Platinum Monolayer Electrocatalysts for Anodic Oxidation of Alcohols

The slow, incomplete oxidation of methanol and ethanol on platinum-based anodes as well as the high price and limited reserves of Pt has hampered the practical application of direct alcohol fuel cells. We describe the electrocatalysts consisting of one Pt monolayer (one atom thick layer) placed on extended or nanoparticle surfaces having the activity and selectivity for the oxidation of alcohol molecules that can be controlled with platinum-support interaction. The suitably expanded Pt monolayer (i.e., Pt/Au(111)) exhibits a factor of 7 activity increase in catalyzing methanol electrooxidation relative to Pt(111). Sizable enhancement is also observed for ethanol electrooxidation. Furthermore, a correlation between substrate-induced lateral strain in a Pt monolayer and its activity/selectivity is established and rationalized by experimental and theoretical studies. The knowledge we gained with single-crystal model catalysts was successfully applied in designing real nanocatalysts. These findings for alcohols are likely to be applicable for the oxidation of other classes of organic molecules

Size- and Composition-Dependent Enhancement of Electrocatalytic Oxygen Reduction Performance in Ultrathin Palladium–Gold (Pd_1–<i>x</i>Au_<i>x</i>) Nanowires

In this report, we examine the composition- and size-dependent performance in hierarchical Pd_1–<i>x</i>Au_<i>x</i> nanowires (NWs) encapsulated with a conformal Pt monolayer shell (Pt∼Pd_1–<i>x</i>Au_<i>x</i>). The ultrathin Pd_1–<i>x</i>Au_<i>x</i> NWs are prepared by a solution-based method wherein the chemical composition can be readily and predictably controlled. Importantly, as-prepared Pd₉Au NWs maintain significantly enhanced oxygen reduction reaction (ORR) activity (0.40 mA/cm²), as compared with elemental Pd NW/C (0.12 mA/cm²) and Pt nanoparticles (NP)/C (0.20 mA/cm²), respectively. After the deposition of a Pt monolayer, a volcano-type composition dependence is observed in the ORR activity of the Pt∼Pd_1–<i>x</i>Au_<i>x</i> NWs as the Au content is increased from 0 to 30% with the activity of the Pt∼Pd₉Au NWs (0.98 mA/cm², 2.54 A/mg_Pt), representing the optimum performance. We note that the platinum group metal activity of the ultrathin 2 nm NWs (0.64 A/mg) is significantly enhanced as compared with that of analogous 50 nm NWs (0.16 A/mg) and commercial Pt NP/C (0.1–0.2 A/mg), thereby highlighting a distinctive size-dependent enhancement in NW performance

Molybdenum Nitrides as Oxygen Reduction Reaction Catalysts: Structural and Electrochemical Studies

Monometallic (δ-MoN, Mo₅N₆, and Mo₂N) and bimetallic molybdenum nitrides (Co_0.6Mo_1.4N₂) were investigated as electrocatalysts for the oxygen reduction reaction (ORR), which is a key half-reaction in hydrogen fuel cells. Monometallic hexagonal molybdenum nitrides are found to exhibit improved activities over rock salt type molybdenum nitride (γ-Mo₂N), suggesting that improvements are due to either the higher molybdenum valence or a more favorable coordination environment in the hexagonal structures. Further enhancements in activity were found for hexagonal bimetallic cobalt molybdenum nitride (Co_0.6Mo_1.4N₂), resulting in a modest onset potential of 0.713 V versus reversible hydrogen electrode (RHE). Co_0.6Mo_1.4N₂ exhibits good stability in acidic environments, and in the potential range lower than 0.5 V versus RHE, the ORR appears to proceed via a four-electron mechanism based on the analysis of rotating disc electrode results. A redetermination of the structures of the binary molybdenum nitrides was carried out using neutron diffraction data, which is far more sensitive to nitrogen site positions than X-ray diffraction data. The revised monometallic hexagonal nitride structures all share many common features with the Co_0.6Mo_1.4N₂ structure, which has alternating layers of cations in octahedral and trigonal prismatic coordination, and are thus not limited to only trigonal prismatic Mo environments (as was originally postulated for δ-MoN)

Highly Enhanced Electrocatalytic Oxygen Reduction Performance Observed in Bimetallic Palladium-Based Nanowires Prepared under Ambient, Surfactantless Conditions

We have employed an ambient, template-based technique that is simple, efficient, and surfactantless to generate a series of bimetallic Pd_1–<i>x</i>Au_<i>x</i> and Pd_1–<i>x</i>Pt_<i>x</i> nanowires with control over composition and size. Our as-prepared nanowires maintain significantly enhanced activity toward oxygen reduction as compared with commercial Pt nanoparticles and other 1D nanostructures, as a result of their homogeneous alloyed structure. Specifically, Pd₉Au and Pd₄Pt nanowires possess oxygen reduction reaction (ORR) activities of 0.49 and 0.79 mA/cm², respectively, which are larger than the analogous value for commercial Pt nanoparticles (0.21 mA/cm²). In addition, core–shell Pt∼Pd₉Au nanowires have been prepared by electrodepositing a Pt monolayer shell and the corresponding specific, platinum mass, and platinum group metal mass activities were found to be 0.95 mA/cm², 2.08 A/mg_Pt, and 0.16 A/mg_PGM, respectively. The increased activity and catalytic performance is accompanied by improved durability toward ORR

Mixed Close-Packed Cobalt Molybdenum Nitrides as Non-noble Metal Electrocatalysts for the Hydrogen Evolution Reaction

A two-step solid-state reaction for preparing cobalt molybdenum nitride with a nanoscale morphology has been used to produce a highly active and stable electrocatalyst for the hydrogen evolution reaction (HER) under acidic conditions that achieves an <i>i</i>R-corrected current density of 10 mA cm^–2 at −0.20 V vs RHE at low catalyst loadings of 0.24 mg/cm² in rotating disk experiments under a H₂ atmosphere. Neutron powder diffraction and pair distribution function (PDF) studies have been used to overcome the insensitivity of X-ray diffraction data to different transition-metal nitride structural polytypes and show that this cobalt molybdenum nitride crystallizes in space group <i>P</i>6₃/<i>mmc</i> with lattice parameters of <i>a</i> = 2.85176(2) Å and <i>c</i> = 10.9862(3) Å and a formula of Co_0.6Mo_1.4N₂. This space group results from the four-layered stacking sequence of a mixed close-packed structure with alternating layers of transition metals in octahedral and trigonal prismatic coordination and is a structure type for which HER activity has not previously been reported. Based on the accurate bond distances obtained from time-of-flight neutron diffraction data, it is determined that the octahedral sites contain a mixture of divalent Co and trivalent Mo, while the trigonal prismatic sites contain Mo in a higher oxidation state. X-ray photoelectron spectroscopy (XPS) studies confirm that at the sample surface nitrogen is present and N–H moieties are abundant

Mixed Close-Packed Cobalt Molybdenum Nitrides as Non-noble Metal Electrocatalysts for the Hydrogen Evolution Reaction

A two-step solid-state reaction for preparing cobalt molybdenum nitride with a nanoscale morphology has been used to produce a highly active and stable electrocatalyst for the hydrogen evolution reaction (HER) under acidic conditions that achieves an <i>i</i>R-corrected current density of 10 mA cm^–2 at −0.20 V vs RHE at low catalyst loadings of 0.24 mg/cm² in rotating disk experiments under a H₂ atmosphere. Neutron powder diffraction and pair distribution function (PDF) studies have been used to overcome the insensitivity of X-ray diffraction data to different transition-metal nitride structural polytypes and show that this cobalt molybdenum nitride crystallizes in space group <i>P</i>6₃/<i>mmc</i> with lattice parameters of <i>a</i> = 2.85176(2) Å and <i>c</i> = 10.9862(3) Å and a formula of Co_0.6Mo_1.4N₂. This space group results from the four-layered stacking sequence of a mixed close-packed structure with alternating layers of transition metals in octahedral and trigonal prismatic coordination and is a structure type for which HER activity has not previously been reported. Based on the accurate bond distances obtained from time-of-flight neutron diffraction data, it is determined that the octahedral sites contain a mixture of divalent Co and trivalent Mo, while the trigonal prismatic sites contain Mo in a higher oxidation state. X-ray photoelectron spectroscopy (XPS) studies confirm that at the sample surface nitrogen is present and N–H moieties are abundant

Mixed Close-Packed Cobalt Molybdenum Nitrides as Non-noble Metal Electrocatalysts for the Hydrogen Evolution Reaction

A two-step solid-state reaction for preparing cobalt molybdenum nitride with a nanoscale morphology has been used to produce a highly active and stable electrocatalyst for the hydrogen evolution reaction (HER) under acidic conditions that achieves an <i>i</i>R-corrected current density of 10 mA cm^–2 at −0.20 V vs RHE at low catalyst loadings of 0.24 mg/cm² in rotating disk experiments under a H₂ atmosphere. Neutron powder diffraction and pair distribution function (PDF) studies have been used to overcome the insensitivity of X-ray diffraction data to different transition-metal nitride structural polytypes and show that this cobalt molybdenum nitride crystallizes in space group <i>P</i>6₃/<i>mmc</i> with lattice parameters of <i>a</i> = 2.85176(2) Å and <i>c</i> = 10.9862(3) Å and a formula of Co_0.6Mo_1.4N₂. This space group results from the four-layered stacking sequence of a mixed close-packed structure with alternating layers of transition metals in octahedral and trigonal prismatic coordination and is a structure type for which HER activity has not previously been reported. Based on the accurate bond distances obtained from time-of-flight neutron diffraction data, it is determined that the octahedral sites contain a mixture of divalent Co and trivalent Mo, while the trigonal prismatic sites contain Mo in a higher oxidation state. X-ray photoelectron spectroscopy (XPS) studies confirm that at the sample surface nitrogen is present and N–H moieties are abundant

Mixed Close-Packed Cobalt Molybdenum Nitrides as Non-noble Metal Electrocatalysts for the Hydrogen Evolution Reaction

A two-step solid-state reaction for preparing cobalt molybdenum nitride with a nanoscale morphology has been used to produce a highly active and stable electrocatalyst for the hydrogen evolution reaction (HER) under acidic conditions that achieves an <i>i</i>R-corrected current density of 10 mA cm^–2 at −0.20 V vs RHE at low catalyst loadings of 0.24 mg/cm² in rotating disk experiments under a H₂ atmosphere. Neutron powder diffraction and pair distribution function (PDF) studies have been used to overcome the insensitivity of X-ray diffraction data to different transition-metal nitride structural polytypes and show that this cobalt molybdenum nitride crystallizes in space group <i>P</i>6₃/<i>mmc</i> with lattice parameters of <i>a</i> = 2.85176(2) Å and <i>c</i> = 10.9862(3) Å and a formula of Co_0.6Mo_1.4N₂. This space group results from the four-layered stacking sequence of a mixed close-packed structure with alternating layers of transition metals in octahedral and trigonal prismatic coordination and is a structure type for which HER activity has not previously been reported. Based on the accurate bond distances obtained from time-of-flight neutron diffraction data, it is determined that the octahedral sites contain a mixture of divalent Co and trivalent Mo, while the trigonal prismatic sites contain Mo in a higher oxidation state. X-ray photoelectron spectroscopy (XPS) studies confirm that at the sample surface nitrogen is present and N–H moieties are abundant

Mixed Close-Packed Cobalt Molybdenum Nitrides as Non-noble Metal Electrocatalysts for the Hydrogen Evolution Reaction

A two-step solid-state reaction for preparing cobalt molybdenum nitride with a nanoscale morphology has been used to produce a highly active and stable electrocatalyst for the hydrogen evolution reaction (HER) under acidic conditions that achieves an <i>i</i>R-corrected current density of 10 mA cm^–2 at −0.20 V vs RHE at low catalyst loadings of 0.24 mg/cm² in rotating disk experiments under a H₂ atmosphere. Neutron powder diffraction and pair distribution function (PDF) studies have been used to overcome the insensitivity of X-ray diffraction data to different transition-metal nitride structural polytypes and show that this cobalt molybdenum nitride crystallizes in space group <i>P</i>6₃/<i>mmc</i> with lattice parameters of <i>a</i> = 2.85176(2) Å and <i>c</i> = 10.9862(3) Å and a formula of Co_0.6Mo_1.4N₂. This space group results from the four-layered stacking sequence of a mixed close-packed structure with alternating layers of transition metals in octahedral and trigonal prismatic coordination and is a structure type for which HER activity has not previously been reported. Based on the accurate bond distances obtained from time-of-flight neutron diffraction data, it is determined that the octahedral sites contain a mixture of divalent Co and trivalent Mo, while the trigonal prismatic sites contain Mo in a higher oxidation state. X-ray photoelectron spectroscopy (XPS) studies confirm that at the sample surface nitrogen is present and N–H moieties are abundant

Mixed Close-Packed Cobalt Molybdenum Nitrides as Non-noble Metal Electrocatalysts for the Hydrogen Evolution Reaction

A two-step solid-state reaction for preparing cobalt molybdenum nitride with a nanoscale morphology has been used to produce a highly active and stable electrocatalyst for the hydrogen evolution reaction (HER) under acidic conditions that achieves an <i>i</i>R-corrected current density of 10 mA cm^–2 at −0.20 V vs RHE at low catalyst loadings of 0.24 mg/cm² in rotating disk experiments under a H₂ atmosphere. Neutron powder diffraction and pair distribution function (PDF) studies have been used to overcome the insensitivity of X-ray diffraction data to different transition-metal nitride structural polytypes and show that this cobalt molybdenum nitride crystallizes in space group <i>P</i>6₃/<i>mmc</i> with lattice parameters of <i>a</i> = 2.85176(2) Å and <i>c</i> = 10.9862(3) Å and a formula of Co_0.6Mo_1.4N₂. This space group results from the four-layered stacking sequence of a mixed close-packed structure with alternating layers of transition metals in octahedral and trigonal prismatic coordination and is a structure type for which HER activity has not previously been reported. Based on the accurate bond distances obtained from time-of-flight neutron diffraction data, it is determined that the octahedral sites contain a mixture of divalent Co and trivalent Mo, while the trigonal prismatic sites contain Mo in a higher oxidation state. X-ray photoelectron spectroscopy (XPS) studies confirm that at the sample surface nitrogen is present and N–H moieties are abundant