Wang Gao

Theoretical Elucidation of the Competitive Electro-oxidation Mechanisms of Formic Acid on Pt(111)

The mechanisms of formic acid (HCOOH) oxidation on Pt(111) under electrochemical conditions have been studied using density functional theory and then compared with the analogous gas-phase reaction. Results show that HCOOH oxidation under a water-covered surface behaves substantially differently than in the gas phase or using a solvation model involving only a few water molecules. Using these models, we evaluated the detailed reaction process, including energies and geometric structures of intermediates and transition states under the influence of different solvation models and electrode potentials. Our calculations indicate that this potential-dependent electrochemical oxidation proceeds via a multipath mechanism (involving both the adsorbed HCOOH and HCOO intermediates), a result succinctly rationalizing conflicting experimental observations. Moreover, this study highlights how subtle changes in electrochemical reaction environments can influence (electro)catalysis.

Nickel Cluster Growth on Defect Sites of Graphene: A Computational Study

The first extraction of graphene in 2004 led to a wide range of experimental and theoretical studies aimed at better understanding and exploiting the unique properties of this novel two-dimensional material. Among the many potential applications, which have been suggested, are uses of graphene as a substrate in high-performance catalysis and as a component in circuit-board technology. In particular, graphene s high surface area and conductivity have motivated proposals to use it as a substrate for growing and/or anchoring metal nanoparticles in high-performance catalysts and other electrochemical devices. However, the activity of such carbon-supported metal catalysts is strongly dependent on the dispersion and stability of the metal clusters on the support (i.e. the ability of the substrate to stabilize metal clusters of various sizes on its surface). Thus, vacancy defects are expected to play a vital role in making graphene suitable for these applications by supplying highly active binding sites for adsorbing and stabilizing metal clusters. Indeed, finite populations of single and double vacancy defects are thermodynamically stable in graphene, and have been studied extensively. Density functional theory (DFT) calculations revealed that vacancy defects resulting from the removal of up to five C atoms reconstruct to form non-hexagonal rings (models are shown in the Supporting Information: Figures S1.b–f). Even larger holes have been observed in electron microscopy experiments. Defects may also play a critical role in using graphene components for circuit fabrication. For example, taking advantage of the Dirac fermions in graphene requires opening up its band gap to convert it from a conductor into a semiconductor. This conversion can be achieved by doping graphene with either B or N atoms; however, another possibility for accomplishing this could be the adsorption of small metal clusters on the surface. Because the adsorption of such clusters can be used to tune additional magnetic and transport properties of the substrate, it might also provide a technique for controlling an additional set of electromagnetic properties. The catalytic nature of Ni is well established, and Ni nanoparticles are commonly used to catalyze the synthesis of carbon nanostructures. Owing to the strong affinity between Ni and C, the incorporation of Ni atoms into carbon nanostructures, grown using Ni catalysts, has been observed. Ushiro et al. reported that X-ray adsorption measurements detect Ni impurities in carbon nanostructures following nickel-catalyzed synthesis, which even treatment with acid is not able to remove. Moreover, Banhart et al. identified Ni impurities wrapped in onion-like graphenic particles by using electron microscopy. The work of Rinaldi et al. is even more supportive. Combining results from DFT calculations and high-resolution transmission electron microcopy measurements (HR-TEM) utilizing several in situ characterization techniques, they concluded that Ni atoms form very stable Ni– C compounds during nickel-catalyzed carbon nanotube (CNT) growth, which are incorporated into the final products. They also found unexpectedly strong adsorption of the Ni clusters on the CNT supports. However, despite the potential advantages of using Ni nanoparticles adsorbed on graphene, their catalytic and electromagnetic properties (with the exception of single and two Ni atoms adsorbates) remain mostly unexplored. Based on these findings, it would be expected that just as Ni nanoparticles might be used to tailor critical properties of defective graphene sheets, a graphene substrate might be used to modify the catalytic properties of nickel nanoparticles as well. To elucidate this potential interplay we employ DFT to study the adsorption of Nin nanoclusters on defective graphene (details in the Supporting Information). As substrate models we select graphene sheets with vacancy defects, resulting from the removal of x atoms (with x 5; see Figure S1 in the Supporting Information). To model the adsorbed Ni nanoparticles, we successively grew Nin clusters with n 10 and focused on the lowest energy adsorption configuration of each Nin cluster on each of these six graphene substrates (with and without vacancy defects). The binding energies (referenced against single Ni atoms and the graphene substrate) for the lowest energy configuration are summarized in Figure 1. The binding energies can be explained by three types of bond contributions. The first type of binding is between Ni atoms. As the cluster size increases the ratio of bulk to surface atoms increases so that the binding energy will asymptotically [*] Dr. W. Gao, Dr. J. E. Mueller, Dr. J. Anton, Prof. Dr. T. Jacob Institut f r Elektrochemie, Universit t Ulm 89081 Ulm (Germany) E-mail: timo.jacob@uni-ulm.de

The Role of Co-Adsorbed CO and OH in the Electrooxidation of Formic Acid on Pt(111)**

The electrooxidation of formic acid (HCOOH) on platinumgroupmetals has been widely studied for its great relevance to electrochemistry as a prototype reaction for the electrooxidation of small organics and its importance in understanding low-temperature fuel cells. 2] It is generally accepted that electrooxidation of HCOOH on Pt proceeds by a dual-path mechanism consisting of indirect and direct paths. In the indirect path, HCOOH is converted into adsorbed CO and then to CO2. In the direct path, HCOOH is converted into CO2 via a reactive intermediate, whose identity is still disputed. Unfortunately, the intermediates from both the indirect and direct paths compete with each another for adsorption sites and the opportunity to react with oxidizers (e.g. OH) on the surface. This competition couples these reaction paths kinetically, hampering the elucidation of their individual reaction mechanisms. In situ infrared reflection-adsorption spectroscopy (IRAS) identifies adsorbed CO, resulting from HCOOH dehydration, as the key reaction intermediate in the indirect path. However, a build-up of CO is observed to poison the system. In contrast, the identity of the reactive intermediate along the direct path is still controversial. Wilhelm and coworkers initially suggested either COH or CHO. Others have long assumed it to be COOH. Using IR spectroscopy, Osawa et al. and Feliu et al. found that formate (HCOO) is the reactive intermediate and that the oxidation of HCOO to CO2 is the rate-determining step for formic acid oxidation. In contrast, Behm et al. argue that weakly adsorbed HCOOH might be the key intermediate, leaving HCOO as a spectator. The electrochemical and spectral data obtained under both static and flow conditions, which provide the basis for these proposed reactive intermediates, are essentially identical. However, different interpretations of the non-linear relationship between the measured current and the formate coverage lead to different conclusions. Cyclic voltammograms (CVs) of HCOOH oxidation (Figure 1) show that the current (I) first peaks around 0.6 V as the potential (U) increases. The current then remains stable or decreases between 0.6 and 0.8 V in what is termed the negative differential resistance (NDR) region. A sharp, increasing around 0.95 V follows the NDR region. 10,12–14] During this process, IR spectroscopy measurements reveal that the polycrystalline Pt surface has a relatively constant coverage of CO below 0.8 V. Above 0.8 V, the coverage of adsorbed CO rapidly decreases due to oxidation, while HCOO quickly increases with increasing potential, until the surface is nearly saturated with HCOO at above 0.9 V. Once the CO coverage is almost completely depleted (ca. 0.95 V), the coverage of HCOO rapidly decreases with increasing potential up to 1.2 V. Thus, these IR measurements suggest that the CV curve can be understood in terms of CO adsorption, desorption, and oxidation by OH. Nevertheless, the detailed roles of adsorbed CO and OH have yet to be elucidated, substantially hindering our understanding of the mechanism of this fundamental reaction. First principles simulations have already been useful in studying HCOOH oxidation. Two density functional theory (DFT) studies indicate that HCOOH oxidation under electrochemical conditions either proceeds via intermediate COOH or initiates from a weakly adsorbed configuration of HCOOH, in which the C H bond is in a “down” configuration. However, several independent theoretical investigations report that the HCOOH adsorption models used in these studies do not correspond to the most energetically favorable structure. More recently, using a gas phase model (i.e. without treating solvation effects), we found that bidentate formate (HCOOB*) is the reactive intermediate in HCOOH oxidation. This result is fully consistent with Figure 1. The potential-dependent rate constants (R) and the experimental CV. The potential sweep rate is 50 mVs .

Facile Synthesis of Non-Graphitizable Polypyrrole-Derived Carbon/Carbon Nanotubes for Lithium-ion Batteries

Graphite is usually used as an anode material in the commercial lithium ion batteries (LIBs). The relatively low lithium storage capacity of 372 mAh g–1 and the confined rate capability however limit its large-scale applications in electrical vehicles and hybrid electrical vehicles. As results, exploring novel carbon-based anode materials with improved reversible capacity for high-energy-density LIBs is urgent task. Herein we present TNGC/MWCNTs by synthesizing tubular polypyrrole (T-PPy) via a self-assembly process, then carbonizing T-PPy at 900 °C under an argon atmosphere (TNGC for short) and finally mixing TNGC with multi-walled carbon nanotubes (MWCNTs). As for TNGC/MWCNTs, the discharge capacity of 561 mAh g−1 is maintained after 100 cycles at a current density of 100 mA g−1. Electrochemical results demonstrate that TNGC/MWCNTs can be considered as promising anode materials for high-energy-density LIBs.

Revealing the active intermediates in the oxidation of formic acid on Au and Pt(111).

The mechanisms of formic acid (HCOOH) oxidation on Au(111) under gas-phase and electrochemical conditions was studied by using density functional theory and then compared with the analogous processes on Pt(111). Our results demonstrate that a mechanism involving a single intermediate molecule is preferred on both Au and Pt(111). Furthermore, under gas-phase conditions, HCOOH oxidation proceeds through the same mechanism (formate pathway) on Au and Pt(111), whereas under electrochemical conditions, it can take place through significantly different mechanisms (formate and/or direct pathways), depending on the applied electrode potential. Our calculations help to rationalize conflicting experimental explanations and are crucial for understanding the mechanism of this fundamental (electro-)catalytic process.

Mechanistic Insights into the Unique Role of Copper in CO2 Electroreduction Reactions.

Cu demonstrates a unique capability towards CO2 electroreduction that can close the anthropogenic carbon cycle; however, its reaction mechanism remains elusive, owing to the obscurity of the solid-liquid interface on Cu surfaces where electrochemical reactions occur. Using a genetic algorithm method in addition to density functional theory, we explicitly identify the configuration of a water bilayer on Cu(2 1 1) and build electrochemical models. These enable us to reveal a mechanistic picture for CO2 electroreduction, finding the key intermediates CCO* for the C2 H4 pathway and CH* for the CH4 pathway, which rationalize a series of experimental observations. Furthermore, we find that the interplay between the Cu surfaces, carbon monomers, and water network (but not the binding of CO*) essentially determine the unique capability of Cu towards CO2 electroreduction, proposing a new and effective descriptor for exploiting optimal catalysts.

Design Principles of Inert Substrates for Exploiting Gold Clusters’ Intrinsic Catalytic Reactivity

Ultralow stability of gold clusters prohibits the understanding of their intrinsic reactivity (that is vital for revealing the origin of gold’s catalytic properties). Using density functional theory including many-body dispersion method, we aim to ascertain effective ways in exploiting gold clusters’ intrinsic reactivity on carbon nanotubes (CNTs). We find that the many body van der Waals interactions are essential for gold clusters’ reactivity on CNTs and even for O2 activation on these supported clusters. Furthermore, curvature and dopant of CNTs are found to qualitatively change the balance between physisorption and chemisorption for gold clusters on CNTs, determining the clusters’ morphology, charge states, stability, and reactivity, which rationalize the experimental findings. Remarkably, N doped small curvature CNTs, which effectively stabilize gold clusters and retain their inherent geometric/electronic structures, can be promising candidates for exploiting gold clusters’ intrinsic reactivity.

Advances in Cathode Materials for High-Performance Lithium-Sulfur Batteries

Lithium-sulfur batteries (LSBs) represent a promising energy storage technology, and they show potential for next-generation high-energy systems due to their high specific capacity, abundant constitutive resources, non-toxicity, low cost, and environment friendliness. Unlike their ubiquitous lithium-ion battery counterparts, the application of LSBs is challenged by several obstacles, including short cycling life, limited sulfur loading, and severe shuttling effect of polysulfides. To make LSBs a viable technology, it is very important to design and synthesize outstanding cathode materials with novel structures and properties. In this review, we summarize recent progress in designs, preparations, structures, and properties of cathode materials for LSBs, emphasizing binary, ternary, and quaternary sulfur-based composite materials. We especially highlight the utilization of carbons to construct sulfur-based composite materials in this exciting field. An extensive discussion of the emerging challenges and possible future research directions for cathode materials for LSBs is provided.

Steric Hindrance in Sulfur Vacancy of Monolayer MoS2 Boosts Electrochemical Reduction of Carbon Monoxide to Methane

The efficient generation of methane by total electroreduction of carbon monoxide (CO) could be of benefit for a more sustainable society. However, a highly efficient and selective catalyst for this process remains to be developed. In this study, density functional theory calculations indicate that steric hindrance in monolayer molybdenum sulfide with 2 S vacancies (DV-MoS2 ) can facilitate the conversion of CO into CH4 with high activity and selectivity under electrochemical reduction at a low potential of -0.53 V vs. RHE and ambient conditions. The potential is a significant improvement on the state-of-the-art Cu electrode (-0.74 V vs. RHE), with less electrical energy. Moreover, the results suggest that such steric hindrance effects are important for structure-sensitive catalytic reactions.

An Integrated Design with new Metal-Functionalized Covalent Organic Frameworks for the Effective Electroreduction of CO2

One of the long-standing issues that prohibits large-scale CO2 reutilization is the low aqueous solubility of CO2 and the incurring inefficient mass transport of CO2 . Herein, we suggest a feasible way to promote the CO2 reutilization by integrating the storage and reduction, with a new covalent organic framework (COF) series constituted by cobalt-phthalocyanine and boronic acid linkers. We find that the porous structure of the cobalt COF is competitive in the CO2 storage and can sustain a high CO2 concentration around the reduction center, whereas the mass transport of CO2 as well as the efficiency of the CO2 reduction is significantly improved. The predicted cobalt COF exhibits an overpotential of 0.27 V and a CO production rate, which is 97.7 times higher than in aqueous solution, for the CO2 reduction. Our work provides a promising candidate for the CO2 reutilization, with valuable insights and an important prototype for future practical design.