3D Bi2O3 fractal nanostructures (f‐Bi2O3) are directly self‐assembled on carbon fiber papers (CFP) using a scalable hot‐aerosol synthesis strategy. This approach provides high versatility in ...modulating the physiochemical properties of the Bi2O3 catalyst by a tailorable control of its crystalline size, loading, electron density as well as providing exposed stacking of the nanomaterials on the porous CFP substrate. As a result, when tested for electrochemical CO2 reduction reactions (CO2RR), these f‐Bi2O3 electrodes demonstrate superior conversion of CO2 to formate (HCOO−) with low onset overpotential and a high mass‐specific formate partial current density of −52.2 mA mg−1, which is ≈3 times higher than that of the drop‐casted control Bi2O3 catalyst (−15.5 mA mg−1), and a high Faradaic efficiency (FEHCOO−) of 87% at an applied potential of −1.2 V versus reversible hydrogen electrode. The findings reveal that the high exposure of roughened β‐phase Bi2O3/Bi edges and the improved electron density of these fractal structures are key contributors in attainment of high CO2RR activity.
Nanostructures of fractal β‐Bi2O3 are uniformly fabricated on carbon fiber paper via a one‐step hot‐aerosol synthesis. The structure then is utilized as an electrocatalyst for CO2 reduction and has high selectivity toward formate production. The high performance is attributed to the exposure of a larger number of active sites and high electron density of the structure.
Closing both the carbon and nitrogen loops is a critical venture to support the establishment of the circular, net‐zero carbon economy. Although single atom catalysts (SACs) have gained interest for ...the electrochemical reduction reactions of both carbon dioxide (CO2RR) and nitrate (NO3RR), the structure–activity relationship for Cu SAC coordination for these reactions remains unclear and should be explored such that a fundamental understanding is developed. To this end, the role of the Cu coordination structure is investigated in dictating the activity and selectivity for the CO2RR and NO3RR. In agreement with the density functional theory calculations, it is revealed that Cu‐N4 sites exhibit higher intrinsic activity toward the CO2RR, whilst both Cu‐N4 and Cu‐N4−x‐Cx sites are active toward the NO3RR. Leveraging these findings, CO2RR and NO3RR are coupled for the formation of urea on Cu SACs, revealing the importance of *COOH binding as a critical parameter determining the catalytic activity for urea production. To the best of the authors’ knowledge, this is the first report employing SACs for electrochemical urea synthesis from CO2RR and NO3RR, which achieves a Faradaic efficiency of 28% for urea production with a current density of −27 mA cm–2 at −0.9 V versus the reversible hydrogen electrode.
Tuning the coordination structure of Cu single atom catalysts is explored for the simultaneous electrochemical conversion of CO2 and NO3− to urea. Cu‐N4 sites achieve a Faradaic efficiency of 28% for urea, demonstrating the potential of single atom catalysts for zero‐carbon fertilizer production from waste carbon dioxide and nitrates.
Photoelectrochemical (PEC) water splitting is considered a promising technology to produce renewable hydrogen, a clean fuel or energy carrier to replace conventional carbon‐based fossil‐fuel sources. ...Nevertheless, the overall solar‐to‐hydrogen efficiency and the cost‐effectiveness of this technology are still unsatisfactory for practical implementation. This can be primarily attributed to the sluggish kinetics of the anodic oxygen evolution reaction (OER) and the relatively low economic value of cogenerated O2 production. Over the past decades, there are extensive efforts to explore more kinetically favorable photooxidation reactions, which coupled with hydrogen evolution reaction (HER) can simultaneously improve H2 production yield and produce higher valuable alternatives to conventional O2. This review aims to present recent progress on the alternative anodic choices to OER. Here, the fundamental of PEC water splitting and the critical components required for this system are first shortly summarized. Then the benefits and issues of alternative photooxidation reactions including photooxidation of water to hydrogen peroxide, chlorine, alcohol, 5‐hydroxymethylfurfural, or urea oxidation when combined with the concurrent HER, are reviewed and analyzed. This review is concluded by presenting a critical evaluation of the challenges and opportunities of these alternative HER‐coupled photooxidation reactions for solar energy production and environmental treatment.
This review article summarizes current progress on alternative reactions to photooxidation of water in a photoelectrochemical cell for solar energy conversion and green H2. The benefits, challenges, and opportunities of the alternatives including photooxidation of water to hydrogen peroxide, of chlorine, alcohol, 5‐hydroxymethylfurfural, and urea when combined with the concurrent hydrogen evolution reaction are analyzed and discussed.
Advance of photonics media is restrained by the lack of structuring techniques for the 3D fabrication of active materials with long‐range periodicity. A methodology is reported for the engineering of ...tunable resonant photonic media with thickness exceeding the plasmonic near‐field enhancement region by more than two orders of magnitude. The media architecture consists of a stochastically ordered distribution of plasmonic nanocrystals in a fractal scaffold of high‐index semiconductors. This plasmonic‐semiconductor fractal media supports the propagation of surface plasmons with drastically enhanced intensity over multiple length scales, overcoming the 2D limitations of established metasurface technologies. The fractal media are used for the fabrication of plasmonic optical gas sensors, achieving a limit of detection of 0.01 vol% at room temperature and sensitivity up to 1.9 nm vol%−1, demonstrating almost a fivefold increase with respect to an optimized planar geometry. Beneficially to their implementation, the self‐assembly mechanism of this fractal architecture allows fabrication of micrometer‐thick media over surfaces of several square centimeters in a few seconds. The designable optical features and intrinsic scalability of these photonic fractal metamaterials provide ample opportunities for applications, bridging across transformation optics, sensing, and light harvesting.
Realization of photonic metamaterials is hindered by the lack of multiscale structuring techniques. The stochastically invariant features of fractals are used for engineering a tunable family of active media, consisting of plasmonic nanoresonators in porous semiconductor matrices. This architecture enhances the plasmonic volume over multiple length‐scales, and excellent room‐temperature sensitivity and low detection limits for volatile molecules are demonstrated.
Environmentally friendly routes from “Power‐to‐X” (P2X) technologies to sustainably harvest and store renewable energy with net‐zero CO2 emission are imperative. The concept of P2X relies on ...(photo)electrolysis of earth‐abundant molecules into value‐added products. For practical utilization, engineering robust, active, albeit inexpensive (photo)electrocatalysts via industrially compatible technologies is indeed crucial. In this context, flame spray pyrolysis (FSP) stands as an emerging approach for one‐step synthesis of ready‐to‐use (photo)electrocatalysts with production rates of Kg h‐1 in lab‐scales. While features of FSP to engineer nanomaterials have been summarised, there is a need for more critical discussions on key factors, modulating properties of flame‐made catalysts. Therefore, this review article will first provide an overview about the concept of the P2X and catalyst development strategies. Unique characteristic of flame‐synthesized nano‐catalysts including compositions, fractal morphologies, defects, and active sites will be then critically discussed. Furthermore, a potential of FSP as an electrode‐assembly technique for one‐step preparation of catalysts on gas diffusion layers for industry‐relevant electrolyser testing will be presented. Finally, perspectives on challenges and opportunities of FSP for renewable energies will be raised. This will provide insights into the versatility and commercial viability of the FSP route for engineering novel nanostructured catalysts for renewable energy applications.
This article reviews the versatility and capability of the flame spray pyrolysis (FSP) in nanocatalyst design and preparation for Power‐to‐X applications. FSP allows modulation of material varieties, chemical compositions, nanostructured morphologies, and defects. Moreover, ready‐to‐use catalysts can be synthesized and assembled directly on gas diffusion layers by one‐step synthesis without nonconductive binders and ink‐based assembly in a time‐frame of seconds.
Over 79 % of 6.3 billion tonnes of plastics produced from 1950 to 2015 have been disposed in landfills or found their way to the oceans, where they will reside for up to hundreds of years before ...being decomposed bringing upon significant dangers to our health and ecosystems. Plastic photoreforming offers an appealing alternative by using solar energy and water to transform plastic waste into value‐added chemical commodities, while simultaneously producing green hydrogen via the hydrogen evolution reaction. This review aims to provide an overview of the underlying principles of emerging plastic photoreforming technologies, highlight the challenges associated with experimental protocols and performance assessments, discuss recent global breakthroughs on the photoreforming of plastics, and propose perspectives for future research. A critical assessment of current plastic photoreforming studies shows a lack of standardised conditions, hindering comparison amongst photocatalyst performance. Guidelines to establish a more accurate evaluation of materials and systems are proposed, with the aim to facilitate the translation of promising fundamental discovery in photocatalysts design.
The experimental procedure generally involves two distinct processes: (1) pre‐treatment of original plastics; and (2) photoreforming of these treated molecules. The initial stage is essential for the breakdown of long carbon chains into smaller components via pulverization and monomerization, while the subsequent stage focuses on transforming these components into economically valuable products through photoreforming. Plastic photoreforming employs a photocatalyst, typically a semiconductor, that absorbs sunlight and initiating chemical reactions. This approach has the capacity to convert plastic waste into high‐value chemicals and generate hydrogen (H2) through water splitting under ambient conditions.
Metal halide perovskite solar cells have an appropriate bandgap (1.5–1.6 eV), and thus output voltage (>1 V), to directly drive solar water splitting. Despite significant progress, their moisture ...sensitivity still hampers their application for integrated monolithic devices. Furthermore, the prevalence of the use of noble metals as co‐catalysts for existing perovskite‐based devices undermines their use for low‐cost H2 production. Here, a monolithic architecture for stable perovskite‐based devices with earth‐abundant co‐catalysts is reported, demonstrating an unassisted overall solar‐to‐hydrogen efficiency of 8.54%. The device layout consists of two monolithically encapsulated perovskite (FA0.80MA0.15Cs0.05PbI2.55Br0.45) solar cells with low‐cost earth‐abundant CoP and FeNi(OH)x co‐catalysts as the photocathode and photoanode, respectively. The CoP‐based photocathode demonstrates more than 17 h of continuous operation, with a photocurrent density of 12.4 mA cm−2 at 0 V and an onset potential as positive as ≈1 V versus reversible hydrogen electrode (RHE). The FeNi(OH)x‐based photoanode achieves a photocurrent of 11 mA cm−2 at 1.23 V versus RHE for more than 13 h continuous operation. These excellent stability and performance demonstrate the potential for monolithic integration of perovskite solar cells and low‐cost earth‐abundant co‐catalysts for efficient direct solar H2 production.
An unassisted overall solar‐to‐hydrogen efficiency of 8.54% is achieved on a monolithic integration of perovskite solar cells with low‐cost earth‐abundant co‐catalysts. The effective encapsulation of the perovskite solar cells and engineering of the co‐catalysts interfaces results in robust monolithic photoelectrodes, demonstrating continuous stable operation over 13 h. The excellent stability and good performance demonstrate the potential for efficient direct solar H2 production.
Photoelectrochemical water splitting is a promising approach for the carbon‐free production of hydrogen using sunlight. Here, robust and efficient WO3 photoanodes for water oxidation were synthesized ...by the scalable one‐step flame synthesis of nanoparticle aerosols and direct gas‐phase deposition. Nanostructured WO3 films with tunable thickness and band gap and controllable porosity were fabricated by controlling the aerosol deposition time, concentration, and temperature. Optimal WO3 films demonstrate superior water oxidation performance, reaching a current density of 0.91 mA at 1.24 V vs. reversible hydrogen electrode (RHE) and an incident photon‐to‐current conversion efficiency (IPCE) of ca. 61 % at 360 nm in 0.1 m H2SO4. Notably, it is found that the excellent performance of these WO3 nanostructures arises from the high in situ restructuring temperature (ca. 1000 °C), which increases oxygen vacancies and decreases charge recombination at the WO3/electrolyte interface. These findings provide a scalable approach for the fabrication of efficient photoelectrodes based on WO3 and other metal oxides for light‐driven water splitting.
Deposits from flames: Efficient WO3 photoanodes were fabricated by a scalable one‐step method by deposition from flame‐made nanoparticle aerosols (see figure). A short deposition time of 10 s was sufficient for the fabrication of robust WO3 films that do not require post‐calcination treatment and achieve a photocurrent density for water oxidation of 0.91 mA cm−2 at 1.24 V vs. RHE in 0.1 m H2SO4, and incident photon to current conversion efficiency of ca. 61 % at 360 nm.
Unlocking the potential of the hydrogen economy is dependent on achieving green hydrogen (H2) production at competitive costs. Engineering highly active and durable catalysts for both oxygen and ...hydrogen evolution reactions (OER and HER) from earth‐abundant elements is key to decreasing costs of electrolysis, a carbon‐free route for H2 production. Here, a scalable strategy to prepare doped cobalt oxide (Co3O4) electrocatalysts with ultralow loading, disclosing the role of tungsten (W), molybdenum (Mo), and antimony (Sb) dopants in enhancing OER/HER activity in alkaline conditions, is reported. In situ Raman and X‐ray absorption spectroscopies, and electrochemical measurements demonstrate that the dopants do not alter the reaction mechanisms but increase the bulk conductivity and density of redox active sites. As a result, the W‐doped Co3O4 electrode requires ≈390 and ≈560 mV overpotentials to reach ±10 and ±100 mA cm−2 for OER and HER, respectively, over long‐term electrolysis. Furthermore, optimal Mo‐doping leads to the highest OER and HER activities of 8524 and 634 A g−1 at overpotentials of 0.67 and 0.45 V, respectively. These novel insights provide directions for the effective engineering of Co3O4 as a low‐cost material for green hydrogen electrocatalysis at large scales.
In this article, the authors study the transformation of the doped Co3O4 during the oxygen and hydrogen evolution reactions using in situ spectroelectrochemical techniques such as in situ X‐ray absorption and Raman spectroscopies. They reveal that the dopants (W, Mo, Sb) improve the bulk conductivity and density of redox active sites but do not alter the reaction mechanisms.
Geometrical structuring of monolithic metal‐organic frameworks (MOFs) components is required for their practical implementation in many areas, including electronic devices, gas storage/separation, ...catalysis, energy storage as well as bio‐medical applications. Despite progress in structuring MOFs, an approach for the precise patterning of MOF functional geometries in the millimeter‐ to micro‐meter depth is lacking. Here, a facile and flexible concept for the microfabrication of complex MOF patterns on large surfaces is reported. The method relies on the engineering of easily‐writable sheets of precursor metal oxide nanoparticles. The gas‐phase conversion of these patterned ceramic nanoparticle sheets results in monolithic MOF objects with arbitrarily shaped geometries and thicknesses of up to hundreds of micrometers. The writing of complex patterns of zeolitic imidazolate framework‐8 (ZIF‐8) is demonstrated by a variety of approaches including ion beam, laser, and hand writing. Nanometer‐scale patterns are achieved by focused ion beam (FIB). Artless handwritings are obtained by using a pen in a similar fashion to writing on a paper. The pure ZIF‐8 composition of the resulting patterns is confirmed by a series of physical and chemical characterization. This facile MOF precursor‐writing approach provides novel opportunities for the design of MOF‐based devices with applications ranging from micro‐fluidics to renewable energy systems.
Rapid and facile micrometer deep patterning of metal‐organic frameworks (MOFs) on the centimeter scale surface. Diverse patterns with pure and crystalline sodalite (sod) Zeolitic Imidazolate Framework‐8 composition have been showcased by the writing of nanoparticle network sheets, providing some unique beneficial features for the design of scalable and well‐defined MOF geometries for various cross‐disciplinary applications.
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