The electrocatalysis of the oxygen reduction reaction (ORR) on five binary Pi alloys (PtCr/C, PtMn/C, PtFe/C, PtCo/C, and PtNi/C) supported on high surface area carbon in a proton exchange membrane fuel cell was investigated. All the alloy electrocatalysts exhibited a high degree of crystallinity with the primary phase of the type Pt3M (LI2 structure with fcc type lattice) and a secondary phase (only minor contribution from this phase) being of the type PtM (LIo structure with tetragonal lattice) as evidenced from x-ray powder diffraction (XRD) analysis. The electrode kinetic studies on the Pt alloys at 95~ and 5 atm pressure showed a two-to threefold increase in the exchange current densities and the current density at 900 mV as well as a decrease in the overvoltage at i0 mA em -2 relative to Pt/C eleetrocatalyst. The PtCr/C alloy exhibited the best performance. In situ EXAFS and XANES analysis at potentials in the double-layer region [0.54 V vs. reversible hydrogen electrode (RHE)] revealed (i) all the alloys possess higher Pt d-band vacancies per atom (with the exception of PtMn/C alloy) relative to Pt/C electrocatalyst and (it) contractions in the Pt-Pt bond distances which confirmed the results from ex situ XRD analysis. A potential excursion to 0.84 V vs. RHE showed that, in contrast to the Pt alloys, the Pt/C electrocatalyst exhibits a significant increase in the Pt d-band vacancies per atom. This increase, in Pt/C has been rationalized as being due to adsorption of OH species from the electrolyte following a Temkin isotherm behavior, which does not occur on the Pt alloys. Correlation of the electronic (Pt d-band vacancies) and geometric (Pt-Pt bond distance) with the electrochemical performance characteristics exhibits a volcano type behavior with the PtCr/C alloy being at the top of the curve. The enhanced electrocatalysis by the alloys therefore can be rationalized on the basis of the interplay between the electronic and geometric factors on one hand and their effect on the chemisorption behavior of OH species from the electrolyte.The role of Pt/C and Pt alloys on the mechanism of the oxygen reduction reaction (ORR) has been investigated previously, 1-4 however the mechanism still remains elusive. One of the first investigations I of the ORR on Pt alloy electrocatalysts was in phosphoric acid; the effect of changes in the Pt-Pt interatomic distances, caused by alloying, was examined. The strength of the [M-HO2]aas bond, the intermediate formed in the rate-determining step of the molecular dioxygen reduction, was shown to depend on the Pt-Pt bond distance in the alloys. A plot of the electrocatalytic activity vs. adsorbate bond strength exhibited a volcano type behavior. 5 It was shown that the lattice contractions due to alloying resulted in a more favorable Pt-Pt distance (while maintaining the favorable Pt electronic properties) for dissociative adsorption of 02. This view was disputed by Glass et al. ~ in their investigation on bulk alloys of PtCr (the binary alloy at the top of the volcano ...

The effect of different alloying conditions (alloying temperature, annealing period) on the electrocatalytic activities for the oxygen reduction reaction (ORR) by three carbon-supported Pt alloy electrocatalysts (WCr, WCo, m i ) was investigated and correlated with electronic and structural parameters determined by in-situ XAS. The results indicate that all the Pt alloys show enhanced ORR activities relative to a W C electrocatalyst. However, the electrocatalytic activity and activation energy for ORR in the case of Pt/Ni and Pt/Co alloys show marked effect due to different alloying conditions. This was in contrast to W C r alloy, where both parameters remained unchanged over the range of alloying conditions. Those electrochemical results were correlated with those obtained from in-situ X-ray absorption spectroscopic (XAS) investigations, which provided information on the electronic (Pt 5d-orbital vacancy, from the X-ray absorption near-edge structure) and geometric (Pt-Pt bond distances, from the extended X-ray absorption fine structure) factors. In-situ XAS results indicate that the supported alloys possess higher P t Sd-orbital vacancies and shorter Pt-Pt bond distances.In addition, the XAS results showed that alloying inhibited chemisorption of oxygenated species (OH) on the Pt at potentials above 0.8 V vs RHE. Correlation of electrocatalytic activities and activation energies for ORR with parameters obtained from in-situ XAS studies indicates that, in the case of m i and Pt/Co alloys, higher alloying temperature and longer annealing periods result in higher Pt 5d-orbital vacancies with the geometric parameters remaining unchanged. The W C r alloy on the other hand revealed no dependence of either the Pt d-orbital vacancies or the geometric parameters on alloying temperature. These observations indicate that the dependence of electrocatalytic activities and activation energy for Pt/Co and Pt/Ni alloys on the thermal history and the absence of such an effect in the W C r alloy could be related to the differences in the Pt Sd-orbital vacancies. IntroductionEnhancement of electrocatalytic activities for an oxygen reduction reaction (ORR) by alloying Pt with first-row transition elements was first reported in phosphoric acid'&nd more recently in proton exchange membrane fuel cells (PEMFCS).~.~ Based on early findings of activity enhancements in phosphoric acid fuel cells (PAFCs), several investigations were conducted to ascertain the role of alloying on the electrocatalytic activity for ORR, detailed reviews of which have been presented e l~e w h e r e .~.~ Recent investigations on five binary carbonsupported Pt alloy electrocatalysts in a PEMFC environment (WCr, Pt/Mn, Pt/Fe, WCo, Pt/Ni) involving both ex-situ XRD and in-situ XAS spectroscopy have revealed interesting correlations between the electronic, geometric, and electrocatalytic activities for ORR by Pt and Pt alloys.' Of special relevance was the application of in-situ XAS spectroscopy, which consists of the near-edge part, X-ray absorption near-edge st...

As functional biomimics of the hydrogen-producing capability of the dinuclear active site in [Fe]H(2)ase, the Fe(I)Fe(I) organometallic complexes, (mu-pdt)[Fe(CO)(2)PTA](2), 1-PTA(2), (pdt = SCH(2)CH(2)CH(2)S; PTA = 1,3,5-triaza-7-phosphaadamantane), and (mu-pdt)[Fe(CO)(3)][Fe(CO)(2)PTA], 1-PTA, were synthesized and fully characterized. For comparison to the hydrophobic (mu-pdt)[Fe(CO)(2)(PMe(3))](2) and [(mu-H)(mu-pdt)[Fe(CO)(2)(PMe(3))](2)](+) analogues, electrochemical responses of 1-PTA(2) and 1-(PTA.H(+))(2) were recorded in acetonitrile and in acetonitrile/water mixtures in the absence and presence of acetic acid. The production of H(2) and the dependence of current on acid concentration indicated that the complexes were solution electrocatalysts that decreased over-voltage for H(+) reduction from HOAc in CH(3)CN by up to 600 mV. The most effective electrocatalyst is the asymmetric 1-PTA species, which promotes H(2) formation from HOAc (pK(a) in CH(3)CN = 22.6) at -1.4 V in CH(3)CN/H(2)O mixtures at the Fe(0)Fe(I) redox level. Functionalization of the PTA ligand via N-protonation or N-methylation, generating (mu-pdt)[Fe(CO)(2)(PTA-H(+))](2), 1-(PTA.H(+))(2), and (mu-pdt)[Fe(CO)(2)(PTA-CH(3)(+))](2), 1-(PTA-Me(+))(2), provided no obvious advantages for the electrocatalysis because in both cases the parent complex is reclaimed during one cycle under the electrochemical conditions and H(2) production catalysis develops from the neutral species. The order of proton/electron addition to the catalyst, i.e., the electrochemical mechanism, is dependent on the extent of P-donor ligand substitution and on the acid strength. Cyclic voltammetric curve-crossing phenomena was observed and analyzed in terms of the possible presence of an eta(2)-H(2)-Fe(II)Fe(I) species, derived from reduction of the Fe(I)Fe(I) parent complex to Fe(0)Fe(I) followed by uptake of two protons in an ECCE mechanism.

A series of binuclear Fe I Fe I complexes, (µ-SEt) 2 [Fe(CO) 2 L] 2 (L = CO (1), PMe 4), PMe 3 (4-P)), that serve as structural models for the active site of Fe-hydrogenase are shown to be electrocatalysts for H 2 production in the presence of acetic acid in acetonitrile. The redox levels for H 2 production were established by spectroelectrochemistry to be Fe 0 Fe 0 for the all-CO complexes and Fe I Fe 0 for the PMe 3 -substituted derivatives. As electrocatalysts, the PMe 3 derivatives are more stable and more sensitive to acid concentration than the all-CO complexes. The electrocatalysis is initiated by electrochemical reduction of these diiron complexes, which subsequently, under weak acid conditions, undergo protonation of the reduced iron center to produce H 2 . An (η 2 -H 2 )Fe II -Fe 0/I intermediate is suggested and probable electrochemical mechanisms are discussed.

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In this study we control the surface structure of Cu thin-film catalysts to probe the relationship between active sites and catalytic activity for the electroreduction of CO 2 to fuels and chemicals. Here, we report physical vapor deposition of Cu thin films on large-format (∼6 cm 2 ) single-crystal substrates, and confirm epitaxial growth in the <100>, <111>, and <751> orientations using X-ray pole figures. To understand the relationship between the bulk and surface structures, in situ electrochemical scanning tunneling microscopy was conducted on Cu(100), (111), and (751) thin films. The studies revealed that Cu(100) and (111) have surface adlattices that are identical to the bulk structure, and that Cu(751) has a heterogeneous kinked surface with (110) terraces that is closely related to the bulk structure. Electrochemical CO 2 reduction testing showed that whereas both Cu(100) and (751) thin films are more active and selective for C-C coupling than Cu(111), Cu(751) is the most selective for >2e− oxygenate formation at low overpotentials. Our results demonstrate that epitaxy can be used to grow singlecrystal analogous materials as large-format electrodes that provide insights on controlling electrocatalytic activity and selectivity for this reaction.carbon dioxide reduction | epitaxy | electrocatalysis | copper T he electrochemical reduction of CO 2 (CO 2 R) is a process that could couple to renewable energy from wind and solar to directly produce fuels and chemicals in a sustainable manner. However, developing catalysts is a major challenge for this reaction, and significant advances are needed to overcome the issues of poor energy efficiency and product selectivity. One reason for these issues is that there are a limited number of catalysts that can effectively convert CO 2 to products that require more than two electrons (>2e − products), e.g., methane, methanol, ethylene, etc. (1, 2). Therefore, developing catalysts that are effective for CO 2 R to >2e − products would greatly improve prospects for utilization, and such an endeavor requires a deeper understanding of the relevant surface chemistry.Out of the polycrystalline metals, Cu is the only one that has shown a propensity for CO 2 R to >2e − products at considerable rates and selectivity (2, 3). To date, its uniqueness is reflected by how nearly all work on catalysts with improved activity and selectivity for >2e − products is based on Cu (4-6). However, polycrystalline Cu is not particularly selective toward any one >2e − reduction product (7). Thus, it is critical to understand what active site motifs lead to this unique selectivity for further reduced products and to apply this knowledge to develop new materials with this electrocatalytic behavior.Single-crystal studies on Cu have shown that CO 2 R activity and selectivity are extremely sensitive to surface structure. In particular, facet sensitivities for C-C coupling are the most widely studied, with experimental reports concluding that Cu(100) terraces and any orientation of step sites are more...

A study based on operando electrochemical scanning tunneling microscopy (EC-STM) has shown that a polycrystalline Cu electrode held at a fixed negative potential, -0.9 V (vs SHE), in the vicinity of CO2 reduction reactions (CO2RR) in 0.1 M KOH, undergoes stepwise surface reconstruction, first to Cu(111) within 30 min, and then to Cu(100) after another 30 min; no further surface transformations occurred after establishment of the Cu(100) surface. The results may help explain the Cu(100)-like behavior of Cu(pc) in terms of CO2RR product selectivity. They likewise suggest that products exclusive to Cu(100) single-crystal electrodes may be generated through the use of readily available inexpensive polycrystalline Cu electrodes. The study highlights the dynamic nature of heterogeneous electrocatalyst surfaces and also underscores the importance of operando interrogations when structure-composition-reactivity correlations are intended.

We report density functional theory (M06L) calculations including Poisson-Boltzmann solvation to determine the reaction pathways and barriers for the hydrogen evolution reaction (HER) on MoS2, using both a periodic two-dimensional slab and a Mo10S21 cluster model. We find that the HER mechanism involves protonation of the electron rich molybdenum hydride site (Volmer-Heyrovsky mechanism), leading to a calculated free energy barrier of 17.9 kcal/mol, in good agreement with the barrier of 19.9 kcal/mol estimated from the experimental turnover frequency. Hydronium protonation of the hydride on the Mo site is 21.3 kcal/mol more favorable than protonation of the hydrogen on the S site because the electrons localized on the Mo-H bond are readily transferred to form dihydrogen with hydronium. We predict the Volmer-Tafel mechanism in which hydrogen atoms bound to molybdenum and sulfur sites recombine to form H2 has a barrier of 22.6 kcal/mol. Starting with hydrogen atoms on adjacent sulfur atoms, the Volmer-Tafel mechanism goes instead through the M-H + S-H pathway. In discussions of metal chalcogenide HER catalysis, the S-H bond energy has been proposed as the critical parameter. However, we find that the sulfur-hydrogen species is not an important intermediate since the free energy of this species does not play a direct role in determining the effective activation barrier. Rather we suggest that the kinetic barrier should be used as a descriptor for reactivity, rather than the equilibrium thermodynamics. This is supported by the agreement between the calculated barrier and the experimental turnover frequency. These results suggest that to design a more reactive catalyst from edge exposed MoS2, one should focus on lowering the reaction barrier between the metal hydride and a proton from the hydronium in solution.