Hydrogen production from water and conversion of carbon dioxide to useful chemicals by room temperature photoelectrocatalysis

Hydrogen Production as a Clean Energy Carrier through Heterojunction Semiconductors for Environmental Remediation

Today, as a result of the advancement of technology and increasing environmental problems, the need for clean energy has considerably increased. In this regard, hydrogen, which is a clean and sustainable energy carrier with high energy density, is among the well-regarded and effective means to deliver and store energy, and can also be used for environmental remediation purposes. Renewable hydrogen energy carriers can successfully substitute fossil fuels and decrease carbon dioxide (CO2) emissions and reduce the rate of global warming. Hydrogen generation from sustainable solar energy and water sources is an environmentally friendly resolution for growing global energy demands. Among various solar hydrogen production routes, semiconductor-based photocatalysis seems a promising scheme that is mainly performed using two kinds of homogeneous and heterogeneous methods, of which the latter is more advantageous. During semiconductor-based heterogeneous photocatalysis, a solid material is stimulated by exposure to light and generates an electron–hole pair that subsequently takes part in redox reactions leading to hydrogen production. This review paper tries to thoroughly introduce and discuss various semiconductor-based photocatalysis processes for environmental remediation with a specific focus on heterojunction semiconductors with the hope that it will pave the way for new designs with higher performance to protect the environment.

Visible-Light Photocatalysts and Their Perspectives for Building Photocatalytic Membrane Reactors for Various Liquid Phase Chemical Conversions

Photocatalytic organic synthesis/conversions and water treatment under visible light are a challenging task to use renewable energy in chemical transformations. In this review a brief overview on the mainly employed visible light photocatalysts and a discussion on the problems and advantages of Vis-light versus UV-light irradiation is reported. Visible light photocatalysts in the photocatalytic conversion of CO2, conversion of acetophenone to phenylethanol, hydrogenation of nitro compounds, oxidation of cyclohexane, synthesis of vanillin and phenol, as well as hydrogen production and water treatment are discussed. Some applications of these photocatalysts in photocatalytic membrane reactors (PMRs) for carrying out organic synthesis, conversion and/or degradation of organic pollutants are reported. The described cases show that PMRs represent a promising green technology that could shift on applications of industrial interest using visible light (from Sun) active photocatalysts.

Overview of Photocatalytic Membrane Reactors in Organic Synthesis, Energy Storage and Environmental Applications

This paper presents an overview of recent reports on photocatalytic membrane reactors (PMRs) in organic synthesis as well as water and wastewater treatment. A brief introduction to slurry PMRs and the systems equipped with photocatalytic membranes (PMs) is given. The methods of PM production are also presented. Moreover, the process parameters affecting the performance of PMRs are characterized. The applications of PMRs in organic synthesis are discussed, including photocatalytic conversion of CO2, synthesis of KA oil by photocatalytic oxidation, conversion of acetophenone to phenylethanol, synthesis of vanillin and phenol, as well as hydrogen production. Furthermore, the configurations and applications of PMRs for removal of organic contaminants from model solutions, natural water and municipal or industrial wastewater are described. It was concluded that PMRs represent a promising green technology; however, before the application in industry, additional studies are still required. These should be aimed at improvement of process efficiency, mainly by development and application of visible light active photocatalysts and novel membranes resistant to the harsh conditions prevailing in these systems.

From CO2methanation to ambitious long-chain hydrocarbons: alternative fuels paving the path to sustainability

Comprehensive insight into the thermochemical, photochemical and electrochemical reduction of CO₂to methane and long-chain hydrocarbons as alternative fuels.

Solar Fuels by Heterogeneous Photocatalysis: From Understanding Chemical Bases to Process Development

The development of sustainable yet efficient technologies to store solar light into high energy molecules, such as hydrocarbons and hydrogen, is a pivotal challenge in 21st century society. In the field of photocatalysis, a wide variety of chemical routes can be pursued to obtain solar fuels but the two most promising are carbon dioxide photoreduction and photoreforming of biomass-derived substrates. Despite their great potentialities, these technologies still need to be improved to represent a reliable alternative to traditional fuels, in terms of both catalyst design and photoreactor engineering. This review highlights the chemical fundamentals of different photocatalytic reactions for solar fuels production and provides a mechanistic insight on proposed reaction pathways. Also, possible cutting-edge strategies to obtain solar fuels are reported, focusing on how the chemical bases of the investigated reaction affect experimental choices.

Solid-state photoelectrochemical cell with TiO2 nanotubes for water splitting

We have fabricated and tested a photoelectrochemical (PEC) cell where the aqueous electrolyte has been replaced by a proton conducting hydrated Nafion® polymer membrane.

Charge Transport in Two-Photon Semiconducting Structures for Solar Fuels

Semiconducting heterostructures are emerging as promising light absorbers and offer effective electron-hole separation to drive solar chemistry. This technology relies on semiconductor composites or photoelectrodes that work in the presence of a redox mediator and that create cascade junctions to promote surface catalytic reactions. Rational tuning of their structures and compositions is crucial to fully exploit their functionality. In this review, we describe the possibilities of applying the two-photon concept to the field of solar fuels. A wide range of strategies including the indirect combination of two semiconductors by a redox couple, direct coupling of two semiconductors, multicomponent structures with a conductive mediator, related photoelectrodes, as well as two-photon cells are discussed for light energy harvesting and charge transport. Examples of charge extraction models from the literature are summarized to understand the mechanism of interfacial carrier dynamics and to rationalize experimental observations. We focus on a working principle of the constituent components and linking the photosynthetic activity with the proposed models. This work gives a new perspective on artificial photosynthesis by taking simultaneous advantages of photon absorption and charge transfer, outlining an encouraging roadmap towards solar fuels.

Monolithic cells for solar fuels

A tutorial review explaining the many processes occurring in photoelectrochemical cells for solar fuel production, and prospects for future developments.

Chronoamperometric study of membrane electrode assembly operation in continuous flow photoelectrochemical water splitting

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 the liquid or vapour 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 optimisation of cell design and development of membrane electrode assemblies.

Bi-component semiconductor oxide photoanodes for the photoelectrocatalytic oxidation of organic solutes and vapours: a short review with emphasis to TiO2-WO3 photoanodes

The use of binary semiconductor oxide anodes for the photoelectrocatalytic oxidation of organic species (both in solution and gas phase) is reviewed. In the first part of the review, the principle of electrically assisted photocatalysis is presented, the preparation methods for the most common semiconductor oxide catalysts are briefly mentioned, while the advantages of appropriately chosen semiconductor combinations for efficient UV and visible (vis) light utilization are highlighted. The second part of the review focuses on the discussion of TiO(2)-WO(3) photoanodes (among the most studied bi-component semiconductor oxide systems) and in particular on coatings prepared by electrodeposition/electrosynthesis or powder mixtures (the focus of the authors' research during recent years). Studies concerning the microscopic, spectroscopic and photoelectrochemical characterization of the catalysts are presented and examples of photoanode activity towards typical dissolved organic contaminants as well as organic vapours are given. Particular emphasis is paid to: (a) The dependence of photoactivity on catalyst morphology and composition and (b) the possibility of carrying out photoelectrochemistry in all-solid cells, thus opening up the opportunity for photoelectrocatalytic air treatment.

Towards artificial leaves for solar hydrogen and fuels from carbon dioxide

The development of an "artificial leaf" that collects energy in the same way as a natural one is one of the great challenges for the use of renewable energy and a sustainable development. To avoid the problem of intermittency in solar energy, it is necessary to design systems that directly capture CO(2) and convert it into liquid solar fuels that can be easily stored. However, to be advantageous over natural leaves, it is necessary that artificial leaves have a higher solar energy-to-chemical fuel conversion efficiency, directly provide fuels that can be used in power-generating devices, and finally be robust and of easy construction, for example, smart, cheap and robust. This review discusses the recent progress in this field, with particular attention to the design and development of 'artificial leaf' devices and some of their critical components. This is a very active research area with different concepts and ideas under investigation, although often the validity of the considered solutions it is still not proven or the many constrains are not fully taken into account, particularly from the perspective of system engineering, which considerably limits some of the investigated solutions. It is also shown how system design should be included, at least at a conceptual level, in the definition of the artificial leaf elements to be investigated (catalysts, electrodes, membranes, sensitizers) and that the main relevant aspects of the cell engineering (mass/charge transport, fluid dynamics, sealing, etc.) should be also considered already at the initial stage because they determine the design and the choice between different options. For this reason, attention has been given to the system-design ideas under development instead of the molecular aspects of the O(2) - or H(2) -evolution catalysts. However, some of the recent advances in these catalysts, and their use in advanced electrodes, are also reported to provide a more complete picture of the field.

Toward solar fuels: photocatalytic conversion of carbon dioxide to hydrocarbons

The past several decades have seen a significant rise in atmospheric carbon dioxide levels resulting from the combustion of hydrocarbon fuels. A solar energy based technology to recycle carbon dioxide into readily transportable hydrocarbon fuel (i.e., a solar fuel) would help reduce atmospheric CO2 levels and partly fulfill energy demands within the present hydrocarbon based fuel infrastructure. We review the present status of carbon dioxide conversion techniques, with particular attention to a recently developed photocatalytic process to convert carbon dioxide and water vapor into hydrocarbon fuels using sunlight.

Towards solar fuels from water and CO2

Solar fuels from water and CO2 are a topic of current large scientific and industrial interest. Research advances on bioroutes, concentrated solar thermal and low-temperature conversion using semiconductors and a photoelectrocatalytic (PEC) approach, are critically discussed and compared in an attempt to define challenges and current limits and to identify the priorities on which focus research and development (R&D). The need to produce fuels that are easy to transport and store, which can be integrated into the existing energy infrastructure, is emphasized. The role of solar fuels produced from CO2 in comparison with solar H2 is analyzed. Solar fuels are complementary to solar to electrical energy conversion, but they still need intensified R&D before possible commercialization.

Large-scale preparation of ordered titania nanorods with enhanced photocatalytic activity

A titania layer with ordered nanostructures is expected to be of high photocatalytic activity due mainly to its high specific surface area. In the present work, large-area films with ordered titania nanorods were deposited on titanium substrates through a solution approach. The nanorods, with the phase composition of a mixture of anatase and rutile, grew on top of a condensed anatase interlayer along mainly the rutile [001]-axis. The photocatalytic activity was evaluated by decomposing rhodamine B in water and compared with the general sol-gel derived titania films and a commercial DP-25 titania coating. It is found that the as-deposited titania nanorods exhibited extremely high initial photocatalytic activity but declined to a poor value after the consumption of beneficial oxidative peroxo complexes coordinated to Ti(IV). A subsequent thermal treatment eliminated such complexes but at the same time improved the crystallinity of the titania nanorods. The photocatalytic activity of the thermally treated titania nanorods was stable and significantly higher than that of the sol-gel derived film and commercial DP-25 coating.