Extremely stable platinum nanoparticles encapsulated in a zirconia nanocage by area-selective atomic layer deposition for the oxygen reduction reaction

Atomic/molecular layer deposition strategies for enhanced CO2 capture, utilisation and storage materials

Elevated levels of carbon dioxide (CO2) in the atmosphere and the diminishing reserves of fossil fuels have raised profound concerns regarding the resulting consequences of global climate change and the future supply of energy. Hence, the reduction and transformation of CO2 not only mitigates environmental pollution but also generates value-added chemicals, providing a dual remedy to address both energy and environmental challenges. Despite notable advancements, the low conversion efficiency of CO2 remains a major obstacle, largely attributed to its inert chemical nature. It is imperative to engineer catalysts/materials that exhibit high conversion efficiency, selectivity, and stability for CO2 transformation. With unparalleled precision at the atomic level, atomic layer deposition (ALD) and molecular layer deposition (MLD) methods utilize various strategies, including ultrathin modification, overcoating, interlayer coating, area-selective deposition, template-assisted deposition, and sacrificial-layer-assisted deposition, to synthesize numerous novel metal-based materials with diverse structures. These materials, functioning as active materials, passive materials or modifiers, have contributed to the enhancement of catalytic activity, selectivity, and stability, effectively addressing the challenges linked to CO2 transformation. Herein, this review focuses on ALD and MLD's role in fabricating materials for electro-, photo-, photoelectro-, and thermal catalytic CO2 reduction, CO2 capture and separation, and electrochemical CO2 sensing. Significant emphasis is dedicated to the ALD and MLD designed materials, their crucial role in enhancing performance, and exploring the relationship between their structures and catalytic activities for CO2 transformation. Finally, this comprehensive review presents the summary, challenges and prospects for ALD and MLD-designed materials for CO2 transformation.

MnF2 Surface Modulated Hollow Carbon Nanorods on Porous Carbon Nanofibers as Efficient Bi-Functional Oxygen Catalysis for Rechargeable Zinc-Air Batteries

Developing highly efficient bi-functional noble-metal-free oxygen electrocatalysts with low-cost and scalable synthesis approach is challenging for zinc-air batteries (ZABs). Due to the flexible valence state of manganese, MnF2 is expected to provide efficient OER. However, its insulating properties may inhibit its OER process to a certain degree. Herein, during the process of converting the manganese source in the precursor of porous carbon nanofibers (PCNFs) to manganese fluoride, the manganese source is changed to manganese acetate, which allows PCNFs to grow a large number of hollow carbon nanorods (HCNRs). Meanwhile, manganese fluoride will transform from the aggregation state into uniformly dispersed MnF2 nanodots, thereby achieving highly efficient OER catalytic activity. Furthermore, the intrinsic ORR catalytic activity of the HCNRs/MnF2@PCNFs can be enhanced due to the charge modulation effect of MnF2 nanodots inside HCNR. In addition, the HCNRs stretched toward the liquid electrolyte can increase the capture capacity of dissolved oxygen and protect the inner MnF2, thereby enhancing the stability of HCNRs/MnF2@PCNFs for the oxygen electrocatalytic process. MnF2 surface-modulated HCNRs can strongly enhance ORR activity, and the uniformly dispersed MnF2 can also provide higher OER activity. Thus, the prepared HCNRs/MnF2@PCNFs obtain efficient bifunctional oxygen catalytic ability and high-performance rechargeable ZABs.

Surface engineering for stable electrocatalysis

In recent decades, significant progress has been achieved in rational developments of electrocatalysts through constructing novel atomistic structures and modulating catalytic surface topography, realizing substantial enhancement in electrocatalytic activities. Numerous advanced catalysts were developed for electrochemical energy conversion, exhibiting low overpotential, high intrinsic activity, and selectivity. Yet, maintaining the high catalytic performance under working conditions with high polarization and vigorous microkinetics that induce intensive degradation of surface nanostructures presents a significant challenge for commercial applications. Recently, advanced operando and computational techniques have provided comprehensive mechanistic insights into the degradation of surficial functional structures. Additionally, various innovative strategies have been devised and proven effective in sustaining electrocatalytic activity under harsh operating conditions. This review aims to discuss the most recent understanding of the degradation microkinetics of catalysts across an entire range of anodic to cathodic polarizations, encompassing processes such as oxygen evolution and reduction, hydrogen reduction, and carbon dioxide reduction. Subsequently, innovative strategies adopted to stabilize the materials' structure and activity are highlighted with an in-depth discussion of the underlying rationale. Finally, we present conclusions and perspectives regarding future research and development. By identifying the research gaps, this review aims to inspire further exploration of surface degradation mechanisms and rational design of durable electrocatalysts, ultimately contributing to the large-scale utilization of electroconversion technologies.

Potential-Dependent Ionomer Rearrangement on the Pt Surface in Polymer Electrolyte Membrane Fuel Cells

The interface between the catalyst and the ionomer in the catalyst layer of polymer electrolyte membrane fuel cells (PEMFCs) has been a subject of keen interest, but its effect on durability has not been fully understood due to the complexity of the catalyst layer structure. Herein, we utilize a Pt nanoparticle (NP) array electrode fabricated using a block copolymer template as the platform for a focused investigation of the interfacial change between the Nafion thin film and the Pt NP under a constant potential. A set of analyses for the electrodes treated with various potentials reveals that the Nafion thin film becomes densely packed at the intermediate potentials (0.4 and 0.7 V), indicating an increased ionomer-catalyst interaction due to the positive charges formed at the Pt surface at these potentials. Even for a practical PEMFC single cell, we demonstrate that the potential holding at the intermediate potentials increases ionomer adsorption to the Pt surface and the oxygen transport resistance, negatively impacting its power performance. This work provides fresh insight into the mechanism behind the performance fade in PEMFCs caused by potential-dependent ionomer rearrangement.

High-Throughput Area-Selective Spatial Atomic Layer Deposition of SiO2 with Interleaved Small Molecule Inhibitors and Integrated Back-Etch Correction for Low Defectivity

A first-of-its-kind area-selective deposition process for SiO₂  is developed consisting of film deposition with interleaved exposures to small molecule inhibitors (SMIs) and back-etch correction steps, within the same spatial atomic layer deposition (ALD) tool. The synergy of these aspects results in selective SiO₂  deposition up to ~23 nm with high selectivity and throughput, with SiO₂  growth area and ZnO nongrowth area. The selectivity is corroborated by both X-ray photoelectron spectroscopy (XPS) and low-energy ion scattering spectroscopy (LEIS). The selectivity conferred by two different SMIs, ethylbutyric acid, and pivalic acid has been compared experimentally and theoretically. Density Functional Theory (DFT) calculations reveal that selective surface functionalization using both SMIs is predominantly controlled thermodynamically, while the better selectivity achieved when using trimethylacetic acid can be explained by its higher packing density compared to ethylbutyric acid. By employing the trimethylacetic acid as SMI on other starting surfaces (Ta₂ O₅ , ZrO₂ , etc.) and probing the selectivity, a broader use of carboxylic acid inhibitors for different substrates is demonstrated. It is believed that the current results highlight the subtleties in SMI properties such as size, geometry, and packing, as well as interleaved back-etch steps, which are key in developing ever more effective strategies for highly selective deposition processes.

Templated encapsulation of platinum-based catalysts promotes high-temperature stability to 1,100 °C

Stable catalysts are essential to address energy and environmental challenges, especially for applications in harsh environments (for example, high temperature, oxidizing atmosphere and steam). In such conditions, supported metal catalysts deactivate due to sintering-a process where initially small nanoparticles grow into larger ones with reduced active surface area-but strategies to stabilize them can lead to decreased performance. Here we report stable catalysts prepared through the encapsulation of platinum nanoparticles inside an alumina framework, which was formed by depositing an alumina precursor within a separately prepared porous organic framework impregnated with platinum nanoparticles. These catalysts do not sinter at 800 °C in the presence of oxygen and steam, conditions in which conventional catalysts sinter to a large extent, while showing similar reaction rates. Extending this approach to Pd-Pt bimetallic catalysts led to the small particle size being maintained at temperatures as high as 1,100 °C in air and 10% steam. This strategy can be broadly applied to other metal and metal oxides for applications where sintering is a major cause of material deactivation.

Perovskite-Carbon Joint Substrate for Practical Application in Proton Exchange Membrane Fuel Cells under Low-Humidity/High-Temperature Conditions

Highly active catalysts with promising water retention are favorable for proton exchange membrane fuel cells (PEMFCs) operating under low-humidity/high-temperature conditions. When PEMFCs operate under low-humidity/high-temperature conditions, performance attenuation rapidly occurs owing to reduced proton conductivity of dehydrated membrane electrode assemblies. Herein, we load platinum onto a perovskite-carbon joint substrate (BaZr_0.1Ce_0.7Y_0.1Yb_0.1O_3-σ-XC-72R) to construct a highly active and durable catalyst with good water retention capacity. We propose that the Pt/(BZCYYb-C) catalyst layer at cathode can promote the water back diffusion of produced water from the cathode to the membrane, thus preventing the decay of fuel-cell performance under low-humidity/high-temperature conditions. The catalyst exhibited outstanding mass activity of 0.542 A mg_pt^-1 at 0.9 V vs RHE. PEMFCs with such a catalyst delivered very high peak power densities (1.70/1.14 W cm^-2 under H₂-O₂/air conditions at 70 °C) and kept 85.3%/92.1% of initial performance values under low-humidity/high-temperature conditions (relative humidity 60%@70 °C/100 °C).

Effect of Precursor Status on the Transition from Complex to Carbon Shell in a Platinum Core-Carbon Shell Catalyst

Encapsulating platinum nanoparticles with a carbon shell can increase the stability of core platinum nanoparticles by preventing their dissolution and agglomeration. In this study, the synthesis mechanism of a platinum core-carbon shell catalyst via thermal reduction of a platinum-aniline complex was investigated to determine how the carbon shell forms and identify the key factor determining the properties of the Pt core-carbon shell catalyst. Three catalysts originating from the complexes with different platinum to carbon precursor ratios were synthesized through pyrolysis. Their structural characteristics were examined using various analysis techniques, and their electrochemical activity and stability were evaluated through half-cell and unit-cell tests. The relationship between the nitrogen to platinum ratio and structural characteristics was revealed, and the effects on the electrochemical activity and stability were discussed. The ratio of the carbon precursor to platinum was the decisive factor determining the properties of the platinum core-carbon shell catalyst.

Carbon Shell on Active Nanocatalyst for Stable Electrocatalysis

Electrocatalysis is a key process for renewable energy conversion and fuel production in future energy systems. Various nanostructures have been investigated to optimize the electrocatalytic activity and realize efficient energy use. However, the long-term stability of electrocatalysts is also crucial for the sustainable and reliable operation of energy devices. Nanocatalysts are degraded by various processes during electrocatalysis, which causes critical performance loss. Recent operando analyses have revealed the mechanisms of electrocatalyst failure, and specific structures have been identified as robust against degradation. Nevertheless, achieving both high activity and robust stability with the same nanostructure is challenging because the structure-property relationships that affect activity and stability are different. The optimization of electrocatalysis is often limited by a large trade-off between activity and stability in catalyst structures. Therefore, it is essential to introduce functional structural units into catalyst design to achieve electrochemical stability while preserving high activity.In this Account, we highlight the strategic use of carbon shells on catalyst surfaces to improve the stability during electrocatalysis. For this purpose, we cover three issues in the use of carbon-shell-encapsulated nanoparticles (CSENPs) as robust and active electrocatalysts: the origin of the improved stability, the identification of active sites, and synthetic routes. Carbon shells can shield catalyst surfaces from both (electro)chemical oxidation and physical agglomeration. By limiting the exposure of the catalyst surface to an oxidizing (electro)chemical environment, carbon shells can preserve the initial active site structure during electrocatalysis. In addition, by providing a physical barrier between nanoparticles, carbon shells can maintain the high surface area of CSENPs by reducing particle agglomeration during electrocatalysis. This barrier effect is also useful for constructing more active or durable structures by annealing without surface area loss. Compared to the clear stabilizing effect, however, the effect of the shell on active sites on the CSENP surface can be puzzling. Even when they are covered by a carbon shell that can block molecular adsorption on active sites, CSENP catalysts remain active and even exhibit unique catalytic behavior. Thus, we briefly cover recent efforts to identify major active sites on CSENPs using molecular probes. Furthermore, considering the membranelike role of the carbon shell, we suggest several remaining issues that should be resolved to obtain a fundamental understanding of CSENP design. Finally, we describe two synthetic approaches for the successful carbon shell encapsulation of nanoparticles: two-step and one-step syntheses. Both the postmortem coating of nanocatalysts (two-step) and the in situ formation via precursor ligands (one step) are shown to produce a durable carbon layer on nanocatalysts in a controlled manner. The strengths and limitations of each approach are also presented to promote the further investigation of advanced synthesis methods.The hybrid structure of CSENPs, that is, the active catalyst surface and the durable carbon shell, provides an interesting opportunity in electrocatalysis. However, our understanding of CSENPs is still highly limited, and further investigation is needed to answer fundamental questions regarding both active site identification and the mechanisms of stability improvement. Only when we start to comprehend the fundamental mechanisms underlying electrocatalysis on CSENPs will electrocatalysts be further improved for sustainable long-term device operation.

Nickel Phosphide Coated with Ultrathin Nitrogen Doped Carbon Shell as a Highly Durable and Active Catalyst towards Hydrogen Evolution Reaction

Developing alternative catalysts to Pt towards hydrogen evolution reaction(HER) is of both high scientific and technique importance for wide spread application of water electrolysis. Herein, Ni 2 P nanoparticles coated with ultra thin N-doped carbon shell were prepared as a highly efficient HER catalysts. Ni 2 P@CN exhibits both enhanced catalytic activity and durability in comparison with the carbon supported Ni 2 P counterpart, and represents 100% faradaic yield for HER in acidic medium. The improved charge transfer of N doped graphitic carbon shells make a contribution to the increase in activity. Meanwhile, the carbon shells suppress the aggregation and exfoliation of Ni 2 P nanoparticles. As a result, the synergistic role of N doped carbon layer confer the Ni 2 P cores with boosted activity and stability.

Heteroatom-Doped Metal-Free Carbon Nanomaterials as Potential Electrocatalysts

In recent years, heteroatom-incorporated specially structured metal-free carbon nanomaterials have drawn huge attention among researchers. In comparison to the undoped carbon nanomaterials, heteroatoms such as nitrogen-, sulphur-, boron-, phosphorous-, etc., incorporated nanomaterials have become well-accepted as potential electrocatalysts in water splitting, supercapacitors and dye-sensitized solar cells. This review puts special emphasis on the most popular synthetic strategies of heteroatom-doped and co-doped metal-free carbon nanomaterials, viz., chemical vapor deposition, pyrolysis, solvothermal process, etc., utilized in last two decades. These specially structured nanomaterials’ extensive applications as potential electrocatalysts are taken into consideration in this article. Their comparative enhancement of electrocatalytic performance with incorporation of heteroatoms has also been discussed.

Catalyst overcoating engineering towards high-performance electrocatalysis

Clean and sustainable energy needs the development of advanced heterogeneous catalysts as they are of vital importance for electrochemical transformation reactions in renewable energy conversion and storage devices. Advances in nanoscience and material chemistry have afforded great opportunities for the design and optimization of nanostructured electrocatalysts with high efficiency and practical durability. In this review article, we specifically emphasize the synthetic methodologies for the versatile surface overcoating engineering reported to date for optimal electrocatalysts. We discuss the recent progress in the development of surface overcoating-derived electrocatalysts potentially applied in polymer electrolyte fuel cells and water electrolyzers by correlating catalyst intrinsic structures with electrocatalytic properties. Finally, we present the opportunities and perspectives of surface overcoating engineering for the design of advanced (electro)catalysts and their deep exploitation in a broad scope of applications.

Molybdenum Carbide-PtCu Nanoalloy Heterostructures on MOF-Derived Carbon toward Efficient Hydrogen Evolution

In this study, PtCu-Mo₂ C heterostructure with charge redistribution is investigated via first-principles theoretical calculations. Mo₂ C can promote the formation of the electron-rich region of PtCu as an active site, displaying an optimized adsorption behavior toward hydrogen in terms of reduced thermodynamic energy barriers. Owing to the attractive density functional theory calculation results, the PtCu-Mo₂ C heterostructure is fabricated via carbonization of the unique metal-organic framework (MOF) followed by the replacement reduction reaction for the first time. Owing to its swift kinetics and outstanding specific activity, it exhibits high hydrogen evolution reaction (HER) catalytic activity (26 mV @ 10 mA cm^-2 ) and superior mass activity (1 A mg_Pt ^-1 at -0.04 V) in acidic media, which is approximately six times that of commercial Pt/C catalysts. The perception of the intrinsic activity origin of the alloy with an excellent structural support can guide the development of Pt-based and other alloy catalysts in future.

Palladium Nanoparticles Hardwired in Carbon Nanoreactors Enable Continually Increasing Electrocatalytic Activity During the Hydrogen Evolution Reaction

Catalysts typically lose effectiveness during operation, with much effort invested in stabilising active metal centres to prolong their functional lifetime for as long as possible. In this study palladium nanoparticles (PdNP) supported inside hollow graphitised carbon nanofibers (GNF), designated as PdNP@GNF, opposed this trend. PdNP@GNF exhibited continuously increasing activity over 30000 reaction cycles when used as an electrocatalyst in the hydrogen evolution reaction (HER). The activity of PdNP@GNF, expressed as the exchange current density, was always higher than activated carbon (Pd/C), and after 10000 cycles PdNP@GNF surpassed the activity of platinum on carbon (Pt/C). The extraordinary durability and self-improving behaviour of PdNP@GNF was solely related the unique nature of the location of the palladium nanoparticles, that is, at the graphitic step-edges within the GNF. Transmission electron microscopy imaging combined with spectroscopic analysis revealed an orchestrated series of reactions occurring at the graphitic step-edges during electrocatalytic cycling, in which some of the curved graphitic surfaces opened up to form a stack of graphene layers bonding directly with Pd atoms through Pd-C bonds. This resulted in the active metal centres becoming effectively hardwired into the electrically conducting nanoreactors (GNF), enabling facile charge transport to/from the catalytic centres resulting in the dramatic self-improving characteristics of the electrocatalyst.

Facet-Selective Deposition of Ultrathin Al2 O3 on Copper Nanocrystals for Highly Stable CO2 Electroreduction to Ethylene

Catalysts based on Cu nanocrystals (NCs) for electrochemical CO₂ -to-C₂₊ conversion with high activity have been a subject of considerable interest, but poor stability and low selectivity for a single C₂₊ product remain obstacles for realizing sustainable carbon-neutral cycles. Here, we used the facet-selective atomic layer deposition (FS-ALD) technique to selectively cover the (111) surface of Cu NCs with ultrathin Al₂ O₃ to increase the exposed facet ratio of (100)/(111), resulting in a faradaic efficiency ratio of C₂ H₄ /CH₄ for overcoated Cu NCs 22 times higher than that for pure Cu NCs. Peak performance of the overcoated catalyst (Cu NCs/Al₂ O₃ -10C) reaches a C₂ H₄ faradaic efficiency of 60.4 % at a current density of 300 mA cm^-2 in 5 M KOH electrolyte, when using a gas diffusion electrode flow cell. Moreover, the Al₂ O₃ overcoating effectively suppresses the dynamic mobility and the aggregation of Cu NCs, which explains the negligible activity loss and selectivity degradations of Cu NCs/Al₂ O₃ -10C shown in stability tests.

Differences in the Electrochemical Performance of Pt-Based Catalysts Used for Polymer Electrolyte Membrane Fuel Cells in Liquid Half- and Full-Cells

A substantial amount of research effort has been directed toward the development of Pt-based catalysts with higher performance and durability than conventional polycrystalline Pt nanoparticles to achieve high-power and innovative energy conversion systems. Currently, attention has been paid toward expanding the electrochemically active surface area (ECSA) of catalysts and increase their intrinsic activity in the oxygen reduction reaction (ORR). However, despite innumerable efforts having been carried out to explore this possibility, most of these achievements have focused on the rotating disk electrode (RDE) in half-cells, and relatively few results have been adaptable to membrane electrode assemblies (MEAs) in full-cells, which is the actual operating condition of fuel cells. Thus, it is uncertain whether these advanced catalysts can be used as a substitute in practical fuel cell applications, and an improvement in the catalytic performance in real-life fuel cells is still necessary. Therefore, from a more practical and industrial point of view, the goal of this review is to compare the ORR catalyst performance and durability in half- and full-cells, providing a differentiated approach to the durability concerns in half- and full-cells, and share new perspectives for strategic designs used to induce additional performance in full-cell devices.

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.

Doping of MoS2 by "Cu" and "V": An Efficient Strategy for the Enhancement of Hydrogen Evolution Activity

To replace Pt-based compounds in the electrocatalytic hydrogen evolution reaction (HER), MoS₂ has already been established as an efficient catalyst. The electrocatalytic activity of MoS₂ is further improved by tuning the morphology and the electronic structure through doping, which helps the band energy position to be modified. Presently, thin sheets of MoS₂ (MoS₂-TSs) are synthesized via a microwave technique. Thin sheets of MoS₂ can outperform nanosheets of MoS₂ in the HER. Further, the efficiency of the thin sheets is improved by doping with different metals like Cu, V, Zn, Mn, Fe, Sn, etc. "Cu"- and "V"-doped MoS₂-TSs are highly efficient for the HER. At a fixed potential of -0.588 V vs RHE, Cu-doped MoS₂ (Cu-MoS₂-TS), V-doped MoS₂ (V-MoS₂-TS), and MoS₂-TS can generate current densities of 327.46, 308.45, and 127.82 mA/cm², respectively. The electrochemically active surface area increases nearly 7.7-fold and 2.5-fold for Cu-MoS₂-TS and V-MoS₂-TS than for MoS₂-TS, respectively. Cu-MoS₂-TS shows exceptionally high electrocatalytic stability up to 140 h in an acidic medium (0.5 M H₂SO₄). First-principles calculations using density functional theory (DFT) are performed, which are well matched with the experimental observations. DFT calculations dictate that after doping with "V" and "Cu" both valance band maxima and conduction band minima are uplifted, which indicates the higher hydrogen-ion-reducing ability of M-MoS₂-TS (M = Cu, V) compared to bare MoS₂-TS.

Low-PGM and PGM-Free Catalysts for Proton Exchange Membrane Fuel Cells: Stability Challenges and Material Solutions

Fuel cells as an attractive clean energy technology have recently regained popularity in academia, government, and industry. In a mainstream proton exchange membrane (PEM) fuel cell, platinum-group-metal (PGM)-based catalysts account for ≈50% of the projected total cost for large-scale production. To lower the cost, two materials-based strategies have been pursued: 1) to decrease PGM catalyst usage (so-called low-PGM catalysts), and 2) to develop alternative PGM-free catalysts. Grand stability challenges exist when PGM catalyst loading is decreased in a membrane electrode assembly (MEA)-the power generation unit of a PEM fuel cell-or when PGM-free catalysts are integrated into an MEA. More importantly, there is a significant knowledge gap between materials innovation and device integration. For example, high-performance electrocatalysts usually demonstrate undesired quick degradation in MEAs. This issue significantly limits the development of PEM fuel cells. Herein, recent progress in understanding the degradation of low-PGM and PGM-free catalysts in fuel cell MEAs and materials-based solutions to address these issues are reviewed. The key factors that degrade the MEA performance are highlighted. Innovative, emerging material concepts and development of low-PGM and PGM-free catalysts are discussed.

Toolbox for atomic layer deposition process development on high surface area powders

Atomic layer deposition (ALD) is an industrially applied technique for thin film deposition. The vast majority of processes target flat substrates rather than powders. For ALD on powders, new processes are needed, as different reaction conditions are required. Here, two setups are described in detail, which enhance the ALD process development for powders. The first setup described is capable of directly measuring the vapor pressure of a given precursor by a capacitance diaphragm gauge. Promising precursors can be pre-selected, and suitable precursor saturation temperatures can be determined. The second setup consists of four parallel reactors with individual temperature zones to screen the optimal ALD temperature window in a time efficient way. Identifying the precursor saturation temperature beforehand and subsequently performing the first ALD half cycle in the parallel setup at four different reactor temperatures simultaneously will drastically reduce process development times. Validation of both setups is shown for the well-known ALD precursors, trimethylaluminum to deposit aluminum oxide and diethyl zinc to deposit zinc oxide, both on amorphous silica powder.

Atomic/molecular layer deposition for energy storage and conversion

Energy storage and conversion systems, including batteries, supercapacitors, fuel cells, solar cells, and photoelectrochemical water splitting, have played vital roles in the reduction of fossil fuel usage, addressing environmental issues and the development of electric vehicles. The fabrication and surface/interface engineering of electrode materials with refined structures are indispensable for achieving optimal performances for the different energy-related devices. Atomic layer deposition (ALD) and molecular layer deposition (MLD) techniques, the gas-phase thin film deposition processes with self-limiting and saturated surface reactions, have emerged as powerful techniques for surface and interface engineering in energy-related devices due to their exceptional capability of precise thickness control, excellent uniformity and conformity, tunable composition and relatively low deposition temperature. In the past few decades, ALD and MLD have been intensively studied for energy storage and conversion applications with remarkable progress. In this review, we give a comprehensive summary of the development and achievements of ALD and MLD and their applications for energy storage and conversion, including batteries, supercapacitors, fuel cells, solar cells, and photoelectrochemical water splitting. Moreover, the fundamental understanding of the mechanisms involved in different devices will be deeply reviewed. Furthermore, the large-scale potential of ALD and MLD techniques is discussed and predicted. Finally, we will provide insightful perspectives on future directions for new material design by ALD and MLD and untapped opportunities in energy storage and conversion.

Electrocatalysis in confined spaces: interplay between well-defined materials and the microenvironment

Catalysis in a confined space has attracted much attention due to the simultaneously designable nature of active sites and their microenvironment, leading to a broad spectrum of highly efficient chemical conversion schemes. Recent work has extended the scope of confined catalysis to electrochemical reactions. Mechanistic studies suggest that the confined environment in electrocatalysis can modulate mechanical, electronic, and geometric effects, stabilizing important charge-transfer intermediates and promoting reaction kinetics. In this minireview, we first discuss the fundamental concepts of confined catalysis by summarizing density functional theory (DFT) calculations and experimental investigations. We then present the rational design and applications of space-confined electrocatalysts with emphasis on the confined environment provided by carbon-based materials. We specifically focus on metal-based materials confined in carbon nanotubes (CNTs) and their applications in emerging electrochemical reactions including the oxygen reduction reaction (ORR), water-splitting reactions, carbon dioxide reduction reaction (CO2RR), and nitrogen reduction reaction (NRR). Finally, the existing challenges, opportunities, and future directions of electrocatalysis in confined spaces are highlighted.

Cu assisted loading of Pt on CeO2 as a carbon-free catalyst for methanol and oxygen reduction reaction

TEM images of the PtCu/CeO2-21 catalyst. The scale bar in image (B) is 5 nm. Image (C) shows the area chosen for elemental mapping; image (D, E, and F) show the mapping of Ce, Cu, and Pt, respectively.

Area-Selective Atomic Layer Deposition Patterned by Electrohydrodynamic Jet Printing for Additive Manufacturing of Functional Materials and Devices

There is an increasing interest in additive nanomanufacturing processes, which enable customizable patterning of functional materials and devices on a wide range of substrates. However, there are relatively few techniques with the ability to directly 3D print patterns of functional materials with sub-micron resolution. In this study, we demonstrate the use of additive electrohydrodynamic jet (e-jet) printing with an average line width of 312 nm, which acts as an inhibitor for area-selective atomic layer deposition (AS-ALD) of a range of metal oxides. We also demonstrate subtractive e-jet printing with solvent inks that dissolve polymer inhibitor layers in specific regions, which enables localized AS-ALD within those regions. The chemical selectivity and morphology of e-jet patterned polymers towards binary and ternary oxides of ZnO, Al₂O₃, and SnO₂ were quantified using X-ray photoelectron spectroscopy, atomic force microscopy, and Auger electron spectroscopy. This approach enables patterning of functional oxide semiconductors, insulators, and transparent conducting oxides with tunable composition, Å-scale control of thickness, and sub-μm resolution in the x-y plane. Using a combination of additive and subtractive e-jet printing with AS-ALD, a thin-film transistor was fabricated using zinc-tin-oxide for the semiconductor channel and aluminum-doped zinc oxide as the source and drain electrical contacts. In the future, this technique can be used to print integrated electronics with sub-micron resolution on a variety of substrates.

Synthesis of a Rationally Designed Multi-Component Photocatalyst Pt:SiO2:TiO2(P25) with Improved Activity for Dye Degradation by Atomic Layer Deposition

Photocatalysts for water purification typically lack efficiency for practical applications. Here we present a multi-component (Pt:SiO₂:TiO₂(P25)) material that was designed using knowledge of reaction mechanisms of mono-modified catalysts (SiO₂:TiO₂, and Pt:TiO₂) combined with the potential of atomic layer deposition (ALD). The deposition of ultrathin SiO₂ layers on TiO₂ nanoparticles, applying ALD in a fluidized bed reactor, demonstrated in earlier studies their beneficial effects for the photocatalytic degradation of organic pollutants due to more acidic surface Si–OH groups which benefit the generation of hydroxyl radicals. Furthermore, our investigation on the role of Pt on TiO₂(P25), as an improved photocatalyst, demonstrated that suppression of charge recombination by oxygen adsorbed on the Pt particles, reacting with the separated electrons to superoxide radicals, acts as an important factor for the catalytic improvement. Combining both materials into the resulting Pt:SiO₂:TiO₂(P25) nanopowder exceeded the dye degradation performance of both the individual SiO₂:TiO₂(P25) (1.5 fold) and Pt:TiO₂(P25) (4-fold) catalysts by 6-fold as compared to TiO₂(P25). This approach thus shows that by understanding the individual materials’ behavior and using ALD as an appropriate deposition technique enabling control on the nano-scale, new materials can be designed and developed, further improving the photocatalytic activity. Our research demonstrates that ALD is an attractive technology to synthesize multicomponent catalysts in a precise and scalable way.

Tailoring the Electronic Structure of Transition Metals by the V2C MXene Support: Excellent Oxygen Reduction Performance Triggered by Metal-Support Interactions

The enhancement of oxygen reduction reaction (ORR) activity can significantly boost the performance of fuel cells. MXene-supported transition metals with strong metal-support interactions (SMSI) are an effective strategy to increase the catalytic activity and durability while decreasing the usage of noble metals. Herein, a series of composites of transition-metal atoms (Ni, Pd, Pt, Cu, Ag, and Au) deposited on V₂C MXene are designed as potential catalysts for ORR using density functional theory. The calculation results demonstrate that all the transition metals prefer to form a monolayer on V₂C (TM_ML/V₂C) with high thermodynamic stability because of SMSI, in which the Pd, Pt, Ag, and Au monolayers exhibit high chemical stability during the ORR process. Pt_ML/V₂C exhibits the highest activity toward ORR with the overpotential down to 0.38 V and the largest energy barrier of 0.48 eV. The excellent catalytic performance originates from the modification of the electronic structure by the V₂C support because of SMSI. Our studies elucidate the SMSI between transition-metal atoms and V₂C MXene from the atomic level and thus rationally design the ORR catalyst at the cathode of fuel cells to enhance the activity while possessing high stability and less Pt usage.

Spatial Ensembles of Copper-Silica with Carbon Nanotubes as Ultrastable Nanostructured Catalysts for Selective Hydrogenation

Catalyst deactivation is one of the most important issues in heterogeneous catalysis. Constructing a stable nanoscale structure that maintains efficient activity and prolonged stability under redox conditions for catalysis, particularly hydrogenation reactions, remains attractive albeit the flourishing nanoscience. This work presents a facile route to synthesize a semi-encapsulated transition metal by assembling three-dimensional transition metal silicate nanotubes onto carbon nanotubes (CNTs) as precursors. The obtained materials expose an active surface of the transition metal for efficient catalysis and form a specific structure to inhibit the migration of metal nanoparticles (NPs) by establishing strong metal-support interactions. Cu@SiO₂ prepared by common precipitation shows an inferior activity, and its performance is easily attenuated because of the aggregation of Cu NPs. The addition of CNTs as a carrier doubles the intrinsic activity of Cu catalysts. This hybrid catalyst, which consists of Cu species, SiO₂, and CNTs, is among the best catalysts for dimethyl oxalate hydrogenation with boosting activity of 25 h^-1 and enhanced stability of more than 200 h.

Uniform Pt nanoparticles supported on urchin-like mesoporous TiO2 hollow spheres as stable electrocatalysts for the oxygen reduction reaction

In order to promote the commercial application of proton exchange membrane fuel cells, it is of great importance to develop Pt-based electrocatalysts with high activity and stability for the oxygen reduction reaction (ORR). Here, urchin-like mesoporous TiO₂ hollow spheres (UMTHS) with a high specific surface area (167.1 m² g^-1) and improved conductivity were designed and applied as supports to disperse Pt nanoparticles (NPs) for the first time. Uniform Pt NPs (∼3.2 nm) on the surface of nanothorns were obtained after heat treatment. The as-prepared product (Pt/UMTHS) exhibited a more positive half-wave potential (E_h) than that of the reference sample Pt@C without UMTHS (0.867 V vs. 0.829 V). The improved performance can be ascribed to the high specific surface area of UMTHS. The Pt/UMTHS also exhibited a much better ORR stability than the commercial Pt/C after long-term cycling at 0.6-1.0 V according to the comparison of E_h, mass activity and electrochemical surface area with Pt/C. The enhanced stability of Pt/UMTHS was mainly derived from the strong metal support interaction between Pt NPs and UMTHS, together with the spatial restriction and the anti-restriction provided by UMTHS.

Surface functionalization on nanoparticles via atomic layer deposition

As an ultrathin film preparation method, atomic layer deposition (ALD) has recently found versatile applications in fields beyond semiconductors, such as energy, environment, catalysis and so on. The design, preparation and characterization of thin film applied in the emerging fields have attracted great interests. The development of ALD technique on particles opens up a broad horizon in the advanced nanofabrication. Pioneering applications are exploring conformal coating, porous coating and selective surface modification of nanoparticles. Conformal encapsulation of particles is a major application to protect materials with ultrathin films from being eroded by the external environment while keeping the original properties of the primary particles. Porous coating has been developed to simultaneously expose the particles' surface and provide nanopores, which is another important method that demonstrates its advantages in modification of electrode materials, catalysis and energy applications, etc. Selective ALD takes the method forward in order to precisely control the directionality of decoration sites on the particles and selectively passivate undesired facets, sites, or defects. Such methods provide practical strategies for atomic scale and precise surface functionalization on particles and greatly expand its potential applications.

Encapsulating Pt Nanoparticles inside a Derived Two-Dimensional Metal-Organic Frameworks for the Enhancement of Catalytic Activity

The development of highly active and stable electrocatalysts toward oxygen reduction reaction (ORR) and hydrogen evolution reaction (HER) is a key for commercial application of fuel cells and water splitting. Here, we report a highly active and stable Pt nanoparticles (NPs) encapsulated in ultrathin two-dimensional (2D) carbon layers derived from the ultrathin 2D metal-organic framework precursor (ZIF-67). Electrochemical tests reveal that our approach not only stabilized Pt NPs successfully but also boosted Pt activities toward ORR and HER. We found that our Pt catalysts encapsulated in ultrathin 2D carbon layers exhibited an ORR activity of 5.9 and 12 times greater than those of the commercial Pt/C and Pt/RGO without 2D carbon layer protection. Our encapsulated Pt catalysts also show more than nine times higher stability than those of Pt/C catalysts. In addition to ORR, our novel encapsulated Pt catalysts display an extraordinary stability and activity toward HER, with a lower overpotential (14.3 mV in acidic media and 37.2 mV in alkaline media) at a current density of 10 mA cm^-2 than Pt/C catalysts (23.1 mV in acidic media and 92.1 mV in alkaline media). The enhanced electrochemical activities and stability of our encapsulated Pt catalysts are attributed to the synergistic effect of Pt-based NPs and ultrathin 2D carbon layers derived from ZIF-67 with enriched active sites Co-N_x. First-principles simulations reveal that the synergistic catalysis of Pt-based NPs and Co-N_x derived from ZIF-67 improves the activity for ORR and HER.

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.

Electrocatalytic Oxidation of Ethanol on the Surface of Graphene Based Nanocomposites: An Introduction and Review to it in Recent Studies

This review gives an overview of the electrochemical investigations about the properties of various types of graphene composites in the ethanol oxidation. Various routes to provide appropriate graphene-based materials required electrochemical techniques for investigation of different types of the materials as well as their performance and efficacy in ethanol oxidation are discussed in detail. Furthermore, it is demonstrated that the incorporation of suitable materials, e. g. noble metals (graphene-supported binary and ternary metal nanoparticles), metal oxides, conductive polymer, etc, with graphene results in excellent electrocatalytic activity, superb durability and selectivity in ethanol oxidation. Immobilization of electrocatalytically active NPs on graphene supports using physical approaches is considered as an effective route to prepare direct ethanol fuel cell (DEFC) anode catalysts.

Highly Dispersed Single-Atom Pt and Pt Clusters in the Fe-Modified KL Zeolite with Enhanced Selectivity for n-Heptane Aromatization

Conversion of straight-chain paraffins into aromatics is particularly attractive but extremely challenging in the oil refining industry. Constructing the Pt-supported catalysts with high aromatic selectivity is vital. Here, we report a strategy to use Fe-modified KL zeolites to improve the Pt atom utilization efficiency and anchor them inside KL zeolite channels via atomic-layer deposition technique. A combination of highly dispersed single-atom Pt and electron-rich Pt clusters is fabricated on the KL zeolite through the creation of proper nucleation sites. The resulted catalyst (PtFe-1/KL) exhibits excellent performance for the n-heptane aromatization (90.1% aromatic selectivity) with an apparent activation energy of 131 kJ/mol and much enhanced stability at a relatively lower temperature (420 °C). Experimental analysis and density functional theory calculation demonstrate that the single-atom Pt might play a key role in the initial dehydrogenation of n-heptane to 1-heptene, and the superior stable Pt clusters encapsulated inside Fe-decorated KL zeolite channels accelerate the 1-heptene dehydrocyclization to aromatics. The synergetic interaction between single-atom Pt and Pt clusters enables the PtFe-1/KL catalyst to be one of the most effective n-heptane aromatization catalysts reported to date.

Oxygen Reduction Reaction Catalyzed by Black-Phosphorus-Supported Metal Nanoparticles: Impacts of Interfacial Charge Transfer

Development of effective catalysts for oxygen reduction reaction (ORR) plays a critical role in the applications of a range of electrochemical energy technologies. In this study, thin-layered black phosphorus (TLBP) was used as a unique supporting substrate for the deposition of metal nanoparticles (MNPs, M = Pt, Ag, Au), and the resulting M-TLBP nanocomposites were found to exhibit apparent ORR activity that was readily manipulated by interfacial charge transfer from TLBP to MNPs. This was confirmed by results from X-ray photoelectron spectroscopic measurements and density functional theory calculations. In comparison to the carbon-supported counterparts, Ag-TLBP and Au-TLBP showed enhanced ORR performance, while a diminished performance was observed with Pt-TLBP. This was consistent with the predictions from the "volcano plot". Results from this study suggest that black phosphorus can serve as a unique addition in the toolbox of manipulating electronic properties of supported metal nanoparticles and their electrocatalytic activity.

Identifying the Activation of Bimetallic Sites in NiCo2 S4 @g-C3 N4 -CNT Hybrid Electrocatalysts for Synergistic Oxygen Reduction and Evolution

Hybrid materials composed of transition-metal compounds and nitrogen-doped carbonaceous supports are promising electrocatalysts for various electrochemical energy conversion devices, whose activity enhancements can be attributed to the synergistic effect between metallic sites and N dopants. While the functionality of single-metal catalysts is relatively well-understood, the mechanism and synergy of bimetallic systems are less explored. Herein, the design and fabrication of an integrated flexible electrode based on NiCo₂ S₄ /graphitic carbon nitride/carbon nanotube (NiCo₂ S₄ @g-C₃ N₄ -CNT) are reported. Comparative studies evidence the electronic transfer from bimetallic Ni/Co active sites to abundant pyridinic-N in underlying g-C₃ N₄ and the synergistic effect with coupled conductive CNTs for promoting reversible oxygen electrocatalysis. Theoretical calculations demonstrate the unique coactivation of bimetallic Ni/Co atoms by pyridinic-N species (a Ni, Co-N₂ moiety), which simultaneously downshifts their d-band center positions and benefits the adsorption/desorption features of oxygen intermediates, accelerating the reaction kinetics. The optimized NiCo₂ S₄ @g-C₃ N₄ -CNT hybrid manifests outstanding bifunctional performance for catalyzing oxygen reduction/evolution reactions, highly efficient for realistic zinc-air batteries featuring low overpotential, high efficiency, and long durability, superior to those of physical mixed counterparts and state-of-the-art noble metal catalysts. The identified bimetallic coactivation mechanism will shed light on the rational design and interfacial engineering of hybrid nanomaterials for diverse applications.