Single Entity Electrocatalysis.

Making a measurement over millions of nanoparticles or exposed crystal facets seldom reports on reactivity of a single nanoparticle or facet, which may depart drastically from ensemble measurements. Within the past 30 years, science has moved toward studying the reactivity of single atoms, molecules, and nanoparticles, one at a time. This shift has been fueled by the realization that everything changes at the nanoscale, especially important industrially relevant properties like those important to electrocatalysis. Studying single nanoscale entities, however, is not trivial and has required the development of new measurement tools. This review explores a tale of the clever use of old and new measurement tools to study electrocatalysis at the single entity level. We explore in detail the complex interrelationship between measurement method, electrocatalytic material, and reaction of interest (e.g., carbon dioxide reduction, oxygen reduction, hydrazine oxidation, etc.). We end with our perspective on the future of single entity electrocatalysis with a key focus on what types of measurements present the greatest opportunity for fundamental discovery.

Exploring the Complex Chemistry and Degradation of Ascorbic Acid in Aqueous Nanoparticle Synthesis.

Ascorbic acid (AA) is the most widely used reductant for noble metal nanoparticle (NP) synthesis. Despite the synthetic relevance, its aqueous chemistry remains misunderstood, due in part to various assumptions about its reduction pathway which are insufficiently supported by experimental evidence. This study aims to provide an understanding of the complex chemistry associated with AA under aqueous conditions. We demonstrate that (i) AA undergoes appreciable degradation in alkaline solution on a timescale relevant to NP synthesis, (ii) contrary to popular belief, AA does not degrade into dehydroascorbic acid (DHA), nor is DHA the oxidized product of AA under noble metal NP synthetic conditions, (iii) DHA, which readily degrades under alkaline conditions, can also effectively reduce metal salt precursors to metal NPs, (iv) neither ascorbate nor dehydroascorbate act as surface capping agents post-synthetically on the NPs (v) AA degradation time greatly affects the morphology and polydispersity of the resultant NP. Results from our mechanistic investigation enabled us to utilize purposefully-aged reductants to achieve control over shape yield and monodispersity in the seed-mediated synthesis of Au nanorods. Our findings have important implications for achieving monodispersed products in the many metal NP synthesis reactions that make use of AA as a reducing agent.

Anodic Electrodeposition of IrO x Nanoparticles from Aqueous Nanodroplets.

Electrodeposition has been used for centuries to create new materials. However, synthetic platforms are still necessary to enrich a variety of nanomaterials that can be electrodeposited. For instance, IrO x is a popular material for the water oxidation reaction, but electrodeposition strategies for the controlled growth of IrO x nanoparticles are lacking. Here, we demonstrate the anodic electrodeposition of IrO x nanoparticles from aqueous nanodroplets. Field emission scanning electron microscopy (FESEM) and scanning transmission electron microscopy (STEM) images confirm the macro- and microstructure of the resulting nanoparticles. IrO x nanoparticles of 43 ± 10 nm in diameter were achieved. X-ray photoelectron spectroscopy (XPS) showed the presence of Ir(III) and Ir(IV) hydrated oxyhydroxide species. The synthesis of IrO x nanoparticles under anodic conditions using water nanodroplets expands the capabilities of our technique and provides a tunable platform for IrO x nanoparticle electrodeposition.

For Zinc Metal Batteries, How Many Electrons go to Hydrogen Evolution? An Electrochemical Mass Spectrometry Study.

Despite the advantages of aqueous zinc (Zn) metal batteries (AZMB) like high specific capacity (820 mAh g-1 and 5,854 mAh cm-3 ), low redox potential (-0.76 V vs. the standard hydrogen electrode), low cost, water compatibility, and safety, the development of practically relevant batteries is plagued by several issues like unwanted hydrogen evolution reaction (HER), corrosion of Zn substrate (insulating ZnO, Zn(OH)2 , Zn(SO4 )x (OH)y , Zn(ClO4 )x (OH)y etc. passivation layer), and dendrite growth. Controlling and suppressing HER activity strongly correlates with the long-term cyclability of AZMBs. Therefore, a precise quantitative technique is needed to monitor the real-time dynamics of hydrogen evolution during Zn electrodeposition. In this study, we quantify hydrogen evolution using in situ electrochemical mass spectrometry (ECMS). This methodology enables us to determine a correction factor for the faradaic efficiency of this system with unmatched precision. For instance, during the electrodeposition of zinc on a copper substrate at a current density of 1.5 mA/cm2 for 600 seconds, 0.3 % of the total charge is attributed to HER, while the rest contributes to zinc electrodeposition. At first glance, this may seem like a small fraction, but it can be detrimental to the long-term cycling performance of AZMBs. Furthermore, our results provide insights into the correlation between HER and the porous morphology of the electrodeposited zinc, unravelling the presence of trapped H2 and Zn corrosion during the charging process. Overall, this study sets a platform to accurately determine the faradaic efficiency of Zn electrodeposition and provides a powerful tool for evaluating electrolyte additives, salts, and electrode modifications aimed at enhancing long-term stability and suppressing the HER in aqueous Zn batteries.

Ultrafine Mix-Phase SnO-SnO2 Nanoparticles Anchored on Reduced Graphene Oxide Boost Reversible Li-Ion Storage Capacity beyond Theoretical Limit.

Tin-based materials with high specific capacity have been studied as high-performance anodes for Li-ion storage devices. Herein, a mix-phase structure of SnO-SnO2@rGO (rGO = reduced graphene oxide) was designed and prepared via a simple chemical method, which leads to the growth of tiny nanoparticles of a mixture of two different tin oxide phases on the crumbled graphene nanosheets. The three-dimensional structure of graphene forms the conductive framework. The as-prepared mix phase SnO-SnO2@rGO exhibits a large Brunauer-Emmett-Teller surface area of 255 m2 g-1 and an excellent ionic diffusion rate. When the resulting mix-phase material was examined for Li-ion battery anode application, the SnO-SnO2@rGO was noted to deliver an ultrahigh reversible capacity of 2604 mA h g-1 at a current density of 0.1 A g-1. It also exhibited superior rate capabilities and more than 82% retention of capacity after 150 charge-discharge cycles at 0.1 A g-1, lasting until 500 cycles at 1 A g-1 with very good retention of the initial capacity. Owing to the uniform defects on the rGO matrix, the formation of LiOH upon lithiation has been suggested to be the primary cause of this very high reversible capacity, which is beyond the theoretical limit. A Li-ion full cell was assembled using LiNi0.5Mn0.3Co0.2O2 (NMC-532) as a high-capacity cathodic counterpart, which showed a very high reversible capacity of 570 mA h g-1 (based on the anode weight) at an applied current density of 0.1 A g-1 with more than 50% retention of capacity after 100 cycles. This work offers a favorable design of electrode material, namely, mix-phase tin oxide-nanocarbon matrix, exhibiting adequate electrochemical performance for Li storage applications.

Search for New Anode Materials for High Performance Li-Ion Batteries.

Owing to an unmatched combination of power and energy density along with cyclic stability, the Li-ion battery has qualified itself to be the highest performing rechargeable battery. Taking both transportable and stationary energy storage requirements into consideration, Li-ion batteries indeed stand tall in comparison to any other existing rechargeable battery technologies. However, graphite, which is still one of the best performing Li-ion anodes, has specific drawbacks in fulfilling the ever-increasing energy and power density requirements of the modern world. Therefore, further research on alternative anode materials is absolutely essential. Equally important is the search for and enhanced use of right earth abundant materials for battery electrodes so as to bring down the costs of the battery systems. In this spotlight article, we discuss the current research progress in the area of alternative anode materials for Li-ion battery, putting our own research work over the past several years into perspective. Starting from conversion anode systems like oxides and sulfides, to insertion cum alloying systems like transition metal carbides, to molecularly engineered open framework systems like metal organic frameworks (MOFs), covalent organic frameworks (COFs), and organic-inorganic hybrid perovskites (OIHPs), this spotlight provides a complete essence of the recent developments in the area of alternative anodes. The possible and potential impact of these new anode materials is detailed and discussed here.

Air exposed 1,4-bis(trimethylsilyl)-1,4-dihydropyrazine: an avant-garde carbonization precursor for multi-functionalized carbon material.

1,4-Bis(trimethysilyl)-1,4-dihydropyrazine 1 has been utilized as a small molecule precursor for carbonization to N,O-containing few-layered carbon sheets 3via the formation of a polymeric material 2 upon simple air exposure at room temperature. Without any further purification, this multi-functionalized carbon material 3 exhibited excellent anode performance in a lithium ion battery.

Tuning the electronic energy level of covalent organic frameworks for crafting high-rate Na-ion battery anode.

Crystalline Covalent Organic Frameworks (COFs) possess ordered accessible nano-channels. When these channels are decorated with redox-active functional groups, they can serve as the anode in metal ion batteries (LIB and SIB). Though sodium's superior relative abundance makes it a better choice over lithium, the energetically unfavourable intercalation of the larger sodium ion makes it incompatible with the commercial graphite anodes used in Li-ion batteries. Also, their sluggish movement inside the electrodes restricts the fast sodiation of SIB. Creating an electronic driving force at the electrodes via chemical manipulation can be a versatile approach to overcome this issue. Herein, we present anodes for SIB drawn on three isostructural COFs with nearly the same Highest Occupied Molecular Orbitals (HOMO) levels but with varying Lowest Unoccupied Molecular Orbitals (LUMO) energy levels. This variation in the LUMO levels has been deliberately obtained by the inclusion of electron-deficient centers (phenyl vs. tetrazine vs. bispyridine-tetrazine) substituents into the modules that make up the COF. With the reduction in the cell-potential, the electrons accumulate in the anti-bonding LUMO. Now, these electron-dosed LUMO levels become efficient anodes for attracting the otherwise sluggish sodium ions from the electrolyte. Also, the intrinsic porosity of the COF favors the lodging and diffusion of the Na+ ions. Cells made with these COFs achieve a high specific capacity (energy density) and rate performance (rapid charging-discharging), something that is not as easy for Na+ compared to the much smaller sized Li+. The bispyridine-tetrazine COF with the lowest LUMO energy shows a specific capacity of 340 mA h g-1 at 1 A g-1 and 128 mA h g-1 at a high current density of 15 A g-1. Only a 24% drop appears on increasing the current density from 0.1 to 1 A g-1, which is the lowest among all the top-performing COF derived Na-ion battery anodes.

Polymer-Assisted Electrophoretic Synthesis of N-Doped Graphene-Polypyrrole Demonstrating Oxygen Reduction with Excellent Methanol Crossover Impact and Durability.

The design and synthesis of metal-free catalysts with superior electrocatalytic activity, high durability, low cost, and under mild conditions is extremely desirable but remains challenging. To address this problem, a polymer-assisted electrochemical exfoliation technique of graphite in the presence of an aqueous acidic medium is reported. This simple, cost-effective, and mass-scale production approach could open the possibility for the synthesis of high-quality nitrogen-doped graphene-polypyrrole (NG-PPy). The NG-PPy catalyst displays an improved half wave potential (E1/2 =0.77 V) in alkaline medium compared with G-PPy (E1/2 =0.66 V). Most importantly, this catalyst demonstrates excellent stability with high methanol tolerance, and it outperforms the commercial Pt/C catalyst and other previously reported metal-free catalysts. The content of graphitic nitrogen atoms is the key factor for the enhancement of electrocatalytic activity towards oxygen reduction reactions (ORR). Interestingly, the NG-PPy catalyst can be used as a cathode material in a zinc-air battery, which demonstrates a higher peak power density (59 mW cm-2 ) than G-PPy (36.6 mW cm-2 ), highlighting the importance of the low-cost material synthesis approach towards the development of metal-free efficient ORR catalysts for fuel cell and metal-air battery applications. Remarkably, the polymer-assisted electrophoretic exfoliation of graphite with a high yield (≈88 wt %) of few-layer graphene flakes could pave the way towards the mass production of high-quality graphene for a variety of applications.

Fe3 SnC@CNF: A 3 D Antiperovskite Intermetallic Carbide System as a New Robust High-Capacity Lithium-Ion Battery Anode.

A 3 D intermetallic anti-perovskite carbide, Fe3 SnC, is reported as a Li-ion battery anode. Single-phase Fe3 SnC showed a reversible Li-ion capacity of 426 mAh g-1 that increased significantly (600 mAh g-1 ) upon its in situ synthesis by electrospinning and pyrolysis to render a conducting carbon nanofibre (CNF) based composite. Importantly, the Fe3 SnC@CNF composite showed excellent stability in up to 1000 cycles with a remarkable 96 % retention of capacity. The rate performance was equally impressive with a high capacity of 500 mAh g-1 delivered at a high current density of 2 A g-1 . An estimation of Li ion diffusion from the electrochemical impedance data showed a major enhancement of the rate by a factor of 2 in the case of Fe3 SnC@CNF compared to the single-phase Fe3 SnC sample. Post-cyclic characterisation revealed that the unit cell was retained despite a volume expansion upon the inclusion of four Li atoms per unit cell, as calculated from the capacity value. The cyclic voltammogram shows four distinctive peaks that could be identified as the sequential incorporation of up to four Li atoms. First-principles DFT calculations were performed to elucidate the favourable sites for the inclusion of 1-4 Li atoms inside the Fe3 SnC unit cell along with the associated strain.

Facile one step synthesis of Cu-g-C3N4 electrocatalyst realized oxygen reduction reaction with excellent methanol crossover impact and durability.

Non-precious metal doped carbonaceous materials are currently the most promising alternative towards oxygen reduction reaction (ORR) electrocatalysts in terms of cost, accessibility, efficiency and durability. In this work, a simple one-step pyrolysis process was used for the synthesis of copper doped graphitic carbon nitride (Cu-g-C3N4) as electrocatalyst. The as-synthesized Cu-g-C3N4 material is displaying excellent electrocatalytic response towards ORR in alkaline medium. In comparison to commercial Pt/C catalyst, Cu-g-C3N4 exhibits high methanol tolerance, long term stability, without compromising (4e-) electron transfer pathway process and attaining less than 4% H2O2 formation. The enhanced electrocatalytic behaviour may be ascribed to the formation of active sites strongly coupled into the nitrogen-rich carbon matrix. Such a low-cost, extremely durable and stable electrocatalyst can therefore be regarded as an efficient cathodic material, which can be utilized for several renewable energy conversion technologies such as fuel cell, biofuel cell and metal-air battery.

CO2 Laser Direct Written MOF-Based Metal-Decorated and Heteroatom-Doped Porous Graphene for Flexible All-Solid-State Microsupercapacitor with Extremely High Cycling Stability.

Over the past decade, flexible and wearable microelectronic devices and systems have gained significant importance. Because portable power source is an essential need of such wearable devices, currently there is considerable research emphasis on the development of planar interdigitated micro energy -torage devices by employing diverse precursor materials to obtain functional materials (functional carbon, oxides, etc.) with the desirable set of properties. Herein we report for the first time the use of metal organic framework (MOF) and zeolitic imidazolate framework (ZIF-67) for high-wavelength photothermal laser direct writing of metal-decorated, heteroatom-doped, porous few-layer graphene electrodes for microsupercapacitor application. We argue that the specific attributes of MOF as a precursor and the high-wavelength laser writing approach (which creates extremely high localized and transient temperature (>2500 °C) due to strong absorption by lattice vibrations) are together responsible for the peculiar interesting properties of the carbon material thus synthesized, thereby rendering extremely high cycling stability to the corresponding microsupercapacitor device. Our device exhibits near 100% retention after 200 000 cycles as well as stability under 150° bending.