There is an urgent global need for electrochemical energy storage that includes materials that can provide simultaneous high power and high energy density. One strategy to achieve this goal is with pseudocapacitive materials that take advantage of reversible surface or near-surface Faradaic reactions to store charge. This allows them to surpass the capacity limitations of electrical double-layer capacitors and the mass transfer limitations of batteries. The past decade has seen tremendous growth in the understanding of pseudocapacitance as well as materials that exhibit this phenomenon. The purpose of this Review is to examine the fundamental development of the concept of pseudocapacitance and how it came to prominence in electrochemical energy storage as well as to describe new classes of materials whose electrochemical energy storage behavior can be described as pseudocapacitive.

We investigate the permeability and selectivity of graphene sheets with designed subnanometer pores using first principles density functional theory calculations. We find high selectivity on the order of 10(8) for H(2)/CH(4) with a high H(2) permeance for a nitrogen-functionalized pore. We find extremely high selectivity on the order of 10(23) for H(2)/CH(4) for an all-hydrogen passivated pore whose small width (at 2.5 A) presents a formidable barrier (1.6 eV) for CH(4) but easily surmountable for H(2) (0.22 eV). These results suggest that these pores are far superior to traditional polymer and silica membranes, where bulk solubility and diffusivity dominate the transport of gas molecules through the material. Recent experimental investigations, using either electron beams or bottom-up synthesis to create pores in graphene, suggest that it may be possible to employ such techniques to engineer variable-sized, graphene nanopores to tune selectivity and molecular diffusivity. Hence, we propose using porous graphene sheets as one-atom-thin, highly efficient, and highly selective membranes for gas separation. Such a pore could have widespread impact on numerous energy and technological applications; including carbon sequestration, fuel cells, and gas sensors.

The 1T phase of transition metal dichalcogenides (TMDs) has been demonstrated in recent experiments to display catalytic activity for hydrogen evolution reaction (HER), but the catalytic mechanism has not been elucidated so far. Herein, using 1T MoS 2 as the prototypical TMD material, we studied the HER activity on its basal plane from periodic density functional theory (DFT) calculations. Compared to the non-reactive basal plane of 2H phase MoS 2 , the catalytic activity of the basal plane of 1T phase MoS 2 mainly arises from its affinity for binding H at the surface S sites. Using the binding free energy (∆G H ) of H as the descriptor, we found that the optimum evolution of H 2 will proceed at surface H coverage of 12.5% ~ 25%. Within this coverage, we examined the reaction energy and kinetic activation barrier for the three elementary steps of the HER process. The Volmer step was found to be facile, while the subsequent Heyrovsky reaction is kinetically more favorable than the Tafel reaction. Our results suggest that at low overpotential, HER can take place readily on the basal plane of 1T MoS 2 via the Volmer-Heyrovsky mechanism. We further screened the dopants for the HER activity and found that substitutional doping of the Mo atom by metals such as Mn, Cr, Cu, Ni, and Fe can make 1T MoS 2 a better HER catalyst.

We report single-atom doping of gold nanoclusters (NCs), and its drastic effects on the optical, electronic, and catalytic properties, using the 25-atom system as a model. In our synthetic approach, a mixture of Pt(1)Au(24)(SC(2)H(4)Ph)(18) and Au(25)(SC(2)H(4)Ph)(18) was produced via a size-focusing process, and then Pt(1)Au(24)(SC(2)H(4)Ph)(18) NCs were obtained by selective decomposition of Au(25)(SC(2)H(4)Ph)(18) in the mixture with concentrated H(2)O(2) followed by purification via size-exclusion chromatography. Experimental and theoretical analyses confirmed that Pt(1)Au(24)(SC(2)H(4)Ph)(18) possesses a Pt-centered icosahedral core capped by six Au(2)(SC(2)H(4)Ph)(3) staples. The Pt(1)Au(24)(SC(2)H(4)Ph)(18) cluster exhibits greatly enhanced stability and catalytic activity relative to Au(25)(SC(2)H(4)Ph)(18) but a smaller energy gap (E(g) ≈ 0.8 eV vs 1.3 eV for the homogold cluster).

The bottom-up assembly of nanoparticles into diverse ordered solids is a challenge because it requires nanoparticles, which are often quasi-spherical, to have interaction anisotropy akin to atoms and molecules. Typically, anisotropy has been introduced by changing the shape of the inorganic nanoparticle core. Here, we present the design, self-assembly, optical properties, and total structural determination of Ag29(BDT)12(TPP)4, an atomically precise tetravalent nanocluster (NC) (BDT, 1,3-benzenedithiol; TPP, triphenylphosphine). It features four unique tetrahedrally symmetrical binding surface sites facilitated by the supramolecular assembly of 12 BDT (wide footprint bidentate thiols) in the ligand shell. When each of these sites was selectively functionalized by a single phosphine ligand, particle stability, synthetic yield, and the propensity to self-assemble into macroscopic crystals increased. The solid crystallized NCs have a substantially narrowed optical band gap compared to that of the solution state, suggesting strong interparticle electronic coupling occurs in the solid state.