Atomically Dispersed Zn/Co-N-C as ORR Electrocatalysts for Alkaline Fuel Cells.

Hydrogen fuel cells have drawn increasing attention as one of the most promising next-generation power sources for future automotive transportation. Developing efficient, durable, and low-cost electrocatalysts, to accelerate the sluggish oxygen reduction reaction (ORR) kinetics, is urgently needed to advance fuel cell technologies. Herein, we report on metal-organic frameworks-derived nonprecious dual metal single-atom catalysts (SACs) (Zn/Co-N-C), consisting of Co-N4 and Zn-N4 local structures. These catalysts exhibited superior ORR activity with a half-wave potential (E1/2) of 0.938 V versus RHE (reversible hydrogen electrode) and robust stability (ΔE1/2 = -8.5 mV) after 50k electrochemical cycles. Moreover, this remarkable performance was validated under realistic fuel cell working conditions, achieving a record-high peak power density of ∼1 W cm-2 among the reported SACs for alkaline fuel cells. Operando X-ray absorption spectroscopy was conducted to identify the active sites and reveal catalytic mechanistic insights. The results indicated that the Co atom in the Co-N4 structure was the main catalytically active center, where one axial oxygenated species binds to form an Oads-Co-N4 moiety during the ORR. In addition, theoretical studies, based on a potential-dependent microkinetic model and core-level shift calculations, showed good agreement with the experimental results and provided insights into the bonding of oxygen species on Co-N4 centers during the ORR. This work provides a comprehensive mechanistic understanding of the active sites in the Zn/Co-N-C catalysts and will pave the way for the future design and advancement of high-performance single-site electrocatalysts for fuel cells and other energy applications.

Effect of Different In2O3(111) Surface Terminations on CO2 Adsorption.

In2O3-based catalysts have shown high activity and selectivity for CO2 hydrogenation to methanol; however, the origin of the high performance of In2O3 is still unclear. To elucidate the initial steps of CO2 hydrogenation over In2O3, we have combined X-ray photoelectron spectroscopy and density functional theory calculations to study the adsorption of CO2 on the In2O3(111) crystalline surface with different terminations, namely, the stoichiometric, reduced, and hydroxylated surface. The combined approach confirms that the reduction of the surface results in the formation of In adatoms and that water dissociates on the surface at room temperature. A comparison of the experimental spectra and the computed core-level shifts (using methanol and formic acid as benchmark molecules) suggests that CO2 adsorbs as a carbonate on all three surface terminations. We find that the adsorption of CO2 is hindered by hydroxyl groups on the hydroxylated surface.

Hydrogen adsorption on In2O3(111) and In2O3(110).

In2O3-Based catalysts have been measured to have a high activity for CO2 hydrogenation to H3COH. Here, we use density functional theory calculations with and without Hubbard-U corrections in combination with ab initio thermodynamics to investigate the dissociative adsorption of H2 over In2O3(111) and In2O3(110). H2 is found to dissociate heterolytically with a moderate barrier on both facets. Diffusion of hydrogen leads to the preferred homolytic adsorption configuration. Vacancy formation by water formation is thermodynamically preferred at high hydrogen coverages. Both surfaces are found to be hydroxylated at typical reaction conditions with the highest coverage predicted for In2O3(110). O 1s core level shifts are calculated for different coverages. The hydroxylated surfaces show two distinct shifts corresponding to different types of OH-groups. The presence of surface oxygen vacancies is not visible in the O 1s signatures. The results show that hydroxylation of the surfaces results in changes of the oxidation state of In-ions, which suggests that the redox properties on In2O3 are important for catalytic reduction of CO2 to added value chemicals.

CO2 adsorption on hydroxylated In2O3(110).

Catalytic synthesis of methanol from CO2 is one route to produce added-value chemicals from a greenhouse gas. Here, density functional theory calculations and ab initio thermodynamics are used to study CO2 adsorption on In2O3(110) in the presence of H2 and H2O. We find that the surface is heavily hydroxylated by either H2 or H2O and that hydroxylation promotes H2-induced vacancy formation. Moreover, CO2 adsorbs rather in a CO2- configuration on hydroxylated In2O3(110) than on oxygen vacancy sites. The results suggest that hydroxylation-induced oxidation-state changes of In-ions play a significant role in CO2 adsorption and activation during methanol synthesis.

Steps Control the Dissociation of CO2 on Cu(100).

CO2 reduction reactions, which provide one route to limit the emission of this greenhouse gas, are commonly performed over Cu-based catalysts. Here, we use ambient pressure X-ray photoelectron spectroscopy together with density functional theory to obtain an atomistic understanding of the dissociative adsorption of CO2 on Cu(100). We find that the process is dominated by the presence of steps, which promote both a lowering of the dissociation barrier and an efficient separation between adsorbed O and CO, reducing the probability for recombination. The identification of steps as sites for efficient CO2 dissociation provides an understanding that can be used in the design of future CO2 reduction catalysts.