The effect of pH on the hydrogen oxidation and evolution reaction (HOR/HER) rates is addressed for the first time for the three most active monometallic surfaces: Pt, Ir, and Pd carbon-supported catalysts. Kinetic data were obtained for a proton exchange membrane fuel cell (PEMFC; pH z 0) using the H 2 -pump mode and with a rotating disk electrode (RDE) in 0.1 M NaOH. Our findings point toward: (i) a similar z100-fold activity decrease on all these surfaces when going from low to high pH; (ii) a reaction rate controlled by the Volmer step on Pt/C; and (iii) the H-binding energy being the unique and sole descriptor for the HOR/HER in alkaline electrolytes. Based on a detailed discussion of our data, we propose a new mechanism for the HOR/HER on Pt-metals in alkaline electrolytes.Fuel cells and electrolyzers are important for renewable energy conversion and storage. They are currently based on protonexchange membranes (PEMs) operating at low pH (pH z 0), which offer high power densities, but require large amounts of platinum for the oxygen reduction reaction (ORR) in fuel cells 1 and of Ir for the oxygen evolution reaction (OER) in electrolyzers. 2 For the hydrogen oxidation/evolution reaction (HOR/HER) only very small amounts of Pt are required due to its extremely high activity for the HOR/HER. 3 The H 2 anode performance in PEMFCs suggested exchange current densities (i 0 ) in the order of 10 2 mA cm Pt À2 , 4 which was conrmed by mass-transport-free fuel cell measurements 3,5 and microelectrode data. 6 Until then, 100-fold lower i 0 -values for Pt in acid were reported erroneously, generally based on rotating disk electrode (RDE) measurements 7,8 from which, however, the kinetics of reactions with i 0 -values much above the diffusion limited RDE current density (z2-3 mA cm disk À2 ) cannot be quantied. 9In an alkaline electrolyte, non-noble metal catalysts are very active for the ORR 10,11 and for the OER, 12,13 so that in conjunction with alkaline membranes (OH À -exchange membranes 14,15 ) a replacement of the noble-metal intensive PEM technology by alkaline membrane technology seems promising. Unfortunately, for yet unclear reasons, the HOR/ HER kinetics on Pt are much slower in alkaline than in acid electrolytes, and large amounts of Pt are needed to catalyze the HOR/HER in an alkaline environment. 9 Therefore, it is critical to develop alternative HOR/HER catalysts for alkaline electrolytes and -to guide this search -to elucidate the reasons for the poor HOR/HER activity of Pt in alkaline electrolytes.Traditionally, the overall reactions have been written either with protons in acid or with hydroxide ions in alkaline media: 16 in acid:in base:The future of electromobility relies on the development of cost effective and durable energy conversion systems such as fuel cells and electrolyzers. These devices, based on proton-exchange membranes (PEMs), operating at pH 0, offer high power densities, but require large amounts of noble metal for the oxygen reduction reaction (ORR) in fuel cells and the oxygen evo...

H(2)-air polymer-electrolyte-membrane fuel cells are electrochemical power generators with potential vehicle propulsion applications. To help reduce their cost and encourage widespread use, research has focused on replacing the expensive Pt-based electrocatalysts in polymer-electrolyte-membrane fuel cells with a lower-cost alternative. Fe-based cathode catalysts are promising contenders, but their power density has been low compared with Pt-based cathodes, largely due to poor mass-transport properties. Here we report an iron-acetate/phenanthroline/zeolitic-imidazolate-framework-derived electrocatalyst with increased volumetric activity and enhanced mass-transport properties. The zeolitic-imidazolate-framework serves as a microporous host for phenanthroline and ferrous acetate to form a catalyst precursor that is subsequently heat treated. A cathode made with the best electrocatalyst from this work, tested in H(2)-O(2,) has a power density of 0.75 W cm(-2) at 0.6 V, a meaningful voltage for polymer-electrolyte-membrane fuel cells operation, comparable with that of a commercial Pt-based cathode tested under identical conditions.

Fe-based catalytic sites for the reduction of oxygen in acidic medium have been identified by 57Fe Mössbauer spectroscopy of Fe/N/C catalysts containing 0.03 to 1.55 wt% Fe, which were prepared by impregnation of iron acetate on carbon black followed by heat-treatment in NH3 at 950°C. Four different Fe-species were detected at all iron concentrations: three doublets assigned to molecular FeN4-like sites with their ferrous ion in low (D1), medium (D2) or high spin state (D3), and two other doublets assigned to a single Fe-species (D4 and D5) consisting of surface oxidized nitride nanoparticles (FexN, with x≤2.1). A fifth Fe-species appears only in those catalysts with Fe-contents ≥ 0.27 wt%. It is characterized by a very broad singlet, which has been assigned to incomplete FeN4-like sites that quickly dissolve in contact with an acid. Among the five Fe-species identified in these catalysts, only D1 and D3 display catalytic activity for the oxygen reduction reaction (ORR) in the acid medium, with D3 featuring a composite structure with a protonated neighbour basic nitrogen and being by far the most active species, with an estimated turn over frequency for the ORR of 11.4 e− site−1 s−1 at 0.8V vs RHE. Moreover, all D1 sites and between 1/2 to 2/3 of the D3 sites are acid-resistant. A scheme for the mechanism of site formation upon heat-treatment is also proposed. This identification of the ORR-active sites in these catalysts is of crucial importance to design strategies to improve the catalytic activity and stability of these materials.

Nine non-noble-metal catalysts (NNMCs) from five different laboratories were investigated for the catalysis of O(2) electroreduction in an acidic medium. The catalyst precursors were synthesized by wet impregnation, planetary ball milling, a foaming-agent technique, or a templating method. All catalyst precursors were subjected to one or more heat treatments at 700-1050 degrees C in an inert or reactive atmosphere. These catalysts underwent an identical set of electrochemical characterizations, including rotating-disk-electrode and polymer-electrolyte membrane fuel cell (PEMFC) tests and voltammetry under N(2). Ex situ characterization was comprised of X-ray photoelectron spectroscopy, neutron activation analysis, scanning electron microscopy, and N(2) adsorption and its analysis with an advanced model for carbonaceous powders. In PEMFC, several NNMCs display mass activities of 10-20 A g(-1) at 0.8 V versus a reversible hydrogen electrode, and one shows 80 A g(-1). The latter value corresponds to a volumetric activity of 19 A cm(-3) under reference conditions and represents one-seventh of the target defined by the U.S. Department of Energy for 2010 (130 A cm(-3)). The activity of all NNMCs is mainly governed by the microporous surface area, and active sites seem to be hosted in pore sizes of 5-15 A. The nitrogen and metal (iron or cobalt) seem to be present in sufficient amounts in the NNMCs and do not limit activity. The paper discusses probable directions for synthesizing more active NNMCs. This could be achieved through multiple pyrolysis steps, ball-milling steps, and control of the powder morphology by the addition of foaming agents and/or sulfur.