Amorphous Cobalt Vanadium Oxide as a Highly Active Electrocatalyst for Oxygen Evolution

The water-splitting reaction provides a promising mechanism to store renewable energies in the form of hydrogen fuel. The oxidation half-reaction, the oxygen evolution reaction (OER), is a complex four-electron process that constitutes an efficiency bottleneck in water splitting. Here we report a highly active OER catalyst, cobalt vanadium oxide. The catalyst is designed on the basis of a volcano plot of metal–OH bond strength and activity. The catalyst can be synthesized by a facile hydrothermal route. The most active pure-phase material (a-CoVOx) is X-ray amorphous and provides a 10 mA cm–2 current density at an overpotential of 347 mV in 1 M KOH electrolyte when immobilized on a flat substrate. The synthetic method can also be applied to coat a high-surface-area substrate such as nickel foam. On this three-dimensional substrate, the a-CoVOx catalyst is highly active, reaching 10 mA cm–2 at 254 mV overpotential, with a Tafel slope of only 35 mV dec–1. This work demonstrates a-CoVOx as a promising electrocatalyst for oxygen evolution and validates M–OH bond strength as a practical descriptor in OER catalysis.

Synthesis and Application of Transition Metal Oxides as Oxygen Evolution Catalysts

Laurent Liardet

Nowadays, mankind is facing an important energy challenge. Depletion in fossil fuels reserves and increasing energy demand, coupled with the problematic greenhouse effect induced by massive anthropogenic release of carbon dioxide into the atmosphere, are driving forces for the development of renewable and carbon-free energy technologies. Hydrogen gas is considered as the energy carrier of the future and is expected to replace fossil fuels in order to create a Hydrogen Economy. Hydrogen is viable for decarbonization of our society only if it is produced from sustainable and renewable energies. Water splitting, the redox reaction producing hydrogen and oxygen gas by reducing and oxidizing water, respectively, is a promising technique to produce clean hydrogen. However, the kinetics of the oxidative half-reaction, namely the oxygen evolution reaction (OER), are slow and catalysts are required in order to increase the global efficiency of water splitting. Moreover, in order to perform water splitting at large scale, catalysts based on Earth-abundant elements should be developed. This thesis focuses on the development of transition metal oxides catalysts, by different synthesis techniques, for electrochemical and photoelectrochemical oxygen evolution. Chapter 2 focuses on the atomic layer deposition (ALD) of different cobalt-based metal oxides and the development of procedures to fabricate bi- and trimetallic oxides catalysts. Cobalt vanadium oxide (CoVOx), cobalt iron oxide (CoFeOx) as well as cobalt vanadium iron oxide (CoVFeOx) are deposited by ALD and evaluated for OER. The ALD deposition method allowed us to apply these material on high-surface area nickel foam substrates. Among them, CoFeOx shows the highest catalytic activity for water oxidation and can compete with state-of-the art electrocatalysts for OER in alkaline solution. In Chapter 3, we focused on developing a photoelectrodeposition method to deposit CoFeOx on nanocauliflower hematite photoanodes. This photoelectrodeposition method allowed the deposition of an ultrathin, amorphous and optically transparent CoFeOx layer that induced a large cathodic shift of the photocurrent onset potential of hematite and a high enhancement of the photocurrent at 1.0 V vs RHE. Additionally, the reason of the improvement of the onset potential was explored in detail in order to differentiate between possible activity enhancement effects such as an increase of charge transfer kinetics, a reduction of surface recombination, a modification of the flat band potential or an increase of the photovoltage. We concluded that CoFeOx improved the OER efficiency of nanocauliflower hematite photoanodes by purely enhancing the kinetics of water oxidation. Chapter 4 details the development of a hydrothermal route to fabricate CoVOx. On the basis of a volcano plot that correlates the OER activity to the M-OH bond strength of metal oxides, CoVOx was predicted to be a highly active electrocatalyst that could compete with the most active OER electrocatalysts in alkaline solution. After characterization, this hydrothermally synthesized cobalt vanadium oxide was seen to be amorphous and provided high catalytic activity towards OER. This chapter, in addition to increasing the library of OER electrocatalysts, demonstrates the usefulness of M-OH bond strength as a simple descriptor for experimental development of OER catalysts.

EPFL2019

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

Oxidatively Electrodeposited Thin-Film Transition Metal (Oxy)hydroxides as Oxygen Evolution Catalysts

Xile Hu, Laurent Liardet, Carlos Gilberto Morales Guio

The electrolysis of water to produce hydrogen and oxygen is a simple and attractive approach to store renewable energies in the form of chemical fuels. The oxygen evolution reaction (OER) is a complex four-electron process that constitutes the most energy-inefficient step in water electrolysis. Here we describe a novel electrochemical method for the deposition of a family of thin-film transition metal (oxy)hydroxides as OER catalysts. The thin films have nanodomains of crystallinity with lattice spacing similar to those of double-layered hydroxides. The loadings of these thin-film catalysts were accurately determined with a resolution of below 1 mu g cm(-2) using an electrochemical quartz microcrystal balance. The loading-activity relations for various catalysts were established using voltammetry and impedance spectroscopy. The thin-film catalysts have up to four types of loading-activity dependence due to film nucleation and growth as well as the resistance of the films. A zone of intrinsic activity has been identified for all of the catalysts where the mass-averaged activity remains constant while the loading is increased. According to their intrinsic activities, the metal oxides can be classified into three categories: NiOx, MnOx, and FeOx belong to category I, which is the least active; CoOx and CoNiOx belong to category II, which has medium activity; and FeNiOx, CoFeOx, and CoFeNiOx belong to category III, which is the most active. The high turnover frequencies of CoFeOx and CoFeNiOx at low overpotentials and the simple deposition method allow the fabrication of high-performance anode electrodes coated with these catalysts. In 1 M KOH and with the most active electrode, overpotentials as low as 240 and 270 mV are required to reach 10 and 100 mA cm(-2), respectively.