vivek murthi - Academia.edu (original) (raw)

Papers by vivek murthi

The advantages of synchrotrons as a unique source of infrared radiation, in the mid and far spect... more The advantages of synchrotrons as a unique source of infrared radiation, in the mid and far spectral region have been clearly demonstrated . In this presentation, we describe techniques involved in studying surface species using infrared microscopy.

The advantages of synchrotrons as a unique source of infrared radiation, in the mid and far spect... more The advantages of synchrotrons as a unique source of infrared radiation, in the mid and far spectral region have been clearly demonstrated . In this presentation, we describe techniques involved in studying surface species using infrared microscopy.

The advantages of synchrotrons as a unique source of infrared radiation, in the mid and far spect... more The advantages of synchrotrons as a unique source of infrared radiation, in the mid and far spectral region have been clearly demonstrated . In this presentation, we describe techniques involved in studying surface species using infrared microscopy.

High cathodic overpotential losses are a common feature of low and medium temperature polymer ele... more High cathodic overpotential losses are a common feature of low and medium temperature polymer electrolyte fuel cells (PEMFCs) which can be attributed in part to electrocatalyst surface poisoning by chemisorbed OH species. Further, other "directly" adsorbed species such as anions from the electrolyte can also poison Pt sites. 1,2 Such directly adsorbed anions also lower the fuel cell current densities by altering the hydrogen oxidation and evolution reactions at the anode. The adsorption of anions are controlled not only by electronic and physical interactions, but also by the geometry of the anion-platinum complex. 3,4 Recently, it has been shown that the Pt adsorbate scattering, visible in the near edge XANES region, is very sensitive to the adsorbate site symmetry. Using a novel analysis technique allows isolation of this Pt-adsorbate scattering, combined with theoretical calculations, allows for the in situ determination of adsorption sites on platinum electrodes utiliz...

Journal of Physical Chemistry C, 2007

X-ray absorption spectroscopy (XAS) is utilized in situ to gain new insights into the electronic ... more X-ray absorption spectroscopy (XAS) is utilized in situ to gain new insights into the electronic and chemical interactions of anions specifically adsorbed on Pt/C. A novel difference methodology was utilized, along with full-multiple scattering calculations using the FEFF8 code, to interpret the X-ray absorption near edge structure (XANES). Significant direct contact ("specific") anion adsorption occurs in 1 M H 2 SO 4 and 6 M TFMSA, while it does not in 1 M HClO 4 and 1 M TFMSA. This specific anion adsorption significantly hinders O(H) chemisorption, particularly formation of subsurface O, causes the Pt nanoparticle to become more round, and weakens the Pt-Pt bonding at the surface. The specific anion adsorption becomes sitespecific only after lateral interactions from other chemisorbed species such as OH on the surface force the anions to adsorb into specific sites. Alloying has a profound effect on the strength of the anion adsorption and whether site-specific or just specific adsorption occurs.

ECS Transactions, 2008

ABSTRACT Nafion thin films (~500 Aå) spin coated onto the Pt and Au layers on Si are investigated... more ABSTRACT Nafion thin films (~500 Aå) spin coated onto the Pt and Au layers on Si are investigated for water uptake and swelling. Neutron Reflectometry data demonstrate that these structures are composed of nafion layers which differ from their bulk properties in terms of their scattering length density and water uptake. However, as the relative humidity is increased, the amount of water in the layers increases for a Nafion on Au and is comparable to that observed on bulk nafion membranes only up to 80 % RH. However, the Nafion on Pt reaches complete hydration at 96% RH. Although, a single layer model was able to fit the NR data obtained on nafion under vapor conditions, only a bilayer model, with an outer less hydrated nafion and an inner more hydrated nafion layer, was able to fit the data in liquid water.

Journal of The Electrochemical Society, 2005

An analysis of X-ray absorption spectroscopy ͑XAS͒ data ͓X-ray absorption near-edge structure ͑XA... more An analysis of X-ray absorption spectroscopy ͑XAS͒ data ͓X-ray absorption near-edge structure ͑XANES͒ and extended X-ray absorption fine structure ͑EXAFS͔͒ at the Pt L 3 edge for Pt-M bimetallic materials ͑M = Co, Cr, Ni, Fe͒ and at the Co K edge for Pt-Co is reported for Pt-M/C electrodes in HClO 4 at different potentials. The XANES data are analyzed using the ⌬ method, which utilizes the spectrum at some potential V minus that at 0.54 V reversible hydrogen electrode ͑RHE͒ representing a reference spectrum. These ⌬ data provide direct spectroscopic evidence for the inhibition of OH chemisorption on the cluster surface in the Pt-M. This OH chemisorption, decreasing in the direction Pt Ͼ Pt-Ni Ͼ Pt-Co Ͼ Pt-Fe Ͼ Pt-Cr, is directly correlated with the previously reported fuel cell performance ͑electrocatalytic activities͒ of these bimetallics, confirming the role of OH poisoning of Pt sites in fuel cells. EXAFS analysis shows that the prepared clusters studied have different morphologies, the Pt-Ni and Pt-Co clusters were more homogeneous with M atoms at the surface, while the Pt-Fe and Pt-Cr clusters had a "Pt skin." The cluster morphology determines which previously proposed OH inhibition mechanism dominates, the electronic mechanism in the presence of the Pt skin, or lateral interactions when M-OH groups exist on the surface.

The advantages of synchrotrons as a unique source of infrared radiation, in the mid and far spect... more The advantages of synchrotrons as a unique source of infrared radiation, in the mid and far spectral region have been clearly demonstrated . In this presentation, we describe techniques involved in studying surface species using infrared microscopy.

The advantages of synchrotrons as a unique source of infrared radiation, in the mid and far spect... more The advantages of synchrotrons as a unique source of infrared radiation, in the mid and far spectral region have been clearly demonstrated . In this presentation, we describe techniques involved in studying surface species using infrared microscopy.

The advantages of synchrotrons as a unique source of infrared radiation, in the mid and far spect... more The advantages of synchrotrons as a unique source of infrared radiation, in the mid and far spectral region have been clearly demonstrated . In this presentation, we describe techniques involved in studying surface species using infrared microscopy.

High cathodic overpotential losses are a common feature of low and medium temperature polymer ele... more High cathodic overpotential losses are a common feature of low and medium temperature polymer electrolyte fuel cells (PEMFCs) which can be attributed in part to electrocatalyst surface poisoning by chemisorbed OH species. Further, other "directly" adsorbed species such as anions from the electrolyte can also poison Pt sites. 1,2 Such directly adsorbed anions also lower the fuel cell current densities by altering the hydrogen oxidation and evolution reactions at the anode. The adsorption of anions are controlled not only by electronic and physical interactions, but also by the geometry of the anion-platinum complex. 3,4 Recently, it has been shown that the Pt adsorbate scattering, visible in the near edge XANES region, is very sensitive to the adsorbate site symmetry. Using a novel analysis technique allows isolation of this Pt-adsorbate scattering, combined with theoretical calculations, allows for the in situ determination of adsorption sites on platinum electrodes utiliz...

Journal of Physical Chemistry C, 2007

X-ray absorption spectroscopy (XAS) is utilized in situ to gain new insights into the electronic ... more X-ray absorption spectroscopy (XAS) is utilized in situ to gain new insights into the electronic and chemical interactions of anions specifically adsorbed on Pt/C. A novel difference methodology was utilized, along with full-multiple scattering calculations using the FEFF8 code, to interpret the X-ray absorption near edge structure (XANES). Significant direct contact ("specific") anion adsorption occurs in 1 M H 2 SO 4 and 6 M TFMSA, while it does not in 1 M HClO 4 and 1 M TFMSA. This specific anion adsorption significantly hinders O(H) chemisorption, particularly formation of subsurface O, causes the Pt nanoparticle to become more round, and weakens the Pt-Pt bonding at the surface. The specific anion adsorption becomes sitespecific only after lateral interactions from other chemisorbed species such as OH on the surface force the anions to adsorb into specific sites. Alloying has a profound effect on the strength of the anion adsorption and whether site-specific or just specific adsorption occurs.

ECS Transactions, 2008

ABSTRACT Nafion thin films (~500 Aå) spin coated onto the Pt and Au layers on Si are investigated... more ABSTRACT Nafion thin films (~500 Aå) spin coated onto the Pt and Au layers on Si are investigated for water uptake and swelling. Neutron Reflectometry data demonstrate that these structures are composed of nafion layers which differ from their bulk properties in terms of their scattering length density and water uptake. However, as the relative humidity is increased, the amount of water in the layers increases for a Nafion on Au and is comparable to that observed on bulk nafion membranes only up to 80 % RH. However, the Nafion on Pt reaches complete hydration at 96% RH. Although, a single layer model was able to fit the NR data obtained on nafion under vapor conditions, only a bilayer model, with an outer less hydrated nafion and an inner more hydrated nafion layer, was able to fit the data in liquid water.

Journal of The Electrochemical Society, 2005

An analysis of X-ray absorption spectroscopy ͑XAS͒ data ͓X-ray absorption near-edge structure ͑XA... more An analysis of X-ray absorption spectroscopy ͑XAS͒ data ͓X-ray absorption near-edge structure ͑XANES͒ and extended X-ray absorption fine structure ͑EXAFS͔͒ at the Pt L 3 edge for Pt-M bimetallic materials ͑M = Co, Cr, Ni, Fe͒ and at the Co K edge for Pt-Co is reported for Pt-M/C electrodes in HClO 4 at different potentials. The XANES data are analyzed using the ⌬ method, which utilizes the spectrum at some potential V minus that at 0.54 V reversible hydrogen electrode ͑RHE͒ representing a reference spectrum. These ⌬ data provide direct spectroscopic evidence for the inhibition of OH chemisorption on the cluster surface in the Pt-M. This OH chemisorption, decreasing in the direction Pt Ͼ Pt-Ni Ͼ Pt-Co Ͼ Pt-Fe Ͼ Pt-Cr, is directly correlated with the previously reported fuel cell performance ͑electrocatalytic activities͒ of these bimetallics, confirming the role of OH poisoning of Pt sites in fuel cells. EXAFS analysis shows that the prepared clusters studied have different morphologies, the Pt-Ni and Pt-Co clusters were more homogeneous with M atoms at the surface, while the Pt-Fe and Pt-Cr clusters had a "Pt skin." The cluster morphology determines which previously proposed OH inhibition mechanism dominates, the electronic mechanism in the presence of the Pt skin, or lateral interactions when M-OH groups exist on the surface.