Atomic Level Insights of Non-Noble Metal Catalysts for the Oxygen Evolution Reaction

The conversion of intermittent renewable energy resources in the form of chemical bond, such as hydrogen production from electrochemical water splitting, is a promising way to satisfy the future global energy demand and address the environmental issues. The oxygen evolution reaction (OER) is the bottleneck among the overall reaction due to its endergonic thermodynamics and complicated 4H+/4e- transfer process. Despite extensive efforts have been made to develop cheap, efficient and robust heterogeneous electrocatalysts for OER, their heterogeneous nature makes the deep investigation of the active sites and reaction mechanisms challenging. Benefiting from high atomic efficiency, excellent intrinsic activity, and well-defined active motifs, the atomically dispersed catalysts provides an opportunity to get insight of the catalysts. The Chapter 2 demonstrates a Co single-atom precatalyst immobilized on N-doped carbon support (Co-N-C) can be transformed to a Co-Fe double-atom catalyst (Co-Fe-N-C) via electrochemical activation in KOH containing Fe impurities. The Co-Fe-N-C exhibited a turnover frequency among one of the highest values of the state-of-the-art OER catalysts. Electrochemical, microscopic, and spectroscopic data, including atomic-resolution electron microscopy and operando X-ray absorption spectroscopy (XAS), reveal an atomically dispersed dimeric Co-Fe moiety as the active site of the Co-Fe-N-C. On the basis of Co-Fe-N-C, the Chapter 3 further describes a general synthesis of Co, Fe, and Ni-containing double-atom catalysts from their single-atom precursors via in situ electrochemical transformation. All these double-atom catalysts have molecule-like bimetallic active sites, which resembles the possible key active centers of bimetallic heterogeneous catalysts. In Chapter 4, a systematic mechanistic insight of the double-atom catalysts and the catalytic process were provided, by combining both electrokinetics analysis and operando XAS results. These mechanisms follow a similar O-O bond forming step and all exhibit bimetallic cooperation, while each mechanisms diverge in the catalytic site and the source of OH- for O-O bond formation as well as the order of proton and electron transfer. The works from Chapter 2-4 suggested that the double-atom catalysts not only provide an attractive molecular platform for fundamental studies of heterogeneous OER electrocatalysts, but also establish a bridge between homogeneous and heterogeneous OER catalysis. The traditional OER catalysts typically occurs via a mechanism involving four consecutive proton-coupled electron transfer (PCET) steps, which has a performance limit imposed by the scaling relationship of various oxygen intermediates. The Chapter 5 describes using operando Raman spectroscopy and electrokinetic analysis to study two active model OER catalysts, FeOOH-NiOOH composite and NiFe layered double hydroxide (LDH). The data that FeOOH-NiOOH operates by a bifunctional mechanism where the rate-determining O-O bond forming step is the OH- attack on a Fe=O coupled with a hydrogen atom transfer to a nearby NiIII-O site, whereas NiFe LDH operates by a conventional mechanism of four consecutive PCET steps. The bifunctional mechanism, hitherto only supported by theoretical computations, has the potential to circumvent the scaling relationship limit of conventional OER mechanism. Thus, this work provides the first experimental evidences to validate such novel mechanisms.

Spectroscopic and Electrokinetic Evidence for a Bifunctional Mechanism of the Oxygen Evolution Reaction**

Lichen Bai, Xile Hu, Seunghwa Lee

A bifunctional oxygen evolution reaction (OER) mechanism, in which the energetically demanding step of the attack of hydroxide on a metal oxo unit is facilitated by a hydrogen atom transfer to a second site, has the potential to circumvent the scaling relationship. However, the bifunctional mechanism has hitherto only been supported by theoretical computations. Here we describe an operando Raman spectroscopic and electrokinetic study of two highly active OER catalysts, FeOOH-NiOOH and NiFe layered double hydroxide (LDH). The data support two distinct mechanisms for the two catalysts: FeOOH-NiOOH operates by a bifunctional mechanism where the rate-determining O-O bond forming step is the OH- attack on a Fe=O coupled with a hydrogen atom transfer to a Ni-III-O site, whereas NiFe LDH operates by a conventional mechanism of four consecutive proton-coupled electron transfer steps. The experimental validation of the bifunctional mechanism enhances the understanding of OER catalysts.

WILEY-V C H VERLAG GMBH2020

Coupling of electrocatalysts to photoabsorbers for solar fuels production

Carlos Gilberto Morales Guio

The direct transformation of sunlight into fuels and chemicals has attracted considerable attention for the potential impact on the storage of solar energies at large scales and the reduction of CO2 emissions. More than 80% of our current global energy consumption comes from the burning of non-renewable fossil fuels which produces enormous quantities of CO2 and contributes to global warming. We believe today that the key to reduce our dependence on exhaustible natural resources and limit the emission of greenhouse gases is the transition to CO2-free renewable energy sources. However, harvesting of energy from renewable sources (i.e. solar, wind, tidal) is intermittent and the deployment of devices for their transformation into useful forms of energy (i.e. electricity, chemicals, fuels, biomass) in a global scale is conditioned upon the improvement in efficiency of the energy storage mechanisms. A promising approach to store energy is the use of sunlight to drive the production of hydrogen fuel from water. Hydrogen produced from water and sunlight can be transformed back to electricity on demand or be used to generate mechanical energy in cars to power the transportation industry. The oxidation of hydrogen generates useable energy and at the same time exhaust clean, carbon-free water as the only product. Therefore, the production of hydrogen using a renewable energy source in an efficient and inexpensive device has the potential to discourage the continued use of fossil fuels and improve our environment. The viability of any mechanism for the storage of sunlight as fuel will depend on the reduction of energy penalties during the transformation process. Electrocatalysts are advanced materials that bring into one active site electrons and molecules in the appropriate orientation to lower activation energy barriers, accelerate rate of transformations and improve overall efficiencies for chemical reactions. The best catalysts for water splitting are precious metals such as Pt, Ir and Ru, which show superior rates of reaction at low overpotentials. However, these noble metals are too expensive and scarce for large scale applications. This thesis summarizes our recent research in the preparation, characterization, and application of Earth-abundant electrocatalysts for solar water splitting. Although in the last decade a great number of efficient and inexpensive electrocatalysts have been reported, methods for the deposition of these materials on the surface of photoabsorbers are rarely reported. This work shows various simple and scalable techniques for the deposition of hydrogen and oxygen evolving electrocatalysts on the surface of photocathodes and photoanodes, respectively. Surface-protected photocahodes were activated for solar hydrogen production in alkaline conditions with MoS2+x, Ni-Mo and Mo2C. A novel method for the deposition of an optically transparent amorphous iron nickel oxide oxygen evolution electrocatalyst is also reported. The catalyst was deposited on both thin film and high-aspect ratio nanostructured hematite photoanodes. The low catalyst loading combined with its high activity at low overpotential results in significant improvement on the onset potential for photoelectrochemical water oxidation. This transparent catalyst further enables the preparation of a stable hematite/perovskite solar cell tandem device, which performs unassisted water splitting.

EPFL2016

Molecular Engineering of Electrode Surfaces for Electrocatalysis

Patrick Eduard Alexa

Electrocatalysts play an important part on the route towards environmental friendly energy sources. They support modern technologies like fuel cells to move further away from the utilization of fossil fuels and the resulting CO2 emission into our atmosphere. The main principle of such energy conversion technologies is to store chemical energy within chemical bonds and to release it as electric energy to fulfill the energy demand of today's society. However energy conversion chemistry in such technologies requires catalysts in order to be effective by providing an alternative reaction pathway that lowers activation energies and maximizes the energy output. Providing suitable catalyst surfaces requires a fundamental knowledge of the interaction of reactants and reaction intermediates with the catalyst surfaces. This concern is addressed in this thesis by fabricating specifically tailored molecular components on metal surfaces, which act as organic/inorganic hybrid electrodes for various energy conversion reactions in electrocatalysis. Characterizing the electrode surface is crucial in order to identify active sites of the catalyst. The molecular structure is investigated by surface sensitive techniques, such as STM and XPS in combination with a home-build transfer system, which enables electrode fabrication in UHV and electrocatalytic experiments in a liquid environment. A detailed characterization of organic polymer components on electrode surfaces is obtained by combining experimental data with MD simulations. The morphology of polymer networks can be successfully mimicked and depending on the chemical composition and the structure, spatial correlations on short length scales are reported. Hybrid electrodes with organic polymer co-catalysts reveal an increased catalytic activity for the HER by providing suitable docking sites that act as catalytic active sites. Reactants and reaction intermediates are stabilized via hydrogen bonding, which results in a higher catalytic turnover. The specific designed structure of the organic co-catalyst helps to identify a fraction of the reaction mechanism and highlights the importance of molecular components in electrocatalysis. The utilization of organic units on electrocatalysts provides also an excellent opportunity in order to use single metal atoms as catalysts. Engineering large pi-systems has no significant impact on the ORR activity, while the distance between active sites and the underlying electrode are crucial for enhancing the catalytic activity. A significant factor in using molecular components in electrocatalysis is the stability of the organics. Using electrografting allows to design organic/inorganic hybrid electrodes with covalent bonds between the molecular component and the metal substrate. Attaching empty porphyrin molecules to substrates introduces a powerful technique in molecular engineering, since empty porphyrin cycles can be metallized with various metal centers that can act as electrocatalysts for different electrochemical reactions. In summary, this thesis provides multiple engineering aspects, which are useful in designing molecular components on metal electrodes for energy conversion. Such hybrid catalyst surfaces are capable of improving the catalytic activity and allow to study the mechanism of catalytic reactions. Implementing organic components provides new routes for tailoring novel materials, which deliver new insight into the complex field of electrocatalysis.