Efficient Metal-Free Electrocatalysts for Oxygen Reduction: Polyaniline-Derived N- and O-Doped Mesoporous Carbons

The selective formation of graphene ranging from two-dimensional sheets to three-dimensional mesoporous nanospheres

This research presents a template-free solvothermal method which offers selective preparation of graphene ranging from two-dimensional sheets to 3-dimensional nanospheres. The thus prepared nanospheres have size-defined mesopores with a huge surface area and, after doping with nitrogen, exhibited stronger electrocatalytic activity toward oxygen reduction than commercial Pt/C catalysts.

Self-template construction of hollow Co3O4 microspheres from porous ultrathin nanosheets and efficient noble metal-free water oxidation catalysts

Developing noble metal-free water oxidation catalysts is essential for many energy conversion/storage processes (e.g., water splitting). Herein, we report the facile synthesis of hollow Co3O4 microspheres composed of porous, ultrathin (<5 nm), single-crystal-like nanosheets via a novel "self-template" route. The successful preparation of these hollow Co3O4 nanomaterials includes three main steps: (1) the synthesis of solid cobalt alkoxide microspheres, (2) their subsequent self-template conversion into hollow cobalt hydroxide microspheres composed of ultrathin nanosheets, and finally (3) thermal treatment of hollow cobalt hydroxide microspheres into the hollow Co3O4 material. The as-obtained hollow Co3O4 nanomaterial possesses a high BET surface area (∼180 m(2) g(-1)), and can serve as an active and stable water oxidation catalyst under both electrochemical and photochemical reaction conditions, owing to its unique structural features. In the electrochemical water oxidation, this catalyst affords a current density of 10 mA cm(-2) (a value related to practical relevance) at an overpotential of ∼0.40 V. Moreover, with the assistance of a sensitizer [Ru(bpy)3](2+) (bpy = 2,2'-bipyridine), this nanomaterial can catalyze water oxidation reactions under visible light irradiation with an O2 evolution rate of ∼12 218 μmol g(-1) h(-1). Our results suggest that delicate nanostructuring can offer unique advantages for developing efficient water oxidation catalysts.

Highly graphitized nitrogen-doped porous carbon nanopolyhedra derived from ZIF-8 nanocrystals as efficient electrocatalysts for oxygen reduction reactions

Nitrogen-doped graphitic porous carbons (NGPCs) have been synthesized by using a zeolite-type nanoscale metal-organic framework (NMOF) as a self-sacrificing template, which simultaneously acts as both the carbon and nitrogen sources in a facile carbonization process. The NGPCs not only retain the nanopolyhedral morphology of the parent NMOF, but also possess rich nitrogen, high surface area and hierarchical porosity with well-conducting networks. The promising potential of NGPCs as metal-free electrocatalysts for oxygen reduction reactions (ORR) in fuel cells is demonstrated. Compared with commercial Pt/C, the optimized NGPC-1000-10 (carbonized at 1000 °C for 10 h) catalyst exhibits comparable electrocatalytic activity via an efficient four-electron-dominant ORR process coupled with superior methanol tolerance as well as cycling stability in alkaline media. Furthermore, the controlled experiments reveal that the optimum activity of NGPC-1000-10 can be attributed to the synergetic contributions of the abundant active sites with high graphitic-N portion, high surface area and porosity, and the high degree of graphitization. Our findings suggest that solely MOF-derived heteroatom-doped carbon materials can be a promising alternative for Pt-based catalysts in fuel cells.

Heteroatom-doped highly porous carbon from human urine

Human urine, otherwise potentially polluting waste, is an universal unused resource in organic form disposed by the human body. We present for the first time "proof of concept" of a convenient, perhaps economically beneficial, and innovative template-free route to synthesize highly porous carbon containing heteroatoms such as N, S, Si, and P from human urine waste as a single precursor for carbon and multiple heteroatoms. High porosity is created through removal of inherently-present salt particles in as-prepared "Urine Carbon" (URC), and multiple heteroatoms are naturally doped into the carbon, making it unnecessary to employ troublesome expensive pore-generating templates as well as extra costly heteroatom-containing organic precursors. Additionally, isolation of rock salts is an extra bonus of present work. The technique is simple, but successful, offering naturally doped conductive hierarchical porous URC, which leads to superior electrocatalytic ORR activity comparable to state of the art Pt/C catalyst along with much improved durability and methanol tolerance, demonstrating that the URC can be a promising alternative to costly Pt-based electrocatalyst for ORR. The ORR activity can be addressed in terms of heteroatom doping, surface properties and electrical conductivity of the carbon framework.

Seaweed-derived heteroatom-doped highly porous carbon as an electrocatalyst for the oxygen reduction reaction

We report the template-free pyrolysis of easily available natural seaweed, Undaria pinnatifida, as a single precursor, which results in "seaweed carbon" (SCup). Interestingly, thus-obtained SCup not only contains heteroatoms such as nitrogen and sulfur in its framework, but it also possesses a well-developed porous structure with high surface area. The heteroatoms in SCup originate from the nitrogen- and sulfur-containing ingredients in seaweed, whereas the porosity is created by removal of salts inherently present in the seaweed. These essential and fundamental properties make seaweed a prime choice as a precursor for heteroatom-containing highly porous carbon as a metal-free efficient electrocatalyst. As-synthesized SCup showed excellent electrocatalytic activity in the oxygen reduction reaction (ORR) in alkaline medium, which can be addressed in terms of the presence of the nitrogen and sulfur heteroatoms, the well-developed porosity, and the electrical conductivity in the carbon framework. The pyrolysis temperature was a key controlling parameter that determined the trade-off between heteroatom doping, surface properties, and electrical conductivity. In particular, SCup prepared at 1000 °C showed the best ORR performance. Additionally, SCup exhibited enhanced durability and methanol tolerance relative to the state of the art commercial Pt catalyst, which demonstrates that SCup is a promising alternative to costly Pt-based catalysts for the ORR.

Biomolecule-doped PEDOT with three-dimensional nanostructures as efficient catalyst for oxygen reduction reaction

Metal macro-cyclic compounds have drawn considerable attention as alternative catalysts for oxygen reduction reaction. However, the continuous pyrolysis process usually needed for improving the performance of these compounds require an elevated temperature and complicated procedures, thus leading to an unpredictable transformation of the chemical structures and limiting their applications. Herein, we develop a new insight to fabricating hemin-doped poly (3,4-ethylenedioxythiophene) (PEDOT) with controllable three-dimensional nanostructures via a one-step, tri-phase, self-assembled polymerization routine. We demonstrate that the hemin-induced synergistic effect results in a very high 4-electron oxygen reduction activity, a better stability, and free from methanol crossover effects even in a neutral phosphate buffer solution (PBS).

Nitrogen and oxygen dual-doped carbon hydrogel film as a substrate-free electrode for highly efficient oxygen evolution reaction

A three-dimensional (3D) electrode composed of nitrogen, oxygen dualdoped graphene-carbon nanotube hydrogel film is fabricated, which greatly favors the transport and access of gas and reaction intermediates, and shows a remarkable oxygen-evolution catalytic performance in both alkaline and acidic solutions.

M(salen)-derived nitrogen-doped M/C (M = Fe, Co, Ni) porous nanocomposites for electrocatalytic oxygen reduction

Carbonaceous materials containing non-precious metal and/or doped nitrogen have attracted tremendous attention in the field of electrochemical energy storage and conversion. Herein, we report the synthesis and electrochemical properties of a new family of nitrogen-doped metal/carbon (M/N/C, M = Fe, Co, Ni) nanocomposites. The M/N/C nanocomposites, in which metal nanoparticles are embedded in the highly porous nitrogen-doped carbon matrix, have been synthesized by simply pyrolyzing M(salen) (salen = N,N′-bis(salicylidene)-ethylenediamine) complex precursors. The prepared Co/N/C and Fe/N/C exhibit remarkable electrocatalytic activity (with onset potential of 0.96 V for Fe/N/C and half-wave potential of 0.80 V for Co/N/C) and high stability for the oxygen reduction reaction (ORR). The superior performance of the nanocomposites is attributed to their bimodal-pore structure, high surface area, as well as uniform distribution of high-density nitrogen and metal active sites.

Synergistically enhanced activity of graphene quantum dot/multi-walled carbon nanotube composites as metal-free catalysts for oxygen reduction reaction

Graphene quantum dots (GQDs), as metal-free carbon nanomaterials, have potential applications in electrochemical fields due to their strong chemical inertness, oxygen-rich functional groups and remarkable quantum confinement and edge effects. Herein, we demonstrate that a novel metal-free electrode composed of GQDs and multi-walled carbon nanotubes (MWCNTs) exhibits a significant synergistic effect on enhanced catalytic activity for oxygen reduction reaction (ORR). Compared to commercially available Pt/C catalysts, enhanced electrocatalytic activity, improved long-term stability and excellent resistance to crossover effect were observed for the novel composite electrode. Interestingly, the amount of GQDs introduced is found to have an apparent effect on the positions of the reduction peaks of the electrodes.

Confined nanospace synthesis of less aggregated and porous nitrogen-doped graphene as metal-free electrocatalysts for oxygen reduction reaction in alkaline solution

A facile and low-emission strategy is used for preparation of porous nitrogen-doped graphene (NGR) in a confined nanospace. The negative charged graphene oxide (GO) serves as a substrate for deposition of electropositive metal amine complex and then thin layer of silica (SiO2) is formed onto the copper amine ion-coated GO. Carbonization of copper amine ion-coated GO in a confined nanospace of SiO2 and the subsequent removal of the Cu particles and SiO2 layer produces less aggregated and porous nitrogen-doped graphene (NGR). NGR materials are highly active, cheap, and selective metal-free electrocatalysts for the oxygen reduction reaction (ORR) in alkaline solution. The electron transfer for ORR at NGR catalysts is found to be around 4 at potentials ranging from -0.35 to -0.70 V. NGR may be further exploited as potentially efficient and inexpensive nonmetal ORR catalysts with good selectivity and long-term stability in alkaline solution.

Metal (metal = Fe, Co), N codoped nanoporous carbon for efficient electrochemical oxygen reduction

Fe, N codoped nanoporous carbon (N–Fe–nC) exhibits superior oxygen reduction activity with lower overpotential and comparable electron transfer number compared with Pt/C.

Advanced oxygen reduction reaction catalyst based on nitrogen and sulfur co-doped graphene in alkaline medium

A novel N and S co-doped graphene, denoted N–S-G, catalyst for ORR was fabricated by a one-step pyrolysis method of GO-supported poly[3-amino-5-mercapto-1,2,4-triazole] (PAMTa) composite.

Graphene quantum dots cut from graphene flakes: high electrocatalytic activity for oxygen reduction and low cytotoxicity

3–8 nm sized high quality graphene quantum dots with zigzag edges and multi-heteroatom doping were synthesized through a green process of electrochemically cutting pristine few-layer graphene flakes.

Electrocatalytic activity of metalloporphyrins grown in situ on graphene sheets toward oxygen reduction reaction in an alkaline medium

A series of novel non-noble-metal catalysts for ORR, based on metalloporphyrins grown on poly(sodium-p-styrenesulfonate) modified reduced graphene oxide sheets, have been successfully fabricated using an in situ solvothermal synthesis method.

Engineering self-assembled N-doped graphene–carbon nanotube composites towards efficient oxygen reduction electrocatalysts

Tuning the composition of N-doped graphene–carbon nanotube composites can boost their electrocatalytic performance for the oxygen reduction reaction.