From Water Oxidation to Reduction: Transformation from NixCo3-xO4 Nanowires to NiCo/NiCoOx Heterostructures

A homologous Ni-Co based nanowire catalyst pair, composed of NixCo3-xO4 nanowires and NiCo/NiCoOx nanohybrid, is developed for efficient overall water splitting. NixCo3-xO4 nanowires are found as a highly active oxygen evolution reaction (OER) catalyst, and they are converted into a highly active hydrogen evolution reaction (HER) catalyst through hydrogenation treatment as NiCo/NiCoOx heteronanostructures. An OER current density of 10 mA cm(-2) is obtained with the NixCo3-xO4 nanowires under an overpotential of 337 mV in 1.0 M KOH, and an HER current density of 10 mA cm(-2) is obtained with the NiCo/NiCoOx heteronanostructures at an overpotential of 155 mV. When integrated in an electrolyzer, these catalysts demonstrate a stable performance in water splitting.

Amorphous molybdenum sulfides as hydrogen evolution catalysts

Xile Hu, Carlos Gilberto Morales Guio

Providing energy for a population projected to reach 9 billion people within the middle of this century is one of the most pressing societal issues. Burning fossil fuels at a rate and scale that satisfy our near-term demand will irreversibly damage the living environment. Among the various sources of alternative and CO2-emission-free energies, the sun is the only source that is capable of providing enough energy for the whole world. Sunlight energy, however, is intermittent and requires an efficient storage mechanism. Sunlight-driven water splitting to make hydrogen is widely considered as one of the most attractive methods for solar energy storage. Water splitting needs a hydrogen evolution catalyst to accelerate the rate of hydrogen production and to lower the energy loss in this process. Precious metals such as Pt are superior catalysts, but they are too expensive and scarce for large-scale applications. In this Account, we summarize our recent research on the preparation, characterization, and application of amorphous molybdenum sulfide catalysts for the hydrogen evolution reaction. The catalysts can be synthesized by electrochemical deposition under ambient conditions from readily available and inexpensive precursors. The catalytic activity is among the highest for nonprecious catalysts. For example, at a loading of 0.2 mg/cm2, the optimal catalyst delivers a current density of 10 mA/cm2 at an overpotential of 160 mV. The growth mechanism of the electrochemically deposited film catalysts was revealed by an electrochemical quartz microcrystal balance study. While different electrochemical deposition methods produce films with different initial compositions, the active catalysts are the same and are identified as a “MoS2+x” species. The activity of the film catalysts can be further promoted by divalent Fe, Co, and Ni ions, and the origins of the promotional effects have been probed. Highly active amorphous molybdenum sulfide particles can also be prepared from simple wet-chemical routes. Electron transport is sometimes slow in the particle catalysts, and an impedance model has been established to identify this slow electron transport. Finally, the amorphous molybdenum sulfide film catalyst has been integrated onto a copper(I) oxide photocathode for photoelectrochemical hydrogen evolution. The conformal catalyst efficiently extracts the excited electrons to give an impressive photocurrent density of −5.7 mA/cm2 at 0 V vs RHE. The catalyst also confers good stability.

American Chemical Society2014

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

Computational Modeling of the Oxygen Evolution Reaction at Semiconductor-Water Interfaces: A Path Towards Breaking Linear Scaling Relationships

Patrick Gono

Photoelectrochemical water splitting is a promising source of clean, renewable fuel in the form of hydrogen. Despite extensive research endeavors, the widespread adoption of this technology is impeded due to suboptimal catalysts for the oxygen evolution reaction (OER) taking place on the anode. Studies have shown that linear scaling relationships exist between the binding energies of the OER intermediates. These relationships are responsible for the emergence of an overpotential which limits the catalytic efficiency of electrochemical cells. This work is devoted to the study of the OER, the emergence of linear scaling relationships, and possible avenues for overcoming them. We employ a selection of ab-initio computational free energy methods based on density functional theory in order to investigate the OER at atomistic model semiconductor interfaces. An approach based on thermodynamic integration and \textit{ab-initio} molecular dynamics simulations is used to isolate the effect of the solvent on the OER free energy steps. The observed effect is twofold. First, the presence of explicit water affects the equilibrium configurations of the reaction intermediates. Second, explicit water modifies the OER free energy steps by up to 0.5 eV. This effect is rationalized in terms of electrostatic interactions at the interface. Next, we verify the existence of the linear scaling relationships across a set of semiconductor and metal interfaces. We establish the robustness of the scaling relationships with respect to the adopted energy functional. Two potential approaches of breaking the linear scaling are considered. First, a bifunctional mechanism that involves two close but functionally different active sites is investigated. The performance of the proposed mechanism is studied on a set of model interfaces, and the results indicate that specific combinations of catalysts may overcome the limitations imposed by the linear scaling relationship. We were able to establish general trends governing the efficiency of the bifunctional mechanism. In particular, a correlation between the valence band maximum and the hydrogen binding energy may be used as a descriptor in future searches for suitable hydrogen acceptors within the bifunctional scheme. Next, a detailed study of NiOOH/FeOOH catalysts identifies features most favorable to the bifunctional mechanism. Overpotentials as low as 0.13 eV are found for specific configurations. These results support the bifunctional mechanism as a means to break the linear scaling relationships. Second, hole polarons at the TiO

_2

surface induced by electronegative adsorbates in the OER intermediates are studied. The computational hydrogen electrode method in conjunction with a hybrid density functional is employed to quantify the effect of the surface polarons on the OER free energy steps. We find that the hole bipolarons reduce the overpotential of the reaction-determining step leading to good agreement with experiment. The stability of the polarons is further confirmed at the hydrated surface through a free energy study involving the explicit treatment of the solvent. Finally, the polaron energy level is aligned with respect to the redox levels in water to better understand the role of the polarons in the OER. Since the occurrence of surface hole polarons is unrelated to the scaling relationships, it offers an additional handle in the search for improved catalysts.

EPFL2020