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Electrochemical transformation processes involving carbon, hydrogen, oxygen, nitrogen, and small-molecule chemistries represent a promising means to store renewable energy sources in the form of chemical energy. However, their widespread deployment is hindered by a lack of efficient, selective, durable, and affordable electrocatalysts. Recently, grain boundary (GB) engineering as one category of defect engineering, has emerged as a viable and powerful pathway to achieve improved electrocatalytic performances. This review presents a timely and comprehensive overview of recent advances in GB engineering for efficient electrocatalysis. The beneficial effects of introducing GBs into electrocatalysts are discussed, followed by an overview of the synthesis and characterization of GB-enriched electrocatalysts. Importantly, the latest developments in leveraging GB engineering for enhanced electrocatalysis are thoroughly examined, focusing on the electrochemical utilization cycles of carbon, hydrogen, oxygen, and nitrogen. Future research directions are proposed to further advance the understanding and application of GB engineering for improved electrocatalysis. (Figure presented.).

One of the effective strategies to pursue the highly durable high-temperature polymer electrolyte membrane fuel cells (HT-PEMFCs) is to introduce inorganic fillers to the phosphoric acid-doped polybenzimidazole (PA/PBI) membranes. Among the inorganic fillers, phosphates such as phosphosilicate are effective in mitigating acid loss at elevated temperatures (200–300 °C). In this paper, the effect of in situ formed phosphosilicate on the performance and stability of SiO2/PA/PBI composite membranes is studied in detail. The mechanical properties and electrochemical performances of the in situ formed SiO2/PA/PBI membranes depend strongly on the content of in situ formed Si5P6O25 fillers and its distribution and microstructure in the membrane. Such in situ formed SiO2/PA/PBI composite membranes show a high conductivity of 53.5 mS cm−1 at 220 °C. The assembled single cell shows a maximum peak power density (PPD) of 530.6 mW cm−2 and excellent stability at elevated temperature of 220 °C for over 130 h. The exceptional stability at 220 °C is most likely due to the existence of predominant amorphous phosphosilicate phases in the in situ formed SiO2/PA/PBI composite membranes, which inhibits the evaporation and leaching of PA at elevated temperatures. The results indicate the practical application of in situ formed SiO2/PA/PBI composite membranes for HT-PEMFCs.

The electrochemical carbon dioxide reduction reaction (CO2RR), which can produce value-added chemical feedstocks, is a proton-coupled-electron process with sluggish kinetics. Thus, highly efficient, cheap catalysts are urgently required. Transition metal oxides such as CoOx, FeOx, and NiOx are low-cost, low toxicity, and abundant materials for a wide range of electrochemical reactions, but are almost inert for CO2RR. Here, we report for the first time that nitrogen doped carbon nanotubes (N-CNT) have a surprising activation effect on the activity and selectivity of transition metal-oxide (MOx where M = Fe, Ni, and Co) nanoclusters for CO2RR. MOx supported on N-CNT, MOx/N-CNT, achieves a CO yield of 2.6–2.8 mmol cm−2 min−1 at an overpotential of −0.55 V, which is two orders of magnitude higher than MOx supported on acid treated CNTs (MOx/O-CNT) and four times higher than pristine N-CNT. The faraday efficiency for electrochemical CO2-to-CO conversion is as high as 90.3% at overpotential of 0.44 V. Both in-situ XAS measurements and DFT calculations disclose that MOx nanoclusters can be hydrated in CO2 saturated KHCO3, and the N defects of N-CNT effectively stabilize these metal hydroxyl species under carbon dioxide reduction reaction conditions, which can split the water molecules and provide local protons to inhibit the poisoning of active sites under carbon dioxide reduction reaction conditions.

3D reconstruction plays an increasingly important role in modern photogrammetric systems. Conventional satellite or aerial-based remote sensing (RS) platforms can provide the necessary data sources for the 3D reconstruction of large-scale landforms and cities. Even with low-altitude UAVs (Unmanned Aerial Vehicles), 3D reconstruction in complicated situations, such as urban canyons and indoor scenes, is challenging due to frequent tracking failures between camera frames and high data collection costs. Recently, spherical images have been extensively used due to the capability of recording surrounding environments from one camera exposure. In contrast to perspective images with limited FOV (Field of View), spherical images can cover the whole scene with full horizontal and vertical FOV and facilitate camera tracking and data acquisition in these complex scenes. With the rapid evolution and extensive use of professional and consumer-grade spherical cameras, spherical images show great potential for the 3D modeling of urban and indoor scenes. Classical 3D reconstruction pipelines, however, cannot be directly used for spherical images. Besides, there exist few software packages that are designed for the 3D reconstruction of spherical images. As a result, this research provides a thorough survey of the state-of-the-art for 3D reconstruction of spherical images in terms of data acquisition, feature detection and matching, image orientation, and dense matching as well as presenting promising applications and discussing potential prospects. We anticipate that this study offers insightful clues to direct future research.

Enhancing charge-carrier dynamics is imperative to achieve efficient photoelectrodes for practical photoelectrochemical devices. However, a convincing explanation and answer for the important question which has thus far been absent relates to the precise mechanism of charge-carrier generation by solar light in photoelectrodes. Herein, to exclude the interference of complex multi-components and nanostructuring, we fabricate bulky TiO2 photoanodes through physical vapor deposition. Integrating photoelectrochemical measurements and in situ characterizations, the photoinduced holes and electrons are transiently stored and promptly transported around the oxygen-bridge bonds and 5-coordinated Ti atoms to form polarons on the boundaries of TiO2 grains, respectively. Most importantly, we also find that compressive stress-induced internal magnetic field can drastically enhance the charge-carrier dynamics for the TiO2 photoanode, including directional separation and transport of charge carriers and an increase of surface polarons. As a result, bulky TiO2 photoanode with high compressive stress displays a high charge-separation efficiency and an excellent charge-injection efficiency, leading to 2 orders of magnitude higher photocurrent than that produced by a classic TiO2 photoanode. This work not only provides a fundamental understanding of the charge-carrier dynamics of the photoelectrodes but also provides a new paradigm for designing efficient photoelectrodes and controlling the dynamics of charge carriers.

As a widely used commodity chemical, ammonia is critical for producing nitrogen-containing fertilizers and serving as the promising zero-carbon energy carrier. Photoelectrochemical nitrogen reduction reaction (PEC NRR) can provide a solar-powered green and sustainable route for synthesis of ammonia (NH3). Herein, an optimum PEC system is reported with an Si-based hierarchically-structured PdCu/TiO2/Si photocathode and well-thought-out trifluoroethanol as the proton source for lithium-mediated PEC NRR, achieving a record high NH3 yield of 43.09 µg cm−2 h−1 and an excellent faradaic efficiency of 46.15% under 0.12 MPa O2 and 3.88 MPa N2 at 0.07 V versus lithium(0/+) redox couple (vs Li0/+). PEC measurements coupled with operando characterization reveal that the PdCu/TiO2/Si photocathode under N2 pressures facilitate the reduction of N2 to form lithium nitride (Li3N), which reacts with active protons to produce NH3 while releasing the Li+ to reinitiate the cycle of the PEC NRR. The Li-mediated PEC NRR process is further enhanced by introducing small amount of O2 or CO2 under pressure by accelerating the decomposition of Li3N. For the first time, this work provides mechanistic understanding of the lithium-mediated PEC NRR process and opens new avenues for efficient solar-powered green conversion of N2-to-NH3.

Development of high temperature polymer electrolyte membrane fuel cells (HT-PEMFCs) at elevated temperatures is important for the enhancement of tolerance toward CO impurities and for the development of non-precious metal catalysts. The key challenge in such HT-PEMFCs is the high temperature polymer electrolyte membranes. Herein, the development of in situ formed metal pyrophosphates (MP2O7, where M = Sn, Ti, and Zr) in phosphoric acid doped polybenzimidazole (PA/PBI) composite membranes for HT-PEMFCs is reported. The formation mechanism of MP2O7, and characteristics of MP2O7/PA/PBI composite membranes are studied in detail. In contrast to the rapid decay in performance of pristine PA/PBI membrane cells, the in situ formed MP2O7/PA/PBI composite membranes show significantly higher proton conductivity, improved performance, and stability at elevated temperatures of 200–250 °C. The best results are obtained on the in situ formed SnP2O7/PA/PBI composite membrane cells, exhibiting a high peak power density of 476 mW cm−2 and proton conductivity of 51.3 mS cm−1 at 250 °C. The excellent durability of SnP2O7/PA/PBI composite membrane is due to the uniform distribution of in situ formed SnP2O7 nanoparticles in PBI membranes and the formation of a gel-like region, thin and irregular amorphous layer on the SnP2O7 with the high acid retention ability. This effectively alleviates the PA leaching at elevated temperatures of the new HT-PEMFCs.

SfM (Structure from Motion) has been extensively used for UAV (Unmanned Aerial Vehicle) image orientation. Its efficiency is directly influenced by feature matching. Although image retrieval has been extensively used for match pair selection, high computational costs are consumed due to a large number of local features and the large size of the used codebook. Thus, this paper proposes an efficient match pair retrieval method and implements an integrated workflow for parallel SfM reconstruction. First, an individual codebook is trained online by considering the redundancy of UAV images and local features, which avoids the ambiguity of training codebooks from other datasets. Second, local features of each image are aggregated into a single high-dimension global descriptor through the VLAD (Vector of Locally Aggregated Descriptors) aggregation by using the trained codebook, which remarkably reduces the number of features and the burden of nearest neighbor searching in image indexing. Third, the global descriptors are indexed via the HNSW (Hierarchical Navigable Small World) based graph structure for the nearest neighbor searching. Match pairs are then retrieved by using an adaptive threshold selection strategy and utilized to create a view graph for divide-and-conquer based parallel SfM reconstruction. Finally, the performance of the proposed solution has been verified using three large-scale UAV datasets. The test results demonstrate that the proposed solution accelerates match pair retrieval with a speedup ratio ranging from 36 to 108 and improves the efficiency of SfM reconstruction with competitive accuracy in both relative and absolute orientation.

3D reconstruction plays an increasingly important role in modern photogrammetric systems. Conventional satellite or aerial-based remote sensing (RS) platforms can provide the necessary data sources for the 3D reconstruction of large-scale landforms and cities. Even with low-altitude UAVs (Unmanned Aerial Vehicles), 3D reconstruction in complicated situations, such as urban canyons and indoor scenes, is challenging due to the frequent tracking failures between camera frames and high data collection costs. Recently, spherical images have been extensively exploited due to the capability of recording surrounding environments from one camera exposure. Classical 3D reconstruction pipelines, however, cannot be used for spherical images. Besides, there exist few software packages for 3D reconstruction of spherical images. Based on the imaging geometry of spherical cameras, this study investigates the algorithms for the relative orientation using spherical correspondences, absolute orientation using 3D correspondences between scene and spherical points, and the cost functions for BA (bundle adjustment) optimization. In addition, an incremental SfM (Structure from Motion) workflow has been proposed for spherical images using the above-mentioned algorithms. The proposed solution is finally verified by using three spherical datasets captured by both consumer-grade and professional spherical cameras. The results demonstrate that the proposed SfM workflow can achieve the successful 3D reconstruction of complex scenes and provide useful clues for the implementation in open-source software packages. The source code of the designed SfM workflow would be made publicly available.

Solar-driven photocatalysts for water splitting and CO2 reduction have been widely studied for dealing with environmental pollution and energy sustainability issues. Among the most promising semiconductor photocatalysts, graphitic carbon nitride (g-C3N4) and TiO2 (anatase) with band gaps of ∼2.7 and ∼3.2 eV, respectively, are investigated extensively. However, the high photogenerated carrier recombination efficiency of g-C3N4 and the relatively wide band gap of TiO2 (responsive to ultraviolet light only) are the factors that can lower the photocatalytic activities of the materials. Thus, one of the prevalent strategies is to construct g-C3N4/TiO2 nanocomposites to promote charge carrier separation and to improve photoabsorption in the visible region for attaining efficient utilization of solar energy in photocatalytic water splitting, CO2 reduction, and organic pollutant photodegradation. Here, a comprehensive overview is made on the exploitation of g-C3N4/TiO2 nanocomposites for photocatalytic applications, emphasizing layered heterostructures, for solar-driven H2 generation and CO2 reduction. Challenges in resolving various issues such as low efficiency, low stability, and noble metal cocatalyst dependency, as well as band gap narrowing accompanied reduction in redox ability of the g-C3N4/TiO2 nanocomposites, are discussed.

Oxygen evolution reaction (OER) is a key half-reaction in many electrochemical transformations, and efficient electrocatalysts are critical to improve its kinetics which is typically sluggish due to its multielectron-transfer nature. Perovskite oxides are a popular category of OER catalysts, while their activity remains insufficient under the conventional adsorbate evolution reaction scheme where scaling relations limit activity enhancement. The lattice oxygen-mediated mechanism (LOM) has been recently reported to overcome such scaling relations and boost the OER catalysis over several doped perovskite catalysts. However, direct evidence supporting the LOM participation is still very little because the doping strategy applied would introduce additional active sites that may mask the real reaction mechanism. Herein, a dopant-free, cation deficiency manipulation strategy to tailor the bulk diffusion properties of perovskites without affecting their surface properties is reported, providing a perfect platform for studying the contribution of LOM to OER catalysis. Further optimizing the A-site deficiency achieves a perovskite candidate with excellent intrinsic OER activity, which also demonstrates outstanding performance in rechargeable Zn–air batteries and water electrolyzers. These findings not only corroborate the key role of LOM in OER electrocatalysis, but also provide an effective way for the rational design of better catalyst materials for clean energy technologies.

UAVs have become one of the widely used remote sensing platforms and played a critical role in the construction of smart cities. However, due to the complex environment in urban scenes, secure and accurate data acquisition brings great challenges to 3D modeling and scene updating. Optimal trajectory planning of UAVs and accurate data collection of onboard cameras are non-trivial issues in urban modeling. This study presents the principle of optimized views photogrammetry and verifies its precision and potential in large-scale 3D modeling. Different from oblique photogrammetry, optimized views photogrammetry uses rough models to generate and optimize UAV trajectories, which is achieved through the consideration of model point reconstructability and view point redundancy. Based on the principle of optimized views photogrammetry, this study first conducts a precision analysis of 3D models by using UAV images of optimized views photogrammetry and then executes a large-scale case study in the urban region of Qingdao city, China, to verify its engineering potential. By using GCPs for image orientation precision analysis and TLS (terrestrial laser scanning) point clouds for model quality analysis, experimental results show that optimized views photogrammetry could construct stable image connection networks and could achieve comparable image orientation accuracy. Benefiting from the accurate image acquisition strategy, the quality of mesh models significantly improves, especially for urban areas with serious occlusions, in which 3 to 5 times of higher accuracy has been achieved. Besides, the case study in Qingdao city verifies that optimized views photogrammetry can be a reliable and powerful solution for the large-scale 3D modeling in complex urban scenes., Comment: 16 pages, 24 figures

Incremental Structure from Motion (ISfM) has been widely used for UAV image orientation. Its efficiency, however, decreases dramatically due to the sequential constraint. Although the divide-and-conquer strategy has been utilized for efficiency improvement, cluster merging becomes difficult or depends on seriously designed overlap structures. This paper proposes an algorithm to extract the global model for cluster merging and designs a parallel SfM solution to achieve efficient and accurate UAV image orientation. First, based on vocabulary tree retrieval, match pairs are selected to construct an undirected weighted match graph, whose edge weights are calculated by considering both the number and distribution of feature matches. Second, an algorithm, termed weighted connected dominating set (WCDS), is designed to achieve the simplification of the match graph and build the global model, which incorporates the edge weight in the graph node selection and enables the successful reconstruction of the global model. Third, the match graph is simultaneously divided into compact and non-overlapped clusters. After the parallel reconstruction, cluster merging is conducted with the aid of common 3D points between the global and cluster models. Finally, by using three UAV datasets that are captured by classical oblique and recent optimized views photogrammetry, the validation of the proposed solution is verified through comprehensive analysis and comparison. The experimental results demonstrate that the proposed parallel SfM can achieve 17.4 times efficiency improvement and comparative orientation accuracy. In absolute BA, the geo-referencing accuracy is approximately 2.0 and 3.0 times the GSD (Ground Sampling Distance) value in the horizontal and vertical directions, respectively. For parallel SfM, the proposed solution is a more reliable alternative., Comment: 14 pages, 11 figures

Stereo matching of high-resolution satellite images (HRSI) is still a fundamental but challenging task in the field of photogrammetry and remote sensing. Recently, deep learning (DL) methods, especially convolutional neural networks (CNNs), have demonstrated tremendous potential for stereo matching on public benchmark datasets. However, datasets for stereo matching of satellite images are scarce. To facilitate further research, this paper creates and publishes a challenging dataset, termed WHU-Stereo, for stereo matching DL network training and testing. This dataset is created by using airborne LiDAR point clouds and high-resolution stereo imageries taken from the Chinese GaoFen-7 satellite (GF-7). The WHU-Stereo dataset contains more than 1700 epipolar rectified image pairs, which cover six areas in China and includes various kinds of landscapes. We have assessed the accuracy of ground-truth disparity maps, and it is proved that our dataset achieves comparable precision compared with existing state-of-the-art stereo matching datasets. To verify its feasibility, in experiments, the hand-crafted SGM stereo matching algorithm and recent deep learning networks have been tested on the WHU-Stereo dataset. Experimental results show that deep learning networks can be well trained and achieves higher performance than hand-crafted SGM algorithm, and the dataset has great potential in remote sensing application. The WHU-Stereo dataset can serve as a challenging benchmark for stereo matching of high-resolution satellite images, and performance evaluation of deep learning models. Our dataset is available at https://github.com/Sheng029/WHU-Stereo

Single-phase perovskite oxides that contain nonprecious metals have long been pursued as candidates for catalyzing the oxygen evolution reaction, but their catalytic activity cannot meet the requirements for practical electrochemical energy conversion technologies. Here a cation deficiency-promoted phase separation strategy to design perovskite-based composites with significantly enhanced water oxidation kinetics compared to single-phase counterparts is reported. These composites, self-assembled from perovskite precursors, comprise strongly interacting perovskite and related phases, whose structure, composition, and concentration can be accurately controlled by tailoring the stoichiometry of the precursors. The composite catalyst with optimized phase composition and concentration outperforms known perovskite oxide systems and state-of-the-art catalysts by 1–3 orders of magnitude. It is further demonstrated that the strong interfacial interaction of the composite catalysts plays a key role in promoting oxygen ionic transport to boost the lattice-oxygen participated water oxidation. These results suggest a simple and viable approach to developing high-performance, perovskite-based composite catalysts for electrochemical energy conversion.

Phosphate poisoning of Pt electrocatalysts is one of the major barriers that constrains the performance of phosphoric acid-doped polybenzimidazole (PA/PBI) membrane fuel cells. Herein, we developed new atomically dispersed bimetallic FeCu coordinated with nitrogen-doped carbon nanotubes (FeCu/N-CNTs) as Pt-free oxygen reduction reaction (ORR) electrocatalysts. The cell with FeCu/N-CNTs cathodes delivers a peak power density of 302 mWcm−2 at 230℃, similar to that using Pt/C electrocatalysts (1 mgPt cm−2) but with a much better stability. In contrast to phosphate poisoning of Pt/C, FeCu/N-CNTs show PA enhanced activities. DFT calcualtions indicate that phosphate promotion effect results from the stronger binding of phosphate on Cu sites, which decreases the activation energy barrier for the cleavage of the O2 double bond and provides local protons to facilitate the proton-coupled electron transfer ORR. The results also show that FeCu/N-CNTs have a much better activity for ORR as comapre to Fe single atom catalysts coordinated with nitrogen-doped carbon nanotubes, Fe/N-CNTs. This study demonstrates the promising potential of bimetallic FeCu/N-CNTs as true Pt-free, highly active and durable cathodes for PA/PBI based high temperature polymer electrolyte fuel cells.

Defective electrocatalysts, especially for intrinsic defective carbon, have aroused a wide concern owing to high spin and charge densities. However, the designated nitrogen species favorable for creating defects by the removal of nitrogen, and the influence of defects for the coordination structure of active site and oxygen reduction reaction (ORR) activity have not been elucidated. Herein, we designed and synthesized a pair of electrocatalysts, denoted as Fe-N/C and Fe-ND/C for coordination sites of atomic iron-nitrogen and iron-nitrogen/defect configuration embedded in hollow carbon spheres, respectively, through direct pyrolysis of their corresponding hollow carbon spheres adsorbed with Fe(acac)3. The nitrogen defects were fabricated via the evaporation of pyrrolic-N on nitrogen doped hollow carbon spheres. Results of comparative experiments between Fe-N/C and Fe-ND/C reveal that Fe-ND/C shows superior ORR activity with an onset potential of 30 mV higher than that of Fe-N/C. Fe-ND sites are more favorable for the enhancement of ORR activity. Density functional theory (DFT) calculation demonstrates that Fe-ND/C with proposed coordination structure of FeN4−x (0

Electrochemical energy storage (EES) plays an important role in personal electronics, electrified vehicles, and smart grid. Lithium-ion batteries (LIBs) and supercapacitors (SCs) are two of the most important EES devices that have been widely used in our daily life. The energy density of LIBs is heavily dependent on the electrode capacity, in which the charge storage proceeds mainly in three different mechanisms, that is, alloying, conversion, and intercalation. Conventional LIBs show high energy density, but the rate performance is usually unfavorable. As a comparison, the SCs, which store energy based on electrochemical double layer capacitance (EDLC) or surface Faradaic redox pseudocapacitance, shows outstanding rate performance, but the energy density is still much worse than LIBs. Recently, intercalation pseudocapacitance appears as a new type of EES mechanism which stores energy into the bulk of electrode through a battery-like intercalation process but behaves similar to an electrode of SCs (fast reaction kinetics). Such intercalation pseudocapacitance can effectively narrow the gap between SCs and LIBs in energy density and power density, providing a new opportunity for the development of advanced energy storage system with both high energy density and power density. Up to now, more and more reports about intercalation pseudocapacitive materials have been appeared in literature, however, a systematic analysis of the recent development in intercalation pseudocapacitance is still lack. In this article, we provided an in-time review of the recent progress in the understanding of intercalation pseudocapacitive process and the development of related electrode materials for EES. Importance was paid to the difference between Faradaic surface-redox pseudocapacitance and intercalation pseudocapacitance, as well between battery-like intercalation and pseudocapacitive intercalation. Both cation interaction (Li+ and Na+) and oxygen anion intercalation pseudocapacitance

Lithium-sulfur (Li-S) batteries are promising next-generation energy storage technologies due to their high theoretical energy density, environmental friendliness, and low cost. However, low conductivity of sulfur species, dissolution of polysulfides, poor conversion from sulfur reduction, and lithium sulfide (Li2S) oxidation reactions during discharge-charge processes hinder their practical applications. Herein, under the guidance of density functional theory calculations, we have successfully synthesized large-scale single atom vanadium catalysts seeded on graphene to achieve high sulfur content (80 wt % sulfur), fast kinetic (a capacity of 645 mAh g-1 at 3 C rate), and long-life Li-S batteries. Both forward (sulfur reduction) and reverse reactions (Li2S oxidation) are significantly improved by the single atom catalysts. This finding is confirmed by experimental results and consistent with theoretical calculations. The ability of single metal atoms to effectively trap the dissolved lithium polysulfides (LiPSs) and catalytically convert the LiPSs/Li2S during cycling significantly improved sulfur utilization, rate capability, and cycling life. Our work demonstrates an efficient design pathway for single atom catalysts and provides solutions for the development of high energy/power density Li-S batteries.

Single-atom catalysts (SACs) have attracted much attentions due to the advantages of high catalysis efficiency and selectivity. However, the controllable and efficient synthesis of SACs remains a significant challenge. Herein, we report a controlled one-pot synthesis of nickel single atoms embedded on nitrogen-doped carbon nanotubes (NiSA−N−CNT) and nitrogen-doped graphene (NiSA−N−G). The formation of NiSA−N−CNT is due to the solid-to-solid rolling up mechanism during the high temperature pyrolysis at 800 °C from the stacked and layered Ni-doped g-C3N4, g-C3N4−Ni structure to a tubular CNT structure. Addition of citric acid introduces an amorphous carbon source on the layered g-C3N4−Ni and after annealing at the same temperature of 800 °C, instead of formation of NiSA−N−CNT, Ni single atoms embedded in planar graphene type supports, NiSA−N−G were obtained. The density functional theory (DFT) calculation indicates the introduction of amorphous carbon source substantially reduces the structure fluctuation or curvature of layered g-C3N4-Ni intermediate products, thus interrupting the solid-to-solid rolling process and leading to the formation of planar graphene type supports for Ni single atoms. The as-synthesized NiSA−N−G with Ni atomic loading of ∼6 wt% catalysts shows a better activity and stability for the CO2 reduction reaction (CO2RR) than NiSA−N−CNT with Ni atomic loading of ∼15 wt% due to the open and exposed Ni single atom active sites in NiSA−N−G. This study demonstrates for the first time the feasibility in the control of the microstructure of carbon supports in the synthesis of SACs.

Single-atom catalysts (SACs) are attracting significant attention due to their exceptional catalytic performance and stability. However, the controllable, scalable, and efficient synthesis of SACs remains a significant challenge. Herein, a new and versatile seeding approach is reported to synthesize SACs supported on different 2D materials such as graphene, boron nitride (BN), and molybdenum disulfide (MoS2). This method is demonstrated on the synthesis of Ni, Co, Fe, Cu, Ag, Pd single atoms as well as binary atoms of Ni and Cu codoped on 2D support materials with the mass loading of single atoms in the range of 2.8–7.9 wt%. In particular, the applicability of the new seeding strategy in electrocatalysis is demonstrate on nickel SACs supported on graphene oxide (SANi-GO), exhibiting excellent catalytic performance for electrochemical CO2 reduction reaction with a turnover frequency of 325.9 h−1 at a low overpotential of 0.63 V and high selectivity of 96.5% for CO production. The facile, controllable, and scalable nature of this approach in the synthesis of SACs is expected to open new research avenues for the practical applications of SACs.

Elevation of operational temperatures of polymer electrolyte membrane fuel cells (PEMFCs) has been demonstrated with phosphoric acid-doped polybenzimidazole (PA/PBI) membranes. The technical perspective of the technology is simplified construction and operation with possible integration with, e.g., methanol reformers. Toward this target, significant efforts have been made to develop acid-base polymer membranes, inorganic proton conductors, and organic-inorganic composite materials. This report is devoted to updating the recent progress of the development particularly of acid-doped PBI, phosphate-based solid inorganic proton conductors, and their composite electrolytes. Long-term stability of PBI membranes has been well documented, however, at typical temperatures of 160 C. Inorganic proton-conducting materials, e.g., alkali metal dihydrogen phosphates, heteropolyacids, tetravalent metal pyrophosphates, and phosphosilicates, exhibit significant proton conductivity at temperatures of up to 300 C but have so far found limited applications in the form of thin films. Composite membranes of PBI and phosphates, particularly in situ formed phosphosilicates in the polymer matrix, showed exceptionally stable conductivity at temperatures well above 200 C. Fuel cell tests at up to 260 C are reported operational with good tolerance of up to 16% CO in hydrogen, fast kinetics for direct methanol oxidation, and feasibility of nonprecious metal catalysts. The prospect and future exploration of new proton conductors based on phosphate immobilization and fuel cell technologies at temperatures above 200 C are discussed.

A rechargeable hybrid zinc battery is developed for reaching high power density and high energy density simultaneously by introducing an alkaline Zn–transition metal compound (Zn–MX) battery function into a Zn–air battery. However, the conventional single-layer electrode design cannot satisfy the requirements of both a hydrophilic interface for facilitating ionic transfer to maximize the Zn–MX battery function and a hydrophobic interface for promoting gas diffusion to maximize the Zn–air battery function. Here, a function-separated design is proposed, which allocates the two battery functions to the two faces of the cathode. The electrode is composed of a hydrophobic MnS layer decorated with Ni–Co–S nanoclusters that allows for smooth gas diffusion and efficient oxygen electrocatalysis and a hydrophilic NixCo1−xS2 layer that favors fast ionic transfer and superior performance for energy storage. The battery with the function-separated electrode shows a high short-term discharge voltage of ≈1.7 V, an excellent high-rate galvanostatic discharge–charge with a power density up to 153 mW cm−2 at 100 mA cm−2, a good round-trip efficiency of 75% at 5 mA cm−2, and a robust cycling stability for 330 h with an excellent voltage gap of ≈0.7 V at 5 mA cm−2.

Elevation of operational temperatures of polymer electrolyte membrane fuel cells (PEMFCs) has been demonstrated with phosphoric acid-doped polybenzimidazole (PA/PBI) membranes. The technical perspective of the technology is simplified construction and operation with possible integration with, e.g., methanol reformers. Toward this target, significant efforts have been made to develop acid-base polymer membranes, inorganic proton conductors, and organic-inorganic composite materials. This report is devoted to updating the recent progress of the development particularly of acid-doped PBI, phosphate-based solid inorganic proton conductors, and their composite electrolytes. Long-term stability of PBI membranes has been well documented, however, at typical temperatures of 160°C. Inorganic proton-conducting materials, e.g., alkali metal dihydrogen phosphates, heteropolyacids, tetravalent metal pyrophosphates, and phosphosilicates, exhibit significant proton conductivity at temperatures of up to 300°C but have so far found limited applications in the form of thin films. Composite membranes of PBI and phosphates, particularly in situ formed phosphosilicates in the polymer matrix, showed exceptionally stable conductivity at temperatures well above 200°C. Fuel cell tests at up to 260°C are reported operational with good tolerance of up to 16% CO in hydrogen, fast kinetics for direct methanol oxidation, and feasibility of nonprecious metal catalysts. The prospect and future exploration of new proton conductors based on phosphate immobilization and fuel cell technologies at temperatures above 200°C are discussed.

Iron single atom catalysts (Fe SACs) are the best-known nonprecious metal (NPM) catalysts for the oxygen reduction reaction (ORR) of polymer electrolyte membrane fuel cells (PEMFCs), but their practical application has been constrained by the low Fe SACs loading (<2 wt%). Here, a one-pot pyrolysis method is reported for the synthesis of iron single atoms on graphene (FeSA-G) with a high Fe SAC loading of ≈7.7 ± 1.3 wt%. The as-synthesized FeSA-G shows an onset potential of 0.950 V and a half-wave potential of 0.804 V in acid electrolyte for the ORR, similar to that of Pt/C catalysts but with a much higher stability and higher phosphate anion tolerance. High temperature SiO 2 nanoparticle-doped phosphoric acid/polybenzimidazole (PA/PBI/SiO 2 ) composite membrane cells utilizing a FeSA-G cathode with Fe SAC loading of 0.3 mg cm −2 delivers a peak power density of 325 mW cm −2 at 230 °C, better than 313 mW cm −2 obtained on the cell with a Pt/C cathode at a Pt loading of 1 mg cm −2 . The cell with FeSA-G cathode exhibits superior stability at 230 °C, as compared to that with Pt/C cathode. Our results provide a new approach to developing practical NPM catalysts to replace Pt-based catalysts for fuel cells.

Liquid ammonia can be the key enabler for being an easily transported energy storage carrier, which is highly desirable to be produced from renewable energy, such as solar or electricity, under eco-friendly and mild conditions. However, innovation in the photoelectrochemical devices with high activity, stability, and selectivity for nitrogen-to-ammonia fixation has proven to be very challenging because nitrogen reduction reaction competes with the hydrogen evolution reaction, which occurs preferentially on the photocathode surface at a comparable thermodynamic potential. Thus, we have designed a unique aerophilic-hydrophilic heterostructured Si-based photocathode for improving the energy conversion efficiency. The aerophilic-hydrophilic heterostructure provides a new insight on designing efficient and robust photocathodes for nitrogen fixation.

Nanoclusters (NCs) have been demonstrated of outstanding performance in electrochemical energy storage and conversion technologies due to their strong quantum confinement effects and strong interaction with supports. Here, we developed a class of ultrafine metal-oxide (MOx, M = Fe, Co and Ni) NCs incorporated with iron phthalocyanine (FePc), MOx/FePc-G, supported on graphene as high-performance catalysts for oxygen reduction reaction (ORR), oxygen evolution reaction (OER), and carbon dioxide reduction (CO2RR). The high activities for ORR and OER are attributed to the electron donation and accepting ability of the highly redox active of FePc-G that could tune the properties of MOx. The FeOx/FePc-G exhibits an extremely positive half-wave potential (E1/2) of 0.888 and 0.610 V for ORR in alkaline and neutral conditions, respectively, which is around 60 mV more positive than that of Pt/C. And NiOx/FePc-G shows similar OER activity with the state-of-the-art catalysts, Ir/C, and better performance than NiFeO NCs supported on graphene. Remarkably, the CoOx/FePc-G and NiOx/FePc-G show high activity and selectivity to reduce CO2 into CO with a low onset potential of −0.22 V (overpotential is 0.11 V).

g-C3N4 crystalline/amorphous lateral-like homostructures were prepared using crystalline g-C3N4 nanosheets as seeds via sequential edge-epitaxy growth. The homojunction effectively separates photogenerated carriers, resulting in high photo- and electro-catalytic activities.

© 2018 Elsevier B.V. Supported single atom catalysts (SACs), emerging as a new class of catalytic materials, have been attracting increasing interests. Here we developed a Ni SAC on microwave exfoliated graphene oxide (Ni-N-MEGO) to achieve single atom loading of ~6.9 wt%, significantly higher than previously reported SACs. The atomically dispersed Ni atoms, stabilized by coordination with nitrogen, were found to be predominantly anchored along the edges of nanopores (< 6 nm) using a combination of X-ray absorption spectroscopy (XAS) and aberration-corrected scanning transmission electron microscopy (AC-STEM). The Ni-N-MEGO exhibits an onset overpotential of 0.18 V, and a current density of 53.6 mA mg-1 at overpotential of 0.59 V for CO2 reduction reaction (CO2RR), representing one of the best non-precious metal SACs reported so far in the literature. Density functional theory (DFT) calculations suggest that the electrochemical CO2-to-CO conversion occurs more readily on the edge-anchored unsaturated nitrogen coordinated Ni single atoms that lead to enhanced activity toward CO2RR.

The practical application of single atom catalysts (SACs) is constrained by the low achievable loading of single metal atoms. Here, nickel SACs stabilized on a nitrogen-doped carbon nanotube structure (NiSA-N-CNT) with ultrahigh Ni atomic loading up to 20.3 wt % have been successfully synthesized using a new one-pot pyrolysis method employing Ni acetylacetonate (Ni(acac)2) and dicyandiamide (DCD) as precursors. The yield and formation of NiSA-N-CNT depends strongly on the Ni(acac)2/DCD ratio and annealing temperature. Pyrolysis at 350 and 650 °C led to the formation of Ni single atom dispersed melem and graphitic carbon nitride (Ni-melem and Ni-g-C3N4). Transition from a stacked and layered Ni-g-C3N4 structure to a bamboo-shaped tubular NiSA-N-CNT structure most likely occurs via a solid-to-solid curling or rolling-up mechanism, thermally activated at temperatures of 700-900 °C. Extended X-ray absorption fine structure (EXAFS) experiments and simulations show that Ni single atoms are stabilized in the N-CNT structure through nitrogen coordination, forming a structure with four nearest N coordination shell surrounded by two carbon shells, Ni-N4. The NiSA-N-CNT catalysts show an excellent activity and selectivity for the electrochemical reduction of CO2, achieving a turnover frequency (TOF) of 11.7 s-1 at -0.55 V (vs RHE), but a low activity for the O2 reduction and O2 evolution reactions, as compared to Ni nanoparticles supported on N-CNTs.

This paper proposes a strategy for efficient geometrical verification in unmanned aerial vehicle (UAV) image matching. First, considering the complex transformation model between correspondence set in the image-space, feature points of initial candidate matches are projected onto an elevation plane in the object-space, with assistant of UAV flight control data and camera mounting angles. Spatial relationships are simplified as a 2D-translation in which a motion establishes the relation of two correspondence points. Second, a hierarchical motion consistency constraint, termed HMCC, is designed to eliminate outliers from initial candidate matches, which includes three major steps, namely the global direction consistency constraint, the local direction-change consistency constraint and the global length consistency constraint. To cope with scenarios with high outlier ratios, the HMCC is achieved by using a voting scheme. Finally, an efficient geometrical verification strategy is proposed by using the HMCC as a pre-processing step to increase inlier ratios before the consequent application of the basic RANSAC algorithm. The performance of the proposed strategy is verified through comprehensive comparison and analysis by using real UAV datasets captured with different photogrammetric systems. Experimental results demonstrate that the generated motions have noticeable separation ability, and the HMCC-RANSAC algorithm can efficiently eliminate outliers based on the motion consistency constraint, with a speedup ratio reaching to 6 for oblique UAV images. Even though the completeness sacrifice of approximately 7 percent of points is observed from image orientation tests, competitive orientation accuracy is achieved from all used datasets. For geometrical verification of both nadir and oblique UAV images, the proposed method can be a more efficient solution., Comment: 31 pages; 11104 words

© 2018 Nanofibrous NiAl2O4/Al2O3 ceramic was prepared by electrospinning and subsequent calcination at 1000 °C. Under reducing atmosphere, Ni nanoparticles in situ grew from and were rooted in nanofibrous support. The anchored Ni-NiOx nanocatalysts showed the strong interaction with Al2O3-NiAl2O4 supports owing to the incompletion of NiAl2O4 and NiO reduction and therefore high resistances to aggregation and carbon formation. The nanofibrous catalysts have the advantages of both metal gauze catalysts (fast mass transfer) and supported catalysts (nanosized catalysts). Compared with conventional supported Ni-based catalysts, the nanofibours catalysts produced the highest syngas production during methane partial oxidation at the highest recorded gas hourly space velocity of 8 × 106 L·Kg-1 h-1. The catalytic reaction was operated for 10 h without noticeable performance degradation and the fibrous structure of the nanocatalysts was retained.

© 2018 The establishment of intimate electrode/electrolyte interface is very important in solid oxide fuel cells (SOFCs), because it plays a critical role in the overall cell performance and durability. In this study, Mn segregation and interface formation between directly assembled La0.8Sr0.2MnO3(LSM) electrode and yttrium-stabilized zirconia (YSZ) or gadolinium-doped ceria (GDC) electrolytes are studied using combined focused ion beam and scanning transmission electron microscopy (FIB-STEM). In the case of LSM/YSZ and LSM/GDC electrodes, a significant reduction in the electrode ohmic resistance is observed after cathodic polarization at 900 °C and 500 mA cm-2, indicating the formation of an intimate interface. However, LSM particles start to disintegrate at the electrode/electrolyte interface with the increase of polarization time in the case of LSM/YSZ electrode. On the other hand, the LSM/GDC interface is very stable with negligible microstructure change at the interface. Mn segregation from the LSM perovskite structure is identified under the influence of polarization in both LSM/YSZ and LSM/GDC electrodes. The results demonstrate that nature of the electrolyte plays a critical role in the electrochemical activity, microstructure, morphology and stability of LSM/electrolyte interface under SOFC operation conditions.

Palladium nanoparticles (Pd NPs) supported on Ni single atoms encapsulated in carbon nanotubes (NiSA) show a significantly enhanced electrocatalytic activity for the oxidation reactions of methanol, ethanol and glycerol in alkaline media due to an unusual electron withdrawal effect of NiSA on Pd NPs.

© 2018 Hydrogen Energy Publications LLC The high CO tolerance or resistance is critical for the practical application of proton exchange membrane fuel cells (PEMFCs) coupled with on board reformers for transportation applications due to the presence of high level of CO in the reformats. Increasing the operating temperature is most effective to enhance the CO tolerance of PEMFCs and therefore is of high technological significance. Here, we report a new PEMFC based on SiO2 nanoparticles doped phosphoric acid/polybenzimidazole (PA/PBI/SiO2) composite membranes for operation at temperatures higher than 200 °C. The phosphoric acid within the polymer matrix is stabilized by PA/phosphosilicate nanoclusters formed via prior polarization treatment of the membrane cells at 250 °C at a cell voltage of 0.6 V for 24 h, achieving a high proton conductivity and excellent stability at temperatures beyond that of conventional PA/PBI membranes. The proton conductivity of PA/PBI/SiO2 composite membranes is in the range of 0.029–0.041 S cm-1 and is stable at 250 °C. The PA/PBI/SiO2 composite membrane cell displays an exceptional CO tolerance with a negligible loss in performance at CO contents as high as 11.7% at 240 °C. The cell delivers a peak power density of 283 mW cm-2 and is stable at 240 °C for 100 h under a cell voltage of 0.6 V in 6.3% CO-contained H2 fuel under anhydrous conditions.

© 2018, The Author(s). The trade-offs between photoelectrode efficiency and stability significantly hinder the practical application of silicon-based photoelectrochemical devices. Here, we report a facile approach to decouple the trade-offs of silicon-based photocathodes by employing crystalline TiO2 with graded oxygen defects as protection layer. The crystalline protection layer provides high-density structure and enhances stability, and at the same time oxygen defects allow the carrier transport with low resistance as required for high efficiency. The silicon-based photocathode with black TiO2 shows a limiting current density of ~35.3 mA cm-2 and durability of over 100 h at 10 mA cm-2 in 1.0 M NaOH electrolyte, while none of photoelectrochemical behavior is observed in crystalline TiO2 protection layer. These findings have significant suggestions for further development of silicon-based, III–V compounds and other photoelectrodes and offer the possibility for achieving highly efficient and durable photoelectrochemical devices.

© 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim An acid pretreatment strategy is developed to enhance the proton transport of polysulfone-polyvinylpyrrolidone (PSF-PVP) membranes for application in vanadium redox flow batteries (VRFB). The acid pretreatment leads to the formation of ionic conducting clusters with a size of around d=15.41 nm in the membrane (p-PSF-PVP). As a result, the proton conductivity and proton/vanadium ion selectivity of the p-PSF-PVP membrane increases to 6.60×10-2 S cm-1 and 10.63×107 S min cm-3, respectively, values significantly higher than 2.30×10-2 S cm-1 and 6.67×107 S min cm-3 of the pristine PSF-PVP membrane. Moreover, a VRFB assembled with the p-PSF-PVP membrane exhibits a high coulombic efficiency of 98.6 % and an outstanding energy efficiency of 88.5 %. The results indicate that treatment with either sulfuric acid or phosphoric acid leads to an improvement of membrane properties, and the acid pretreatment is a promising strategy to significantly enhance the performance of the PSF-PVP membrane for VRFB application.

© Copyright 2018 American Chemical Society. Bismuth-based oxides exhibit outstanding oxygen ionic conductivity and fast oxygen surface kinetics and have shown great potential as a highly active component for electrode materials in solid oxide fuel cells (SOFCs). Herein, a Nb-doped La0.6Sr0.4Co0.2Fe0.7Nb0.1O3-d (LSCFNb) electrode with 40% Er0.4Bi1.6O3 (ESB) composite electrode was successfully fabricated by decoration method and directly assembled on barrier-layer-free yttrium-stabilized zirconia (YSZ) electrolyte cells, achieving a peak power density of 1.32 W cm-2 and excellent stability at 750 °C and 250 mA cm-2 for 100 h. ESB decoration also significantly reduces the activation energy from 214 kJ mol-1 for the O2 reduction on pristine LSCFNb electrode to 98 kJ mol-1. Further microstructural analysis reveals that there is a redistribution and migration of the ESB phase in the ESB-LSCFNb composite toward the YSZ electrolyte under the influence of cathodic polarization, forming a thin ESB layer at the cathode/YSZ electrolyte interface. The in situ formed ESB layer not only prevents the direct contact and subsequent reaction between segregated SrO and YSZ electrolytes, but also remarkably promotes the oxygen migration/diffusion at the interface for O2 reduction reaction, resulting in a remarkable increase in power output and a decrease in activation energy. The present study clearly demonstrated the in situ formation of a highly functional and active ESB protective layer at LSCFNb cobaltite cathode and YSZ electrolyte interface via ESB-decorated LSCFNb composite cathode under SOFC operation conditions.

Silicon (Si) requires a protection layer to maintain stable and long-time photoanodic reaction. However, poor charge separation and transfer are key constraint factors in protection layer/Si photoanodes that reduce their water-splitting efficiency. Here, a simultaneous enhancement of charge separation and transfer in Nb-doped NiOx /Ni/black-Si photoanodes induced by plasma treatment is reported. The optimized photoanodes yield the highest charge-separation efficiency (?sep ) of ˜81% at 1.23 V versus reversible hydrogen electrode, corresponding to the photocurrent density of ˜29.1 mA cm-2 . On the basis of detailed characterizations, the concentration and species of oxygen defects in the NiOx -based layer are adjusted by synergistic effect of Nb doping and plasma treatment, which are the dominating factors for forming suitable band structure and providing a favorable hole-migration channel. This work elucidates the important role of oxygen defects on charge separation and transfer in the protection layer/Si-based photoelectrochemical systems and is encouraging for application of this synergistic strategy to other candidate photoanodes.

© 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. Single-atom catalysts (SACs) are the smallest entities for catalytic reactions with projected high atomic efficiency, superior activity, and selectivity; however, practical applications of SACs suffer from a very low metal loading of 1-2 wt%. Here, a class of SACs based on atomically dispersed transition metals on nitrogen-doped carbon nanotubes (MSA-N-CNTs, where M = Ni, Co, NiCo, CoFe, and NiPt) is synthesized with an extraordinarily high metal loading, e.g., 20 wt% in the case of NiSA-N-CNTs, using a new multistep pyrolysis process. Among these materials, NiSA-N-CNTs show an excellent selectivity and activity for the electrochemical reduction of CO 2 to CO, achieving a turnover frequency (TOF) of 11.7 s -1 at -0.55 V (vs reversible hydrogen electrode (RHE)), two orders of magnitude higher than Ni nanoparticles supported on CNTs.

© 2017 Fuel cell reactors or electrolyzers based on alkaline anion-exchange membrane and proton exchange membrane (AAEM & PEM) are investigated for gas phase electrochemical conversion of humidified CO 2 under identical conditions, using conventional Pt/C, Pd/C and Cu/CNTs as cathodic catalysts. Humidified CO 2 can be converted to H 2 and CO via the AAEM based fuel cell reactors with an estimated onset potential of -1.0 V using Cu/CNT as cathodic catalyst, while PEM based fuel cell reactors can only produce H 2 under identical conditions. This can be attributed to the suppressed hydrogen evolution and enhanced CO 2 reduction in the case of AAEM based reactor. Remarkably, AAEM based reactor with Cu/CNT cathode catalysts shows a higher activity (8.88 µmol h -1 cm -2 ) to convert CO 2 to CO than that of Pd/C (7.59 µmol h -1 cm -2 ) and Pt/C (0.75 µmol h -1 cm -2 ) at the same catalyst loading. This study indicates that AAEM based reactor is able to realize the gas phase electrochemical conversion of CO 2 , providing a viable strategy for CO 2 utilization and conversion to useful fuels.

© 2018 American Chemical Society. Carbon nanotubes (CNTs) are excellent supports for electrocatalysts because of their large surface area, excellent electronic conductivity, and high chemical and structural stability. In the present study, the activity of CNTs on direct electron transfer (DET) and on immobilized glucose oxidase (GO X ) is studied as a function of number of walls of CNTs. The results indicate that the GO X immobilized by the CNTs maintains its electrocatalytic activity toward glucose; however, the DET and electrocatalytic activity of GO X depend strongly on the number of inner tubes of CNTs. The GO X immobilized on triple-walled CNTs (TWNTs) has the highest electron-transfer rate constant, 1.22 s -1 , for DET, the highest sensitivity toward glucose detection, 66.11 ± 5.06 µA mM -1 cm -2 , and the lowest apparent Michaelis-Menten constant, 6.53 ± 0.58 mM, as compared to GO X immobilized on single-walled and multiwalled CNTs. The promotion effect of CNTs on the GO X electrocatalytic activity and DET is most likely due to the electron-tunneling effect between the outer wall and inner tubes of TWNTs. The results of this study have general implications for the fundamental understanding of the role of CNT supports in DET processes and can be used for the better design of more effective electrocatalysts for biological processes including biofuel cells and biosensors.