Atomic-scale engineering of advanced catalytic and energy materials via atomic layer deposition for eco-friendly vehicles

Zero-emission eco-friendly vehicles with partly or fully electric powertrains have exhibited rapidly increased demand for reducing the emissions of air pollutants and improving the energy efficiency. Advanced catalytic and energy materials are essential as the significant portions in the key technologies of eco-friendly vehicles, such as the exhaust emission control system, power lithium ion battery and hydrogen fuel cell. Precise synthesis and surface modification of the functional materials and electrodes are required to satisfy the efficient surface and interface catalysis, as well as rapid electron/ion transport. Atomic layer deposition (ALD), an atomic and close-to-atomic scale manufacturing method, shows unique characteristics of precise thickness control, uniformity and conformality for film deposition, which has emerged as an important technique to design and engineer advanced catalytic and energy materials. This review has summarized recent process of ALD on the controllable preparation and modification of metal and oxide catalysts, as well as lithium ion battery and fuel cell electrodes. The enhanced catalytic and electrochemical performances are discussed with the unique nanostructures prepared by ALD. Recent works on ALD reactors for mass production are highlighted. The challenges involved in the research and development of ALD on the future practical applications are presented, including precursor and deposition process investigation, practical device performance evaluation, large-scale and efficient production, etc.


Introduction
Eco-friendly vehicles are the sustainable future of vehicle industry by reducing the emissions of air pollutants and improving the energy efficiency, which are powered by advanced engine technologies with matched zero-emission control system or partly and fully powered by clean energy technologies, such as battery and fuel cell [1][2][3][4][5]. Nowadays the sales of electric vehicles have grown year by year, yet the continuous efforts will be made to develop efficient catalysts for exhaust emission control in the coming decades, as internal combustion engines are still widely used in fossil fuel vehicles and transitional hybrid electric vehicles [6]. Both lithium-ion battery and hydrogen fuel cell are impressive power supplies for electric vehicles. Owing to the increasing specific energy and decreasing cost in the past decades, lithium-ion battery with lightweight, compactness and affordability has almost exclusively powered commercial electric vehicles at present [7,8]. Different from lithium-ion battery, hydrogen fuel cell produces electricity from the electrochemical oxidation of hydrogen [9]. Since the first commercialization of hydrogen fuel cell vehicle in 2014 by Toyota, much attention has been paid to overcoming the technical barriers for large scale applications, such as cost, performance and durability [10].
Advanced catalytic and energy materials are essential for the key components of eco-friendly vehicles, such as exhaust emission control system, lithium ion battery and hydrogen fuel cell. Although the roles of these materials are different in the components, improving the performance of catalysis and electron/ion transport is important for development of advanced catalytic and energy materials. As shown in figure 1, precious metals or metal oxides are commonly applied as exhaust catalysts to eliminate carbon monoxide (CO), hydrocarbons, nitrogen oxides (NO x ) and particulate matter. These catalytic reactions at gas/solid two-phase interfaces are sensitive to the surface and interface structures of exhaust catalysts [11,12]. As an energy conversion component, hydrogen fuel cell involves the electrochemical catalysis at gas/solid/liquid threephase interfaces, which also needs highly efficient catalysts to accelerate the sluggish kinetics of catalytic reactions on electrodes [13,14]. Moreover, the stability of exhaust gas catalysts and hydrogen fuel cell catalysts has also attracted much attention, which is closely related to their nanostructures [15]. Furthermore, the electron/ion transport is important for energy storage and conversion for both lithium-ion battery and hydrogen fuel cell. For instance, the high energy density of lithiumion battery usually brings out the problems of low cycle life and poor safety that requires surface modifications of cathode and anode materials to improve their stability and simultaneously ensure electron/ion transport [16,17]. The transport of electron/ion and reactants in the membrane electrode assembly (MEA) can affect the power density and efficiency of hydrogen fuel cell, which requires the precise control and modification of electrochemical interfaces [18,19]. Overall, the precise control of surface and interfacial structure of catalytic and energy material is of great significance to satisfy the highly efficient surface and interface catalysis, as well as rapid electron/ion transport.
Atomic layer deposition (ALD), an atomic and close-toatomic scale manufacturing method, is capable of designing and engineering advanced catalysts and electrodes, which exhibits higher accuracy than conventional wet-chemistry methods [20]. As shown in figure 1, ALD is a thin film technology based on alternately self-limited saturated adsorption and chemical reaction of gas-phase precursors on substrate surface, which is firstly invented by Tuomo Suntola in 1970s [21]. In a typical ALD cycle, an inert gas is introduced to purge out unreacted precursor A or B to separate two self-limited chemical reactions on substrate. The self-limited chemical reactions between precursors and substrate result in the superior capability of ALD for preparing uniform and conformal films on substrate with high aspect ratio micro-nano structures. The thickness of films prepared by ALD can also be precisely controlled by tuning the ALD cycles. ALD has achieved industrial applications in displays, microelectronics and solar energy with its unique characteristics of precise thickness control, uniformity and conformality [22][23][24]. For vehicles, ALD has been performed to prepare precious metal or metal oxide nano coatings to improve the performance of micro-electro-mechanical system, such as capacitive sensor, micro-resonator and highpower radar [25][26][27]. The structure components can be modified by ALD coatings to increase the mechanic strength and corrosion resistance [28][29][30][31]. As a promising energy storage device, supercapacitor also requires surface modification on the electrodes to increase the energy density and cyclic stability. ALD technique has not only been utilized to coat active materials on high-surface-area supports to improve the energy density, but also played an important role in preparing passivation layer to enhance electrochemical stability and minimize self-discharge [32][33][34][35]. Moreover, ALD is also considered as a thin-film micromanufacturing technique for designing and preparing microbatteries and electrochemical energy storage devices [36,37].
Indeed, applications of ALD for catalysts, batteries and fuel cells have also attracted widespread attention to the enhancement of their chemical and electrochemical performance [38,39]. Since the catalysts for exhaust gas cleaning and energy conversion in fuel cells, as well as electrode materials in batteries are powder materials with high specific surface areas, the ALD modifications on these catalytic and energy materials suffer from agglomeration problem and usually exhibit lower efficiency than that of conventional ALD on planer substrates. In order to achieve efficient and uniform gas-solid contact between precursors and particles, continuous fluidization or mechanical agitation of particle bed has been introduced into the special reactors for ALD on porous materials [40][41][42]. The ALD processes for surface modifications on powder materials have also been summarized in previously reported reviews [43][44][45][46]. Since the invention of ALD in 1970s, it has been applied to prepare supported heterogeneous catalysts [47]. In the past decade, advanced catalysts with atomically precise control over the active sites and composite structures have been designed and prepared by ALD, such as supported uniform metal particles, clusters and single atoms, as well as various unique overcoating structures, which exhibit enhanced activity, selectivity and stability for a variety of catalytic reactions [48,49]. Since the early work of ALD coating on LiCoO 2 cathodes to improve the durability, ALD is proposed to modify the surface and interface of electrode materials and separators for batteries and fuel cells, which can result in the improvements of structural and chemical stability, electronic and ionic conductivity, as well as mass transport for energy storage and conversion [32,[50][51][52]. Overall, ALD has emerged as an important technique for the surface and interface engineering of advanced catalytic and energy materials, which is of significance for the research and development for eco-friendly vehicles.
In this review, the ALD processes on materials for catalysis and energy, as well as their enhanced performance for exhaust emission control, power lithium ion battery and hydrogen fuel cell are comprehensively introduced. In the following sections, the catalysts such as precious metal, non-precious metal and oxide prepared by ALD for exhaust emission control are reviewed initially. Then, an overview of ALD for the surface and interface modification of lithium ion battery materials, such as cathode, anode and separator is provided. As a controllable preparation and modifications methods for catalysts and electrodes, ALD processes applied for hydrogen fuel cell are reviewed. At the end of this review, the ALD reactors for mass production are introduced, while the challenges involved in the research and development of ALD on the future practical applications in eco-friendly vehicles are discussed.

Catalysts for exhaust emission control
In order to satisfy the current near-zero vehicle emission regulations for vehicle exhaust pollutants, such as CO, hydrocarbons, NO x and particulate matter, the activity of catalyst at low temperature is the key to eliminate the exhaust pollutants at cold start stage, while the high temperature stability determines the lifetime of catalyst at working conditions. Since the catalytic reactions of exhaust pollutants occur on the surface of catalysts, the catalytic activities are sensitive to the atomic structures of surface and interface of exhaust catalysts. Table 1 has summarized the recently reported catalysts prepared by ALD for exhaust emission control. Supported Pt group metal catalysts are commonly used as the three-way catalyst and diesel oxide catalyst for gasoline and diesel aftertreatment, respectively. Composite catalysts with smaller precious metal size and stronger metal-support interaction have been prepared to improve the low temperature activity. A promising strategy has emerged to encapsulate Ptbased nanoparticles in a protective layer to enhance the high temperature stability. Supported non-precious metal or oxide  [75] (Continued.) catalysts have been prepared by ALD for SCR of NO x and soot combustion.

Supported Pt group metal catalysts
By utilizing the initial island nucleation stage of metal ALD, ALD has been applied to prepare metallic nanoparticles for catalysis [48,88]. Since the self-limiting chemical reactions of sequential precursors on the surface of supports, ALD provides the opportunity to deposit uniform metal nanoparticles with precisely controlled size, while the mass loading of metal is affected by the nature of metal precursors, deposition temperature and surface structures of supports. As another factor to affect the catalytic activity, the density of metal particles is also closely related to the metal nucleation behavior and surface structures of supports. For Pt ALD with trimethyl-(methylcyclopentadienyl)platinum (Me 3 (MeCp)Pt), using H 2 as the second reagent shows smaller Pt size than the use of O 2 , which was attributed the activation of H 2 on Pt nanoparticles inducing the reduction of Pt and the depletion of surface hydroxyl groups [89]. Decreasing the size of noble metal particle is a direct method to increase the metal atom efficiency. Especially, single metal atom on support shows the maximum metal utilization. Supported single atom catalysts can also provide an ideal model for investigating the interfacial effects on catalyst activity, which can shed light on designing more efficient automotive catalyst. As shown in figure 2(a), Lu et al utilized the surface defects on CeO 2 nanorods to anchor Pt single atoms during Pt ALD [53,54]. Pt single atoms on the defects exhibited high stability due to the Pt-O-Ce bond interactions. Water in the feed gas could form surface hydroxyl groups on Pt 1 /CeO 2 that accelerated CO oxidation following Mars-van Krevelen mechanism by yielding the carboxyl intermediate. Moreover, they reported the bottom-up precise synthesis of Pt dimers by selective deposition of additional Pt atoms on the Pt single atoms like Lego building blocks [90,91]. The activity and stability of single atom catalysts are closely related to their local coordination environment [92][93][94]. For instance, the unique O lattice [H] could be created in the vicinity of Pt 2+ on CeO 2 after steam treatment at 750 • C, which not only greatly enhanced the low-temperature CO oxidation activity of CeO 2 supported Pt single atoms, but also exhibited outstanding hydrothermal stability [11,95]. The migration and aggregation of single atoms are closely related to the free energy and density of adatoms on supports. By creating surface defective sites on supports or constructing atom ensemble catalysts, the single atom catalysts can exhibit superior activity and durability after aging at 900 • C [96,97]. Recently, Liu et al reported a redox-coupled ALD method to prepare Pt single atoms and sub-nanoclusters on Cu doped CeO 2 supports (figure 2(b)) [55]. The coordination environment of interfacial Pt atom could affect the activation of O 2 and adsorption strength of CO, while a moderate CO adsorption strength at the interface could facilitate the low-temperature CO oxidation performance. The sub-nanoclusters were formed by the aggregation of supported Pt single atoms under a reduced atmosphere, which showed remarkable CO oxidation performance with an onset temperature (T onset ) below room temperature and much higher turnover frequency (TOF) than atomically-dispersed Pt catalysts. Gao et al prepared size-controlled Pt nanoparticles on TiO 2 nano-arrays and CeO 2 -based nanoflake arrays integrated on cordierite honeycombs, which exhibit good catalytic oxidation activities under the simulated exhaust condition of low-temperature diesel combustion [56,57]. The 90% conversion temperature of CO and hydrocarbon over Pt/CeO 2 was about 180 • C. Li et al prepared highly dispersed Pt nanoparticles with the average size of 1.9 nm on SiO 2 supported by 5 cycles ALD of Pt, the TOF of which at 200 • C reached to 1.8 s −1 [58]. Oxide supports are important for the dispersion of noble metal catalysts, the surface and bulk stability of which can also directly affect the durability of composite catalysts for frequently consuming and replenishing of surface oxygen species, as well as the high temperature working conditions. Compared with the simple transition metal oxides, such as CeO 2 , FeO x , Co 3 O 4 , the complex metal oxides with perovskite, mullite phase structures have been adopted for exhaust catalysis, due to their intrinsic activities and more stable atomic structures based on the backbone structure of connected metaloxygen polyhedra [98][99][100]. Enterkin et al deposited size controlled Pt nanoparticles on perovskite SrTiO 3 nanocuboids by controlling the number of ALD cycles, which exhibited a lower 50% conversion temperature (T 50 ) and higher stability for propane oxidation than a conventional Pt/Al 2 O 3 catalyst [59]. By controlling the morphology of single crystal SrTiO 3 supports, Chen et al found that Pd ALD on SrTiO 3 nanocuboids with TiO 2 -(001) exposed facet was more significant in size with the increase of the number of ALD cycles, while that on SrTiO 3 nanododecahedra with (110) exposed facet exhibited secondary nucleation and the density of Pd nanoparticles changed significantly [60]. Since the more edge and corner sites, smaller Pd nanoparticles (∼2 nm) on SiTiO 3 supports exhibited higher TOF for CO oxidation than larger Pd nanoparticles with the size of ∼3 nm. Different with perovskite oxides, Mn-based mullite oxide could expose the unique Mn-Mn dimer structure on the surface that was active for O 2 dissociation, which exhibited excellent NO oxidation activity [99,100]. As shown in figure 2(c), Liu et al reported the SmMn 2 O 5 mullite-type oxide supported Pt sub-nanoclusters catalyst by one cycle of Pt ALD [61]. The composite catalyst exhibited superior CO oxidation activity with the T 50 of 86 • C and activation energy of 43.87 kJ mol −1 due to the high activity of O 2 dissociation at Pt/SmMn 2 O 5 bifunctional interface. The constructed Pt/SmMn 2 O 5 bifunctional interface could provide spatially separated sites for CO and O 2 , which served as an efficient poison-free CO oxidation site. Based on first-principles based microkinetics analysis, they designed the interfacial structure with Mn-Fe hetero-dimer that was predicted to further enhance the low temperature CO oxidation activity of Pt/SmMn 2 O 5 catalyst [101]. In order to stabilize simple transition metal oxides, Onn and Gorte et al coated highsurface-area Al 2 O 3 supports by CeO 2 or Co 3 O 4 thin films to produce composite catalyst supports for Pd [63][64][65]. The composite catalysts showed similar activity to CeO 2 or Co 3 O 4 supported Pd catalysts for both CO and CH 4 oxidation, yet they can maintain their surface areas to much higher temperatures. Furthermore, Gorte et al synthesized high-surfacearea LaFeO 3 , CaTiO 3 , LaCoO 3 perovskite-type coatings on MgAl 2 O 4 supports by controlling the ALD cycles of simple oxides, Pt and Pd nanoparticles on which were found to be stabilized and switch between 'active' and 'inactive' states after high-temperature reduction or oxidation (figure 2(d)) [66][67][68][69]. These intelligent catalysts were closely related to the interaction between noble metal with the perovskite oxide films [102]. For instance, supported Pt nanoparticles could become part of the lattice of high-surface-area CaTiO 3 films under high temperature oxidizing condition due to the strong interaction between Pt with CaTiO 3 , while support Pd catalysts on CaTiO 3 did not show this phenomenon [67]. Their recent work revealed the different interaction between Rh and three types of perovskite films, while the interaction strength affected the size of Rh after high temperature redox treatment, which followed the sequence of Rh/CaTiO 3

Encapsulated Pt group metal catalysts
Since the surface active sites are predominant in catalytic activity, the thermal stability of Pt group metal nanoparticle is an important factor leading to the decrease of active sites due to sintering at high temperature. Oxide overcoatings are created as physical barriers to minimize the agglomeration of Pt group metal nanoparticles at elevated working temperature, which need certain trade-off between stabilization and reactivity by controlling the process for the oxide shell growth. Owing to the precisely controlling of ALD on the film thickness, many encapsulation strategies have been developed to enhance the stability of Pt group metal nanoparticles and keep the exposure of surface active sites to reactants. The number of Al 2 O 3 ALD cycles was controlled to prepare porous coatings on Pd nanoparticles by utilizing the nucleation stage of Al 2 O 3 [104]. Lu et al reported a calcination post-treatment method to create ∼2 nm micropores in ∼8 nm Al 2 O 3 coatings on Pd nanoparticles by removing the carbon residues caused by Al 2 O 3 ALD process [105]. Based on this method, Duan et al coated Pd/SiO 2 catalyst by porous Al 2 O 3 overlayers (figure 3(a)), large amounts of pentacoordinated Al 3+ sites in which can stabilize the PdO x phases resulting in enhanced activity and stability for methane combustion [70]. Cui et al also coated porous Al 2 O 3 on Pd/Al 2 O 3 by ALD, which effectively reduced the deactivation of Pd after sintering at 700 • C [71]. Furthermore, a porous Al 2 O 3 coating layer was prepared by thermal decomposition of organic carbon chains in the alucone layer deposited by molecular layer deposition (MLD), which stabilized the supported Pt nanoparticles even at 800 • C [72]. Besides the thick and porous Al 2 O 3 coating, some ultrathin films with the thickness smaller than 1 nm have been reported to stabilize Pt group metal nanoparticles. As shown in figure 3(b), Onn et al created a semicore-shell-like structure on PdO/Al 2 O 3 catalyst by ∼1 nm thick ZrO 2 coating after calcination at 800 • C, which prevented the sintering of Pd and stabilized methane oxidation rates [73]. Onn et al modified Pd/CeO 2 by 0.4 nm ZrO 2 film, which maintained the activity for methane oxidation upon calcination to 800 • C [74]. The agglomeration of Pt particles on Al 2 O 3 supports was mitigated by ∼0.83 nm porous MnO x coatings prepared by ALD [75].
In order to decrease the blocking of surface sites on Pt group metal nanoparticles by porous oxides, the selective ALD method has been developed to construct encapsulated nanostructures by control the nucleation and deposition of metal oxide on the desired areas or sites on the surface of catalysts. A low-temperature ABC-type ALD was reported to simultaneously deposit protected Pd nanoparticles and new support surface on TiO 2 supports. Pd nanoparticles were activated by removing the protective ligands after multiple ABC cycles [106]. Based on the unreactive template molecule or ligands, shape-selective sieving layers were prepared to stabilize single atom catalysts [107][108][109]. As shown in figure 3(c), Liu et al fabricated the Co 3 O 4 -nanotraps-anchored Pt nanoparticle structures on Al 2 O 3 supports based on the blocked Co 3 O 4 deposition on 1-octadecanethiol selectively covered Pt nanoparticles [76]. The surface Pt sites were re-exposed after removing the 1-octadecanethiol blocking agents via calcination in air. The formed Co 3 O 4 nanotraps were regarded as physical barriers and exhibited strong interfacial interactions with Pt, which greatly improved the low temperature CO oxidation activity and thermal stability of Pt nanoparticles simultaneously. Without using the blocking agents, Chen et al have also found the inherent selective deposition behaviors of some metal oxides on different surface sites of Pt group metal nanoparticles that depend on the activity of precursors and deposition conditions. For instance, CeO x was selectively deposited onto Pt (111) while leaving the Pt (100) surface intact, which was utilized to form nanofences around Pt nanoparticles (figure 3(d)). CeO 2 nanofence coated Pt nanoparticles exhibited sintering resistance at 700 • C in air and the excellent activity of the catalysts were retained after calcination [77]. Based on theoretical calculations, Wen et al found that the metal cyclopentadienyl precursors preferred to decomposition on the edge of Pt nanoparticles, indicating that edges were naturally selected to be covered during ALD of metal oxide using metal cyclopentadienyl precursors [110]. Nevertheless, AlO x ALD with dimethylaluminum isopropoxide as Al precursor preferentially coated the Pt (111) facets [111]. The selective growth of FeO x and NiO x on the low coordination edges sites of Pt nanoparticles were achieved using the FeCp 2 and CoCp 2 precursors [78,112]. Moreover, the NiO x passivated Pt nanoparticles exhibited significantly enhanced sintering resistance after calcination at 750 • C in air due to the stabilization of volatile atoms at low coordinated sites on Pt nanoparticles. Besides selective ALD method, the confined structures of metal nanoparticles were reported to be constructed with the assistance of templates [113]. The micropores in a KL zeolite were utilized to confine Pt nanoclusters by controlling the size during Pt ALD, which prevented the agglomeration of Pt catalysts [114]. Qin et al developed a template-assisted ALD method to construct confined metal nanoparticles. The metal oxide coated metal nanoparticles were firstly prepared on carbon nanocoils by ALD [115,116]. As sacrificial templates, carbon nanocoils were subsequently removed by calcination treatment, resulting in the metal oxide nanotubes confined metal nanoparticles with enhanced stability.

Supported non-precious metal or oxide catalysts
Besides Pt group metal catalysts, many supported nonprecious metal or oxide catalysts have also been prepared by ALD. For instance, Liang et al synthesized Fe single atoms on SiO 2 supports by optimizing the dose time of Fe precursor during ALD (figure 4(a)) [79]. SiO 2 supported Fe single atoms with high mass loading of 1.49 wt% exhibited much higher CO oxidation activity than other reported Fe-based catalysts with the TOF of ∼0.011 s −1 at 360 • C. Highly dispersed NiO x nanoparticles with the average size of ∼1 nm were deposited on mesoporous Al 2 O 3 supports by ALD and subsequently annealing process, which exhibited high CO oxidation activity near room temperature and enhanced stability after calcination above 500 • C [80]. Jackson et al developed a CeO 2 and ZrO 2 coated Cu-exchanged ZSM-5 catalysts by ALD for NO x removal from lean-diesel emissions using hydrocarbon catalyzed SCR [81]. Cu-exchanged ZSM-5 supports were modified by ZrO 2 deposited within the pore volume and ZrO 2 /CeO 2 coated on the surface, which exhibited ∼40% NO x conversion at 350 • C. Amorphous TiO 2 domains supported highly dispersed VO x species were prepared on mesoporous silica SBA-15 by ALD, which produced Si-O-V and Ti-O-V linkages [82]. The VO x /TiO 2 /SBA-15 catalyst exhibited 61.1% NO x conversion with N 2 selectivity  [79]. Copyright (2020) American Chemical Society. (b) SiO 2 coated Cu-SSZ-13 for NH 3 SCR of NOx. Reprinted from [84], Copyright (2021), with permission from Elsevier. (c) Fe 2 O 3 coated CeO 2 /TiO 2 for NH 3 SCR of NOx. Reprinted with permission from [85]. Copyright (2022) American Chemical Society. of 98.1% at 250 • C due to the moderate acidity of highly dispersed VO x on TiO 2 /SBA-15. Sun et al selectively deposited TiO 2 on the oxygen-containing functional groups of graphene oxide to stabilize graphene oxide scrolled MnO 2 nanowires for SCR of NO x [83]. TiO 2 modified graphene oxide greatly improved the steam and SO 2 resistance of MnO 2 catalyst for SCR of NO x at low temperature by delaying the oxidizability of MnO 2 . Furthermore, they coated SiO 2 on Cu-SSZ-13 to improve the high-temperature hydrothermal stability by suppressing the leaching of alumina from SSZ-13 molecular sieve (figure 4(b)) [84]. SiO 2 coated Cu-SSZ-13 showed the similar initial activity as Cu-SSZ-13 and remained considerable activity after ageing under moist air condition at 800 • C. As shown in figure 4(c), Qi et al coated Fe 2 O 3 on CeO 2 /TiO 2 catalysts to construct surface iron sulfate and subsurface CeO 2 structure by ALD and pre-sulfation method [85]. The electron transfer between surface Fe species and subsurface Ce species was tailored to improve the SO 2 tolerance of catalysts for NO x reduction. Soot combustion is an important catalytic reaction for controlling the particulate soot emission from diesel vehicles. Ivanova et al deposited CeO 2 based thin films by ALD and found that the CeO 2 film deposited at 300 • C showed a better activity than CeO 2 deposited at low temperatures with the complete soot conversion temperature of 450 • C [86,87].

Power lithium ion battery
At present, new energy vehicles powered by lithium ion battery are developing rapidly all over the world. Although the power lithium ion batteries used in current electric vehicles can meet the basic requirements on energy density, there are still problems such as poor cycle stability and safety, which are closely related to the failure of battery materials. The performance of battery can be enhanced by atomic scale coatings on cathode, anode and separator by ALD that cannot only improve the electrochemical stability, but also ensure the transport of electrons and Li ions. The nanocoatings on the surface of cathode and anode materials can improve the structural stability of materials and suppress the side reactions between electrode and electrolyte during charge-discharge cycles. The enhanced mechanical performance and thermal stability of separator have also been reported after modified by ultrathin oxide films prepared by ALD.

Cathodes coated by ALD
Cathode can usually limit the specific energy of lithium ion battery that is determined by the specific capacity and nominal voltage of battery. At present, the commonly used cathode materials for power lithium ion battery are olivinetype LiFePO 4 and Ni-rich layer transition metal oxides (LiNi x Co y Mn 1−y−x O 2 , NCM, x ⩾ 5). Since the high working voltage and energy density, spinel type LiNi 0.5 Mn 1.5 O 4 is promising as the next generation cathode materials for electric vehicles. Table 2 has summarized the recently reported cathode materials modified by ALD and their electrochemical performances. Although LiFePO 4 exhibits excellent cyclic stability and intrinsic thermal stability, it suffers from poor rate performance and electrochemical stability at high Li 3 PO 4 -TiO 2 5 nm 81.2% capacity retention rate after 300 cycles at 0.5 C rate [147] temperatures due to its inherent low electronic-and ionicconductivity. Liu  that exhibited an improvement in both specific capacity and rate capability compared to those of uncoated LiFePO 4 [117]. From the scanning electron microscopy morphologies of uncoated and TiN coated LiFePO 4 after 1000 cycles at 2 C rate, the sample without ALD treatment showed a thick solid permeable interface layer and obvious cracks on the surface. Moreover, the TiN coating on LiFePO 4 can also improve the cyclic stability at a high temperature of 55 • C, which could suppress the dissolution of Fe at high temperature [118]. In the recent research, Jin et al carried out the deposition of ZrO 2 on LiFePO 4 , which exhibited enhanced electrochemical performance in all solid state lithium battery due to the reduced undesirable side reactions between cathode and solid state electrolyte [119]. With the advantage of higher capacity than LiFePO 4 , layered NCM materials are also widely used in electric vehicles. However, the poor intrinsic thermal stability of NCM and interface stability between NCM and electrolyte lead to the problems of cycling stability and safety, which become more severe with the increase of Ni content. Therefore, the surface modification by ALD is capable of improving the electrochemical performance of NCM materials. Al 2 O 3 was the most common used coating on NCM materials, which improved the cycling performance of LiNi 0.5 Co 0.2 Mn 0.3 O 2 by restraining the dissolution of transition metal in NCM [120,121]. ZrO 2 and ZnO were reported to enhance the capacity retention and rate capability of LiNi 0.5 Co 0.2 Mn 0.3 O 2 at high temperature or high voltage operation due to the suppressed undesirable side reaction [122][123][124]. For LiNi 0.6 Co 0.2 Mn 0.2 O 2 , conformal Al 2 O 3 coatings were deposited on the cathode electrodes, which were served as physical barriers to prevent the surface corrosion of NCM particles and enhance the cycling performance at high charge voltages [125,126]. The carbon and Al 2 O 3 hybrid coating was prepared by the pyrolysis of organic carbon chains in alucone coatings prepared by MLD, which improved the electrochemical kinetics of LiNi 0.6 Co 0.2 Mn 0.2 O 2 by decreasing both charge and ion transfer resistance [127]. As shown in figure 5 [128]. The post-annealing process induced the diffusion of Al atoms in cathode, which achieved 92.2% capacity retention after 300 cycles at 1 C rate. In addition to using Al 2 O 3 as coating material, TiO 2 coating was also used to improve the cycling performance of LiNi 0.6 Mn 0.2 Co 0.2 O 2 cathodes, which suppressed the interfacial parasitic reactions [129,149]. When the content of Ni reaches to 80%, the capacity fading during cycling becomes more severe. Many researches have also reported that Al 2 O 3 coatings on high Ni-content cathode materials can improve the capacity retention by suppressing phase transition and hydrofluoric acid-induced transition metal dissolution [130][131][132][133]. With the higher electrical conductivity than metal oxide, ultra-thin TiN layer was coated on LiNi 0.8 Co 0.1 Mn 0.1 O 2 cathodes by ALD to accelerate the electron transport during the cycling [134]. Moreover, some lithium-containing coatings, such as LiAlF 4 and Li x Zr y PO z , were deposited to improve the rate performance and capacity retention due to their high lithium ion conductivity [135,136].
Since the high working voltage of 4.7 V vs Li/Li + , absence of Co and good safety performance, spinel LiNi 0.5 Mn 1.5 O 4 has been regarded as a good alternative to NCM materials for next generation battery in electric vehicles [150]. However, such high working voltage induces many surface chemistry issues, such as Mn dissolution and side reactions between cathode and electrolyte, which lead to the capacity loss during cycling. Ultrathin Al 2 O 3 coatings were prepared to improve the electrochemical stability at elevated temperature and suppress the self-discharge due to the mitigation of  [142]. Moreover, they found that FeO x ALD followed by post-annealing treatment induced Fe doping in LiMn 1.5 Ni 0.5 O 4 , which improved the Li + transport and cyclic stability [143,151]. In addition to coating these oxides, some lithium containing coatings (LiAlO 2 , LiF) and AlPO 4 have been reported to suppress the dissolution of Mn and undesirable side reactions between LiNi 0.5 Mn 1.5 O 4 and electrolyte [144][145][146]. As shown in figure 5(c), a hybrid Li 3 PO 4 -TiO 2 coating was designed and prepared on LiNi 0.5 Mn 1.5 O 4 by ALD, in which Li 3 PO 4 was considered as an ion conductive layer and TiO 2 was considered as an electron conductive layer. The hybrid coating not only provided sufficient ion and electronic conductivity, but also prevented the side reactions between cathode and electrolyte, which enhanced the cycling and rate performance of LiNi 0.5 Mn 1.5 O 4 [147].

Anodes coated by ALD
Although many high performance materials such as carbonbased, silicon-based and alloy materials are the promising anodes for power lithium ion battery, graphite is still the most widely used anode materials now, due to its low cost, high reversible capacity and moderate volume change [152]. In order to enhance the cycling stability of lithium ion battery with graphite anode, ALD has also been performed to coat graphite by ultrathin oxide that can act as artificial solid electrolyte interface. For instance, Al 2 O 3 was used to prevent electrolyte decomposition on the surface of natural graphite during initial charge-discharge [153]. As shown in figure 6(a), Lee et al compared the electrochemical performance of graphite electrodes with Al 2 O 3 coated on graphite powder and Al 2 O 3 directly coated on electrode. They found that Al 2 O 3 coated on graphite powder exhibited poor capacity retention compared to bare materials due to the inhibited electron conduction between graphite powder and current collector, while the capacity retention was significantly enhanced after Al 2 O 3 direct deposition on graphite electrode that enabled the electron transport. As a potential anode material, TiO 2 was also reported to modify the graphite electrode, which not only improved the cycling stability at elevated temperature, but also increased the initial discharge capacity [154]. Besides acting as artificial solid electrolyte interface, metal oxide coatings on graphite can also inhibit the formation of soluble byproducts, such as radicals, ROLi, which can diffuse to positive electrode and participate in the side reactions [155]. As shown in figure 6(b), the ultra-thin Al 2 O 3 coating on graphite (ALD-graphite) drastically improved the cycling stability of LiNi 0.5 Mn 1.5 O 4 /graphite full cell due to the isolation of graphite from electrolyte and inhibition of undesired side reactions [156]. Furthermore, the time-of-flight-secondary ions mass spectrometry analysis was performed to study the electrode/electrolyte interfaces after charge-discharge cycling, the results of which revealed that Al 2 O 3 coating on graphite anode reduced the formation of HF by decomposing LiPF 6 and inhibited the attack of HF on both graphite anode and LiNi 0.5 Mn 1.5 O 4 cathode. In addition to graphite, Si-based materials have also attracted much attention to lithium ion battery due to the high theoretical capacity. However, the large volume change makes Si-based materials suffer from rapid capacity fading during charge-discharge cycling. Si/C composite with buffer effect on volume change and enhanced electron conductivity is considered as the most promising anode materials to make their way into the large-scale market. Conformal Al 2 O 3 coating was deposited on Si/C composite nanofiber anodes, which increased the capacity retention after 100 charge-discharge cycles by improving the mechanical integrity of electrode structure and preventing the side reactions between anode materials and electrolyte [157]. Furthermore, Al 2 O 3 was also directly deposited on Si-graphene composite electrode to improve the cycling performance by suppressing the side reactions [158].

Separator modified by ALD
Although the separator does not directly participate in the electrochemical processes, it is responsible for the improvement of lithium ion battery performance and safety. Since ALD is based on the alternative reactions between precursors and substrate including pore walls, many membranes are reported to be modified by ALD to adjust the pore sizes or to establish new separation layers to enhance selectivity and permeability [159]. At present, polyolefin-type separators, such as polyethylene and polypropylene membranes are commonly used due to their good mechanical strength and chemical stability at mild temperature, which can meet the basic needs of separator for lithium ion battery. However, these polyolefintype separators are prone to shrinkage and deformation at high temperature due to their low melting points, resulting in the internal short circuit and speeding up thermal runaway of lithium ion battery [160,161]. Some metal oxides deposited by ALD have been applied to modify the surface of separator to enhance the thermal stability, which can keep the thickness of separator. As shown in figure 7(a), Al 2 O 3 nano coatings were deposited on the surface of polypropylene microframework, which resulted in the unchanged thickness compared to the conventional ceramic coated separator. Meanwhile, the Al 2 O 3 coated polypropylene separator exhibited improved stability at 160 • C and wettability in polar electrolyte, as well as unchanged electrochemical performance [50]. In order to decrease the thickness of ALD coating, Wang et al introduced plasma pretreatment to increase the active groups on polypropylene separator that resulted in the conformally coating of TiO 2 by 20 ALD cycles. The thin TiO 2 coating not only suppressed the thermal shrinkage and improved the wettability of polypropylene separator, but also increased the specific discharge capacities at variable rates due to on expense of pore size in polypropylene separator [162]. Besides polyolefin-type separators, the polyvinylidene fluoride-hexafluoropropylene nonwoven separator has also been modified by ALD coatings. As shown in figure 7(b), Al 2 O 3 ALD was performed on nanofiber separator after O 2 plasma treatment. Al 2 O 3 coated separator exhibited drastically improved safety performance with the stable morphology after heat treatment at 270 • C, while the bare separator turned into ribbon type at 160 • C [163,164]. In order to increase ALD coating rate on separator, a roll-to-roll ALD system was designed including ALD shower arrays, oxygen radio-frequency plasma reactor, gas delivering sub-systems and vacuum pumping units [165]. As shown in figure 7(c), the moving speed of separator in roll-to-roll ALD system was investigated, which could affect the uniformity and mass loading of ALD coatings, as well as the contact angle of electrolyte on separator.

Hydrogen fuel cell
Although hydrogen fuel cell a promising alternative due to its high theoretical efficiency and power density, as well as environmental friendliness, the high cost and poor durability are the most obstacles for its scale commercial application in vehicle. As the core component of hydrogen fuel cell, MEA is widely studied to evaluate the performance of functional components, such as catalyst layer, gas diffusion layer and proton exchange membrane. Since the sluggish kinetics of oxygen reduction reaction (ORR) at the cathode of hydrogen fuel cell, large amounts of Pt catalyst are required in the catalyst layer, resulting in the high cost of hydrogen fuel cell. Highly dispersed Pt catalysts with controlled size have been prepared by ALD method, which can not only increase the utilization of Pt, but also decrease the O 2 transfer resistance in the catalyst layer. Furthermore, ALD has been utilized to modify Pt catalysts and conductive supports to enhance the intrinsic catalytic activity and stability. In the MEA level, the surface modification of catalyst layer, gas diffusion layer and proton exchange membrane has been performed to increase the mass transport and durability of hydrogen fuel cell.

Preparation of supported Pt catalysts
Different from exhaust gas catalysts, Pt catalysts for hydrogen fuel cells are usually supported by carbon based supports that can satisfy the requirements of high electron conductivity and corrosion resistance under fuel cell condition as listed in table 3. Although well dispersed Pt catalysts with controllable size are achieved by ALD on oxide supports, carbon based supports with high specific surface area or low surface nucleation sites need some pre-treatments and special ALD processes [184,185]. By using long pulse and purge times to maximize penetration of the ALD precursors into and removal of byproducts out of supports, King et al deposited Pt nanoparticles with narrow size distribution on carbon aerogels with appreciable surface area of ∼480 m 2 g −1 [186]. Since bare nucleation sites on carbon nanotubes, Perng et al deposited uniform and well-distributed Pt nanoparticles on acidtreated carbon nanotubes, which exhibited a higher specific power density in MEA than commercial Pt/C catalysts [166,167]. Hsieh et al studied the two self-limiting reactions during Pt ALD using methylcyclopentadienyl-(trimethyl) platinum (MeCpPtMe 3 ) and oxygen and found a linear increase of Pt mass loading on carbon nanotubes with the ALD cycles [168,169]. By comparing the growth behaviors of Pt using O 2 and H 2 as the second precursor, Lubers et al found that oxygen induced the combustion of carbon supports and the aggregation of Pt after Pt precursor attached on the surface of carbon supports, while hydrogen removed the ligands of Pt precursor by hydrogenation reactions that did not recreate surface functionalization, leading to the smaller size of Pt nanoparticles [187].
In order to uniformly deposit Pt nanoparticles on carbon black supports, Lee et al filtered the carbon supports and controlled their size within the range of 60-100 µm that ensured the homogenous fluidization during ALD process ( figure 8(a)) [170]. The size of Pt nanoparticles were controlled by changing the number of ALD cycles, while the highly dispersed and dense Pt nanoparticles with the size of ∼1 nm were prepared by 5 cycles of Pt ALD. The further increased ALD cycles could result in size increase and agglomerate of Pt due to preferential adsorption and reaction of Pt precursor occurring on previously deposited Pt nanoparticles. Nonetheless, the number of ALD cycle was optimized to about 15-20 for the best fuel cell performance, which ensured the Pt diameter of 2-3 nm that could exhibit the highest mass activity and satisfy the requirement for mass transfer [170,171]. The optimized Pt size was also reported as ∼2 nm on carbon nanotubes for ORR [172]. In order to increase the utilization of Pt, Xu et al reported a passivation-gas incorporated ALD method using CO molecules as the growth inhibitors to suppress the thickness increase of Pt nanoparticles [173]. CO molecules promoted two-dimensional growth of Pt nanoparticles with a more than 40% improvement in Pt  [183] surface-to-volume ratio that resulted in the high mass activity for ORR. Liu et al prepared Pd 3 Au@Pt core@shell structure by selective ALD to increase Pt usage efficiency. Owing to the block of surfactant molecules on carbon supports and the catalytic decomposition of Pt precursors on Pd 3 Au, a Pt shell was selectively deposited on Pd 3 Au nanoparticles instead of on carbon, which exhibited a significantly improved mass activity for ORR compared with commercial Pt/C [174]. As shown in figures 8(b) and (c), Sun et al deposited Pt single atoms on Pd nanoparticles or metal-organic framework (MOF)-derived N-doped carbon to maximum the utilization of Pt, which delivered 4 or 6.5 times higher mass activity than that of Pt nanoparticles for ORR [175,176]. The Pt single atoms were anchored by the pyridinc N sites on the MOF-derived Ndoped carbon supports. Due to the low-coordination environment and interaction between Pt atoms and nitrogen-doping sites, the electronic structures of Pt single atoms were versatile with different adsorption intermediates, while the multichannel reaction mechanism of oxygen reduction reaction on Pt single atoms exhibited lowered free energy change for the rate-determining step.
Although carbon black is widely used as support for Pt catalysts, it usually suffers from low resistance to electrochemical corrosion under the working condition of hydrogen fuel cell, leading to the detachment and aggregation of Pt catalysts [188,189]. Lots of carbon-alternative materials, such as carbides, nitrides and metal oxides, have been studied, which own sufficient electrical conductivity, high resistance to electrochemical corrosion, as well as strong interaction between the support and Pt. Similar as carbon black, Pt ALD on carbide is also following an island growth mechanism [190]. Cheng et al deposited uniform Pt nanoparticles with the average size of 3.2 nm on corrosion-resistant ZrC supports by controlling the number of Pt ALD cycles [177]. Since the strong interactions between Pt nanoparticles and ZrC supports, Pt nanoparticles deposited by ALD showed high total unoccupied density of states of Pt 5d character, which exhibited higher ORR activity and durability than Pt nanoparticles prepared by a conventional chemical reduction method. The performance of Pt/Mo 2 C prepared by ALD was evaluated in a MEA, which showed higher power density than commercial Pt/C after accelerated degradation test [178,191]. Besides carbides, TiN was also used as a support of Pt nanoparticles prepared by ALD ( figure 8(d)), which exhibited enhanced ORR performance due to the good conductivity and corrosion resistance of TiN, as well as the strong interactions between Pt and TiN support [179,180]. Nb doped TiO 2 and Sb doped SnO 2 were also reported to promote the ORR catalytic activity and stability of uniform Pt nanoparticles prepared by ALD [181,182]. Especially, a triple junction interface composed by carbon support, oxide and Pt nanoparticle was constructed by ALD, which could overcome the problems of oxide, such as limited conductivity and small specific surface area [183,192,193].

Modifications of Pt and supports by ALD
Compared with pure Pt nanoparticles, Pt catalysts modified by strain engineering or alloying have been designed and achieved by ALD, which exhibit enhanced intrinsic catalytic activity and stability for hydrogen fuel cell. As shown in figure 9(a), Xu et al sequentially deposited CoO x and Pt on supports, and prepared the lattice strained Pt catalysts by dissolving the CoO x core with acid [194]. The Pt lattice compression was confirmed by the Pt-Pt distance analysis from the results of x-ray absorption spectroscopy and transmission electron microscopy, which resulted in the mass activity at 0.9 V for MEA close to 0.8 A mg Pt −1 . By varying the number and order of Pt and Co precursor cycles, the PtCo alloy nanoparticles with narrow particle size distribution and high metal dispersion were prepared by ALD, which exhibited positive onset potential and larger limiting current than Pt/C [195]. Pt 75 [196]. Based on the precisely control over the deposition of single atoms, the prepared Co single atoms modified Pt nanoparticles exhibited significantly improved activity and stability for ORR compared to commercial Pt/C [197]. Besides the sequentially deposition of Pt and metal, ALD of Pt and metal oxide in combination with a subsequent reduction step was also performed to prepare Pt alloys. For instance, Pt 3 Ti and PtZn alloy nanoparticles were synthesized by reducing Pt and TiO 2 or ZnO at different temperature, the average sizes of which were ∼10 nm and ∼3 nm, respectively [198,199]. As shown in figure 9(b), Chen et al recently reported a strategy to construct uniform sub-3 nm Pt-based intermetallic nanocrystals based on selectively ultrathin metal oxide coating on Pt nanoparticles via ALD [200]. Area-selective and thickness controllable ZnO coatings were performed to prepare uniform PtZn intermetallic nanocrystals with the size of 2.50 ± 0.65 nm, which could provide Zn atoms for alloying and prevent the sintering of Pt nanoparticles during ordering reduction. The outstanding MEA performance with peak power density of 1.56 W cm −2 and 10.42% loss in mass activity after 30 000 voltage cycles was achieved, which was attributed to the decreased binding energy of Ptoxygen intermediates for weakly polarized surface Pt atoms and suppressed electrochemical Ostwald ripening for uniform PtZn intermetallic nanocrystals.
Since the deactivation of Pt catalysts is resulted by the agglomeration, coalescence and Ostwald ripening of nanoparticles due to the dissolution of surface Pt atoms and weak interaction between Pt and supports, surface overcoating engineering is widely investigated to develop advanced electrocatalysts [203]. ALD has been utilized to modify the surface of Pt nanoparticles and carbon support materials due to the precise thickness control of overcoatings. In order to satisfy the work condition of hydrogen fuel cell, some metal oxides and nitrides with high corrosion resistance under acidic and oxidizing conditions have been chosen as overcoatings, such as TiO 2 , ZrO 2 , SnO 2 and WN. For instance, ultrathin TiO 2 was coated on Pt/C catalysts by ALD, which could achieve a good balance between the durability and activity by controlling the thickness of TiO 2 overcoating [204][205][206]. Liu et al achieved the stabilization of Pt low-coordinated sites on a commercial Pt/C catalyst by nitrogen doped TiO 2 by coupling selective ALD of TiO 2 and following nitrogen doping process [207]. With good conductivity and corrosion resistance, SnO 2 and WN coatings were also performed to increase the durability of Pt/C catalysts by mitigating the migration and agglomeration of Pt nanoparticles [208,209]. As shown in figure 9(c), Lim et al prepared ZrO 2 coating on Pt catalysts with different ALD cycles, which were confirmed to be porous or to have a thickness of several nanometers or less [201]. MEA durability tests showed that 2 ALD cycles of ZrO 2 coating decreased the deterioration rate to one-fourth of that of the pure Pt catalyst, with a minor decrease in the fuel cell power. In order to protect the surface of Pt from coated metal oxide, Sun et al demonstrated an approach to stabilized Pt catalysts for ORR by area selective ALD of ZrO 2 and TaO x , which was assisted by the blocking agent of oleylamine preventing the attachment of Zr and Ta precursors on Pt nanoparticles [210,211]. Besides Pt nanoparticles, the modifications of supports have also been investigated to improve the corrosion resistance of supports and enhance the interaction between Pt and supports. For instance, Tammeveski et al decorated acidtreated multi-walled carbon nanotubes by TiO 2 that exhibited strong metal-support interaction with subsequently deposited Pt nanoparticles [212,213]. As shown in figure 9(d), Zhang et al prepared Al based coatings with enriched pores on nitrogen-doped carbon nanotubes via MLD, which was effective stabilizer for anchoring the Pt nanoparticles [202]. They also deposited highly dispersed nitrogen doped Ta 2 O 5 nanoparticles on carbon black supports to prevent Pt nanocrystals from migration and aggregation [214].

MEA modification by ALD
In the MEA level, the performance of hydrogen fuel cell is not only related to the activity and stability of Pt catalysts, but also attributed to the electron and proton conductivity, the mass transport of gas reactants and management of water. The catalyst layer composited by supported Pt catalysts and ionomer is the key component for electrochemical reaction of reactants, where the nanostructure and distribution of ionomer are important for mass transport. As shown in figure 10(a), Sabarirajan et al designed and prepared a Pt nanoelectrode array for hydrogen fuel cell [215]. The Pt nanoelectrode array was prepared by Pt ALD onto anodized aluminium oxide discs with subsequently etching process, which was used as an ionomer-free electrode for transport and reaction kinetics studies, indicating the surface migration mechanism for proton transport. Bottom-up fabrication of cathode electrodes for ORR was developed with ALD technology to improve the Pt catalyst-ionomer interface and accelerate mass transport [216,217]. Furthermore, an anode catalyst layer was prepared by directly depositing Pt nanoparticles with uniform size on gas diffusion layer, which exhibited excellent activity and stability compared with the anode prepared using commercial carbon supported Pt catalysts and a conventional screen printing method [218,219]. Although ALD was a powerful technique to prepared highly dispersed Pt catalysts on supports as presented above, Weimer et al found that the residual functional groups after Pt ALD could decrease the hydrophobicity of catalyst layer, which resulted in the poor MEA performance by inducing water flooding [220,221]. They modified Pt/C catalysts by sub-monolayer WN films and subsequently thermal treatment that improved the catalyst performance in the mass transport region. ALD was employed to maintain MEA with a satisfactory water content and distribution. For instance, ultrathin layer of hydrophilic TiO 2 was coated on hydrophobic microporous layer of gas diffusion layer at the cathode, which resulted in high MEA performance at low humidity operation [222]. As shown in figure 10(b), different numbers of HfO 2 ALD cycle were used to modify the microporous layer on gas diffusion layer, which could control the water contact angle [223]. 25 ALD cycles of HfO 2 exhibited the smallest charge transfer resistance and mass transport resistance in low and high humidity conditions. As the widely used proton conducting media, Nafion membrane was coated by Al 2 O 3 ALD to improve the mechanical strength and decrease the membrane permeability to gas reactants (figure 10(c)) [224]. The above achieved progresses indicate that the modifications of functional components in MEA by ALD are critical for the application of hydrogen fuel cell.

ALD reactors for catalytic and energy materials
ALD has been demonstrated to play an important role in designing and preparing advanced catalytic and energy materials by atomically precise controlling the surface and interface structures. Compared with conventional liquid and solid processes for surface coating or modification, ALD exhibits the advantages of high-precise control of coating thickness, good uniformity and conformality, as well as strong adhesion between coatings and substrates due to the characteristic of self-limiting chemical reaction. Nevertheless, as mentioned in previous section, the catalysts and electrode materials in batteries are powder materials with high specific surface areas, which suffer from particle agglomeration problem, much slow diffusion of precursors in agglomerates and lower deposition rate than that on planer substrates. Furthermore, the popularization of eco-friendly vehicles requires large amounts of catalytic and energy materials that ranges from tons to 10 000 tons. High mass production reactors with low cost ALD processes are the key to meet the practicality of powder ALD for industrialization. Based on conventional ALD reactors, lab scale powder ALD was usually performed by uniformly spreading powder on a tray that was just capable of coating grams of powder [225]. As shown in figure 11(a), the static powder bed with height large than 1 mm was loaded in a powder tray that incorporated in a viscous flow ALD reactor, which required much longer dose and pure times to ensure adequate penetration of precursors. In order to monitor the reaction processes during ALD, Naumann d' Strempel et al integrated a magnetically suspended balance in fixed-bed reactor to record the mass changes of powder [226]. Recently, a fixed-bed ALD reactor was used to prepare Ni catalysts on mesoporous ZrO 2 supports [227]. The precursors and inert gas were led downwards through the fixed bed with 5 g of ZrO 2 supports and an ALD cycle needs several hours. To address the limitation of precursor diffusion in static powder bed, various methods have been reported previously to disperse powder by overcoming the interparticle cohesion, such as fluidization, rotation and vibration [228][229][230]. Compared with static bed ALD reactor, larger quantities of powder can be coated in these ALD reactors with shorter precursor dose and purge times. For instance, Lu et al performed Al 2 O 3 ALD on 10 g of Pt/SiO 2 catalyst in a commercial fluidized bed reactor [231]. By in situ monitoring the surface reactions of ALD, the optimized precursor dose times for trimethylaluminum and water were 28 min and 17 min, respectively, while the precursor utilization of trimethylaluminum was 91.8 ± 14.6%.
In order to promote the industrialization of powder ALD, much attention has been paid to developing new powder ALD reactors to further increase the mass production and coating efficiency. Increasing the volume of reactor was a direct strategy to increase the throughput based on previously reported fluidized bed reactor or rotary reactor, while the corresponding ALD processes should also be optimized. As shown in figure 11(a), high-capacity rotary drum was inserted into a viscous-flow tubular ALD reactor, which can provide homogenous coating on tens of grams of powder with less than 2 min saturation of trimethylaluminum [232,233]. A rotary reactor was incorporated with a moveable dual-zone furnace to change the temperature of powder for precursor adsorption and oxidative removal of the precursor ligands, which can coat a large quantity of powder with high specific surface area in a relatively short period of time [234]. The disperse powder bed coupled with external force field was another strategy to increase the mass production and coating efficiency by enhancing the gas-solid contact between precursors and powder. As shown in figure 11(b), Duan et al designed a fluidized bed coupled rotary reactor for powder ALD [235]. The double-layer powder cartridge can ensure precursors flowing through the particle bed exclusively to achieve high utilization without static exposure operation. Recently, they reported an ultrasonic vibration-assisted fluidized bed ALD reactor to promote the deagglomeration of powder and facilitate the heat transfer and precursor diffusion in high-volume powder bed (figure 11(c)) [236]. Different with the batchwise operation of fluidized bed or rotary reactors, spatial ALD was also applied for powder coating to achieve the continuous operation. A high-throughput semi-continuous ALD system was developed by connecting several fluidized bed reactors and feeding powder on the one side and removing them at the other side, which can achieve the production of hundreds of kilograms of powder [130]. As shown in figure 11(d), van Ommen et al designed a lab-scale pneumatic transport reactor for spatial powder ALD that deposited Pt nanoclusters on TiO 2 supports with a production rate of 1 g min −1 [237]. Based on the linear vibration to convey particles through alternating  [237]. (e) Continuous vibrating bed reactor for spatial powder ALD. Reprinted from [238], Copyright (2021), with permission from Elsevier. regions of precursor gas, a continuous vibrating bed reactor was developed for spatial powder ALD, the powder flow behavior in which can be controlled by adjusting the frequency and amplitude of excitation waveform for horizontal conveyors. (figure 11(e)) [238]. Besides spatial powder ALD, the porous electrodes or separator can also be directly modified by spatial ALD, which is more compatible with the flow line production [239,240]. Much longer precursor exposure time was required for the diffusion of precursors in porous structures, which limited the moving speed of electrodes or separator [241,242]. Furthermore, the uniformity of precursor distribution in the width direction and the utilization of precursor could be further optimized by developing the ALD reactors and processes.

Summary and perspectives
In this review, recent progress of ALD on the controllable preparation and modification of advanced catalytic and energy materials, as well as their enhanced performance for exhaust emission control, power lithium ion battery and hydrogen fuel cell have been summarized and discussed. The low temperature activity of supported metal and oxide catalysts are improved by controlling their nucleation, size and distribution, as well as the metal-support interactions, while the promising strategies by encapsulating Pt-based nanoparticles in ALD deposited overlayers are developed to enhance the sintering resistance under high temperature conditions of exhaust emission control system. ALD has also been performed to prepare and modify highly dispersed Pt catalysts and conductive supports to enhance the intrinsic catalytic activity and stability for cathode ORR in hydrogen fuel cell. For power lithium ion battery, the ALD coatings with controllable thickness on cathode, anode and separator cannot only improve the electrochemical and structural stability by suppressing undesirable side reactions during charge-discharge cycling, but also ensure the transport of electrons and Li ions to keep the discharge capacity and rate performance. The electron and proton conductivity, as well as the mass transport of gas reactants of MEA are enhanced by ALD modifying the functional components, such as catalyst layer, gas diffusion layer and proton exchange membrane.
Although ALD has been applied to enhance the chemical and electrochemical performance of catalytic and energy materials for exhaust emission control, power lithium ion battery and hydrogen fuel cell, there are still several challenges that involved in the research and development of ALD on the future practical applications for eco-friendly vehicles. One of the challenges is the development of new precursors. For instance, the cost of precursors should be decreased for large-scale applications of ALD, especially for the expensive precious metal precursors. At present, metal oxides are the most common materials for ALD overcoatings on catalytic and energy materials. New precursors with high chemical activity and the corresponding ALD processes for other metal compounds such as carbides, nitrides are also important for the performance enhancement of catalytic and energy materials. Furthermore, much attention needs to be paid to the performance evaluation of ALD modified materials in practical device level. For instance, the performance of oxide supported Pt group metal catalysts for three-way catalysis or diesel oxidation catalysis can be evaluated under the practical reaction atmosphere and gas hourly space velocity condition. Pouch cell is needed to study the effect of ALD coatings on cathode or anode materials for lithium ion battery, while the electrochemical performance of modified MEA can be evaluated by hydrogen fuel cell stack under dynamic cycle working condition.
Large-scale and efficient production of ALD modified materials is also a challenge for the practical applications. For rotary ALD reactor, the production rate is closely dependent on the volume of rotary drum, which is usually reported less than hundreds of grams of powder per batch. When fluidized ALD reactor is coupled with external force field, such as mechanical vibration, rotation and ultrasound, the production rate can reach to hundreds of kilograms of powder per batch due to the promoted disaggregation and uniform distribution of catalysts or cathode/anode particles. However, the production volume still cannot satisfy the requirement of tons of materials per year in industrial applications. In order to further increase the production volume of powder ALD, the comprehensive reaction mechanism and kinetics of ALD in dynamic agglomerates in fluidized bed should be studied. The diffusion and reaction kinetics model of precursor in dynamic agglomerates should be developed to investigate the effect of external force field on ALD process on particles, which can give guidance to reactor design and ALD parameters optimization. Furthermore, large amounts of precursors need to be delivered into reactor for mass production of particles, which should be quantitative and concentration controllable. In situ monitoring of reaction process of ALD is necessary to be coupled to control the supply of precursors and balance the reaction efficiency and precursor utilization. Spatial ALD is a promising technique to achieve mass production, which can not only be applied on particle coatings, but also continuously deposit materials on flexible electrode or separator. Since the diffusion and reaction of precursors in porous substrate is much slow than that on planner substrate, the moving speed of porous substrate should be compatible with industrial flow line production by designing highly efficient and uniform injector for precursor distribution. Above all, the powder and spatial ALD reactors, as well as the corresponding ALD processes are of great significance to scale up the manufacturing process by increasing the deposition efficiency and precursor utilization, which can facilitate the industrialization of ALD for advanced catalytic and energy materials.