Search

Electrolytic formic acid synthesis delivers green formic acid at the right concentration for emerging applications.

ispartof: Green and Sustainable Chemistry vol:39 status: published

We report on thin-film coatings and catalyst nanoparticles both deposited by atomic layer deposition (ALD) on silicon (Si) photocathodes that simultaneously enhance their performance and stability. Regarding thin-film coatings, we scan various materials (SiO2, TiO2 and Al2O3) appropriate for the electrical passivation of Si surface defects, which result in high minority carrier lifetimes (up to 100 μs). However, these passivating layers seem to act as a barrier for the photogenerated carriers obstructing them from participating in the hydrogen evolution reaction, thus limiting the onset potential (Von). Regarding platinum (Pt) nanoparticles, two deposition approaches are followed based on using different co-reactants during the ALD process, i.e. an O2 pulse or a N2-plasma pulse, with the latter resulting in a higher Von (505 mV). By combining thin-film SiO2 coatings of various thicknesses deposited on top of ALD Pt nanoparticles, a synergetic effect of performance and stability enhancement is observed, with Von values reaching 525 mV. Finally, by systematically studying the Si photocathodes in a day/night cycle operation and using the case of electroless deposited Pt nanoparticles as a benchmark, the N2-plasma deposited Pt nanoparticles coated by ultra-thin SiO2 film show an enhanced stability (85 h-4 days/night cycles).

Recirculation is a chemical engineering concept generally applied to enhance the conversion of reagents. In common applications, by-products are removed together with the desired products and only the unconverted feed is recirculated. In the concept proposed here to enhance the electrochemical reduction of CO2 to formate or formic acid, by-products are recirculated to the reactor together with unconverted CO2. Initial accumulation of by-products like H2 in the reactor shifts the electrochemical formation potential of these undesired by-products to higher values and favours in this way the formation of formic acid, for which the potential is unaffected. Mathematical simulation of this recirculation process revealed formate selectivity to be substantially improved when by-products are recirculated, even at low pressure. The concept was experimentally validated in an experiment with a membrane electrode assembly reactor. In gas mixtures containing 0.07 MPa of H2 and 0.03 MPa of CO2, the hydrogen selectivity of the electrochemical reduction was decreased by ca. 40% as opposed to pure CO2. This leads to a significant increase in the formate selectivity and energy efficiency. By-product recirculation to enhance the reaction selectivity may be advantageous to other applications of electrocatalysis suffering from low selectivity of the electrocatalyst.

© 2018 Elsevier B.V. The production of renewable hydrogen gas with a vapor-fed solar hydrogen generator is an appealing strategy. In such all solid-state standalone device no liquid electrolyte is required. The use of an anion exchange membrane separating cathode and anode compartments is particularly advantageous and its performance matches the performance of liquid phase electrolysis. In this work, a vapor-fed water electrolyzer was investigated comprising a poly(vinyl alcohol) anion exchange membrane impregnated with 4M KOH solution and Ni-based catalysts. The performance was evaluated over time periods of hours to days. The vapor fed device showed similar initial activity as an electrolysis setup with the same electrodes immersed in 1M KOH liquid electrolyte. To sustain a current density of 10 mA/cm² it was observed that the required potential initially increased but eventually reached a steady state at ca. 1.9 V. From a D2O isotope labelling experiment of the vapor phase, it was noticed that the system initially consumed H2O absorbed in the membrane to produce H2. After this water of the membrane was consumed, water to be split is taken from the vapor phase and water mass transfer limitation starts occurring. Once water mass transfer and splitting reactions are matched, a steady state potential is reached. The results of this work are encouraging and pave the way to efficient stable operation of vapor-fed solar hydrogen generators. ispartof: Catalysis Today vol:334 pages:243-248 ispartof: location:BELGIUM, Univ Catholique Louvain, Louvain la Neuve status: published

ispartof: INDUSTRIAL & ENGINEERING CHEMISTRY RESEARCH vol:60 issue:31 pages:11812-11823 status: published

The development of active and stable earth-abundant catalysts for hydrogen and oxygen evolution is one of the requirements for successful production of solar fuels. Atomic Layer Deposition (ALD) is a proven technique for conformal coating of structured (photo)electrode surfaces with such electrocatalyst materials. Here, we show that ALD can be used for the deposition of iron and cobalt phosphate electrocatalysts. A PE-ALD process was developed to obtain cobalt phosphate films without the need for a phosphidation step. The cobalt phosphate material acts as a bifunctional catalyst, able to also perform hydrogen evolution after either a thermal or electrochemical reduction step. ispartof: Nanoscale Advances vol:1 issue:10 pages:4166-4172 ispartof: location:England status: published

Nafion proton exchange membranes dehydrate when they are used in the gas phase and in high-temperature applications, such as fuel cells and (photo)electrolysis. Retaining a high level of membrane hydration under such conditions can be achieved by using inorganic fillers, but has never been demonstrated for thin films. Herein, several types of siliceous nanoparticles were incorporated for the first time into Nafion thin films. For composite Nafion materials, increased water uptake does not always induce increased proton conductivity. Here, increased water uptake did result in higher proton conductivity due to a synergistic effect within the composite film. The nanocomposites displayed a higher water uptake than could be expected based on the water uptake of the individual materials. Excess water present at the Nafion-filler interface was found to cause the proton conductivity of nanocomposite Nafion/Ludox AS-40 thin films to double compared with pristine Nafion at low relative humidity (from 2 to 4 mS cm-2 ). Knowledge about the properties of such interfaces will allow for the better design of self-humidifying nanocomposite Nafion membranes, films, and catalyst layers.

The concept of an all-gas-phase photo-electrochemical cell (PEC) producing hydrogen gas from volatile organic contaminated gas and light is presented. Without applying any external bias, organic contaminants are degraded and hydrogen gas is produced in separate electrode compartments. The system works most efficiently with organic pollutants in inert carrier gas. In the presence of oxygen gas, the cell performs less efficiently but still significant photocurrents are generated, showing the cell can be run on organic contaminated air. The purpose of this study is to demonstrate new application opportunities of PEC technology and to encourage further advancement toward photo-electrochemical remediation of air pollution with the attractive feature of simultaneous energy recovery and pollution abatement.

Extraction of water vapour from atmospheric air and condensing it to liquid water for human usage is an imaginative solution to the water scarcity problem. Atmospheric water vapour is a large and readily accessible fresh water source able to fulfil human water needs. Many systems that draw water vapour from the air with water collecting surfaces, desiccant materials such as zeolites, silica gels, MOFs, polymers and salts and aids such as membranes have been proposed. Much progress has been made in increasing water harvesting efficiency, reducing cost and improving applicability especially in the extreme atmospheric conditions of arid regions. But all these systems are energy intensive and this energy demand for water production is an important element of the water-energy nexus. In this paper the intrinsic energy requirements of water vapour capturing processes in different atmospheric conditions are quantified as the specific water yield (L kW−1 h−1). Distinction is made between passive systems that use natural phenomena like solar energy directly, and active systems with human transformation of the energy vector. The generation of thermoelectric energy involves water use and may even lead to overall water consumption instead of production. Technologies involving air cooling to provoke condensation of the water vapour reach specific water yields of 1–4 L kW−1 h−1 but their application is strongly dependent on atmospheric conditions. A specific water yield of 0.1–1 L kW−1 h−1 is commonly achieved for an ad/absorption–desorption cycle with a desiccant material. Depending on climate conditions, either passive systems with desiccants or active cooling of condensation surfaces is energy wise the optimum choice. The intrinsic energy requirements of atmospheric water harvesting are more than hundred times larger than seawater desalination. Fundamentally new concepts are needed to make atmospheric water an affordable fresh water source.

Early career (doctoral and postdoctoral) researchers often lack experience with transdisciplinary research despite their interest in tackling societal challenges with colleagues. Engagement in transdisciplinary research may not be an obvious choice because of limited support from their academic environment, difficulties of publishing, or a lack of suitable methods. In this work, we focus on the last. In order to evaluate several possible methodologies, we brought together a group of 10 young researchers from various disciplines to consider the question ‘What is progress?’. They examined this question via essay writing, a workshop, and a full-day colloquium, using methods that were based on examples from literature. After this process, input from the participants was gathered by means of a survey. Here, we provide an evaluation of existing methods and introduce four new methods: orientation exercise, census, individual reflection, walking consensus. Our results show that such a transdisciplinary exercise can readily be performed by a group of young researchers if the process is methodologically well structured, opening up opportunities for integrating such transdisciplinary insights in early career research.

Advances in the synthesis and assembly of nanomaterials offer a unique opportunity to purposefully design structures according to the requirements of the targeted applications. This paper shows a process to create robust 3D carbon nanotube (CNT) structures, which provide an electrically conductive support for nanoparticle coating. We describe a process to reliably fabricate robust honeycomb structures with walls made out of aligned CNTs. We present a design of experimental analysis of this fabrication process and discuss methods to coat these honeycombs with BiVO4 for solar fuel applications. The proposed honeycomb structure allows for an efficient transport of electrons through the electrode, as well as an enhanced light-electrode interaction. Finally, we demonstrate that the developed CNT electrodes can survive harsh BiVO4 synthesis conditions and can subsequently be used as photoelectrodes for solar water splitting. ispartof: NANOSCALE vol:11 issue:47 pages:22964-22970 ispartof: location:England status: published

Advances in the synthesis and assembly of nanomaterials offer a unique opportunity to purposefully design structures according to the requirements of the targeted applications. This paper shows a process to create robust 3D carbon nanotube (CNT) structures, which provide an electrically conductive support for nanoparticle coating. We describe a process to reliably fabricate robust honeycomb structures with walls made out of aligned CNTs. We present a design of experimental analysis of this fabrication process and discuss methods to coat these honeycombs with BiVO

© 2019 Elsevier B.V. Under alkaline conditions mixed NiFe oxides/oxyhydroxides are among the most active catalysts for the oxygen evolution reaction (OER). Here we investigate Ni 0.5 Fe 0.5 clusters as a well-defined model highly active electrocatalyst system for the OER. The electrodes were prepared using gas-phase deposition of mixed Ni 0.5 Fe 0.5 metallic clusters produced by cluster beam deposition (CBD), a technique offering precise control of composition and loading. Highly dispersed Ni 0.5 Fe 0.5 clusters were deposited at 1 equivalent monolayer and used as OER catalyst in 1 M KOH. The low loading allows assessment of the intrinsic catalytic activity and their extensive structural characterization by XAFS and XPS spectroscopies. Ni 0.5 Fe 0.5 clusters demonstrate high stability as there is no apparent potential increase after a 12 h constant current density anodization test. After electrochemical aging, the lowest overpotential was achieved and amounts to 372 mV at 10 mA cm −2 . The Tafel slope reaches 37 mV dec −1 . The measured electrochemical activity is also compared with other systems. After OER reaction in KOH the cluster surface is enriched in NiOOH, while concomitant Fe depletion from the catalyst surface is observed. These findings help to shed light on the formed active phase at the nanoscale. ispartof: Catalysis Today vol:334 pages:59-67 ispartof: location:BELGIUM, Univ Catholique Louvain, Louvain la Neuve status: published

Metal halide perovskites are actively pursued as photoelectrodes to drive solar fuel synthesis. However, currently, these photocathodes suffer from limited stability in water, which hampers their practical application. Here, we report a high-performance solution-processable photocathode composed of cesium formamidinium methylammonium triple-cation lead halide perovskite protected by an Al-doped ZnO (AZO) layer combined with a Field's metal encapsulation. Careful selection of charge transport layers resulted in an improvement in photocurrent, fill factor, device stability and reproducibility. The dead pixels count reduced from 25 to 6% for the devices with an AZO layer, and in photocathodes with an AZO layer the photocurrent density increased by almost 20% to 14.3 mA cm-2. In addition, we observed a 5-fold increase in the device lifetime for photocathodes with AZO, which reached up to 18 h before complete failure. Finally, the photocathodes are fabricated using low-cost and scalable methods, which have promise to become compatible with standard solution-based processes. ispartof: ACS APPLIED MATERIALS & INTERFACES vol:11 issue:26 pages:23198-23206 ispartof: location:United States status: published

The development of active and stable earth-abundant catalysts for hydrogen and oxygen evolution is one of the requirements for successful production of solar fuels. Atomic Layer Deposition (ALD) is a proven technique for conformal coating of structured (photo)electrode surfaces with such electrocatalyst materials. Here, we show that ALD can be used for the deposition of iron and cobalt phosphate electrocatalysts. A PE-ALD process was developed to obtain cobalt phosphate films without the need for a phosphidation step. The cobalt phosphate material acts as a bifunctional catalyst, able to also perform hydrogen evolution after either a thermal or electrochemical reduction step.

Solar hydrogen devices combine the power of photovoltaics and water electrolysis to produce hydrogen in a hybrid form of energy production. To engineer these into integrated devices (i.e. a water splitting catalyst on top of a PV element), the need exists for thin film catalysts that are both transparent for solar light and efficient in water splitting. Manganese oxides have already been shown to exhibit good water splitting performance, which can be further enhanced by conformal coating on high surface-area structures. The latter can be achieved by atomic layer deposition (ALD). However, to optimize the catalytic and transparency properties of the water splitting layer, an excellent control over the oxidation state of the manganese in the film is required. So far MnO, Mn3O4 and MnO2 ALD have been shown, while Mn2O3 is the most promising catalyst. Therefore, we investigated the post-deposition oxidation and reduction of MnO and MnO2 ALD films, and derived strategies to achieve every phase in the MnO–MnO2 range by tuning the ALD process and post-ALD annealing conditions. Thin film Mn2O3 is obtained by thermal reduction of ALD MnO2, without the need for oxidative high temperature treatments. The obtained Mn2O3 is examined for solar water splitting devices, and compared to the as-deposited MnO2. Both thin films show oxygen evolution activity and good solar light transmission.

Solar driven water splitting can be achieved by coupling electrolyzers with PhotoVoltaics (PV). Integration of both functions in a compact PhotoElectroChemical (PEC) cell is an attractive option but presents significant scientific challenges. In this work, the design of single- and dual-compartment PEC cells for research purposes is discussed. The fabrication of separator-electrode assemblies is an important aspect, and upscaling of these architectures even to centimeter scale is not trivial. The layout of a new dual-compartment compact PEC cell with in-situ monitoring of pH, temperatures, and oxygen and hydrogen evolution for research purposes is presented. Finally, a prospect of future PEC cells for practical applications is presented. ispartof: Oil and Gas Science and Technology - Revue de l'Institut Français du Pétrôle vol:70 issue:5 pages:877-889 ispartof: location:FRANCE, Solaize status: published

Photocatalytic thin films composed of titanium dioxide and poly(styrene sulfonate) with hierarchical porosity and high transparency were prepared by the layer-by-layer self-assembly method. The influence of preparation conditions on the structure and photocatalytic activity of the films was investigated. It was found there is no need to reach adsorption equilibrium during layer deposition. Using very short deposition times, films with better transparency and higher activity for methylene blue photo-oxidation were obtained. Diffusion limitations due to strong adsorption of dye molecules and ammonia to the polyelectrolyte were revealed. These results indicate that films could be devised to selectively degrade specific target molecules in a mixture. The films were deposited on a three-dimensional carbon substrate and successfully demonstrated in a photoelectrochemical water splitting experiment.

We propose a new design for monolithic solar water splitting based on porous multijunction solar cells. Porosity, causing minimal solar cell degradation, minimizes the ohmic losses associated to ion transport, maintaining high efficiencies when up-scaling.

Monolithic solar water splitting devices consist of photovoltaic materials integrated with electrocatalysts and produce solar hydrogen by water splitting upon solar illumination in one device. Upscaling of monolithic solar water splitting devices is obstructed by high ohmic losses in the electrolyte due to long ionic transport distances. A new design overcomes the problem by introducing micron sized pores in a silicon wafer substrate coated with electrocatalysts. A porous solar hydrogen device was simulated by applying a current corresponding to ca. 10% solar-to-hydrogen efficiency. Porous monoliths of 550 μm thickness with varying pore size and spacing were fabricated by laser ablation and electrochemically characterized. Ohmic losses well below 100 mV were reached at 14.4% porosity with 77 μm pores spaced 250 μm apart in 0.25 M KOH electrolyte. In 1 M KOH, 100 mV was reached at 6% porosity with 1 mm pore spacing. Our results suggest ohmic losses below 50 mV can be achieved when using 10 μm thick substrates at 0.2% porosity. These findings make it possible for monolithic solar water splitting devices to be scaled without loss of efficiency. ispartof: Journal of Physical Chemistry C vol:120 issue:38 pages:21242-21247 status: published

Review: design of photoelectrochemical cells that combine water oxidation and CO2 reduction; 50 refs.

A system is demonstrated that autonomously produces hydrogen gas using sunlight and outside air as the only inputs. Oxygen and hydrogen formation reactions occur on either side of a monolithic "solar membrane" inserted in a two-compartment photoelectrochemical cell. A surface film of Nafion® serves as a solid electrolyte. This proof of concept invites further development of air-based cells. © 2014 The Royal Society of Chemistry. ispartof: RSC ADVANCES vol:4 issue:55 pages:29286-29290 status: published

Synthetic methods that allow for the controlled design of well-defined Pt nanoparticles are highly desirable for fundamental catalysis research. In this work, we propose a strategy that allows precise and independent control of the Pt particle size and coverage. Our approach exploits the versatility of the atomic layer deposition (ALD) technique by combining two ALD processes for Pt using different reactants. The particle areal density is controlled by tailoring the number of ALD cycles using trimethyl(methylcyclopentadienyl)platinum and oxygen, while subsequent growth using the same Pt precursor in combination with nitrogen plasma allows for tuning of the particle size at the atomic level. The excellent control over the particle morphology is clearly demonstrated by means of in situ and ex situ X-ray fluorescence and grazing incidence small angle X-ray scattering experiments, providing information about the Pt loading, average particle dimensions, and mean center-to-center particle distance., The performance of supported nanoparticle catalysts is closely related to their size, shape and interparticle distance. Here, the authors introduce an atomic layer deposition-based strategy to independently tune the size and coverage of platinum nanoparticles with atomic-level precision.

not Available.

Water splitting was performed in a photoelectrochemical cell (PEC) with water oxidation and hydrogen formation reactions in two separate compartments. A photoanode consisting of carbon paper loaded with TiO2 and a cathode made of Pt dispersed on carbon black spread also on carbon paper were fixed on both sides of a Nafion membrane and electrically coupled via an external circuit. Anode and cathode compartments with serpentine flow field were operated either in liquid or vapor phase. Electrical current was monitored with chronoamperometry and D2 formation from deuterated water using mass spectrometry. Mapping the photocurrent under a variety of reaction conditions enabled identification of the limiting factors related to proton and photocarrier transport and reaction product evacuation. This comprehensive research approach to the operation of a PEC will assist future optimization of cell design and development of membrane electrode assemblies. http://pubs.rsc.org/en/content/articlelanding/2013/cp/c3cp50890k ispartof: Physical Chemistry Chemical Physics vol:15 issue:23 pages:9315-9325 ispartof: location:England status: published

© J. Rongé et al., published by IFP Energies nouvelles, 2015. Increasingly vast research efforts are devoted to the development of materials and processes for solar hydrogen production by light-driven dissociation of water into oxygen and hydrogen. Storage of solar energy in chemical bonds resolves the issues associated with the intermittent nature of sunlight, by decoupling energy generation and consumption. This paper investigates recent advances and prospects in solar hydrogen processes that are reaching market readiness. Future energy scenarios involving solar hydrogen are proposed and a case is made for systems producing hydrogen from water vapor present in air, supported by advanced modeling. ispartof: Oil and Gas Science and Technology - Revue de l'Institut Français du Pétrôle vol:70 issue:5 pages:863-876 ispartof: location:FRANCE, Solaize status: published

A combined experimental-numerical approach was used to study transient phenomena occurring in a photoelectrochemical cell using a membrane-separated porous TiO2-based photoanode and a dark Pt-based cathode. The effects of three parameters (pH in the anodic compartment, operating cell temperature, and cathode compartment preconditioning with hydrogen) on the photocurrent was systematically investigated using design of experiments and analysis of variance. A theoretical model was developed able to accurately reproduce and predict the measurements. The model indicated that the electrochemical reaction uses two parallel pathways on the anodic interface. The first pathway represented the rapid charging of surface states and the subsequent formation of acidic titanol groups at the TiO2/H2O interface which, upon illumination, caused an anodic overshoot at a short timescale. These states recombined with the formed O2 at a longer timescale which resulted in a current decrease after the overshoot. The second pathway was governed by transfer processes of H+ ions at the TiO2/Nafion® interface and caused the observed current increase under illumination and positive relaxation in the dark, both at long timescales. A negative undershoot was observed when the reverse electrolysis reaction was preferred.