Carbon Corrosion Induced by Surface Defects Accelerates Degradation of Platinum/Graphene Catalysts in Oxygen Reduction Reaction.

Graphene supported electrocatalysts have demonstrated remarkable catalytic performance for oxygen reduction reaction (ORR). However, their durability and cycling performance are greatly limited by Oswald ripening of platinum (Pt) and graphene support corrosion. Moreover, comprehensive studies on the mechanisms of catalysts degradation under 0.6-1.6 V versus RHE (Reversible Hydrogen Electrode) is still lacking. Herein, degradation mechanisms triggered by different defects on graphene supports are investigated by two cycling protocols. In the start-up/shutdown cycling (1.0-1.6 V vs. RHE), carbon oxidation reaction (COR) leads to shedding or swarm-like aggregation of Pt nanoparticles (NPs). Theoretical simulation results show that the expansion of vacancy defects promotes reaction kinetics of the decisive step in COR, reducing its reaction overpotential. While under the load cycling (0.6-1.0 V vs. RHE), oxygen containing defects lead to an elevated content of Pt in its oxidation state which intensifies Oswald ripening of Pt. The presence of vacancy defects can enhance the transfer of electrons from graphene to the Pt surface, reducing the d-band center of Pt and making it more difficult for the oxidation state of platinum to form in the cycling. This work will provide comprehensive understanding on Pt/Graphene catalysts degradation mechanisms.

Regulating the Pore Structure of Activated Carbon by Pitch for High-Performance Sodium-Ion Storage.

The pore structure of carbon anodes plays a crucial role in enhancing the sodium storage capacity. Designing more confined pores in carbon anodes is accepted as an effective strategy. However, current design strategies for confined pores in carbon anodes fail to achieve both high capacity and initial Coulombic efficiency (ICE) simultaneously. Herein, we develop a strategy for utilizing the repeated impregnation and precarbonization method of liquid pitch to regulate the pore structure of the activated carbon (AC) material. Driven by capillary coalescence, the pitch is impregnated into the pores of AC, which reduces the specific surface area of the material. During the carbonization process, numerous pores with diameters less than 1 nm are formed, resulting in a high capacity and improved ICE of the carbon anode. Moreover, the ordered carbon layers derived from the liquid pitch also enhance the electrical conductivity, thereby improving the rate capability of as-obtained carbon anodes. This enables the fabricated material (XA-4T-1300) to have a high ICE of 91.1% and a capacity of 383.0 mA h g-1 at 30 mA g-1. The capacity retention is 95.5% after 300 cycles at 1 A g-1. This study proposes a practical approach to adjust the microcrystalline and pore structures to enhance the performance of sodium-ion storage in materials.

Releasing Free Radicals in Precursor Triggers the Formation of Closed Pores in Hard Carbon for Sodium-Ion Batteries.

Increasing closed pore volume in hard carbon is considered to be the most effective way to enhance the electrochemical performance in sodium-ion batteries. However, there is a lack of systematic insights into the formation mechanisms of closed pores at molecular level. In this study, a regulation strategy of closed pores via adjustment of the content of free radicals is reported. Sufficient free radicals are exposed by part delignification of bamboo, which is related to the formation of well-developed carbon layers and rich closed pores. In addition, excessive free radicals from nearly total delignification lead to more reactive sites during pyrolysis, which competes for limited precursor debris to form smaller microcrystals and therefore compact the material. The optimal sample delivers a large closed pore volume of 0.203 cm3 g-1, which leads to a high reversible capacity of 350 mAh g-1 at 20 mA g-1 and enhanced Na+ transfer kinetics. This work provides insights into the formation mechanisms of closed pores at molecular level, enabling rational design of hard carbon pore structures.

Biomass-based controllable morphology of carbon microspheres with multi-layer hollow structure for superior performance in supercapacitors.

The electrochemical properties of corn starch (CS)-based hydrothermal carbon microsphere (CMS) electrode materials for supercapacitor are closely related to their structures. Herein, cetyltrimethyl ammonium bromide (CTAB) was used as a soft template to form the corn starch (CS)-based carbon microspheres with radial hollow structure in the inner and middle layers by hydrothermal and sol-gel method. Due to the introduction of multi-layer hollow structure of carbon microsphere, more micropores were produced during CO2 activation, which increased the specific surface area and improved the capacitance performance. Compared to commercial activated carbon, the four different morphologies of corn starch CMS had better electrochemical performances. Consequently, the proposed CO2-(CTAB)-CS-CS exhibits a high discharge specific capacitance of 242.5F/g at 1 A/g in three-electrode system with 6 M KOH electrolyte, better than commercial activated carbon with 208.5F/g. Moreover, excellent stability is achieved for CO2-(CTAB)-CS-CS with approximately 97.14 % retention of the initial specific capacitance value after 10,000 cycles at a current density of 2 A/g, while the commercial activated carbon has 86.96 % retention. This implies that the corn starch-based multilayer hollow CMS could be a promising electrode material for high-performance supercapacitors.

Whole Landscape of the Origin and Evolution of Gassing in Supercapacitors at a High Voltage.

Although supercapacitors with acetonitrile-based electrolytes (AN-based SCs) have realized high-voltage (3.0 V) applications by manufacturers, gas generation at high voltages is a critical issue. Also, the exact origins and evolution mechanisms of gas generation during SC aging at 3.0 V still lack a whole landscape. In this work, floating tests under realistic working conditions are conducted by 22450-type cylindrical cells with an AN-based commercial electrolyte. Comprehensive insights into the origins and evolution mechanisms of gas species at 2.7 and 3.0 V are acquired, which involves multiple side reactions related to the electrode, current collector, and electrolyte. Both experimental evidence and density functional theory calculations demonstrate that the primary reasons for gas generation are residual water and oxygen-containing functional groups, especially hydroxyl and carboxyl. More importantly, additional types of gas (such as CO2, NH3, and alkenes) can only be detected at a higher voltage of 3.0 V rather than 2.7 V after failure, suggesting that these gas species can be regarded as the failure signatures at 3.0 V. This breakthrough analysis will provide fundamental guidance for failure evaluation and designing AN-based SCs with extended lifetime at 3.0 V.

Constructing a Low-Cost Si-NSs@C/NG Composite by a Ball Milling-Catalytic Pyrolysis Method for Lithium Storage.

Silicon-based composites are promising candidates as the next-generation anode materials for high-performance lithium-ion batteries (LIBs) due to their high theoretical specific capacity, abundant reserves, and reliable security. However, expensive raw materials and complicated preparation processes give silicon carbon anode a high price and poor batch stability, which become a stumbling block to its large-scale practical application. In this work, a novel ball milling-catalytic pyrolysis method is developed to fabricate a silicon nanosheet@amorphous carbon/N-doped graphene (Si-NSs@C/NG) composite with cheap high-purity micron-size silica powder and melamine as raw materials. Through systematic characterizations such as XRD, Raman, SEM, TEM and XPS, the formation process of NG and a Si-NSs@C/NG composite is graphically demonstrated. Si-NSs@C is uniformly intercalated between NG nanosheets, and these two kinds of two-dimensional (2D) materials are combined in a surface-to-surface manner, which immensely buffers the stress changes caused by volume expansion and contraction of Si-NSs. Attributed to the excellent electrical conductivity of graphene layer and the coating layer, the initial reversible specific capacity of Si-NSs@C/NG is 807.9 mAh g-1 at 200 mA g-1, with a capacity retention rate of 81% in 120 cycles, exhibiting great potential for application as an anode material for LIBs. More importantly, the simple and effective process and cheap precursors could greatly reduce the production cost and promote the commercialization of silicon/carbon composites.

High-Voltage Redox Mediator of an Organic Electrolyte for Supercapacitors by Lewis Base Electrocatalysis.

Redox electrolytes for supercapacitors (SCs) have recently sparked widespread interest. Due to the redox reactions within electrolytes, they can achieve high capacitance and long cycle stability. However, the energy density of SCs with redox electrolytes is limited by the narrow applied electrochemical window due to the irreversible side reaction of redox mediators at high potential. To overcome this issue, a redox mediator with a high redox potential, tetrachloridehydroquinone (TCHQ), is added to organic electrolytes to obtain a broad electrochemical window. TCHQ is designed to undergo a dehydrogenation reaction catalyzed by N-doped activated carbon to provide capacitance. The pyrrole N atoms have the highest electrocatalytic activity based on the theoretical calculation of reaction overpotential with predicted reaction pathways due to their Lewis basicity. Benefitting from that, TCHQ shows promising reversibility with a larger electrochemical window (up to 2.7 V). As a result, a higher energy density is obtained when compared to commercial SCs. This study proposes a strategy for designing redox mediators and interfaces of SCs with high energy density and a calculation method of dehydrogenation reaction electrocatalysis.

Modification of Nitrate Ion Enables Stable Solid Electrolyte Interphase in Lithium Metal Batteries.

The lifespan of high-energy-density lithium metal batteries (LMBs) is hindered by heterogeneous solid electrolyte interphase (SEI). The rational design of electrolytes is strongly considered to obtain uniform SEI in working batteries. Herein, a modification of nitrate ion (NO3 - ) is proposed and validated to improve the homogeneity of the SEI in practical LMBs. NO3 - is connected to an ether-based moiety to form isosorbide dinitrate (ISDN) to break the resonance structure of NO3 - and improve the reducibility. The decomposition of non-resonant -NO3 in ISDN enriches SEI with abundant LiNx Oy and induces uniform lithium deposition. Lithium-sulfur batteries with ISDN additives deliver a capacity retention of 83.7 % for 100 cycles compared with rapid decay with LiNO3 after 55 cycles. Moreover, lithium-sulfur pouch cells with ISDN additives provide a specific energy of 319 Wh kg-1 and undergo 20 cycles. This work provides a realistic reference in designing additives to modify the SEI for stabilizing LMBs.

Semi-Immobilized Molecular Electrocatalysts for High-Performance Lithium-Sulfur Batteries.

Lithium-sulfur (Li-S) batteries constitute promising next-generation energy storage devices due to the ultrahigh theoretical energy density of 2600 Wh kg-1. However, the multiphase sulfur redox reactions with sophisticated homogeneous and heterogeneous electrochemical processes are sluggish in kinetics, thus requiring targeted and high-efficient electrocatalysts. Herein, a semi-immobilized molecular electrocatalyst is designed to tailor the characters of the sulfur redox reactions in working Li-S batteries. Specifically, porphyrin active sites are covalently grafted onto conductive and flexible polypyrrole linkers on graphene current collectors. The electrocatalyst with the semi-immobilized active sites exhibits homogeneous and heterogeneous functions simultaneously, performing enhanced redox kinetics and a regulated phase transition mode. The efficiency of the semi-immobilizing strategy is further verified in practical Li-S batteries that realize superior rate performances and long lifespan as well as a 343 Wh kg-1 high-energy-density Li-S pouch cell. This contribution not only proposes an efficient semi-immobilizing electrocatalyst design strategy to promote the Li-S battery performances but also inspires electrocatalyst development facing analogous multiphase electrochemical energy processes.

New Insights into the Mechanism of LiDFBOP for Improving the Low-Temperature Performance via the Rational Design of an Interphase on a Graphite Anode.

The high impedance of the solid electrolyte interphase (SEI) is one of the important factors that deteriorate the charge behavior of lithium-ion batteries (LIBs) at low temperatures, which hinders their practical application in portable electronic products and electric vehicles under extreme conditions. Based on this consideration, a LiF-rich SEI film with low impedance, using lithium difluorobis(oxalato)phosphate (LiDFBOP) as an electrolyte additive and a blank electrolyte without commercial additives, is constructed on a graphite surface. The decomposition mechanism of LiDFBOP is further deduced by density functional theory calculations. This additive inhibits the decomposition of the electrolyte and then forms a thin SEI film with more LiF. LiF, possessing high Young's modulus, makes the SEI film dense and stable. At the same time, more LiF/Li2CO3 interfaces are formed to increase the ionic conductivity. Benefiting from the components and the structure of the SEI, the graphite/Li cells exhibit excellent cycling stability (ca. 85.5% initial capacity retention for 200 cycles at 1 C) and an impressive low-temperature performance (ca. 200% capacity for electrolytes without LiDFBOP at -20 °C). This work presents an effective strategy for developing a functional electrolyte to meet the requirement of LIBs with enhanced low-temperature performance.

Removal of azo dye from aqueous solution by a low-cost activated carbon prepared from coal: adsorption kinetics, isotherms study, and DFT simulation.

The high-risk organic pollutants produced by industries are of growing concern. The highly porous coal-based activated carbon (AC) having a specific surface area of 3452.8 m2/g is used for the adsorption of azo dye from synthetic solution. The sorbent is characterized through BET, SEM, TEM, XRD, FT-IR, TGA, and zeta potential. The sorbent exhibits - 18.7 mV surface charge, which is high enough for making suspension. The maximum dye uptake of 333 mg/g is observed in sorbent under acidic medium. The thermodynamics parameters like ∆G, ∆H, and ΔS were found to be - 12.40 kJ mol-1, 39.66 kJ mol-1, and 174.55 J mol-1 K-1 at 293 K, respectively, revealing that the adsorption mechanism is spontaneous, endothermic, and feasible. The experimental data follows the Langmuir and D-R models. The adsorption follows pseudo 2nd-order kinetics. DFT investigation shows that the dye sorption onto AC in configuration No. 4 (CFG-4) is more effective, as this configuration has high ∆H (enthalpy change) and adsorption energy (Eads). This is confirmed by Mullikan atomic charge transfer phenomenon.

Critical Role of Surface Defects in the Controllable Deposition of Li2S on Graphene: From Molecule to Crystallite.

Uncontrollable electrochemical deposition of Li2S has negative impacts on the electrochemical performance of lithium-sulfur batteries, but the relationship between the deposition and the surface defects is rarely reported. Herein, ab initio molecular dynamics (AIMD) and density functional theory (DFT) approaches are used to study the Li2S deposition behaviors on pristine and defected graphene substrates, including pyridinic N (PDN) doped and single vacancy (SV), as well as the interfacial characteristics, in that such defects could improve the polarity of the graphene material, which plays a vital role in the cathode. The result shows that due to the constraint of molecular vibration, Li2S molecules tend to form stable adsorption with PDN atoms and SV defects, followed by the nucleation of Li2S clusters on these sites. Moreover, the clusters are more likely to grow near these sites following a spherical pattern, while a lamellar pattern is favorable on pristine graphene substrates. It is also discovered that PDN atoms and SV defects provide atomic-level pathways for the electronic transfer within the Li2S-electrode interface, further improving the electrochemical performance of the Li-S battery. It is found for the first time that surface defects also have strong impacts on the deposition pattern of Li2S and provide electronic pathways simultaneously. Our work demonstrated the interior relationship between the surface defects in carbon substrates and the stability of Li2S precipitates, which is of high significance to understand the electrochemical kinetics and design Li-S battery with long cycle life.

Constructing Ni12P5/Ni2P Heterostructures to Boost Interfacial Polarization for Enhanced Microwave Absorption Performance.

Heterostructures with a rich phase boundary are attractive for surface-mediated microwave absorption (MA) materials. However, understanding the MA mechanisms behind the heterogeneous interface remains a challenge. Herein, a phosphine (PH3) vapor-assisted phase and structure engineering strategy was proposed to construct three-dimensional (3D) porous Ni12P5/Ni2P heterostructures as microwave absorbers and explore the role of the heterointerface in MA performance. The results indicated that the heterogeneous interface between Ni12P5 and Ni2P not only creates sufficient lattice defects for inducing dipolar polarization but also triggers uneven spatial charge distribution for enhancing interface polarization. Furthermore, the porous structure and proper component could provide an abundant heterogeneous interface to strengthen the above polarization relaxation process, thereby greatly optimizing the electromagnetic parameters and improving the MA performance. Profited by 3D porous heterostructure design, P400 could achieve the maximum reflection loss of -50.06 dB and an absorption bandwidth of 3.30 GHz with an ultrathin thickness of 1.20 mm. Furthermore, simulation results confirmed its superior ability (14.97 dB m2 at 90°) to reduce the radar cross section in practical applications. This finding may shed light on the understanding and design of advanced heterogeneous MA materials.

CoP/RGO-Pd Hybrids with Heterointerfaces as Highly Active Catalysts for Ethanol Electrooxidation.

The ethanol oxidation reaction is of critical importance to the commercial viability of direct ethanol fuel cell technology. However, owing to the poor C-C bond cleavage capability, almost all ethanol oxidation is incomplete and suffers from low selectivity toward the C1 pathway. Herein, under the support of theoretical calculations that the heterointerfaces between CoP and Pd can reduce the energy barrier of C-C bond cleavage, rich heterointerfaces in CoP/RGO-Pd hybrids were designed to improve ethanol electrooxidation performance through enhancing the selectivity toward the C1 pathway. The experimental results show that the faradaic efficiency of the C1 pathway of CoP/RGO-Pd hybrids is as high as 27.6%, surpassing most reported catalysts in the literature. As a result of this enhancement, CoP/RGO-Pd10 exhibits mass activity as high as 4597 mA·mgPd-1 and specific activity as high as 10 mA·cm-2, which are much higher than those of other Pd-based electrocatalysts.

Activated Carbon Based Supercapacitors with a Reduced Graphene Oxide Additive: Preparation and Properties.

We have successfully enhanced the performance of commercial supercapacitors that use Japan Kuraray 80F activated carbon and Super-P conductive carbon black as the conductive agent with reduced graphene oxide (rGO) additive. The ratios of conductive carbon black to rGO studied are 3:1, 5:1, 10:1, 15:1 and 1:0. The enhancement is most pronounced at 15:1, and the specific capacitance being 137.5 F g-1, which is a 23.8% improvement over the 1:0 control. The specific capacitance retention is 70.1% after 10000 cycles. The impedance resistance is also reduced to 1.5 Ω, which is 3.3 times lower than the 1:0 control. Additionally, the rGO additive does not alter the favorable pore size distribution of the primary matrix and successfully preserves its small mesoporous structure, which facilitates facile transport of electrolyte.

Defect formation-induced tunable evolution of oxygen functional groups for sodium storage in porous graphene.

The coinciding effects of carbon defects and oxygen functional groups in porous graphene were demonstrated in this work. The species and distributions of oxygen functional groups evolved with the types of defects, especially those containing C[double bond, length as m-dash]O bonds mainly distributed along the edge of ring defects, and enhanced Na+ storage.

A novel hafnium-graphite oxide catalyst for the Meerwein-Ponndorf-Verley reaction and the activation effect of the solvent.

Construction and application of novel hydrogenation catalysts is important for the conversion of carbonyl or aldehyde compounds into alcohols in the field of biomass utilization. In this work, a novel, efficient, and easily prepared hafnium-graphite oxide (Hf-GO) catalyst was constructed via the coordination between Hf4+ and the carboxylic groups in GO. The catalyst was applied into the hydrogenation of biomass derived carbonyl compounds via the Meerwein-Ponndorf-Verley (MPV) reaction. The catalyst gave high efficiency under mild conditions. An interesting phenomenon was found whereby the activity of the catalyst increased gradually in the initial stage during reaction. The solvent, isopropanol, was proved to have an activation effect on the catalyst, and the activation effect varied with different alcohols and temperatures. Further characterizations showed that isopropanol played the activation effect via replacing the residual solvent (DMF) in micro- and mesopores during the preparation process, which was hard to be completely removed by common drying process.

Structural Evolution of Phosphorus Species on Graphene with a Stabilized Electrochemical Interface.

Phosphorus doping is an effective approach to tailor the surface chemistry of carbon materials. In this work, two-dimensional graphene, as a simplified model for all sp2 hybrid carbon allotropes, is employed to explore the surface chemistry of P-doped carbon materials. Thermally reduced graphene oxide, with abundant residual oxygen functionalities, is doped by phosphorus heteroatoms through H3PO4 activation, followed by passivation in an inert atmosphere. The structural evolution of the phosphorus species in the carbon lattice during the thermal treatment is systematically studied by Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, X-ray diffraction, and Raman spectroscopy with the assistance of first-principles calculations. The C3-P═O configuration is identified as the most stable structure in the graphene lattice and plays a key role in stabilizing the electrochemical interface between the electrode and electrolyte. These features enable an electrode based on P-doped graphene to exhibit an enlarged potential window of 1.5 V in an aqueous electrolyte, a remarkable improved cycling stability, and an ultralow leak current. Therefore, this contribution provides insights for designing phosphorus-doped carbon materials toward electrocatalysis, energy-related applications, and so forth.

Air cathode of zinc-air batteries: a highly efficient and durable aerogel catalyst for oxygen reduction.

In this communication, N-doped carbon shell encapsulated Co3O4@Co nanoparticles (NPs) were assembled on N-doped reduced graphene oxide to form non-precious metal aerogels (Co3O4@Co/N-r-GO) for oxygen reduction. In an alkaline medium, the aerogels exhibit comparable oxygen reduction reaction (ORR) performances to the commercial Pt/C with respect to half-wave potential, current density and durability. Zn-air batteries (ZABs) assembled with Co3O4@Co/N-r-GO as the air cathode display high average voltages of ∼1.38 and ∼1.3 V at a current density of 1 and 5 mA cm-2, respectively, and show a specific capacity of ∼800 mA h g-1. These results demonstrate that Co3O4@Co/N-r-GO materials are highly active and durable ORR catalysts for ZABs. The unique structure of the composite aerogel, i.e. the carbon shell encapsulated Co3O4@Co NPs and the 3D interconnected macroporous N-r-GO matrix, is responsible for its high ORR activity and stability.

Electronic Structure Tuning in Ni3FeN/r-GO Aerogel toward Bifunctional Electrocatalyst for Overall Water Splitting.

Searching for the highly active, stable, and high-efficiency bifunctional electrocatalysts for overall water splitting, e.g., for both oxygen evolution (OER) and hydrogen evolution (HER), is paramount in terms of bringing future renewable energy systems and energy conversion processes to reality. Herein, three-dimensional (3D) Ni3FeN nanoparticles/reduced graphene oxide (r-GO) aerogel electrocatalysts were fabricated using precursors of (Ni,Fe)/r-GO alginate hydrogels through an ion-exchange process, followed by a convenient one-step nitrogenization treatment in NH3 at 700 °C. The resultant materials exhibited excellent electrocatalytic performance for OER and HER in alkaline media, with only small overpotentials of 270 and 94 mV at a current density of 10 mA cm-2, respectively. The good performance was attributed to abundant active sites and high electrical conductivity of the bimetallic nitrides and efficient mass transport of the 3D r-GO aerogel framework. Furthermore, an alkaline electrolyzer was set up using Ni3FeN/r-GO as both the cathode and the anode, which achieved a 10 mA cm-2 current density at 1.60 V with durability of 100 h for overall water splitting. Density functional theory calculations support that Ni3FeN (111)/r-GO is more favorable for overall water splitting since the surface electronic structure of Ni3FeN is tuned by transferring electrons from Ni3FeN cluster to the r-GO through interaction of two metal species. Thus, the currently developed Ni3FeN/r-GO with superior water-splitting performance may potentially serve as a material for use in industrial alkaline water electrolyzers.