Beyond Extended Surfaces: Understanding the Oxygen Reduction Reaction on Nanocatalysts.

Increasing the platinum utilization efficiency is the key to the advancement and broad dissemination of proton-exchange-membrane fuel cells (PEMFCs). Central to the task is the creation of highly active and durable Pt-based catalysts for the cathodic oxygen reduction reaction (ORR), which demands a comprehensive understanding of the ORR processes on these catalysts under reaction conditions. Past efforts have accumulated a vast wealth of knowledge of the ORR on extended Pt and Pt-alloy model surfaces. While the knowledge has been applied to understanding and designing ORR catalysts, it has also been recognized that these understandings cannot always translate into nanoscale systems. In this Perspective, we will review the progress that the theoretical descriptor has evolved to reconcile the observed differences between extended and nanoscale Pt surfaces, and we highlight the needs in advancing both characterizations and theories in order to understand ORR in the more complex Pt-alloy nanocatalysts. Particularly, understanding the dynamic structure-composition-function relation of Pt-alloy nanocatalysts during ORR demands concerted efforts in precision synthesis, advanced atomistic-scale in situ characterization, and comprehensive computational models.

Molecular Vise Approach to Create Metal-Binding Sites in MOFs and Detection of Biomarkers.

We report a new approach to create metal-binding site in a series of metal-organic frameworks (MOFs), where tetratopic carboxylate linker, 4',4'',4''',4''''-methanetetrayltetrabiphenyl-4-carboxylic acid, is partially replaced by a tritopic carboxylate linker, tris(4-carboxybiphenyl)amine, in combination with monotopic linkers, formic acid, trifluoroacetic acid, benzoic acid, isonicotinic acid, 4-chlorobenzoic acid, and 4-nitrobenzoic acid, respectively. The distance between these paired-up linkers can be precisely controlled, ranging from 5.4 to 10.8 Å, where a variety of metals, Mg2+ , Al3+ , Cr3+ , Mn2+ , Fe3+ , Co2+ , Ni2+ , Cu2+ , Zn2+ , Ag+ , Cd2+ and Pb2+ , can be placed in. The distribution of these metal-binding sites across a single crystal is visualized by 3D tomography of laser scanning confocal microscopy with a resolution of 10 nm. The binding affinity between the metal and its binding-site in MOF can be varied in a large range (observed binding constants, Kobs from 1.56×102 to 1.70×104  L mol-1 ), in aqueous solution. The fluorescence of these crystals can be used to detect biomarkers, such as cysteine, homocysteine and glutathione, with ultrahigh sensitivity and without the interference of urine, through the dissociation of metal ions from their binding sites.

Labile Coordination Interphase for Regulating Lean Ion Dynamics in Reversible Zn Batteries.

Rechargeability in zinc (Zn) batteries is limited by anode irreversibility. The practical lean electrolytes exacerbate the issue, compromising the cost benefits of zinc batteries for large-scale energy storage. In this study, a zinc-coordinated interphase is developed to avoid chemical corrosion and stabilize zinc anodes. The interphase promotes Zn2+ ions to selectively bind with histidine and carboxylate ligands, creating a coordination environment with high affinity and fast diffusion due to thermodynamic stability and kinetic lability. Experiments and simulations indicate that interphase regulates dendrite-free electrodeposition and reduces side reactions. Implementing such labile coordination interphase results in increased cycling at 20 mA cm-2 and high reversibility of dendrite-free zinc plating/stripping for over 200 hours. A Zn||LiMn2 O4 cell with 74.7 mWh g-1 energy density and 99.7% Coulombic efficiency after 500 cycles realized enhanced reversibility using the labile coordination interphase. A lean-electrolyte full cell using only 10 µL mAh-1 electrolyte is also demonstrated with an elongated lifespan of 100 cycles, five times longer than bare Zn anodes. The cell offers a higher energy density than most existing aqueous batteries. This study presents a proof-of-concept design for low-electrolyte, high-energy-density batteries by modulating coordination interphases on Zn anodes.

Bubble-Mediated Large-Scale Hierarchical Assembly of Ultrathin Pt Nanowire Network Monolayer at Gas/Liquid Interfaces.

Extensive macroscale two-dimensional (2-D) platinum (Pt) nanowire network (NWN) sheets are created through a hierarchical self-assembly process with the aid of biomolecular ligands. The Pt NWN sheet is assembled from the attachment growth of 1.9 nm-sized 0-D nanocrystals into 1-D nanowires featuring a high density of grain boundaries, which then interconnect to form monolayer network structures extending into centimeter-scale size. Further investigation into the formation mechanism reveals that the initial emergence of NWN sheets occurs at the gas/liquid interfaces of the bubbles produced by sodium borohydride (NaBH4) during the synthesis process. Upon the rupture of these bubbles, an exocytosis-like process releases the Pt NWN sheets at the gas/liquid surface, which subsequently merge into a continuous monolayer Pt NWN sheet. The Pt NWN sheets exhibit outstanding oxygen reduction reaction (ORR) activities, with specific and mass activities 12.0 times and 21.2 times greater, respectively, than those of current state-of-the-art commercial Pt/C electrocatalysts.

Graphene-nanopocket-encaged PtCo nanocatalysts for highly durable fuel cell operation under demanding ultralow-Pt-loading conditions.

The proton exchange membrane fuel cell (PEMFC) as an attractive clean power source can promise a carbon-neutral future, but the widespread adoption of PEMFCs requires a substantial reduction in the usage of the costly platinum group metal (PGM) catalysts. Ultrafine nanocatalysts are essential to provide sufficient catalytic sites at a reduced PGM loading, but are fundamentally less stable and prone to substantial size growth in long-term operations. Here we report the design of a graphene-nanopocket-encaged platinum cobalt (PtCo@Gnp) nanocatalyst with good electrochemical accessibility and exceptional durability under a demanding ultralow PGM loading (0.070 mgPGM cm-2) due to the non-contacting enclosure of graphene nanopockets. The PtCo@Gnp delivers a state-of-the-art mass activity of 1.21 A mgPGM-1, a rated power of 13.2 W mgPGM-1 and a mass activity retention of 73% after an accelerated durability test. With the greatly improved rated power and durability, we project a 6.8 gPGM loading for a 90 kW PEMFC vehicle, which approaches that used in a typical catalytic converter.

A general one-step plug-and-probe approach to top-gated transistors for rapidly probing delicate electronic materials.

The miniaturization of silicon-based electronics has motivated considerable efforts in exploring new electronic materials, including two-dimensional semiconductors and halide perovskites, which are usually too delicate to maintain their intrinsic properties during the harsh device fabrication steps. Here we report a convenient plug-and-probe approach for one-step simultaneous van der Waals integration of high-k dielectrics and contacts to enable top-gated transistors with atomically clean and electronically sharp dielectric and contact interfaces. By applying the plug-and-probe top-gate transistor stacks on two-dimensional semiconductors, we demonstrate an ideal subthreshold swing of 60 mV per decade. Using this approach on delicate lead halide perovskite, we realize a high-k top-gate CsPbBr3 transistor with a low operating voltage and a very high two-terminal field-effect mobility of 32 cm2 V-1 s-1. This approach can be extended to centimetre-scale MoS2 and perovskite and generate top-gated transistor arrays, offering a rapid and convenient way of accessing intrinsic properties of delicate emerging materials.

Toward Rational Design of Single-Atom Catalysts.

Downscaling catalyst size has long been used to promote the atomic utilization efficiency of catalysts. Single-atom catalysts (SACs) are the current end of this downscaling road, offering the potential of 100% metal atom utilization and excellent catalytic behavior compared with traditional nanoparticle catalysts. However, most development of SACs still relies on trial-and-error experiments because of the insufficient understanding of the distinctive properties of SACs and their structure-activity relationships. This Perspective discusses the path forward toward the rational design of SACs through a summary of understanding regarding the distinctive properties of single-atom active sites, their dynamic changes during the reactions, and the corresponding reaction mechanisms. Major challenges and opportunities for future research on SACs are identified in precisely controlled synthesis, advanced operando characterizations, and discovering the unconventional catalytic mechanisms.

Synthesis of zinc phthalocyanine with large steric hindrance and its photovoltaic performance for dye-sensitized solar cells.

A zinc phthalocyanine (ZnPc) derivative (Zn-tri-PcNc-8) containing tri-benzonaphtho-condensed porphyrazine with one carboxylic and six diphenylphenoxy peripheral substitutions was designed and synthesized as a sensitizer for dye-sensitized solar cells (DSSCs). For the purpose of extending the absorption spectra while minimizing the formation of ZnPc molecular aggregates, bulky 2,6-diphenylphenoxy groups were used as electron donor moieties, and the carboxylic group as an anchoring group to graft the sensitizer onto the semiconductor. It was found that a TiO2-based solar cell sensitized by Zn-tri-PcNc-8 shows a maximum incident photon-to-current conversion efficiency in the red/near-IR light range (650-750 nm), and a solar cell sensitized at near room temperature (30 °C) for 48 h exhibits the best efficiency (3.01%). The efficiency was much higher than that (1.96%) for a solar cell sensitized by its analogue (Zn-tri-PcNc-2) having one carboxyl and three tert-butyl groups without chenodeoxycholic acid (CDCA), indicating that the introduction of six bulky diphenylphenoxy substitutions with large steric hindrance in the ZnPc macrocycle can effectively suppress the molecular aggregates, thus resulting in an improved conversion efficiency. The present results shed light on an effective solution to adjust the ZnPc property via chemical modification such as changing the "push-pull" effect and adding large steric hindrance substituents to further improve the efficiency of the phthalocyanine-sensitized solar cell.