Catalysts for oxygen reduction and evolution reactions are at the heart of key renewable-energy technologies including fuel cells and water splitting. Despite tremendous efforts, developing oxygen electrode catalysts with high activity at low cost remains a great challenge. Here, we report a hybrid material consisting of Co₃O₄ nanocrystals grown on reduced graphene oxide as a high-performance bi-functional catalyst for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Although Co₃O₄ or graphene oxide alone has little catalytic activity, their hybrid exhibits an unexpected, surprisingly high ORR activity that is further enhanced by nitrogen doping of graphene. The Co₃O₄/N-doped graphene hybrid exhibits similar catalytic activity but superior stability to Pt in alkaline solutions. The same hybrid is also highly active for OER, making it a high-performance non-precious metal-based bi-catalyst for both ORR and OER. The unusual catalytic activity arises from synergetic chemical coupling effects between Co₃O₄ and graphene.

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Co3O4 nanocrystals on graphene as a synergistic catalyst for oxygen reduction reaction

Fuel cells powered by hydrogen from secure and renewable sources are the ideal solution for non-polluting vehicles, and extensive research and development on all aspects of this technology over the past fifteen years has delivered prototype cars with impressive performances. But taking the step towards successful commercialization requires oxygen reduction electrocatalysts--crucial components at the heart of fuel cells--that meet exacting performance targets. In addition, these catalyst systems will need to be highly durable, fault-tolerant and amenable to high-volume production with high yields and exceptional quality. Not all the catalyst approaches currently being pursued will meet those demands.

Electrocatalyst approaches and challenges for automotive fuel cells

The mass production of proton exchange membrane (PEM) fuel-cell-powered light-duty vehicles requires a reduction in the amount of Pt presently used in fuel cells. This paper quantifies the activities and voltage loss modes for state-of-the-art MEAs (membrane electrode assemblies), specifies performance goals needed for automotive application, and provides benchmark oxygen reduction activities for state-of-the-art platinum electrocatalysts using two different testing procedures to clearly establish the relative merit of candidate catalysts. A pathway to meet the automotive goals is charted, involving the further development of durable, high-activity Pt-alloy catalysts. The history, status in recent experiments, and prospects for Pt-alloy cathode catalysts are reviewed. The performance that would be needed for a cost-free non-Pt catalyst is defined quantitatively, and the behaviors of several published non-Pt catalyst systems (and logical extensions thereof), are compared to these requirements. Critical research topics are listed for the Pt-alloy catalysts, which appear to represent the most likely route to automotive fuel cells.

Activity benchmarks and requirements for Pt, Pt-alloy, and non-Pt oxygen reduction catalysts for PEMFCs

A fundamental change has been achieved in understanding surface electrochemistry due to the profound knowledge of the nature of electrocatalytic processes accumulated over the past several decades and to the recent technological advances in spectroscopy and high resolution imaging. Nowadays one can preferably design electrocatalysts based on the deep theoretical knowledge of electronic structures, via computer-guided engineering of the surface and (electro)chemical properties of materials, followed by the synthesis of practical materials with high performance for specific reactions. This review provides insights into both theoretical and experimental electrochemistry toward a better understanding of a series of key clean energy conversion reactions including oxygen reduction reaction (ORR), oxygen evolution reaction (OER), and hydrogen evolution reaction (HER). The emphasis of this review is on the origin of the electrocatalytic activity of nanostructured catalysts toward the aforementioned reactions by correlating the apparent electrode performance with their intrinsic electrochemical properties. Also, a rational design of electrocatalysts is proposed starting from the most fundamental aspects of the electronic structure engineering to a more practical level of nanotechnological fabrication.

Design of electrocatalysts for oxygen- and hydrogen-involving energy conversion reactions

Since the discovery of mechanically exfoliated graphene in 2004, research on ultrathin two-dimensional (2D) nanomaterials has grown exponentially in the fields of condensed matter physics, material science, chemistry, and nanotechnology. Highlighting their compelling physical, chemical, electronic, and optical properties, as well as their various potential applications, in this Review, we summarize the state-of-art progress on the ultrathin 2D nanomaterials with a particular emphasis on their recent advances. First, we introduce the unique advances on ultrathin 2D nanomaterials, followed by the description of their composition and crystal structures. The assortments of their synthetic methods are then summarized, including insights on their advantages and limitations, alongside some recommendations on suitable characterization techniques. We also discuss in detail the utilization of these ultrathin 2D nanomaterials for wide ranges of potential applications among the electronics/optoelectronics, electrocat...

Recent Advances in Ultrathin Two-Dimensional Nanomaterials

Fe-based catalysts for O2 reduction under proton exchange membrane fuel cell conditions were prepared on a commercial N234 carbon black support using both a “classical” and a step-by-step procedure to determine if parameters other than microporosity and nitrogen loading of the carbon support are important in the synthesis of Fe/N/C electrocatalysts. The “classical” procedure for obtaining Fe/N/C electrocatalysts is to use a single-step synthesis, in which a carbon support loaded with a metal precursor is heat treated at high temperatures (900−950 °C) in pure NH3. In the step-by-step procedure, microporosity is first etched into the carbon support followed, if necessary, by the addition of N-bearing functionalities and, last, the loading of the metal precursor. Similar maximum microporous contents can be etched into N234, using either NH3 or O2 (air). However, unlike O2 (air), etching with NH3 has the added benefit of creating N-bearing functionalities on the carbon surface. For carbon supports etched in O...

Step-by-Step Synthesis of Non-Noble Metal Electrocatalysts for O2 Reduction under Proton Exchange Membrane Fuel Cell Conditions

Activity and stability in proton exchange membrane fuel cells of iron-based cathode catalysts synthesized with addition of carbon fibers

The fuel cell principle was discovered in 1839 concurrently by Christian Friedrich Schçnbein and Sir William Grove, who noticed that a current flowed between two platinum electrodes in acid medium in their respective anodic and cathodic compartments after collecting hydrogen and oxygen by water electrolysis. In doing so, they demonstrated that water electrolysis and hydrogen–air (oxygen) fuel cells are based on opposite reactions. Two centuries later, the electrochemistry of acidic fuel cells has remained predominantly based on platinum electrodes. In contrast, iron has been nature’s main choice to perform the oxygen reduction reaction (ORR) in living organisms. Dioxygen has not always been part of the earth’s atmosphere. It appeared only about 2.4 billion years ago when it was produced as a metabolic waste by photosynthetic bacteria. The presence of dioxygen in the atmosphere gave rise to aerobic respiration also practiced by bacteria that later evolved into mitochondria in multicellular organisms. Aerobic respiration is a non-photosynthetic form of metabolism. It proceeds by enzyme-catalyzed electron transfer starting with strong reductants and ending with the weakest one: a terminal oxidase. The latter reacts with atmospheric oxygen, completing the oxygen cycle started by the photosynthetic bacteria. Although oxygen is a powerful 4 e /4 H oxidant, it is kinetically inert if in direct contact with most of the organic matter used as food by living organisms. The reduction of oxygen to water requires a catalyst, the most common being cytochrome c oxidase. This contains a binuclear iron(heme)–copper–porphyrin site at which oxygen is reduced. The presence of iron–porphyrin in the cytochrome c active site for oxygen reduction is the inspiration for producing iron–porphyrin-like non-noble catalysts for ORR in proton-exchange membrane fuel cells. If iron–porphyrins impregnated on carbon are used as ORR catalysts, the reduction reaction produces water, but some hydrogen peroxide as well. If hydrogen peroxide is not reduced to water or disproportionated on the catalyst quickly enough, it may decompose the iron–porphyrins to release free iron ions, which are known to react with hydrogen peroxide to produce a strong oxidant (Fenton’s reagent) that further speeds up the decomposition of the catalyst. It has been found that a high-temperature heat treatment of iron–porphyrin catalysts impregnated on carbon in an inert atmosphere improves both the catalytic activity and stability. This finding, however, triggered a debate over the exact identity of the active ORR site. It was also found that, to produce an iron-based catalyst for ORR, all that was needed was an iron precursor (e.g. , iron–porphyrin or an iron salt), a nitrogen precursor (e.g. , iron–porphyrin or another nitrogen-containing solid or gaseous compound), a carbon support (e.g. , carbon black), and a pyrolysis of the material obtained either by the impregnation of iron and nitrogen solid precursors onto the carbon support or by pyrolysis, in the presence of the gaseous nitrogen precursor, of the iron precursor impregnated on the carbon support. The latter may also be replaced with a carbon precursor that thermally decomposes into the carbon support upon pyrolysis. Today, there seems to be a consensus about the nature of the ORR active sites in iron-based catalysts. Three main types of sites have been identified: 1) FeN4/C, consisting of pyrrolic nitrogen atoms; 2) FeN4/C, consisting of pyridinic nitrogen atoms; and 3) CNx, nitrogen-doped carbon. The activity of the CNx sites stems from the presence of nitrogen atoms in the carbon support. All three sites coexist in iron-based catalysts. Their relative proportions depend on the nature of the precursors used for their synthesis. Of the three sites, the most active is FeN4/C with pyridinic nitrogen, followed by that with pyrrolic nitrogen, and finally CNx. The catalytic activity depends on the number of each type of site present by unit volume in the catalyst layer. The activities of four iron-based catalyst are presented in Figure 1 A. Except for the data for catalyst e, which are new results, all other curves have already been reported. Catalyst a was made by ball milling the iron(II)–1,10-phenanthroline complex with highly microporous Black Pearls 2000, followed by a heat-treatment in argon at 1050 8C and a second pyrolysis in ammonia at 950 8C. Catalyst b was made by impregnation of iron(II)–1,10-phenanthroline complex on non-porous carbon black followed by a pyrolysis step at 950 8C in ammonia. Catalyst c was 46 wt% Pt/C from Tanaka Kikinzoku. Catalyst d was made by pyrolyzing Black Pearls 2000 in NH3 at 950 8C. Catalyst e was made by ball milling the iron(II)–1,10-phenanthroline complex with metal–organic framework ZIF-8 then heat-treating the material in argon at 1050 8C, followed by a second pyrolysis step in ammonia at 950 8C. It is clear from Figure 1 A that iron-based catalysts are the most active and catalyst e has the highest number of FeN4/C sites (pyridinic and pyrrolic nitrogen combined), followed by catalysts a and b, whereas the activity of CNx sites is rather low. Recently, Xing and Li et al. reported the catalytic properties of iron-based catalysts that, according to their work, do not have FeNx ORR sites, rather ORR catalytic sites made of iron carbide encased in graphitic layers. All Fe3C/C and the S2-700 catalysts in Figure 1 B were synthesized by a high-pressure py[a] J.-P. Dodelet, R. Chenitz, L. Yang INRSnergie, Mat riaux et T l communications Varennes, Qu bec, J3X 1S2 (Canada) E-mail : dodelet@emt.inrs.ca [b] M. Lef vre Canetique Electrocatalysis Inc. Varennes, Qu bec, J3X 1S2 (Canada)

A New Catalytic Site for the Electroreduction of Oxygen

We have used pulsed lased deposition (PLD) of platinum onto uncatalyzed E-TEK gas-diffusion electrodes (GDE) to prepare very low loading electrodes for polymer electrolyte membrane fuel cells (PEMFCs). When tested at the anode side, the Pt PLD electrodes containing as little as 0.017 mg Pt/cm 2 perform as well as standard E-TEK electrodes that contain about twenty-five times more Pt (0.4 mg/cm 2 ). X-ray diffraction analysis and scanning electron micrographs of Pt PLD electrodes indicate that the deposits are made of 13 nm diam crystallites evenly dispersed at the surface of the GDE backing.

http://esl.ecsdl.org/content/6/7/A125.full.pdf

PEMFC Anode with Very Low Pt Loadings Using Pulsed Laser Deposition

Polymer electrolyte membrane fuel cells (PEMFC) are electrical power generators for a wide range of possible applications, but are still considered too expensive for many. To reduce their cost, much research has focused on replacing the expensive Pt-based electrocatalysts in PEMFCs with a lower-cost alternative. Fe-based cathode catalysts are a promising alternative. To compete with Pt-based cathode catalysts, non-precious metal catalysts must meet three key criteria: have high catalytic activity, allow for high power density at meaningful cell voltages and have adequate operational stability/durability. Over the last three years our research group at INRS-EMT has made significant progress in achieving these criteria including a cathode catalyst with a power density of 0.75W cm−2 at 0.6V under H2/O2, a meaningful voltage for PEMFC operation, comparable with that of a commercial Pt-based cathode tested under identical conditions.

Michel Lefèvre

Papers

Step-by-Step Synthesis of Non-Noble Metal Electrocatalysts for O2 Reduction under Proton Exchange Membrane Fuel Cell Conditions

Activity and stability in proton exchange membrane fuel cells of iron-based cathode catalysts synthesized with addition of carbon fibers

A New Catalytic Site for the Electroreduction of Oxygen

PEMFC Anode with Very Low Pt Loadings Using Pulsed Laser Deposition

Recent Advances in Non-Precious Metal Electrocatalysts for Oxygen Reduction in PEM Fuel Cells