Identifying and Tuning the In Situ Oxygen-Rich Surface of Molybdenum Nitride Electrocatalysts for Oxygen Reduction

Rigorous in situ studies of electrocatalysts are required to enable the design of higher performing materials. Nonplatinum group metals for oxygen reduction reaction (ORR) catalysis containing light elements such as O, N, and C are known to be susceptible to both ex situ and in situ oxidation, leading to challenges associated with ex situ characterization methods. We have previously shown that the bulk O content plays an important role in the activity and selectivity of Mo–N catalysts, but further understanding of the role of composition and morphological changes at the surface is needed. Here, we report the measurement of in situ surface changes to a molybdenum nitride (MoN) thin film under ORR conditions using grazing incidence X-ray absorption and reflectivity. We show that the half-wave potential of MoN can be improved by ∼90 mV by potential conditioning up to 0.8 V versus RHE. Utilizing electrochemical analysis, dissolution monitoring, and surface-sensitive X-ray techniques, we show that under moderate polarization (0.3–0.7 V vs RHE) there is local ligand distortion, O incorporation, and amorphization of the MoN surface, without changes in roughness. Furthermore, with a controlled potential hold procedure, we show that the surface changes concurrent with potential conditioning are stable under ORR relevant potentials. Conversely, at higher potentials (≥0.8 V vs RHE), the film incorporates O, dissolves, and roughens, suggesting that in this higher potential regime, the performance enhancements are due to increased access to active sites. Density functional theory calculations and Pourbaix analysis provide insights into film stability and O incorporation as a function of potential. These findings coupled with in situ electrochemical surface-sensitive X-ray techniques demonstrate an approach to studying nontraditional surfaces in which we can leverage our understanding of surface dynamics to improve performance with the rational, in situ tuning of active sites

Nitride or Oxynitride? Elucidating the Composition–Activity Relationships in Molybdenum Nitride Electrocatalysts for the Oxygen Reduction Reaction

Molybdenum nitride (Mo−N) catalysts have shown promising activity and stability for the oxygen reduction reaction (ORR) in acid. However, the effect of oxygen (O) incorporation (from synthesis, catalysis, or exposure to air) on their activity remains elusive. Here, we use reactive sputtering to synthesize three compositions of thin-film catalysts and use extensive materials characterization to investigate the depth-dependent structure and incorporated O. We show that the as-deposited Mo−N films are highly oxidized both at the surface (>30% O) and in the bulk (3− 21% O) and that the ORR performance is strongly correlated with the bulk structure and composition. Activity for 4e− ORR is highest for compositions with the highest N/O and N/Mo ratio. Furthermore, H2O2 production for the films with moderate O content is comparable to or higher than the most H2O2-selective nonprecious metal catalysts in acidic electrolyte, on a moles per mass or surface area of catalyst basis. Density functional theory provides insight into the energetics of O incorporation and vacancy formation, and we hypothesize that activity trends with O/N ratios can be traced to the varying crystallite phases and their interactions with ORR adsorbates. This work demonstrates the prevalence and significance of O in metal nitride electrocatalysts and motivates further investigation into the role of O in other nonprecious metal materials

Holographic Renormalization for Asymptotically Lifshitz Spacetimes

A variational formulation is given for a theory of gravity coupled to a massive vector in four dimensions, with Asymptotically Lifshitz boundary conditions on the fields. For theories with critical exponent z=2 we obtain a well-defined variational principle by explicitly constructing two actions with local boundary counterterms. As part of our analysis we obtain solutions of these theories on a neighborhood of spatial infinity, study the asymptotic symmetries, and consider different definitions of the boundary stress tensor and associated charges. A constraint on the boundary data for the fields figures prominently in one of our formulations, and in that case the only suitable definition of the boundary stress tensor is due to Hollands, Ishibashi, and Marolf. Their definition naturally emerges from our requirement of finiteness of the action under Hamilton-Jacobi variations of the fields. A second, more general variational principle also allows the Brown-York definition of a boundary stress tensor.Comment: 34 pages, Added Reference

Using Single-Case Experimental Design and Patient-Reported Outcome Measures to Evaluate the Treatment of Cancer-Related Cognitive Impairment in Clinical Practice

Cancer-related cognitive impairment (CRCI) affects a large proportion of cancer survivors and has significant negative effects on survivor function and quality of life (QOL). Treatments for CRCI are being developed and evaluated. Memory and attention adaptation training (MAAT) is a cognitive-behavioral therapy (CBT) demonstrated to improve CRCI symptoms and QOL in previous research. The aim of this article is to describe a single-case experimental design (SCED) approach to evaluate interventions for CRCI in clinical practice with patient-reported outcome measures (PROs). We illustrate the use of contemporary SCED methods as a means of evaluating MAAT, or any CRCI treatment, once clinically deployed. With the anticipated growth of cancer survivorship and concurrent growth in the number of survivors with CRCI, the treatment implementation and evaluation methods described here can be one way to assess and continually improve CRCI rehabilitative services

Application of Nanoparticle Antioxidants to Enable Hyperstable Chloroplasts for Solar Energy Harvesting

The chloroplast contains densely stacked arrays of light-harvesting proteins that harness solar energy with theoretical maximum glucose conversion efficiencies approaching 12%. Few studies have explored isolated chloroplasts as a renewable, abundant, and low cost source for solar energy harvesting. One impediment is that photoactive proteins within the chloroplast become photodamaged due to reactive oxygen species (ROS) generation. In vivo, chloroplasts reduce photodegradation by applying a self-repair cycle that dynamically replaces photodamaged components; outside the cell, ROS-induced photodegradation contributes to limited chloroplast stability. The incorporation of chloroplasts into synthetic, light-harvesting devices will require regenerative ROS scavenging mechanisms to prolong photoactivity. Herein, we study ROS generation within isolated chloroplasts extracted from Spinacia oleracea directly interfaced with nanoparticle antioxidants, including dextran-wrapped nanoceria (dNC) previously demonstrated as a potent ROS scavenger. We quantitatively examine the effect of dNC, along with cerium ions, fullerenol, and DNA-wrapped single-walled carbon nanotubes (SWCNTs), on the ROS generation of isolated chloroplasts using the oxidative dyes, 2’,7’- dichlorodihydrofluorescein diacetate (H_2DCF-DA) and 2,3-bis(2-methoxy-4-nitro-5-sulfophenyl)-2H-tetrazolium-5-carboxanilide sodium salt (XTT). Electrochemical measurements confirm that chloroplasts processed from free solution can generate power under illumination. We find dNC to be the most effective of these agents for decreasing oxidizing species and superoxide concentrations whilst preserving chloroplast photoactivity at concentrations below 5 μM, offering a promising mechanism for maintaining regenerative chloroplast photoactivity for light-harvesting applications

Application of nanoparticle antioxidants to enable hyperstable chloroplasts for solar energy harvesting

The chloroplast contains densely stacked arrays of light-harvesting proteins that harness solar energy with theor. max. glucose conversion efficiencies approaching 12%. Few studies have explored isolated chloroplasts as a renewable, abundant, and low cost source for solar energy harvesting. One impediment is that photoactive proteins within the chloroplast become photodamaged due to reactive oxygen species (ROS) generation. In vivo, chloroplasts reduce photodegrdn. by applying a self-repair cycle that dynamically replaces photodamaged components; outside the cell, ROS-induced photodegrdn. contributes to limited chloroplast stability. The incorporation of chloroplasts into synthetic, light-harvesting devices will require regenerative ROS scavenging mechanisms to prolong photoactivity. Herein, we study ROS generation within isolated chloroplasts extd. from Spinacia oleracea directly interfaced with nanoparticle antioxidants, including dextran-wrapped nanoceria (dNC) previously demonstrated as a potent ROS scavenger. We quant. examine the effect of dNC, along with cerium ions, fullerenol, and DNA-wrapped single-walled carbon nanotubes (SWCNTs), on the ROS generation of isolated chloroplasts using the oxidative dyes, 2',7'- dichlorodihydrofluorescein diacetate (H2DCF-DA) and 2,3-bis(2-methoxy-4-nitro-5-sulfophenyl)-2H-tetrazolium-5-carboxanilide sodium salt (XTT). Electrochem. measurements confirm that chloroplasts processed from free soln. can generate power under illumination. We find dNC to be the most effective of these agents for decreasing oxidizing species and superoxide concns. while preserving chloroplast photoactivity at concns. below 5 μM, offering a promising mechanism for maintaining regenerative chloroplast photoactivity for light-harvesting applications. [on SciFinder(R)

Application of Nanoparticle Antioxidants to Enable Hyperstable Chloroplasts for Solar Energy Harvesting

The chloroplast contains densely stacked arrays of light-harvesting proteins that harness solar energy with theoretical maximum glucose conversion efficiencies approaching 12%. Few studies have explored isolated chloroplasts as a renewable, abundant, and low cost source for solar energy harvesting. One impediment is that photoactive proteins within the chloroplast become photodamaged due to reactive oxygen species (ROS) generation. In vivo, chloroplasts reduce photodegradation by applying a self-repair cycle that dynamically replaces photodamaged components; outside the cell, ROS-induced photodegradation contributes to limited chloroplast stability. The incorporation of chloroplasts into synthetic, light-harvesting devices will require regenerative ROS scavenging mechanisms to prolong photoactivity. Herein, we study ROS generation within isolated chloroplasts extracted from Spinacia oleracea directly interfaced with nanoparticle antioxidants, including dextran-wrapped nanoceria (dNC) previously demonstrated as a potent ROS scavenger. We quantitatively examine the effect of dNC, along with cerium ions, fullerenol, and DNA-wrapped single-walled carbon nanotubes (SWCNTs), on the ROS generation of isolated chloroplasts using the oxidative dyes, 2',7'- dichlorodihydrofluorescein diacetate (H2DCF-DA) and 2,3-bis(2-methoxy-4-nitro-5-sulfophenyl)-2H-tetrazolium-5-carboxanilide sodium salt (XTT). Electrochemical measurements confirm that chloroplasts processed from free solution can generate power under illumination. We find dNC to be the most effective of these agents for decreasing oxidizing species and superoxide concentrations whilst preserving chloroplast photoactivity at concentrations below 5 M, offering a promising mechanism for maintaining regenerative chloroplast photoactivity for light-harvesting applications