Drew Higgins

A review on non-precious metal electrocatalysts for PEM fuel cells

With the approaching commercialization of PEM fuel cell technology, developing active, inexpensive non-precious metal ORR catalyst materials to replace currently used Pt-based catalysts is a necessary and essential requirement in order to reduce the overall system cost. This review paper highlights the progress made over the past 40 years with a detailed discussion of recent works in the area of non-precious metal electrocatalysts for oxygen reduction reaction, a necessary reaction at the PEM fuel cell cathode. Several important kinds of unsupported or carbon supported non-precious metal electrocatalysts for ORR are reviewed, including non-pyrolyzed and pyrolyzed transition metal nitrogen-containing complexes, conductive polymer-based catalysts, transition metal chalcogenides, metal oxides/carbides/nitrides/oxynitrides/carbonitrides, and enzymatic compounds. Among these candidates, pyrolyzed transition metal nitrogen-containing complexes supported on carbon materials (M–Nx/C) are considered the most promising ORR catalysts because they have demonstrated some ORR activity and stability close to that of commercially available Pt/C catalysts. Although great progress has been achieved in this area of research and development, there are still some challenges in both their ORR activity and stability. Regarding the ORR activity, the actual volumetric activity of the most active non-precious metal catalyst is still well below the DOE 2015 target. Regarding the ORR stability, stability tests are generally run at low current densities or low power levels, and the lifetime is far shorter than targets set by DOE. Therefore, improving both the ORR activity and stability are the major short and long term focuses of non-precious metal catalyst research and development. Based on the results achieved in this area, several future research directions are also proposed and discussed in this paper.

A review of graphene and graphene oxide sponge: material synthesis and applications to energy and the environment

This paper gives a comprehensive review about the most recent progress in synthesis, characterization, fundamental understanding, and the performance of graphene and graphene oxide sponges. Practical applications are considered including use in composite materials, as the electrode materials for electrochemical sensors, as absorbers for both gases and liquids, and as electrode materials for devices involved in electrochemical energy storage and conversion. Several advantages of both graphene and graphene oxide sponges such as three dimensional graphene networks, high surface area, high electro/thermo conductivities, high chemical/electrochemical stability, high flexibility and elasticity, and extremely high surface hydrophobicity are emphasized. To facilitate further research and development, the technical challenges are discussed, and several future research directions are also suggested in this paper.

Highly Active and Durable Core−Corona Structured Bifunctional Catalyst for Rechargeable Metal−Air Battery Application

A new class of core-corona structured bifunctional catalyst (CCBC) consisting of lanthanum nickelate centers supporting nitrogen-doped carbon nanotubes (NCNT) has been developed for rechargeable metal-air battery application. The nanostructured design of the catalyst allows the core and corona to catalyze the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR), respectively. These materials displayed exemplary OER and ORR activity through half-cell testing, comparable to state of the art commercial lanthanum nickelate (LaNiO(3)) and carbon-supported platinum (Pt/C), with added bifunctional capabilities allowing metal-air battery rechargeability. LaNiO(3) and Pt/C are currently the most accepted benchmark electrocatalyst materials for the OER and ORR, respectively; thus with comparable activity toward both of these reactions, CCBC are presented as a novel, inexpensive catalyst component for the cathode of rechargeable metal-air batteries. Moreover, after full-range degradation testing (FDT) CCBC retained excellent activity, retaining 3 and 13 times greater ORR and OER current upon comparison to state of the art Pt/C. Zinc-air battery performances of CCBC is in good agreement with the half-cell experiments with this bifunctional electrocatalyst displaying high activity and stability during battery discharge, charge, and cycling processes. Owing to its outstanding performance toward both the OER and ORR, comparable with the highest performing commercial catalysts to date for each of the respective reaction, coupled with high stability and rechargeability, CCBC is presented as a novel class of bifunctional catalyst material that is very applicable to future generation rechargeable metal-air batteries.

Multifunctional TiO2–C/MnO2 Core–Double-Shell Nanowire Arrays as High-Performance 3D Electrodes for Lithium Ion Batteries

The unique TiO2-C/MnO2 core-double-shell nanowires are synthesized for the first time using as anode materials for lithium ion batteries (LIBs). They combine both advantages from TiO2 such as excellent cycle stability and MnO2 with high capacity (1230 mA h g(-1)). The additional C interlayer intends to improve the electrical conductivity. The self-supported nanowire arrays grown directly on current-collecting substrates greatly simplify the fabrication processing of electrodes without applying binder and conductive additives. Each nanowire is anchored to the current collector, leading to fast charge transfer. The unique one-dimensional core-double-shell nanowires exhibit enhanced electrochemical performance with a higher discharge/charge capacity, superior rate capability, and longer cycling lifetime.

Biologically Inspired Highly Durable Iron Phthalocyanine Catalysts for Oxygen Reduction Reaction in Polymer Electrolyte Membrane Fuel Cells

In the present work, we have designed and synthesized a new highly durable iron phtalocyanine based nonprecious oxygen reduction reaction (ORR) catalyst (Fe-SPc) for polymer electrolyte membrane fuel cells (PEMFCs). The Fe-SPc, with a novel structure inspired by that of naturally occurring oxygen activation catalysts, is prepared by a nonpyrolyzing method, allowing adequate control of the atomic structure and surface properties of the material. Significantly improved ORR stability of the Fe-SPc is observed compared with the commercial Fe-Pc catalysts. The Fe-SPc has similar activity to that of the commercial Fe-Pc initially, while the Fe-SPc displays 4.6 times higher current density than that of the commercial Fe-Pc after 10 sweep potential cycles, and a current density that is 7.4 times higher after 100 cycles. This has been attributed to the incorporation of electron-donating functional groups, along with a high degree of steric hindrance maintaining active site isolation. Nonprecious Fe-SPc is promising as a potential alternative ORR electrocatalyst for PEMFCs.

The application of graphene and its composites in oxygen reduction electrocatalysis: a perspective and review of recent progress

The pressing necessity of a sustainable energy economy renders electrochemical energy conversion technologies, such as polymer electrolyte fuel cells or metal–air batteries, of paramount importance. The implementation of these technologies at scale still faces cost and operational durability challenges that stem from the conventionally used oxygen reduction reaction (ORR) electrocatalysts. While years of progress in ORR catalyst research has yielded some very attractive material designs, further advances are still required. Graphene entered the picture over 10 years ago, and scientists have only recently achieved a level of understanding regarding how its specific properties can be fine-tuned for electrocatalyst applications. This paper provides a critical review of the knowledge generated and progress realized over these past years for the development of graphene-based ORR catalysts. The first section discusses the application potential of graphene or modified graphene as platinum nanoparticle catalyst supports. The second section discusses the important role that graphene has played in the development of non-precious metal ORR catalysts, and more particularly its role in pyrolyzed transition metal–nitrogen–carbon complexes or as a support for inorganic nanoparticles. Finally the development of heteroatom doped graphene species is discussed, as this has been demonstrated as an excellent method to fine-tune the physicochemical properties and induce catalytic activity. Throughout this paper, clear differentiation is made between acidic and alkaline ORR catalysts, and some common misconceptions or improper testing practices used throughout the literature are revealed. Synthesis strategies and how they pertain to the resulting structure and electrochemical performance of graphene are discussed. In light of the large body of work done in this area, specific strategies are suggested for perpetuating the advancement of graphene-based ORR electrocatalysts. With concerted efforts it is one day likely that graphene-based catalysts will be a staple of electrochemical energy systems.

Metal–Organic Framework-Derived Bamboo-like Nitrogen-Doped Graphene Tubes as an Active Matrix for Hybrid Oxygen-Reduction Electrocatalysts

In this work, large size (i.e., diameter > 100 nm) graphene tubes with nitrogen-doping are prepared through a high-temperature graphitization process of dicyandiamide (DCDA) and Iron(II) acetate templated by a novel metal-organic framework (MIL-100(Fe)). The nitrogen-doped graphene tube (N-GT)-rich iron-nitrogen-carbon (Fe-N-C) catalysts exhibit inherently high activity towards the oxygen reduction reaction (ORR) in more challenging acidic media. Furthermore, aiming to improve the activity and stability of conventional Pt catalysts, the ORR active N-GT is used as a matrix to disperse Pt nanoparticles in order to build a unique hybrid Pt cathode catalyst. This is the first demonstration of the integration of a highly active Fe-N-C catalyst with Pt nanoparticles. The synthesized 20% Pt/N-GT composite catalysts demonstrate significantly enhanced ORR activity and H(2) -air fuel cell performance relative to those of 20% Pt/C, which is mainly attributed to the intrinsically active N-GT matrix along with possible synergistic effects between the non-precious metal active sites and the Pt nanoparticles. Unlike traditional Pt/C, the hybrid catalysts exhibit excellent stability during the accelerated durability testing, likely due to the unique highly graphitized graphene tube morphologies, capable of providing strong interaction with Pt nanoparticles and then preventing their agglomeration.

Direct atomic-level insight into the active sites of a high-performance PGM-free ORR catalyst

Replacing platinum in air-fed fuel cells Replacing expensive and scarce platinum catalysts in polymer electrolyte membrane fuel cells for the oxygen reduction reaction (ORR) with ones based on non-noble metals would speed up the adoption of hydrogen fuel vehicles. Most of the candidate replacement catalysts that have shown high performance do so only when running on pure oxygen. Chung et al. developed an iron-nitrogen-carbon catalyst from two nitrogen precursors that forms a high-porosity structure and exhibits high ORR performance when running on air. The proposed catalytically active site is FeN4. Science, this issue p. 479 A hierarchically structured iron-nitrogen-carbon catalyst for the oxygen reduction reaction is highly active in air. Platinum group metal–free (PGM-free) metal-nitrogen-carbon catalysts have emerged as a promising alternative to their costly platinum (Pt)–based counterparts in polymer electrolyte fuel cells (PEFCs) but still face some major challenges, including (i) the identification of the most relevant catalytic site for the oxygen reduction reaction (ORR) and (ii) demonstration of competitive PEFC performance under automotive-application conditions in the hydrogen (H2)–air fuel cell. Herein, we demonstrate H2-air performance gains achieved with an iron-nitrogen-carbon catalyst synthesized with two nitrogen precursors that developed hierarchical porosity. Current densities recorded in the kinetic region of cathode operation, at fuel cell voltages greater than ~0.75 V, were the same as those obtained with a Pt cathode at a loading of 0.1 milligram of Pt per centimeter squared. The proposed catalytic active site, carbon-embedded nitrogen-coordinated iron (FeN4), was directly visualized with aberration-corrected scanning transmission electron microscopy, and the contributions of these active sites associated with specific lattice-level carbon structures were explored computationally.

Multigrain Platinum Nanowires Consisting of Oriented Nanoparticles Anchored on Sulfur‐Doped Graphene as a Highly Active and Durable Oxygen Reduction Electrocatalyst

Direct growth of multigrain platinum nanowires on sulfur-doped graphene (PtNW/SG) is reported. The growth mechanism, including Pt nanoparticle nucleation on SG, followed by nanoparticle attachment with orientation along the <111> direction is highlighted. PtNW/SG demonstrates improved Pt mass and specific activity compared with commercial catalysts toward oxygen reduction, in addition to dramatically improved stability through accelerated durability testing.

Controlled Growth of Platinum Nanowire Arrays on Sulfur Doped Graphene as High Performance Electrocatalyst

Graphene supported Pt nanostructures have great potential to be used as catalysts in electrochemical energy conversion and storage technologies; however the simultaneous control of Pt morphology and dispersion, along with ideally tailoring the physical properties of the catalyst support properties has proven very challenging. Using sulfur doped graphene (SG) as a support material, the heterogeneous dopant atoms could serve as nucleation sites allowing for the preparation of SG supported Pt nanowire arrays with ultra-thin diameters (2–5 nm) and dense surface coverage. Detailed investigation of the preparation technique reveals that the structure of the resulting composite could be readily controlled by fine tuning the Pt nanowire nucleation and growth reaction kinetics and the Pt-support interactions, whereby a mechanistic platinum nanowire array growth model is proposed. Electrochemical characterization demonstrates that the composite materials have 2–3 times higher catalytic activities toward the oxygen reduction and methanol oxidation reaction compared with commercial Pt/C catalyst.