High-Valent Nickel Promoted by Atomically Embedded Copper for Efficient Water Oxidation

Efficient and low-cost electrocatalysts for oxygen evolution reaction (OER), particularly in neutral conditions, are of significant importance for renewable energy technologies such as CO2 reduction and seawater splitting electrolysis. High-valent transition-metal sites have been considered as OER active sites; however, the rational design and construction of these sites remain a big challenge. Here, we report a trimetallic NiFeCu oxyhydroxide electrocatalyst, in which high-valent Ni sites are promoted and stabilized by the atomically embedded Cu, as evidenced by X-ray photoelectron spectroscopy and X-ray absorption spectroscopy. Through compositional optimization, Ni6Fe1Cu1 catalysts achieved an overpotential of 385 mV at 10 mA cm(-2), a Tafel slope of 164 mV dec(-1), and a stability of 100 h at pH = 7.2. Density function theory calculations demonstrated that the Cu-doping facilitates the formation of high-valent Ni and thus promotes OER electrocatalysis through modulating the d-band center of Ni and reducing the adsorption energy of oxygenated intermediates on the surface of the catalyst. This work paves a promising avenue for the construction of desired high-valent metal OER catalysts by embedding redox inactive metals.

Metal-organic framework derived copper catalysts for CO2 to ethylene conversion

Jun Li, Ning Wang

The electrochemical reduction of CO2 to ethylene provides a carbon-neutral avenue for the conversion of CO2 to value-added fuels and feedstocks, so contributing to the storage of intermittent renewable electricity. The exploration of efficient electrocatalysts with high ethylene selectivity and productivity is highly desirable but remains challenging. Here, we present a Cu-based catalyst derived from a metal-organic framework (Cu-MOF) which shows enhanced performance due to its porous morphology, complex oxidation states and strong lattice strain. X-ray diffraction, X-ray photoelectron spectroscopy, and X-ray absorption spectroscopy are utilized to track the evolution of the crystal structure and oxidation states during the reaction, and the results reveal that Cu2+ ions are rapidly reduced to Cu+ and then slowly to Cu-0, resulting in a Cu@CuxO core@shell structure. The tensile strain caused by the distorted grain is beneficial for the activation of CO2. Cu+/Cu-0 interfaces formed through stabilized Cu+ facilitate *CO-CO dimerization, promoting conversion to C2+ products and suppressing conversion to C-1 products. The optimized catalyst exhibits a 51% Faraday efficiency (FE) for ethylene and a 70% FE for C2+ products, with 20 h operational stability in an H-cell configuration, and a partial ethylene current density of 150mA cm(-2) in a flow-cell configuration.

ROYAL SOC CHEMISTRY2020

Interpretation of x-ray absorption spectroscopy in the presence of surface hybridization

Katharina Johanna Diller

X-ray absorption spectroscopy (XAS) yields direct access to the electronic and geometric structure of hybrid inorganic-organic interfaces formed upon adsorption of complex molecules at metal surfaces. The unambiguous interpretation of corresponding spectra is challenged by the intrinsic geometric flexibility of the adsorbates and the chemical interactions with the interface. Density-functional theory (DFT) calculations of the extended adsorbate-substrate system are an established tool to guide peak assignment in X-ray photoelectron spectroscopy of complex interfaces. We extend this to the simulation and interpretation of XAS data in the context of functional organic molecules on metal surfaces using dispersion-corrected DFT calculations within the transition potential approach. For the prototypical case of 2H-porphine adsorbed on Ag(111) and Cu(111) substrates, we follow the two main effects of the molecule/surface interaction onto the X-ray absorption signatures: (1) the substrate-induced chemical shift of the 1s core levels that dominates in physisorbed systems and (2) the hybridization-induced broadening and loss of distinct resonances that dominate in more chemisorbed systems. Published by AIP Publishing.

American Institute of Physics2017

In situ nanoscale studies of copper-based catalysts for carbon dioxide electroreduction

Karla Banjac

The urgency of climate change demands the simultaneous removal of carbon dioxide from the atmosphere and the transition to renewable energy sources. This aim is realizable through electrochemical reduction of carbon dioxide (CO2RR), which is a promising route for synthesizing renewable carbon-based fuels and chemicals.Among many existing CO2RR catalysts, copper is the only earth-abundant metal that produces energy-rich molecules. Practical application is hindered by its poor product selectivity and modest long-term catalyst stability. To overcome these limitations, an atomistic understanding of the catalytic processes and identification of CO2RR active sites on Cu are needed.Current research implies that the active sites form upon the catalysts' evolution during the reaction. However, fast Cu oxidation hinders their identification in conventional ex situ studies. The following questions remain unanswered: What are the morphology and chemical composition of the active sites? How do they form?This thesis explores the in situ evolution of Cu-based CO2RR catalysts. We study this phenomenon down to the nanoscale through a synergy of in situ microscopy and spectroscopy studies. We employ graphene as a 2D protecting layer on Cu to study the Cu nanoscale structural evolution in situ via electrochemical scanning tunneling microscopy (EC-STM). We use in situ Raman and X-ray photoelectron spectroscopies to study the evolution of the Cu catalysts' oxidation state.First, EC-STM studies reveal the formation of few nanometer-sized cuboids on Cu catalysts as the main structural evolution during CO2RR. We show that this evolution occurs in aqueous electrolytes over a wide range of pH and compositions. Second, we show that metallic Cu is the active phase. Reduction of the Cu oxide, formed either through chemical synthesis or through the exposure of Cu catalysts to air, is unavoidable prior to CO2RR.To understand the nanocuboid formation mechanism, we present a comparative study of the Cu nanocuboids formed upon CO2RR and the Cu2O cubes prepared through electrochemical cycling synthesis. Morphological and chemical composition differences between them allow us to conclude on distinct formation mechanisms. We show that the metallic Cu nanocuboids form during a potential-driven re-organization of atoms in the topmost surface layers.Furthermore, we present EC-STM studies revealing the atomistic details of the Cu nanocuboid formation. By comparing these processes with the multilayer mound growth in Cu homoepitaxy, we show that the nanocuboids are (100) facet multilayers formed upon reduction of Cu oxides. We propose that the nanocuboids are CO2RR active sites, ultimately forming on any Cu catalyst regardless of its initial macroscopic shape and oxidation state.The in situ identification of active sites, presented here, offers a unique framework for the re-interpretation of the existing literature, invites further fundamental theoretical and experimental research, and paves the way for rational catalyst design. Moreover, this thesis further demonstrates the importance of in situ studies across different length scales in understanding key catalytic processes in renewable energy schemes.

EPFL2021

In situ nanoscale studies of copper-based catalysts for carbon dioxide electroreduction

Karla Banjac

EPFL2021