The composition-dependent hydrogen evolution reaction (HER) activity of Co 2−x Rh x P nanoparticles in acid is reported. The motivation for the current study stems from (1) prior work demonstrating that, although costly, Rh 2 P nanoparticles are highly active and stable toward the HER process and (2) the expectation that diluting Rh sites with Co will result in catalytic synergism while also lowering the overall cost of the material. Here, we establish that the HER activity of Co 2−x Rh x P nanoparticles in acidic media is composition-dependent, with Rh-rich electrocatalysts showing superior activity as compared to those that are Co-rich. Additionally, compositions of Co 2−x Rh x P for which x ≥ 1.25, where the materials adopt the cubic antifluorite structure, deliver comparable initial catalytic activities to pure Rh 2 P, suggesting that the crystal structure of the material may play a more significant role in driving the overall HER activity than the composition. Despite comparable activity to Rh 2 P, Co 2−x Rh x P systems do not have the stability associated with Rh 2 P but undergo a drop from 10 to 5 mA/cm 2 within the first hour of stability testing, associated with Co loss from the surface. In cases where Pt is used as the counter electrode, no such drop in current density is observed, despite the loss of Co, with Pt transfer to the working electrode compensating for the Co depletion. Firstprinciples calculations based on density functional theory show that both the hydrogen binding energies and the Gibbs free energies of hydrogen adsorption increase linearly with x, with Co 0.75 Rh 1.25 P exhibiting a ΔG value that is closest to zero, suggesting that this composition is the most active for HER in this series. Double-layer capacitance data, from which electrochemical surface area (ECSA) data for all of the compositions are computed, are used to demonstrate that the quality and quantity of active sites among different compositions of Co 2−x Rh x P can vary significantly, even when the morphologies and particle sizes are similar.

Interfaces formed between monolayer transition metal dichalcogenides and (metallo)phthalocyanine molecules are promising in energy applications and provide a platform for studying mixed-dimensional molecule-semiconductor heterostructures in general. An accurate characterization of the frontier energy level alignment at these interfaces is key in the fundamental understanding of the charge transfer dynamics between the two photon absorbers. Here, we employ the first-principles substrate screening GW approach to quantitatively characterize the quasiparticle electronic structure of a series of interfaces: metal-free phthalocyanine (H2Pc) adsorbed on monolayer MX2 (M = Mo, W; X = S, Se) and zinc phthalocyanine (ZnPc) adsorbed on MoX2 (X = S, Se). Furthermore, we reveal the dielectric screening effect of the commonly used α-quartz (SiO2) substrate on the H2Pc:MoS2 interface using the dielectric embedding GW approach. Our calculations furnish a systematic set of GW results for these interfaces, providing the structure–property relationship across a series of similar systems and benchmarks for future experimental and theoretical studies.

Quasi-two-dimensional (2D) fully π–d conjugated metal–organic frameworks (MOFs) have been widely employed as active materials of secondary batteries; however, the origin of their high charge storage capacity is still unknown. Some reports have proposed a mechanism by assuming the formation of multiple radicals on one organic ligand, although there is no firm evidence for such a mechanism, which would run counter to the resonance theory. In this work, we utilized various magnetometric techniques to monitor the formation and concentration of paramagnetic species during the electrochemical process of 2D π–d conjugated Cu-THQ MOF (THQ = tetrahydroxy-1,4-benzoquinone). The spin concentration of the fully reduced (discharged 1.5 V) electrode was estimated to be around only 0.1 spin-1/2 per CuO4 unit, which is much lower than that of the expected “diradical” form. More interestingly, a significant elevation of the temperature-independent paramagnetic term was simultaneously observed, which indicates the presence of delocalized π electrons in this discharged state. Such results were corroborated by first-principles density functional theory calculations and the electrochemically active density of states, which reveal the microscopic mechanism of the charge storage in the Cu-THQ MOF. Hence, a graphite-like charge storage mechanism, where the π-electron band accepts/donates electrons during the charge/discharge process, was suggested to explain the excessive charge storage of Cu-THQ. This graphite-like charge storage mechanism revealed by magnetic studies can be readily generalized to other π–d conjugated MOFs.

A prototypical organic photovoltaic material is a heterojunction composed of the blend of regioregular poly(3hexylthiophene) (P3HT) and [6,6]-phenyl-C 61 -butyric acid methyl ester (PCBM). Microscopic understanding of the energy conversion mechanism in this system involves the relationship between the electronic structure and the atomistic geometry of P3HT:PCBM interfaces. In this work, the effect of the number of P3HT layers on the electronic structure of the P3HT:PCBM interface is studied by means of first-principles GW. We apply the substrate screening approach to accelerate such calculations and to better understand the many-body dielectric screening at the interface. The quasiparticle band gap of the entire interface is found to decrease as the number of P3HT layers increases. The gaps of the individual components of the interface are found to be smaller than those of their isolated counterparts, with strong dependence on the number of P3HT layers. Importantly, when comparing the P3HT:PCBM system, where a single interface is present, and the P3HT:PCBM:P3HT system, where an interface is formed on either side of PCBM, we find that the two systems exhibit very different quasiparticle energy level alignments. We discuss the possible implications of our findings in related experiments. The observed trends in the layer-dependent quasiparticle electronic structures of P3HT:PCBM interfaces provide computational insight into energy conversion pathways in these materials.

Excited-state processes at organic-inorganic interfaces consisting of molecular crystals are essential in energy conversion applications. While advances in experimental methods allow direct observation and detection of exciton transfer across such junctions, a detailed understanding of the underlying excitonic properties due to crystal packing and interface structure is still largely lacking. In this work, we use many-body perturbation theory to study structure-property relations of excitons in molecular crystals upon adsorption on a gold surface. We explore the case of the experimentally-studied octyl perylene diimide (C8-PDI) as a prototypical system, and use the GW and Bethe-Salpeter equation (BSE) approach to quantify the change in quasiparticle and exciton properties due to intermolecular and substrate screening. Our findings provide a close inspection of both local and environmental structural effects dominating the excitation energies and the exciton binding and nature, as well as their modulation upon the metal-organic interface composition.