Computational Quantum Chemistry Insights into the Mechanism of VO2 + Reduction on Graphene-Based Electrodes.

The identity of active sites for redox reactions within vanadium redox flow batteries (VRFBs) isstill controversial despite decades of research into the matter. Here, we use density functional theory to examine the premise of selected surface functional groups as active sites and provide mechanistic insights into the reaction pathway for the positive electrode reaction. The adsorption of electroactive species on phenol and carbene-like edge carbon sites was compared using model aromatic clusters. Phenol groups were not favorable sites for the chemisorption of VO2 + in either V-down or O-down approach In contrast, carbene-like edge carbon sites readily adsorbed VO2 + via an oxygen-down approach, mimicking gas-phase CO2 adsorption mechanisms. Subsequent steps to complete the reaction pathway are a series of proton adsorptions and reaction products desorption. The rate-determining step for a reaction pathway using an edge site is VO2+ desorption step with a Gibbs energy of activation of +84 kcal mol-1 .

IRC data for a mechanistic route starting with H2O adsorption and finishing with H2 desorption from graphene.

Intrinsic reaction coordinate (IRC) data regarding the interactions of water with a carbene-like active site located at the edge of a polyaromatic hydrocarbon [1-3] has been obtained using density functional theory (DFT) and the 6-31g(d) basis set as implemented in the Gaussian 16 software [4]. The data is presented as two videos (frontal and lateral mechanism views) combining four consecutive IRC calculations corresponding to the four different transition states presented on "https://doi.org/10.1016/j.carbon.2020.01.011" [3] (Figure 6, side approach). These videos provide powerful insights on two key aspects: a) the rotational process that occurs during water adsorption and b) the hydrogen gas desorption process during water gasification of carbons.

Spin density distributions on graphene clusters and ribbons with carbene-like active sites.

Aiming to better understand the reactivity of graphene-based materials, the present work employs density functional theory that provides detailed information about spin-density distributions for single and contiguous pairs of carbene-like active sites. In order to examine the extent to which different models, methodologies, and approximations affect the outcome, our calculations employ the AIMPRO, QuantumEspresso and Gaussian program packages. Models are in the form of polycyclic aromatic hydrocarbons (PAHs) and graphene nanoribbons (GNRs), both isolated and within supercells with periodic boundary conditions. Benchmarking calculations for the phenyl radical and cation are also presented. General agreement is found among the methods and also with previous studies. A significant electron spin polarization (spin density >1.096 electron spin) on the active sites is seen in both periodic and cluster systems, but it tends to be lower for GNRs than graphene clusters. The effect of the functional seems to be much more important than the position of singularities at the edges of the GNRs. Finally, we show the interactions and effects on spin density when a single site lies at the edge of a bilayer GNR, where bonding between layers may occur under specific circumstances.

Gate-voltage control of oxygen diffusion on graphene.

We analyze the diffusion of oxygen atoms on graphene and its dependence on the carrier density controlled by a gate voltage. We use density functional theory to determine the equilibrium adsorption sites, the transition state, and the attempt frequency for different carrier densities. The ease of diffusion is strongly dependent on carrier density. For neutral graphene, we calculate a barrier of 0.73 eV; however, upon electron doping the barrier decreases almost linearly to reach values as low as 0.15 eV for densities of -7.6×10(13) cm(-2). This implies an increase of more than 9 orders of magnitude in the diffusion coefficient at room temperature. This dramatic change is due to a combined effect of bonding reduction in the equilibrium state and bonding increase at the transition state and can be used to control the patterning of oxidized regions by an adequate variation of the gate voltage.

Active sites in graphene and the mechanism of CO2 formation in carbon oxidation.

Over the past decade we have witnessed a steady rise in contributions of computational quantum chemistry to the understanding of reactivity of carbon materials. Several litmus tests must be applied to this evolving body of work before it can be viewed with a sufficient degree of confidence. The results of a crucial test are presented here: formulation of thermodynamically and kinetically plausible paths for CO(2) formation in the deceivingly simple reaction C + (1 - y/2)O(2) = (1 - y)CO(2) + yCO. A mechanism is proposed that clarifies the nature of atoms responsible for adsorption and reaction of molecular oxygen on the surface of sp(2)-hybridized carbons, both flat and curved, and is also consistent with the postulate that the (re)active sites are carbene- and carbyne-type carbon atoms at graphene edges. Using density functional theory and representative two-dimensional graphene clusters, a direct and an indirect route to CO(2) formation were identified as both necessary and sufficient to account for key experimental observations. The former involves single-site O(2) adsorption on carbene-type zigzag edges. The latter includes the presence of mobile epoxide-type oxygen on the basal plane and its insertion into an edge hexagon, analogous to the conversion of benzene oxide to oxepin; such "unzipping" of graphene and CO(2) desorption is favored at oxygen-saturated edges, thus accounting for the well-documented phenomenon of induced heterogeneity of carbon reactive sites.

On the chemical nature of graphene edges: origin of stability and potential for magnetism in carbon materials.

Heretofore disconnected experimental observations are combined with a theoretical study to develop a model of the chemical composition of the edges of graphene sheets in both flat and curved sp(2)-hybridized carbon materials. It is proposed that under ambient conditions a significant fraction of the oxygen-free edge sites are neither H-terminated nor unadulterated sigma free radicals, as universally assumed. The zigzag sites are carbene-like, with the triplet ground state being most common. The armchair sites are carbyne-like, with the singlet ground state being most common. This proposal is not only consistent with the key electronic properties and surface (re)activity behavior of carbons, but it can also explain the recently documented and heretofore puzzling ferromagnetic properties of some impurity-free carbon materials.