Facile synthesis of CNT interconnected PVP-ZIF-8 derived hierarchically porous Zn/N co-doped carbon frameworks for oxygen reduction.

In this work, a strategy has been adopted to construct an architecture through the coordination of polyvinylpyrrolidone (PVP) and a monodisperse zeolitic imidazolate framework (ZIF-8), which was entwined by carbon nanotubes (CNTs) firstly, followed by a pyrolysis process to obtain the hybrid catalyst. The meticulous design of the hybrid material using CNTs to interconnect the PVP assisted ZIF-8 derived porous carbon frameworks together produces a hierarchical pore structure and dual-heteroatom (Zn/N) doping (Zn-N/PC@CNT). Without further acid treatment, the hybrid material prepared after pyrolysis at 900 °C (PVP-ZIF-8@CNT-900) has been demonstrated as an efficient non-precious metal catalyst for the oxygen reduction reaction (ORR) with its superior stability compared to the commercial 20 wt% Pt/C catalyst in alkaline media. The catalyst shows better performance towards the ORR, with its more positive onset and half-wave potentials (Eonset = 0.960 V vs. RHE and E1/2 = 0.795 V vs. RHE) than the counterpart system which is free of both CNT and PVP. The high performance of the hybrid catalyst can be ascribed to the co-existence of dual-active sites with hierarchical pore structures originating from the synergistic effects between Zn/N co-doped porous carbon and CNTs. We further demonstrated the single-cell performance by using the homemade system as the cathode catalyst for the Alkaline Exchange Membrane Fuel Cell (AEMFC) system, which showed a maximum power density of 45 mW cm-2 compared to 60 mW cm-2 obtained from the 40 wt% Pt/C catalyst.

Enhanced electrocatalytic activity of PtRu/nitrogen and sulphur co-doped crumbled graphene in acid and alkaline media.

The low mass activity and high price of pure platinum (Pt)-based catalysts predominantly limit their large-scale utilization in electrocatalysis. Therefore, the reduction of Pt amount while preserving the electrocatalytic efficiency represents a viable alternative. In this work, we prepared new PtRu2 nanoparticles supported on sulphur and nitrogen co-doped crumbled graphene with trace amounts of iron (PtRu2/PF) electrocatalysts. The PtRu2/PF catalysts exhibited enhanced electrocatalytic performance and stability for the hydrogen evolution reaction (HER) at pH = 0. Moreover, the prepared PtRu2/PF electrocatalyst displayed higher HER activity than commercial 20% Pt/C. The PtRu2/PF catalyst achieved a current density of 10 mA cm-2 at an overpotential value of only 22 mV for HER, performing better activity than many other Pt-based electrocatalysts. Besides, the PtRu2/PF revealed a good performance for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) in alkaline media. The PtRu2/PF catalyst recorded a current density of 10 mA cm-2 at an overpotential of only 270 mV for OER in KOH (1.0 M) solution and an onset potential of 0.96 V vs. RHE (at 1 mA cm-2) for ORR in KOH (0.1 M) solution.

A 3-D nanoribbon-like Pt-free oxygen reduction reaction electrocatalyst derived from waste leather for anion exchange membrane fuel cells and zinc-air batteries.

Fe-Nx and Fe-S-based ORR electrocatalysts have emerged as rightful candidates to replace Pt in fuel cells to make the technology cheap and sustainable. Fe-N-C catalysts are generally prepared by the pyrolysis of conducting polymers, metal-organic frameworks, aerogels, etc., and the combination of multiple heteroatoms and metal precursors. These precursors are mostly expensive and their synthesis involves multiple steps. In this report, we have demonstrated the synthesis of a Fe-N-C catalyst from the waste leather obtained from the footwear and other leather-consuming industries. The pyrolysis of leather with FeCl3 (metal source) results in the formation of a highly thin and porous nano-ribbon like morphology. Waste leather acts as a cost-free single source of heteroatoms like N, S and carbon. The catalyst synthesized at a temperature of 900 °C shows an overpotential of 40 mV and better durability compared to the commercial Pt/C catalyst. The catalyst is demonstrated as the cathode for alkaline exchange membrane fuel cell (AEMFC) and zinc-air battery (ZAB) applications. In the AEMFC, a power density of 50 mW cm-2 and an OCV of 0.92 V are obtained whereas, in the ZAB, it exhibited a power density of 174 mW cm-2 compared to 160 mW cm-2 of the system based on the Pt/C catalyst.

NiCo2O4 nanoarray on CNT sponge: a bifunctional oxygen electrode material for rechargeable Zn-air batteries.

Ni- and Co-based materials have of late gained prominence over conventional noble metal-based ones as catalysts for energy devices. Here, a high performance catalyst which can facilitate both the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) was developed by anchoring a NiCo2O4 nanowire array on a carbon nanotube sponge (NCS). The three-dimensional morphology of NCS ensured efficient transport of the reactants and products on the catalyst surface, thereby improving the activity of the material. The prepared catalyst showed remarkable OER activity, requiring an overpotential of 280 mV, which is comparable to that of noble-metal catalysts. In addition to the noteworthy OER performance, the catalyst performed well with respect to the ORR. The total oxygen electrode activity overpotential of the catalyst was found to be 0.83 V, which is lower than that of commercial electrodes such as Pt/C and RuO2. A rechargeable Zn-air battery constructed with NCS had an open circuit voltage of 1.42 V, a maximum power density of 160 mW cm-2, and an energy density of 706 W h kg-1. NCS exhibited bifunctional electrocatalytic activity and high stability for both the OER and ORR, proving to be a good replacement for noble metal electrodes in rechargeable metal-air batteries.

Superprotonic Conductivity in Flexible Porous Covalent Organic Framework Membranes.

Poor mechanical stability of the polymer electrolyte membranes remains one of the bottlenecks towards improving the performance of the proton exchange membrane (PEM) fuel cells. The present work proposes a unique way to utilize crystalline covalent organic frameworks (COFs) as a self-standing, highly flexible membrane to further boost the mechanical stability of the material without compromising its innate structural characteristics. The as-synthesized p-toluene sulfonic acid loaded COF membranes (COFMs) show the highest proton conductivity (as high as 7.8×10-2  S cm-1 ) amongst all crystalline porous organic polymeric materials reported to date, and were tested under real PEM operating conditions to ascertain their practical utilization as proton exchange membranes. Attainment of 24 mW cm-2 power density, which is the highest among COFs and MOFs, highlights the possibility of using a COF membrane over the other state-of-the-art crystalline porous polymeric materials reported to date.

Nitrogen-doped graphene anchored with mixed growth patterns of CuPt alloy nanoparticles as a highly efficient and durable electrocatalyst for the oxygen reduction reaction in an alkaline medium.

A highly active and durable CuPt alloy catalyst with trigonal bipyramidal and truncated cube-type mixed morphologies, anchored on the nitrogen-doped graphene (NGr) surface (CuPt-TBTC/NGr), was prepared by a simple and fast method. The obtained CuPt alloy showed improved oxygen reduction reaction (ORR) activity, with a 30 mV positive shift in the half-wave potential value, as compared to the state-of-the-art Pt/C catalyst in a 0.1 M KOH solution. The CuPt alloy with the trigonal bipyramidal morphology possesses porous type inter-connected sides, which help to achieve improved mass transport of oxygen during the ORR. The exposure of the (111) plane of the CuPt alloy further improved the catalytic activity towards the dioxygen reduction in alkaline media. The ORR activity of the NGr-supported CuPt alloy was found to be dependent on the reaction time, and improved activity was obtained on the material derived at a reaction time of 90 min (CuPt-TBTC/NGr-90). The material synthesized at a lower or higher reaction time than 90 min resulted in a partially formed trigonal bipyramidal morphology with more truncated cubes or agglomerated trigonal bipyramidal and truncated cubes with closed type structures, respectively. Along with the high intrinsic ORR activity, CuPt-TBTC/NGr-90 displayed excellent electrochemical stability. Even after repeated 1000 potential cycling in a window ranging from 0.10 to 1.0 V (vs. RHE), the system clearly outperformed the state-of-the-art Pt/C catalyst with 15 and 60 mV positive shifts in the onset and half-wave potentials, respectively. CuPt-TBTC/NGr-90 also exhibited 2.1 times higher mass activity and 2.2 times higher specific activity, compared to Pt/C at 0.90 V (vs. RHE). Finally, a zinc-air battery fabricated with the alloy catalyst as the air electrode displayed a peak power density of 300 mW cm-2, which is much higher than the peak power density of 253 mW cm-2 obtained for the state-of-the-art Pt/C catalyst as the air electrode.

Pt- and TCO-Free Flexible Cathode for DSSC from Highly Conducting and Flexible PEDOT Paper Prepared via in Situ Interfacial Polymerization.

Here, we report the preparation of a flexible, free-standing, Pt- and TCO-free counter electrode in dye-sensitized solar cell (DSSC)-derived from polyethylenedioxythiophene (PEDOT)-impregnated cellulose paper. The synthetic strategy of making the thin flexible PEDOT paper is simple and scalable, which can be achieved via in situ polymerization all through a roll coating technique. The very low sheet resistance (4 Ω/□) obtained from a film of 40 µm thick PEDOT paper (PEDOT-p-5) is found to be superior to the conventional fluorine-doped tin oxide (FTO) substrate. The high conductivity (357 S/cm) displayed by PEDOT-p-5 is observed to be stable under ambient conditions as well as flexible and bending conditions. With all of these features in place, we could develop an efficient Pt- and TCO-free flexible counter electrode from PEDOT-p-5 for DSSC applications. The catalytic activity toward the tri-iodide reduction of the flexible electrode is analyzed by adopting various electrochemical methodologies. PEDOT-p-5 is found to display higher exchange current density (7.12 mA/cm(2)) and low charge transfer resistance (4.6 Ω) compared to the benchmark Pt-coated FTO glass (2.40 mA/cm(2) and 9.4 Ω, respectively). Further, a DSSC fabricated using PEDOT-p-5 as the counter electrode displays a comparable efficiency of 6.1% relative to 6.9% delivered by a system based on Pt/FTO as the counter electrode.

Carbon Nanohorn-Derived Graphene Nanotubes as a Platinum-Free Fuel Cell Cathode.

Current low-temperature fuel cell research mainly focuses on the development of efficient nonprecious electrocatalysts for the reduction of dioxygen molecule due to the reasons like exorbitant cost and scarcity of the current state-of-the-art Pt-based catalysts. As a potential alternative to such costly electrocatalysts, we report here the preparation of an efficient graphene nanotube based oxygen reduction electrocatalyst which has been derived from single walled nanohorns, comprising a thin layer of graphene nanotubes and encapsulated iron oxide nanoparticles (FeGNT). FeGNT shows a surface area of 750 m(2)/g, which is the highest ever reported among the metal encapsulated nanotubes. Moreover, the graphene protected iron oxide nanoparticles assist the system to attain efficient distribution of Fe-Nx and quaternary nitrogen based active reaction centers, which provides better activity and stability toward the oxygen reduction reaction (ORR) in acidic as well as alkaline conditions. Single cell performance of a proton exchange membrane fuel cell by using FeGNT as the cathode catalyst delivered a maximum power density of 200 mW cm(-2) with Nafion as the proton exchange membrane at 60 °C. The facile synthesis strategy with iron oxide encapsulated graphitic carbon morphology opens up a new horizon of hope toward developing Pt-free fuel cells and metal-air batteries along with its applicability in other energy conversion and storage devices.

Effect of B site coordination environment in the ORR activity in disordered brownmillerites Ba2In2-xCexO5+δ.

Ba2In2O5 brownmillerites in which the In site is progressively doped with Ce exhibit excellent oxygen reduction activity under alkaline conditions. Ce doping leads to structural changes advantageous for the reaction. Twenty-five percent doping retains the ordered structure of brownmillerite with alternate layers of tetrahedra and octahedra, whereas further increase in Ce concentration creates disorder. Structures with disordered oxygen atoms/vacancies are found to be better oxygen reduction reaction catalysts probably aided by isotropic ionic conduction, and Ba2In0.5Ce1.5O5+δ is the most active. This enhanced activity is correlated to the more symmetric Ce site coordination environment in this compound. Stoichiometric perovskite BaCeO3 with the highest concentration of Ce shows very poor activity emphasizing the importance of oxygen vacancies, which facilitate O2 adsorption, in tandem with catalytic sites in oxygen reduction reactions.

Nitrogen-induced surface area and conductivity modulation of carbon nanohorn and its function as an efficient metal-free oxygen reduction electrocatalyst for anion-exchange membrane fuel cells.

Nitrogen-doped carbon morphologies have been proven to be better alternatives to Pt in polymer-electrolyte membrane (PEM) fuel cells. However, efficient modulation of the active sites by the simultaneous escalation of the porosity and nitrogen doping, without affecting the intrinsic electrical conductivity, still remains to be solved. Here, a simple strategy is reported to solve this issue by treating single-walled carbon nanohorn (SWCNH) with urea at 800 °C. The resulting nitrogen-doped carbon nanohorn shows a high surface area of 1836 m2 g(-1) along with an increased electron conductivity, which are the pre-requisites of an electrocatalyst. The nitrogen-doped nanohorn annealed at 800 °C (N-800) also shows a high oxygen reduction activity (ORR). Because of the high weight percentage of pyridinic nitrogen coordination in N-800, the present catalyst shows a clear 4-electron reduction pathway at only 50 mV overpotential and 16 mV negative shift in the half-wave potential for ORR compared to Pt/C along with a high fuel selectivity and electrochemical stability. More importantly, a membrane electrode assembly (MEA) based on N-800 provides a maximum power density of 30 mW cm(-2) under anion-exchange membrane fuel cell (AEMFC) testing conditions. Thus, with its remarkable set of physical and electrochemical properties, this material has the potential to perform as an efficient Pt-free electrode for AEMFCs.

Enhanced catalytic activity of polyethylenedioxythiophene towards tri-iodide reduction in DSSCs via 1-dimensional alignment using hollow carbon nanofibers.

Here, we report a highly conducting 1-dimensionally (1-D) aligned polyethylenedioxythiophene (PEDOT) along the inner and outer surfaces of a hollow carbon nanofiber (CNF) and its application as a counter electrode in a dye sensitized solar cell (DSSC). The hybrid material (CP-25) displays a conversion efficiency of 7.16% compared to 7.30% for the standard Pt counter electrode, 4.48% for bulk PEDOT and 5.56% for CNF. The enhanced conversion efficiency of CP-25 is attributed to the accomplishment of high conductivity and surface area of PEDOT through the 1-D alignment compared to its bulk counterpart. Reduced charge transfer resistance and high conductivity of CP-25 could be proven by cyclic voltammetry, impedance analysis and Tafel experiments. Further, through a long-term stability test involving efficiency profiling for 20 days, it is observed that CP-25 possesses excellent durability compared to the bulk PEDOT.

Electrodeposited polyethylenedioxythiophene with infiltrated gel electrolyte interface: a close contest of an all-solid-state supercapacitor with its liquid-state counterpart.

We report the design of an all-solid-state supercapacitor, which has charge storage characteristics closely matching that of its liquid-state counterpart even under extreme temperature and humidity conditions. The prototype is made by electro-depositing polyethylenedioxythiophene (PEDOT) onto the individual carbon fibers of a porous carbon substrate followed by intercalating the matrix with polyvinyl alcohol-sulphuric acid (PVA-H2SO4) gel electrolyte. The electrodeposited layer of PEDOT maintained a flower-like growth pattern along the threads of each carbon fiber. This morphology and the alignment of PEDOT led to an enhanced surface area and electrical conductivity, and the pores in the system enabled effective intercalation of the polymer-gel electrolyte. Thus, the established electrode-electrolyte interface nearly mimics that of its counterpart based on the liquid electrolyte. Consequently, the solid device attained very low internal resistance (1.1 Ω cm(-2)) and a high specific capacitance (181 F g(-1)) for PEDOT at a discharge current density of 0.5 A g(-1). Even with a high areal capacitance of 836 mF cm(-2) and volumetric capacitance of 28 F cm(-3), the solid device retained a mass-specific capacitance of 111 F g(-1) for PEDOT. This is in close agreement with the value displayed by the corresponding liquid-state system (112 F g(-1)), which was fabricated by replacing the gel electrolyte with 0.5 M H2SO4. The device also showed excellent charge-discharge stability for 12 000 cycles at 5 A g(-1). The performance of the device was consistent even under wide-ranging humidity (30-80%) and temperature (-10 to 80 °C) conditions. Finally, a device fabricated by increasing the electrode area four times was used to light an LED, which validated the scalability of the process.

Design of a high performance thin all-solid-state supercapacitor mimicking the active interface of its liquid-state counterpart.

Here we report an all-solid-state supercapacitor (ASSP) which closely mimics the electrode-electrolyte interface of its liquid-state counterpart by impregnating polyaniline (PANI)-coated carbon paper with polyvinyl alcohol-H2SO4 (PVA-H2SO4) gel/plasticized polymer electrolyte. The well penetrated PVA-H2SO4 network along the porous carbon matrix essentially enhanced the electrode-electrolyte interface of the resulting device with a very low equivalent series resistance (ESR) of 1 Ω/cm(2) and established an interfacial structure very similar to a liquid electrolyte. The designed interface of the device was confirmed by cross-sectional elemental mapping and scanning electron microscopy (SEM) images. The PANI in the device displayed a specific capacitance of 647 F/g with an areal capacitance of 1 F/cm(2) at 0.5 A/g and a capacitance retention of 62% at 20 A/g. The above values are the highest among those reported for any solid-state-supercapacitor. The whole device, including the electrolyte, shows a capacitance of 12 F/g with a significantly low leakage current of 16 µA(2). Apart from this, the device showed excellent stability for 10000 cycles with a coulombic efficiency of 100%. Energy density of the PANI in the device is 14.3 Wh/kg.

Redox-Mediated Synthesis of Functionalised Graphene: A Strategy towards 2D Multifunctional Electrocatalysts for Energy Conversion Applications.

A simple, one-step synthetic route for developing a two-dimensional multifunctional electrocatalyst is reported, by the functionalisation of graphene using oxidised ethylenedioxythiophene (O-EDOT). The mutually assisted redox reaction between graphene oxide (GO) and EDOT facilitates the reduction of GO to graphene with a concomitant deposition of O-EDOT on the surface of the graphene. The oxidised surface of GO catalyses the reaction without using an added reducing agent, so a controlled and uniform deposition of O-EDOT is ensured on the surface of graphene, which essentially prevents the restacking of the layers. UV/Visible, IR, Raman and X-ray photoelectron spectroscopy give valid evidence for the reduction and functionalisation of graphene sheets. The functional groups present on the surface of graphene are found to tune the physical and chemical properties of graphene. Consequently, the functionalised material displays enhanced electrocatalytic activity for the reduction of oxygen to water and I3- to I- relative to pristine graphene. These distinct property characteristics make the material a versatile cathode electrocatalyst for both alkaline anion-exchange membrane fuel cells and dye-sensitised solar cells.