Régis Chenitz

Recent Advances in Electrocatalysts for Oxygen Reduction Reaction

The recent advances in electrocatalysis for oxygen reduction reaction (ORR) for proton exchange membrane fuel cells (PEMFCs) are thoroughly reviewed. This comprehensive Review focuses on the low- and non-platinum electrocatalysts including advanced platinum alloys, core-shell structures, palladium-based catalysts, metal oxides and chalcogenides, carbon-based non-noble metal catalysts, and metal-free catalysts. The recent development of ORR electrocatalysts with novel structures and compositions is highlighted. The understandings of the correlation between the activity and the shape, size, composition, and synthesis method are summarized. For the carbon-based materials, their performance and stability in fuel cells and comparisons with those of platinum are documented. The research directions as well as perspectives on the further development of more active and less expensive electrocatalysts are provided.

Recent advances in non-precious metal catalysis for oxygen-reduction reaction in polymer electrolyte fuel cells

Hydrogen produced from water and renewable energy could fuel a large fleet of proton-exchange-fuel-cell vehicles in the future. However, the dependence on expensive Pt-based electrocatalysts in such fuel cells remains a major obstacle for a widespread deployment of this technology. One solution to overcome this predicament is to reduce the Pt content by a factor of ten by replacing the Pt-based catalysts with non-precious metal catalysts at the oxygen-reducing cathode. Fe- and Co-based electrocatalysts for this reaction have been studied for over 50 years, but they were insufficiently active for the high efficiency and power density needed for transportation fuel cells. Recently, several breakthroughs occurred that have increased the activity and durability of non-precious metal catalysts (NPMCs), which can now be regarded as potential competitors to Pt-based catalysts. This review focuses on the new synthesis methods that have led to these breakthroughs. A modeling analysis is also conducted to analyze the improvements required from NPMC-based cathodes to match the performance of Pt-based cathodes, even at high current density. While no further breakthrough in volume-specific activity of NPMCs is required, incremental improvements of the volume-specific activity and effective protonic conductivity within the fuel-cell cathode are necessary. Regarding durability, NPMCs with the best combination of durability and activity result in ca. 3 times lower fuel cell performance than the most active NPMCs at 0.80 V. Thus, major tasks will be to combine durability with higher activity, and also improve durability at cell voltages greater than 0.60 V.

A New Catalytic Site for the Electroreduction of Oxygen

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 specific demetalation of Fe–N4 catalytic sites in the micropores of NC_Ar + NH3 is at the origin of the initial activity loss of the highly active Fe/N/C catalyst used for the reduction of oxygen in PEM fuel cells

In this study, we explored the behavior of NC_Ar + NH3, an initially highly active catalyst for oxygen electroreduction, in H2/air fuel cells from 0.8 to 0.2 V at 80 °C and 25 °C, in order to find the causes of its instability. We discovered that the decay of the current density always involves the superposition of fast and slow first order kinetics, for which half-lives were obtained. The half-life of the fast decay was practically the same at all potentials and temperatures with a value of around 138 ± 55 min, while the half-life of the slow decay greatly varied from a minimum of ≈2400 min (40 h) to infinity. From the adsorption–desorption isotherm of NC_Ar + NH3, it was deduced that the Fe/N/C carbonaceous catalyst is characterized by interconnected open-end slit-shaped micropores, in which water (with dissolved H+ and O2) quickly flows in the fuel cells if their width is ≥0.7 nm as it has no interaction with the hydrophobic walls of the micropores. The driving force of this quick water flow is the humidified air streaming through the working cathode. Fe–N4-like active sites are thermodynamically stable in stagnant acidic conditions, but according to the Le Chatelier principle, they demetalate in the flux of water running into the micropores. This specific demetalation is the cause of the initial loss of ORR activity of NC_Ar + NH3 catalysts assigned to the fast current decay in fuel cells.

Formic Acid Oxidation on Composite Catalysts Made of Pt/C or Pd/C Impregnated with Phthalocyanines

The oxidation of formic acid at the surface of two families of composite catalysts, Pd/C-Pc and Pt/C-Pc, where Pc represents a phthalocyanine (H 2 -, Mn-, Fe-, Co-, Ni-, or CuPc), was studied in 10 mM HCOOH in 0.1 M HClO 4 electrolyte and in a fuel cell with 10 M HCOOH. For Pd/C-Pc catalysts; the impregnation of Pc onto Pd/C only resulted in lower mass activities compared to Pd/C alone. For Pt/C-Pc catalysts, the impregnation of CoPc, MnPc, or FePc onto Pt/C strongly enhanced the activity of the composite catalyst for formic acid oxidation at low potential (≤0.5 V vs reference hydrogen electrode) compared to Pt/C, whereas H 2 Pc, NiPc, and CuPc had no influence. The most beneficial effect of Pc impregnation was obtained with CoPc impregnation onto Pt/C. Fuel cell testing was performed for Pd/C, Pd/C-CoPc, Pt/C, Pt/C-CoPc, and Pt-Ru/C. Similar with the results obtained in the solution, the best performing catalyst in fuel cell was Pd/C followed by Pd/C-CoPc, Pt/C-CoPc, Pt-Ru/C, and finally Pt/C. However, only Pt/C-CoPc and Pt-Ru/C were stable in fuel cell testing.

3D Porous Fe/N/C Spherical Nanostructures As High-Performance Electrocatalysts for Oxygen Reduction in Both Alkaline and Acidic Media

Exploring inexpensive and high-performance nonprecious metal catalysts (NPMCs) to replace the rare and expensive Pt-based catalyst for the oxygen reduction reaction (ORR) is crucial for future low-temperature fuel cell devices. Herein, we developed a new type of highly efficient 3D porous Fe/N/C electrocatalyst through a simple pyrolysis approach. Our systematic study revealed that the pyrolysis temperature, the surface area, and the Fe content in the catalysts largely affect the ORR performance of the Fe/N/C catalysts, and the optimized parameters have been identified. The optimized Fe/N/C catalyst, with an interconnected hollow and open structure, exhibits one of the highest ORR activity, stability and selectivity in both alkaline and acidic conditions. In 0.1 M KOH, compared to the commercial Pt/C catalyst, the 3D porous Fe/N/C catalyst exhibits ∼6 times better activity (e.g., 1.91 mA cm-2 for Fe/N/C vs 0.32 mA cm-2 for Pt/C, at 0.9 V) and excellent stability (e.g., no any decay for Fe/N/C vs 35 mV negative half-wave potential shift for Pt/C, after 10000 cycles test). In 0.5 M H2SO4, this catalyst also exhibits comparable activity and better stability comparing to Pt/C catalyst. More importantly, in both alkaline and acidic media (RRDE environment), the as-synthesized Fe/N/C catalyst shows much better stability and methanol tolerance than those of the state-of-the-art commercial Pt/C catalyst. All these make the 3D porous Fe/N/C nanostructure an excellent candidate for non-precious-metal ORR catalyst in metal-air batteries and fuel cells.

Influence of 2H- or Metal-phthalocyanine Impregnation of Pt/C and Pd/C Catalysts on Formic Acid Electro-oxidation

Direct formic acid fuel cells (DFAFCs), alike direct methanol or ethanol fuel cells (DMFCs and DEFCs), are liquid fuel cells newly developed for portable applications. The optimization of DFAFCs has made it possible to attain power densities >200 mW/cm, even at room temperature [1]; a power density never before achieved by other direct liquid fuel cells under the same conditions. Pd has a superior activity compared to Pt for the electro-oxidation of formic acid, but there is evidence of catalytic activity loss over time for Pd as opposed to Pt and PdPt alloys [2]. We have impregnated 2Hand various metal-phthalocyanines (Mn, Fe, Co, Ni and Cu) on commercial Pt/C and Pd/C catalysts to determine whether this improves their catalytic activity toward formic acid oxidation in DFAFCs. Furthermore, heat treatments of these 2Hor metal-phthalocyanineimpregnated catalysts were also performed in Ar atmosphere for 30 min at different temperatures (300, 600, and 950°C).