Design Rules for Oxygen Evolution Catalysis at Porous Iron Oxide Electrodes: A 1000-Fold Current Density Increase

Applications of Atomic Layer Deposition in Design of Systems for Energy Conversion

There is a huge demand for clean energy conversion in all industries. The clean energy production processes include electrocatalytic and photocatalytic conversion of water to hydrogen, carbon dioxide reduction, nitrogen conversion to ammonia, and oxygen reduction reaction and require novel cheap and efficient photo- and electrocatalysts and their scalable methods of fabrication. Atomic layer deposition is a thin film deposition method that allows to deposit thin layers of catalysts on virtually any surface of any shape, size, and porosity in an even and easy to control manner. Here the state of the art in applications of atomic layer deposition in the clean energy production and the opportunities it represents for the whole field of the photo- and electrocatalysis for a sustainable future are reviewed.

Stabilizing an ultrathin MoS2 layer during electrocatalytic hydrogen evolution with a crystalline SnO2 underlayer

Amorphous MoS₂ has been investigated abundantly as a catalyst for hydrogen evolution. Not only its performance but also its chemical stability in acidic conditions have been reported widely. However, its adhesion has not been studied systematically in the electrochemical context. The use of MoS₂ as a lubricant is not auspicious for this purpose. In this work, we start with a macroporous anodic alumina template as a model support, add an underlayer of SnO₂ to provide electrical conduction and adhesion, then provide the catalytically active, amorphous MoS₂ material by atomic layer deposition (ALD). The composition, morphology, and crystalline or amorphous character of all layers are confirmed by spectroscopic ellipsometry, X-ray photoelectron spectroscopy, grazing incidence X-ray diffractometry, scanning electron microscopy and energy dispersive X-ray spectroscopy. The electrocatalytic water reduction performance of the macroporous AAO/SnO₂/MoS₂ electrodes, quantified by voltammetry, steady-state chronoamperometry and electrochemical impedance spectroscopy, is improved by annealing the SnO₂ layer prior to MoS₂ deposition. Varying the geometric parameters of the electrode composite yields an optimized performance of 10 mA cm⁻² at 0.22 V overpotential, with a catalyst loading of 0.16 mg cm⁻². The electrode's stability is contingent on SnO₂ crystallinity. Amorphous SnO₂ allows for a gradual dewetting of the originally continuous MoS₂ layer over wide areas. In stark contrast to this, crystalline SnO₂ maintains the continuity of MoS₂ until at least 0.3 V overpotential.

A molybdenum disulfide coating deposited on a macroporous substrate as an electrocatalyst is mobile on an underlying amorphous tin dioxide substrate, but remains continuous and impervious to acidic conditions on crystalline tin dioxide.

Influence of Chiral Compounds on the Oxygen Evolution Reaction (OER) in the Water Splitting Process

Results are presented concerning the influence on the water splitting process of enantiopure tartaric acid present in bulk solution. Stainless steel and electrodeposited nickel are used as working electrode (WE) surface. The latter is obtained by electrodeposition on the two poles of a magnet. The influence and role played by the chiral compound in solution has been assessed by comparing the current values, in cyclic voltammetry (CV) experiments, recorded in the potential range at which oxygen evolution reaction (OER) occurs. In the case of tartaric acid and nickel WE a spin polarization of about 4% is found. The use of the chiral environment (bulk solution) and ferromagnetic chiral Ni electrode allows for observing the OER at a more favorable potential: About 50 mV (i.e., a cathodic, less positive, shift of the potential at which the oxygen evolution is observed).

Fabrication of γ-Fe2O3 Nanowires from Abundant and Low-cost Fe Plate for Highly Effective Electrocatalytic Water Splitting

Water splitting is thermodynamically uphill reaction, hence it cannot occur easily, and also highly complicated and challenging reaction in chemistry. In electrocatalytic water splitting, the combination of oxygen and hydrogen evolution reactions produces highly clean and sustainable hydrogen energy and which attracts research communities. Also, fabrication of highly active and low cost materials for water splitting is a major challenge. Therefore, in the present study, γ-Fe₂O₃ nanowires were fabricated from highly available and cost-effective iron plate without any chemical modifications/doping onto the surface of the working electrode with high current density. The fabricated nanowires achieved the current density of 10 mA/cm² at 1.88 V vs. RHE with the scan rate of 50 mV/sec. Stability measurements of the fabricated Fe₂O₃ nanowires were monitored up to 3275 sec with the current density of 9.6 mA/cm² at a constant potential of 1.7 V vs. RHE and scan rate of 50 mV/sec.

Mechanistic studies of atomic layer deposition on oxidation catalysts – AlOx and POx deposition

Atomic layer deposition of phosphorus oxide on divanadium pentoxide powder undergoes controllable redox chemistry.

Additive Nano-Lithography with Focused Soft X-rays: Basics, Challenges, and Opportunities

Focused soft X-ray beam induced deposition (FXBID) is a novel technique for direct-write nanofabrication of metallic nanostructures from metal organic precursor gases. It combines the established concepts of focused electron beam induced processing (FEBIP) and X-ray lithography (XRL). The present setup is based on a scanning transmission X-ray microscope (STXM) equipped with a gas flow cell to provide metal organic precursor molecules towards the intended deposition zone. Fundamentals of X-ray microscopy instrumentation and X-ray radiation chemistry relevant for FXBID development are presented in a comprehensive form. Recently published proof-of-concept studies on initial experiments on FXBID nanolithography are reviewed for an overview on current progress and proposed advances of nanofabrication performance. Potential applications and advantages of FXBID are discussed with respect to competing electron/ion based techniques.

Electrocatalytic Performance of Titania Nanotube Arrays Coated with MoS2 by ALD toward the Hydrogen Evolution Reaction

The electrochemical splitting of water provides an elegant way to store renewable energy, but it is limited by the cost of the noble metals used as catalysts. Among the catalysts used for the reduction of water to hydrogen, MoS₂ has been identified as one of the most promising materials as it can be engineered to provide not only a large surface area but also an abundance of unsaturated and reactive coordination sites. Using Mo[NMe₂]₄ and H₂S as precursors, a desired thickness of amorphous MoS₂ can be deposited on TiO₂ nanotubes by atomic layer deposition. The identity and structure of the MoS₂ film are confirmed by spectroscopic ellipsometry, X-ray photoelectron spectroscopy, scanning electron microscopy, and energy dispersive X-ray spectroscopy. The electrocatalytic performance of MoS₂ is quantified as it depends on the tube length and the MoS₂ layer thickness through voltammetry, steady-state chronoamperometry, and electrochemical impedance spectroscopy. The best sample reaches 10 mA/cm² current density at 189 mV overpotential in 0.5 M H₂SO₄. All of the various geometries of our nanostructured electrodes reach an electrocatalytic proficiency comparable with the state-of-the-art MoS₂ electrodes, and the dependence of performance parameters on geometry suggests that the system can even be improved further.

Nanoporous water oxidation electrodes with a low loading of laser-deposited Ru/C exhibit enhanced corrosion stability

For the oxidation of water to dioxygen, oxide-covered ruthenium metal is known as the most efficient catalyst, however, with limited stability. Herein, we present a strategy for incorporating a Ru/C composite onto a novel nanoporous electrode surface with low noble metal loading and improved stability. The Ru/C is coated on the pore walls of anodic alumina templates in a one-step laser-induced deposition method from Ru₃(CO)₁₂ solutions. Scanning electron microscopy proves the presence of a continuous Ru/C layer along the inner pore walls. The amorphous material consists of metallic Ru incorporated in a carbonaceous C matrix as shown by X-ray diffraction combined with Raman and X-ray photoelectron spectroscopies. These porous electrodes reveal enhanced stability during water oxidation as compared to planar samples at pH 4. Finally, their electrocatalytic performance depends on the geometric parameters and is optimized with 13 μm pore length, which yields 2.6 mA cm^-2, or 49 A g^-1, at η = 0.20 V.

Direct oxygen isotope effect identifies the rate-determining step of electrocatalytic OER at an oxidic surface

Understanding the mechanism of water oxidation to dioxygen represents the bottleneck towards the design of efficient energy storage schemes based on water splitting. The investigation of kinetic isotope effects has long been established for mechanistic studies of various such reactions. However, so far natural isotope abundance determination of O₂ produced at solid electrode surfaces has not been applied. Here, we demonstrate that such measurements are possible. Moreover, they are experimentally simple and sufficiently accurate to observe significant effects. Our measured kinetic isotope effects depend strongly on the electrode material and on the applied electrode potential. They suggest that in the case of iron oxide as the electrode material, the oxygen evolution reaction occurs via a rate-determining O−O bond formation via nucleophilic water attack on a ferryl unit.

Understanding reaction mechanisms is crucial for catalyst design. Here, natural-abundance isotope quantifications of O₂ yield mechanistically significant reaction kinetic isotope effects for water oxidation over metal oxide electrodes, the bottleneck step of water electrolysis.

Local Activities of Hydroxide and Water Determine the Operation of Silver-Based Oxygen Depolarized Cathodes

Local ion activity changes in close proximity to the surface of an oxygen depolarized cathode (ODC) were measured by scanning electrochemical microscopy (SECM). While the operating ODC produces OH^- ions and consumes O₂ and H₂ O through the electrocatalytic oxygen reduction reaction (ORR), local changes in the activity of OH^- ions and H₂ O are detected by means of a positioned Pt microelectrode serving as an SECM tip. Sensing at the Pt tip is based on the pH-dependent reduction of PtO and obviates the need for prior electrode modification steps. It can be used to evaluate the coordination numbers of OH^- ions and H₂ O, and the method was exploited as a novel approach of catalyst activity assessment. We show that the electrochemical reaction on highly active catalysts can have a drastic influence on the reaction environment.