Recent Progress in Energy-Driven Water Splitting

A Realistic Approach for Photoelectrochemical Hydrogen Production

The production of hydrogen by water splitting has been a very attractive idea for several decades. However, the energy consumption that is necessary for water oxidation is too high for practical applications. On the contrary, the oxidation of organics is a much easier and less energy-demanding process. In addition, it may be used to consume organic wastes with a double environmental benefit: renewable energy production with environmental remediation. The oxidation of organics in a photoelectrochemical cell, which in that case is also referenced as a photocatalytic fuel cell, has the additional advantage of providing an alternative route for solar energy conversion. With this in mind, the present work describes a realistic choice of materials for the Pt-free photoelectrochemical production of hydrogen, by employing ethanol as a model organic fuel. The photoanode was made of a combination of titania with cadmium sulfide as the photosensitizer in order to enhance visible light absorbance. The cathode electrode was a simple carbon paper. Thus, it is shown that substantial hydrogen can be produced without electrocatalysts by simply exploiting carbon electrodes. Even though an ion transfer membrane was used in order to allow for an oxygen-free cathode environment, the electrolyte was the same in both the anode and cathode compartments. An alkaline electrolyte has been used to allow high hydroxyl concentration, thus facilitating organic fuel (photocatalytic) oxidation. Hydrogen production was then obtained by water reduction at the cathode (counter) electrode.

Donor-Acceptor Supramolecular Organic Nanofibers as Visible-Light Photoelectrocatalysts for Hydrogen Production

Perylene tetracarboxylic diimide (PTCDI) derivatives have been extensively studied for one-dimensional (1D) self-assembled systems and for applications in photocatalysis. Herein, we constructed a PTCDI-based donor-acceptor (D-A) supramolecular system via in situ self-assembly on an indium tin oxide conductive glass surface. The self-assembled PTCDI nanostructures exhibit well-defined nanofibril morphologies and strong photocurrents. Interestingly, a strong and reversible electrochromic color change was observed during cyclic voltammetry. The color of the nanofibers changed from red to blue and then to violet as the reduction progressed to the radical anion and then to the dianion. This series of one-electron reductions was confirmed by UV absorption, electron paramagnetic resonance spectroscopy, and hydrazine reduction. Most importantly, these PTCDI nanofibers exhibit efficient photoelectrocatalytic hydrogen production with remarkable stability under xenon lamp illumination (λ ≥ 420 nm). Among the three nanofibers prepared, the fibers assembled from PTCDI molecule 2 were found to be the most effective catalyst with 30% Faradaic efficiency. In addition, the nanofibers produced hydrogen at a steady-state for more than 8 h and produced repeatable results in 3 consecutive testing cycles, giving them great potential for practical industrial applications. Under an applied bias voltage, the 1D intermolecular stacking along the long axis of the nanofibers affords efficient separation and migration of photogenerated charge carriers, which play a crucial role in the photoelectrocatalytic process. As a proof-of-concept, the D-A-structured PTCDI nanofibers presented herein may guide future research on photoelectrocatalysis based on self-assembled supramolecular systems by providing more options for material design of the catalysts to achieve greater efficiencies.

Self-Powered Wearable Electronics Based on Moisture Enabled Electricity Generation

Most state-of-the-art electronic wearable sensors are powered by batteries that require regular charging and eventual replacement, which would cause environmental issues and complex management problems. Here, a device concept is reported that can break this paradigm in ambient moisture monitoring-a new class of simple sensors themselves can generate moisture-dependent voltage that can be used to determine the ambient humidity level directly. It is demonstrated that a moisture-driven electrical generator, based on the diffusive flow of water in titanium dioxide (TiO₂ ) nanowire networks, can yield an output power density of up to 4 µW cm^-2 when exposed to a highly moist environment. This performance is two orders of magnitude better than that reported for carbon-black generators. The output voltage is strongly dependent on humidity of ambient environment. As a big breakthrough, this new type of device is successfully used as self-powered wearable human-breathing monitors and touch pads, which is not achievable by any existing moisture-induced-electricity technology. The availability of high-output self-powered electrical generators will facilitate the design and application of a wide range of new innovative flexible electronic devices.

Oxidized Laser-Induced Graphene for Efficient Oxygen Electrocatalysis

An efficient metal-free catalyst is presented for oxygen evolution and reduction based on oxidized laser-induced graphene (LIG-O). The oxidation of LIG by O₂ plasma to form LIG-O boosts its performance in the oxygen evolution reaction (OER), exhibiting a low onset potential of 260 mV with a low Tafel slope of 49 mV dec^-1 , as well as an increased activity for the oxygen reduction reaction. Additionally, LIG-O shows unexpectedly high activity in catalyzing Li₂ O₂ decomposition in Li-O₂ batteries. The overpotential upon charging is decreased from 1.01 V in LIG to 0.63 V in LIG-O. The oxygen-containing groups make essential contributions, not only by providing the active sites, but also by facilitating the adsorption of OER intermediates and lowering the activation energy.

Recent Advances in Sensitized Photocathodes: From Molecular Dyes to Semiconducting Quantum Dots

The increasing demand for sustainable and environmentally benign energy has stimulated intense research to establish highly efficient photo-electrochemical (PEC) cells for direct solar-to-fuel conversion via water splitting. Light absorption, as the initial step of the catalytic process, is regarded as the foundation of establishing highly efficient PEC systems. To make full use of visible light, sensitization on photoelectrodes using either molecular dyes or semiconducting quantum dots provides a promising method. In this field, however, there remain many fundamental issues to be solved, which need in-depth study. Here, fundamental knowledge of PEC systems is introduced to enable readers a better understanding of this field. Then, the development history and current state in both molecular dye- and quantum dot-sensitized photocathodes for PEC water splitting are discussed. A systematical comparison between the two systems has been made. Special emphasis is placed on the research of quantum dot-sensitized photocathodes, which have shown superiority in both efficiency and durability towards PEC water splitting at the present stage. Finally, the opportunities and challenges in the future for sensitized PEC water-splitting systems are proposed.

Metal-Organic Framework-Derived ZnO/ZnS Heteronanostructures for Efficient Visible-Light-Driven Photocatalytic Hydrogen Production

Developing highly active, recyclable, and inexpensive photocatalysts for hydrogen evolution reaction (HER) under visible light is significant for the direct conversion of solar energy into chemical fuels for various green energy applications. For such applications, it is very challenging but vitally important for a photocatalyst to simultaneously enhance the visible-light absorption and suppress photogenerated electron-hole recombination, while also to maintain high stability and recyclability. Herein, a metal-organic framework (MOF)-templated strategy has been developed to prepare heterostructured nanocatalysts with superior photocatalytic HER activity. Very uniquely, the synthesized photocatalytic materials can be recycled easily after use to restore the initial photocatalytic activity. It is shown that by controlling the calcination temperature and time with MOF-5 as a host and guest thioacetamide as a sulfur source, the chemical compositions of the formed heterojunctions of ZnO/ZnS can be tuned to further enhance the visible-light absorption and photocatalytic activity. The nanoscale heterojunction ZnO/ZnS structural feature serves to reduce the average free path of charge carriers and improve the charge separation efficiency, thus leading to significantly enhanced HER activity under visible-light irradiation (λ > 420 nm) with high stability and recyclability without any cocatalyst.

Noble Metal-Free Nanocatalysts with Vacancies for Electrochemical Water Splitting

The fast development of nanoscience and nanotechnology has significantly advanced the fabrication of nanocatalysts and the in-depth study of the structural-activity characteristics of materials at the atomic level. Vacancies, as typical atomic defects or imperfections that widely exist in solid materials, are demonstrated to effectively modulate the physicochemical, electronic, and catalytic properties of nanomaterials, which is a key concept and hot research topic in nanochemistry and nanocatalysis. The recent experimental and theoretical progresses achieved in the preparation and application of vacancy-rich nanocatalysts for electrochemical water splitting are explored. Engineering of vacancies has shown to open up a new avenue beyond the traditional morphology, size, and composition modifications for the development of nonprecious electrocatalysts toward efficient energy conversion. First, an introduction followed by discussions of different types of vacancies, the approaches to create vacancies, and the advanced techniques widely used to characterize these vacancies are presented. Importantly, the correlations between the vacancies and activities of the vacancy-rich electrocatalysts via tuning the electronic states, active sites, and kinetic energy barriers are reviewed. Finally, perspectives on the existing challenges along with some opportunities for the further development of vacancy-rich noble metal-free electrocatalysts with high performance are discussed.

Synergism of Geometric Construction and Electronic Regulation: 3D Se-(NiCo)Sx /(OH)x Nanosheets for Highly Efficient Overall Water Splitting

The exploration of highly efficient electrocatalysts for both oxygen and hydrogen generation via water splitting is receiving considerable attention in recent decades. Up till now, Pt-based catalysts still exhibit the best hydrogen evolution reaction (HER) performance and Ir/Ru-based oxides are identified as the benchmark for oxygen evolution reaction (OER). However, the high cost and rarity of these materials extremely hinder their large-scale applications. This paper describes the construction of the ultrathin defect-enriched 3D Se-(NiCo)S_x /(OH)_x nanosheets for overall water splitting through a facile Se-induced hydrothermal treatment. Via Se-induced fabrication, highly efficient Se-(NiCo)S_x /(OH)_x nanosheets are successfully fabricated through morphology optimization, defect engineering, and electronic structure tailoring. The as-prepared hybrids exhibit relatively low overpotentials of 155 and 103 mV at the current density of 10 mA cm^-2 for OER and HER, respectively. Moreover, an overall water-splitting device delivers a current density of 10 mA cm^-2 for ≈66 h without obvious degradation.

Unique Features of the Photocatalytic Reduction of H2O and CO2 by New Catalysts Based on the Analogues of CdS, Cd4P2X3 (X = Cl, Br, I)

Photochemical reduction of H₂O and CO₂ has been investigated with a new family of catalysts of the formula Cd₄P₂X₃ (X= Cl, Br, I), obtained by the complete aliovalent substitution of the sulfide ions in CdS by P and X (Cl, Br, I). Unlike CdS, the Cd₄P₂X₃ compounds exhibit hydrogen evolution and CO₂ reduction from water even in the absence of a sacrificial agent or a cocatalyst. Use of Ni_xP_y as the cocatalyst, enhances hydrogen evolution, reaching 3870 (apparent quantum yield (AQY) = 4.11) and 9258 (AQY = 9.83) μmol h^-1 g^-1, respectively, under artificial and natural (sunlight) irradiation, in the case of Cd₄P₂Br₃/Ni_xP_y. Electrochemical and spectroscopic studies have been employed to understand the photocatalytic activity of this family of compounds. Unlike most of the semiconductor-based photocatalysts, Cd₄P₂X₃ catalysts reduce CO₂ to CO and CH₄ in the absence of sacrificial-agent or cocatalyst using water as the electron source. CO, CH₄, and H₂ have been obtained with these catalysts under artificial as well as sun-light irradiation. First-principles, calculations have been carried out to understand the electronic structure and catalytic features of these new catalysts.

Monitoring surface transformations of metal carbodiimide water oxidation catalysts by operando XAS and Raman spectroscopy

Chemical and structural transformations at the electrode surface of metal carbodiimides MNCN (M = Co, Ni, Mn, Cu), were studied by operando Raman and XAS spectroscopy during electrocatalytic water oxidation

Copper-mediated reduction of azides under seemingly oxidising conditions: catalytic and computational studies

The reduction of aryl azides in the presence of water and air and without an obvious reducing agent is reported.

Simple eco-friendly synthesis of the surfactant free SnS nanocrystal toward the photoelectrochemical cell application

A simple, low cost, non-toxic and eco-friendly pathway for synthesizing efficient sunlight-driven tin sulfide photocatalyst was studied. SnS nanocrystals were prepared by using mechanical method. The bulk SnS was obtained by evaporation of SnS nanocrystal solution. The synthesized samples were characterized by using XRD, SEM, TEM, UV-vis, and Raman analyses. Well crystallized SnS nanocrystals were verified and the electrochemical characterization was also performed under visible light irradiation. The SnS nanocrystals have shown remarkable photocurrent density of 7.6 mA cm^-2 under 100 mW cm^-2 which is about 10 times larger than that of the bulk SnS under notably stable operation conditions. Furthermore, the SnS nanocrystals presented higher stability than the bulk form. The IPCE(Incident photon to current conversion efficiency) of 9.3% at 420 nm was obtained for SnS nanocrystal photoanode which is strikingly higher than that of bulk SnS, 0.78%. This work suggests that the enhancement of reacting area by using SnS nanocrystal absorbers could give rise to the improvement of photoelectrochemical cell efficiency.

CO2 Reduction: From the Electrochemical to Photochemical Approach

Increasing CO2 concentration in the atmosphere is believed to have a profound impact on the global climate. To reverse the impact would necessitate not only curbing the reliance on fossil fuels but also developing effective strategies capture and utilize CO2 from the atmosphere. Among several available strategies, CO2 reduction via the electrochemical or photochemical approach is particularly attractive since the required energy input can be potentially supplied from renewable sources such as solar energy. In this Review, an overview on these two different but inherently connected approaches is provided and recent progress on the development, engineering, and understanding of CO2 reduction electrocatalysts and photocatalysts is summarized. First, the basic principles that govern electrocatalytic or photocatalytic CO2 reduction and their important performance metrics are discussed. Then, a detailed discussion on different CO2 reduction electrocatalysts and photocatalysts as well as their generally designing strategies is provided. At the end of this Review, perspectives on the opportunities and possible directions for future development of this field are presented.

Porous Structured Ni-Fe-P Nanocubes Derived from a Prussian Blue Analogue as an Electrocatalyst for Efficient Overall Water Splitting

Exploring nonprecious metal electrocatalysts to replace the noble metal-based catalysts for full water electrocatalysis is still an ongoing challenge. In this work, porous structured ternary nickel-iron-phosphide (Ni-Fe-P) nanocubes were synthesized through one-step phosphidation of a Ni-Fe-based Prussian blue analogue. The Ni-Fe-P nanocubes exhibit a rough and loose porous structure on their surface under suitable phosphating temperature, which is favorable for the mass transfer and oxygen diffusion during the electrocatalysis process. As a result, Ni-Fe-P obtained at 350 °C with poorer crystallinity offers more unsaturated atoms as active sites to expedite the absorption of reactants. Additionally, the introduction of nickel improved the electronic structure and then reduced the charge-transfer resistance, which would result in a faster electron transport and an enhancement of the intrinsic electrocatalytic activities. Benefiting from the unique porous nanocubes and the chemical composition, the Ni-Fe-P nanocubes exhibit excellent hydrogen evolution reaction and oxygen evolution reaction activities in alkaline medium, with low overpotentials of 182 and 271 mV for delivering a current density of 10 mA cm^-2, respectively. Moreover, the Ni-Fe-P nanocubes show outstanding stability for sustained water splitting in the two-electrode alkaline electrolyzer. This work not only provides a facile approach for designing bifunctional electrocatalysts but also further extends the application of metal-organic frameworks in overall water splitting.