolerance to methanol crossover effects, when compared to the state-of-the-art Pt/C catalyst. Concomitantly, the catalyst also exhibits remarkable oxygen evolution reaction activity. Such a remarkable electrocatalytic activity of the BNS/rGO–Co nanocomposite over its N,S-bi-doped counterpart is due to the importance of boron synergy with the N and S sites in the rGO, and also to the presence of the cobalt oxide interface for better conversion.