Kiran Pal Singh

Fe–P: A New Class of Electroactive Catalyst for Oxygen Reduction Reaction

It has been long thought that Fe-N-C structure, where Fe is bonded with an electronegative heteroatom N, plays a key role as nonprecious metal catalyst for oxygen reduction reaction (ORR) in fuel cells. However, electrocatalytic activity of Fe bonded with electropositive heteroatom P has never been considered for ORR. Herein we report the electrocatalytic activity for ORR of new Fe-P-C.

A new class of electroactive Fe- and P-functionalized graphene for oxygen reduction

While metal and electronegative N-containing carbon has aroused great interest as an efficient catalyst towards the oxygen reduction reaction (ORR), no combination of metal with other heteroatom-containing carbon has received considerable attention. This has motivated us to explore the performance of carbon functionalized with metal and electropositive phosphorous. Herein, we present the first report on the synthesis of a new class of electroactive Fe- and P-functionalized graphene (GPFe) and its electrocatalytic properties in alkaline and acidic media. The introduction of Fe causes remarkable synergistic effects on P-doped reduced graphene oxide by increasing surface area, enhancing the P doping level due to the interaction between Fe and P and generating electrochemically active Fe–P species. N-oxides are known to be in-active for ORR in Fe–N systems, whereas in present Fe–P systems, oxides of Fe and P are found to be beneficial for ORR. Interestingly, after the introduction of Fe, mostly inactive P-doped carbon becomes active in acidic medium. We propose that this study will surely provide renewed insights into active sites for ORR in metal and heteroatom-doped carbon systems.

Iodine-treated heteroatom-doped carbon: conductivity driven electrocatalytic activity

A high conductivity and surface area are the most highly desired properties of an electrocatalyst. Herein, we report a novel technique to synthesize highly conductive and microporous N and S-doped carbon from polyaniline (PANI) via a simple, template-free hydrothermal method followed by carbonization in the presence of iodine. The iodine treatment removes a large amount of the attached oxygen atoms and other heteroatoms and, as a consequence, increases the carbon content. Thus, the iodine treatment decreases the doping of catalytically active heteroatoms, which is unfavourable for the ORR, but at the same time, significantly increases the electrical conductivity, which is beneficial for the ORR. In particular, iodine-treated carbonized PANI (CPANI) shows an exceptionally high conductivity i.e., about 3 times that of untreated CPANI. Iodine treatment is also found to enhance the micropore surface area of the PANI during carbonization without using a harmful activating agent or a hard template. An electrocatalytic study indicates that the activity of the iodine-treated sample is considerably higher than that of an untreated sample. This remarkable upsurge in activity is mainly attributed to the large increase in the conductivity and surface area of the iodine-treated sample. The ORR activity is discussed in terms of the heteroatom content, surface area and conductivity of the carbon. This convenient, innovative approach can offer new possibilities for the design of future highly efficient fuel cell electrocatalysts.

Iron–polypyrrole electrocatalyst with remarkable activity and stability for ORR in both alkaline and acidic conditions: a comprehensive assessment of catalyst preparation sequence

A new facile template-free method is presented to synthesize Fe-treated N-doped carbon (Fe/N–C) catalysts for oxygen reduction reaction (ORR) by employing a synthesis protocol of pyrolysis–leaching–stabilization (PLS) sequence of polypyrrole in the presence of ferric source, which serves dual purposes of an oxidant for pyrrole polymerization and an iron source. Each step in the PLS sequence is assessed in detail in terms of the related structural properties of the resulting carbon catalysts, and their effects on ORR activities are elaborated to confirm the validity of the current synthesis protocol. It is found that the as-prepared carbon catalyst exhibits outstanding high catalytic activity in both alkaline and acidic conditions. The carbon catalyst prepared at a pyrolysis temperature of 900 °C (FePPyC-900) shows remarkably high ORR activity with onset potential of 0.96 V (vs. RHE), which is similar to that of Pt/C, whereas the half-wave potential (E1/2) of FePPyC-900 is 0.877 V, more positive than that of Pt/C at the same catalyst loading amount under alkaline conditions. Furthermore, the FePPyC-900 catalyst also illustrates exceptionally high activity under acidic conditions with onset and half-wave potentials of 0.814 and 0.740 V, respectively, which are almost comparable to those (0.817 and 0.709 V) of the state-of-the-art Pt/C catalyst, which is rarely observed for non-Pt-based carbon catalysts. In addition, the FePPyC-900 catalyst displays much better stability and methanol tolerance than the Pt/C and exhibits a four electron transfer pathway under both alkaline and acidic conditions. Such extraordinary high ORR activity and stability of the FePPyC-samples can be attributed to the implementation of extra stabilization step in addition to conventional sample preparation steps of pyrolysis and subsequent leaching in current PLS synthesis protocol as well as to the use of highly conducting PPy as a single precursor of carbon and nitrogen in the presence of Fe.

Displacive-type ferroelectricity from magnetic correlations within spin-chain

Observation of ferroelectricity among non-d0 systems, which was believed for a long time an unrealistic concept, led to various proposals for the mechanisms to explain the same (i.e. magnetically induced ferroelectricity) during last decade. Here, we provide support for ferroelectricity of a displacive-type possibly involving magnetic ions due to short-range magnetic correlations within a spin-chain, through the demonstration of magnetoelectric coupling in a Haldane spin-chain compound Er2BaNiO5 well above its Néel temperature of (TN = ) 32 K. There is a distinct evidence for electric polarization setting in near 60 K around which there is an evidence for short-range magnetic correlations from other experimental methods. Raman studies also establish a softening of phonon modes in the same temperature (T) range and T-dependent x-ray diffraction (XRD) patterns also reveal lattice parameters anomalies. Density-functional theory based calculations establish a displacive component (similar to d0-ness) as the root-cause of ferroelectricity from (magnetic) NiO6 chain, thereby offering a new route to search for similar materials near room temperature to enable applications.

Active sites and factors influencing them for efficient oxygen reduction reaction in metal-N coordinated pyrolyzed and non-pyrolyzed catalysts: a review

With increasing demand for clean energy and approaching commercialization of polymer electrolyte membrane fuel cells (PEMFCs), replacing expensive Pt-based cathode catalysts with much cheaper non-precious metal (NPM) catalysts has become absolutely essential. This review highlights the parameters that have been considered vital to improving the overall performance of the NPM-based catalysts for oxygen reduction reaction (ORR). In the present review, we focus on well-known catalytic systems in three categories of NPM catalysts, i.e. biomimetic heme–copper oxidase enzymes, non-pyrolyzed/polymeric systems, and pyrolyzed NPM–nitrogen-doped carbon (M–N/C) (M = Fe, Ni, Co, etc.) catalysts. The ORR mechanism on the reported active sites and the effect of varying their local environments are considered and discussed in detail. Among all the catalysts, only pyrolyzed M–N/C catalysts have shown activity and stability much closer to that of the state-of-the-art commercial carbon-supported platinum (Pt/C) catalyst. Although great heights have been climbed in pyrolyzed M–N/C-based catalysts, still general consensuses need to be established regarding the active sites in the NMP-based M–N/C catalysts to help enhance the activity and stability of the catalytic system. By comparing the ORR mechanisms of the three studied systems, various similarities between the active sites are identified and reported comprehensively. On the basis of the information amassed, some future directions for improving the activity, selectivity, and durability of the NPM-based catalysts are also discussed.

Simple approach to advanced binder-free nitrogen-doped graphene electrode for lithium batteries

A simple binder-free synthesis approach of just rubbing nitrogen-doped reduced graphene oxide (N-RGO) powder on a mechanically grinded Cu-foil substrate with a rough surface is proposed for a lithium ion battery (LIB). The nitrogen content of N-RGO is found to be 2.1 wt%. The binder-free N-RGO electrode shows excellent reversible capacity of 551 mA h g−1 as compared to 433 mA h g−1 of the binder-added N-RGO electrode at a current density of 50 mA g−1 after 100 cycles. The process is not only highly reproducible and successful, but also results in high LIB performance, proposing easy scaling-up of such an electrode for commercial application.

Effect of pristine graphene incorporation on charge storage mechanism of three-dimensional graphene oxide: superior energy and power density retention

In the race of gaining higher energy density, carbon’s capacity to retain power density is generally lost due to defect incorporation and resistance increment in carbon electrode. Herein, a relationship between charge carrier density/charge movement and supercapacitance performance is established. For this purpose we have incorporated the most defect-free pristine graphene into defective/sacrificial graphene oxide. A unique co-solvent-based technique is applied to get a homogeneous suspension of single to bi-layer graphene and graphene oxide. This suspension is then transformed into a 3D composite structure of pristine graphene sheets (GSs) and defective N-doped reduced graphene oxide (N-RGO), which is the first stable and homogenous 3D composite between GS and RGO to the best of our knowledge. It is found that incorporation of pristine graphene can drastically decrease defect density and thus decrease relaxation time due to improved associations between electrons in GS and ions in electrolyte. Furthermore, N doping is implemented selectively only on RGO and such doping is shown to improve the charge carrier density of the composite, which eventually improves the energy density. After all, the novel 3D composite structure of N-RGO and GS greatly improves energy and power density even at high current density (20 A/g).

Urine to highly porous heteroatom-doped carbons for supercapacitor: A value added journey for human waste

Obtaining functionalized carbonaceous materials, with well-developed pores and doped heteroatoms, from waste precursors using environmentally friendly processes has always been of great interest. Herein, a simple template-free approach is devised to obtain porous and heteroatom-doped carbon, by using the most abundant human waste, “urine”. Removal of inherent mineral salts from the urine carbon (URC) makes it to possess large quantity of pores. Synergetic effect of the heteroatom doping and surface properties of the URC is exploited by carrying out energy storage application for the first time. Suitable heteroatom content and porous structure can enhance the pseudo-capacitance and electric double layer capacitance, eventually generating superior capacitance from the URC. The optimal carbon electrode obtained particularly at 900 °C (URC-900) possesses high BET surface area (1040.5 m2g−1), good conductivity, and efficient heteroatom doping of N, S, and P, illustrating high specific capacitance of 166 Fg−1 at 0.5 Ag−1 for three-electrode system in inorganic electrolyte. Moreover, the URC-900 delivers outstanding cycling stability with only 1.7% capacitance decay over 5,000 cycles at 5 Ag−1. Present work suggests an economical approach based on easily available raw waste material, which can be utilized for large-scale production of new age multi-functional carbon nanomaterials for various energy applications.