Showing papers on "Overpotential published in 2020"

A review on fundamentals for designing oxygen evolution electrocatalysts

Abstract: Electricity-driven water splitting can facilitate the storage of electrical energy in the form of hydrogen gas. As a half-reaction of electricity-driven water splitting, the oxygen evolution reaction (OER) is the major bottleneck due to the sluggish kinetics of this four-electron transfer reaction. Developing low-cost and robust OER catalysts is critical to solving this efficiency problem in water splitting. The catalyst design has to be built based on the fundamental understanding of the OER mechanism and the origin of the reaction overpotential. In this article, we summarize the recent progress in understanding OER mechanisms, which include the conventional adsorbate evolution mechanism (AEM) and lattice-oxygen-mediated mechanism (LOM) from both theoretical and experimental aspects. We start with the discussion on the AEM and its linked scaling relations among various reaction intermediates. The strategies to reduce overpotential based on the AEM and its derived descriptors are then introduced. To further reduce the OER overpotential, it is necessary to break the scaling relation of HOO* and HO* intermediates in conventional AEM to go beyond the activity limitation of the volcano relationship. Strategies such as stabilization of HOO*, proton acceptor functionality, and switching the OER pathway to LOM are discussed. The remaining questions on the OER and related perspectives are also presented at the end.

Interfacial Engineering of MoO 2 -FeP Heterojunction for Highly Efficient Hydrogen Evolution Coupled with Biomass Electrooxidation

Abstract: Simultaneous highly efficient production of hydrogen and conversion of biomass into value-added products is meaningful but challenging. Herein, a porous nanospindle composed of carbon-encapsulated MoO2 -FeP heterojunction (MoO2 -FeP@C) is proposed as a robust bifunctional electrocatalyst for hydrogen evolution reaction (HER) and biomass electrooxidation reaction (BEOR). X-ray photoelectron spectroscopy analysis and theoretical calculations confirm the electron transfer from MoO2 to FeP at the interfaces, where electron accumulation on FeP favors the optimization of H2 O and H* absorption energies for HER, whereas hole accumulation on MoO2 is responsible for improving the BEOR activity. Thanks to its interfacial electronic structure, MoO2 -FeP@C exhibits excellent HER activity with an overpotential of 103 mV at 10 mA cm-2 and a Tafel slope of 48 mV dec-1 . Meanwhile, when 5-hydroxymethylfurfural is chosen as the biomass for BEOR, the conversion is almost 100%, and 2,5-furandicarboxylic acid (FDCA) is obtained with the selectivity of 98.6%. The electrolyzer employing MoO2 -FeP@C for cathodic H2 and anodic FDCA production requires only a low voltage of 1.486 V at 10 mA cm-2 and can be powered by a solar cell (output voltage: 1.45 V). Additionally, other BEORs coupled with HER catalyzed by MoO2 -FeP@C also have excellent catalytic performance, implying their good versatility.

Reversible planar gliding and microcracking in a single-crystalline Ni-rich cathode.

Abstract: High-energy nickel (Ni)-rich cathode will play a key role in advanced lithium (Li)-ion batteries, but it suffers from moisture sensitivity, side reactions, and gas generation. Single-crystalline Ni-rich cathode has a great potential to address the challenges present in its polycrystalline counterpart by reducing phase boundaries and materials surfaces. However, synthesis of high-performance single-crystalline Ni-rich cathode is very challenging, notwithstanding a fundamental linkage between overpotential, microstructure, and electrochemical behaviors in single-crystalline Ni-rich cathodes. We observe reversible planar gliding and microcracking along the (003) plane in a single-crystalline Ni-rich cathode. The reversible formation of microstructure defects is correlated with the localized stresses induced by a concentration gradient of Li atoms in the lattice, providing clues to mitigate particle fracture from synthesis modifications.

Interfacial Engineering of W 2 N/WC Heterostructures Derived from Solid-State Synthesis: A Highly Efficient Trifunctional Electrocatalyst for ORR, OER, and HER.

Abstract: To meet the practical demand of overall water splitting and regenerative metal-air batteries, highly efficient, low-cost, and durable electrocatalysts for the oxygen reduction reaction (ORR), oxygen evolution reaction (OER), and hydrogen evolution reaction (HER) are required to displace noble metal catalysts. In this work, a facile solid-state synthesis strategy is developed to construct the interfacial engineering of W2 N/WC heterostructures, in which abundant interfaces are formed. Under high temperature (800 °C), volatile CNx species from dicyanodiamide are trapped by WO3 nanorods, followed by simultaneous nitridation and carbonization, to form W2 N/WC heterostructure catalysts. The resultant W2 N/WC heterostructure catalysts exhibit an efficient and stable electrocatalytic performance toward the ORR, OER, and HER, including a half-wave potential of 0.81 V (ORR) and a low overpotential at 10 mA cm-2 for the OER (320 mV) and HER (148.5 mV). Furthermore, a W2 N/WC-based Zn-air battery shows outstanding high power density (172 mW cm-2 ). Density functional theory and X-ray absorption fine structure analysis computations reveal that W2 N/WC interfaces synergistically facilitate transport and separation of charge, thus accelerating the electrochemical ORR, OER, and HER. This work paves a novel avenue for constructing efficient and low-cost electrocatalysts for electrochemical energy devices.

Ultrahigh-Loading of Ir Single Atoms on NiO Matrix to Dramatically Enhance Oxygen Evolution Reaction.

Abstract: Engineering single-atom electrocatalysts with high-loading amount holds great promise in energy conversion and storage application. Herein, we report a facile and economical approach to achieve an unprecedented high loading of single Ir atoms, up to ∼18wt%, on the nickel oxide (NiO) matrix as the electrocatalyst for oxygen evolution reaction (OER). It exhibits an overpotential of 215 mV at 10 mA cm-2 and a remarkable OER current density in alkaline electrolyte, surpassing NiO and IrO2 by 57 times and 46 times at 1.49 V vs RHE, respectively. Systematic characterizations, including X-ray absorption spectroscopy and aberration-corrected Z-contrast imaging, demonstrate that the Ir atoms are atomically dispersed at the outermost surface of NiO and are stabilized by covalent Ir-O bonding, which induces the isolated Ir atoms to form a favorable ∼4+ oxidation state. Density functional theory calculations reveal that the substituted single Ir atom not only serves as the active site for OER but also activates the surface reactivity of NiO, which thus leads to the dramatically improved OER performance. This synthesis method of developing high-loading single-atom catalysts can be extended to other single-atom catalysts and paves the way for industrial applications of single-atom catalysts.

Uncovering near-free platinum single-atom dynamics during electrochemical hydrogen evolution reaction.

Abstract: Single-atom catalysts offering intriguing activity and selectivity are subject of intense investigation. Understanding the nature of single-atom active site and its dynamics under working state are crucial to improving their catalytic performances. Here, we identify at atomic level a general evolution of single atom into a near-free state under electrocatalytic hydrogen evolution condition, via operando synchrotron X-ray absorption spectroscopy. We uncover that the single Pt atom tends to dynamically release from the nitrogen-carbon substrate, with the geometric structure less coordinated to support and electronic property closer to zero valence, during the reaction. Theoretical simulations support that the Pt sites with weakened Pt–support interaction and more 5d density are the real active centers. The single-atom Pt catalyst exhibits very high hydrogen evolution activity with only 19 mV overpotential in 0.5 M H2SO4 and 46 mV in 1.0 M NaOH at 10 mA cm−2, and long-term durability in wide-pH electrolytes. Understanding the structural dynamics of single-atom site in electrochemical reactions is crucial for design of an efficient catalyst. Here, the authors develop highly active Pt single-atom electrocatalyst, and reveal the dynamic evolution of active sites by using operando synchrotron spectroscopy.

Metal Atom-Doped Co 3 O 4 Hierarchical Nanoplates for Electrocatalytic Oxygen Evolution

Abstract: Electrocatalysts based on hierarchically structured and heteroatom-doped non-noble metal oxide materials are of great importance for efficient and low-cost electrochemical water splitting systems Herein, the synthesis of a series of hierarchical hollow nanoplates (NPs) composed of ultrathin Co3 O4 nanosheets doped with 13 different metal atoms is reported The synthesis involves a cooperative etching-coordination-reorganization approach starting from zeolitic imidazolate framework-67 (ZIF-67) NPs First, metal atom decorated ZIF-67 NPs with unique cross-channels are formed through a Lewis acid etching and metal species coordination process Afterward, the composite NPs are converted to hollow Co3 O4 hierarchical NPs composed of ultrathin nanosheets through a solvothermal reaction, during which the guest metal species is doped into the octahedral sites of Co3 O4 Density functional theory calculations suggest that doping of small amount of Fe atoms near the surface of Co3 O4 can greatly enhance the electrocatalytic activity toward the oxygen evolution reaction (OER) Benefiting from the structural and compositional advantages, the obtained Fe-doped Co3 O4 hierarchical NPs manifest superior electrocatalytic performance for OER with an overpotential of 262 mV at 10 mA cm-2 , a Tafel slope of 43 mV dec-1 , and excellent stability even at a high current density of 100 mA cm-2 for 50 h

Designed Formation of Double-Shelled Ni–Fe Layered-Double-Hydroxide Nanocages for Efficient Oxygen Evolution Reaction

Abstract: Delicate design of nanostructures for oxygen-evolution electrocatalysts is an important strategy for accelerating the reaction kinetics of water splitting. In this work, Ni-Fe layered-double-hydroxide (LDH) nanocages with tunable shells are synthesized via a facile one-pot self-templating method. The number of shells can be precisely controlled by regulating the template etching at the interface. Benefiting from the double-shelled structure with large electroactive surface area and optimized chemical composition, the hierarchical Ni-Fe LDH nanocages exhibit appealing electrocatalytic activity for the oxygen evolution reaction in alkaline electrolyte. Particularly, double-shelled Ni-Fe LDH nanocages can achieve a current density of 20 mA cm-2 at a low overpotential of 246 mV with excellent stability.

Li metal deposition and stripping in a solid-state battery via Coble creep.

Abstract: Solid-state lithium metal batteries require accommodation of electrochemically generated mechanical stress inside the lithium: this stress can be1,2 up to 1 gigapascal for an overpotential of 135 millivolts. Maintaining the mechanical and electrochemical stability of the solid structure despite physical contact with moving corrosive lithium metal is a demanding requirement. Using in situ transmission electron microscopy, we investigated the deposition and stripping of metallic lithium or sodium held within a large number of parallel hollow tubules made of a mixed ionic-electronic conductor (MIEC). Here we show that these alkali metals—as single crystals—can grow out of and retract inside the tubules via mainly diffusional Coble creep along the MIEC/metal phase boundary. Unlike solid electrolytes, many MIECs are electrochemically stable in contact with lithium (that is, there is a direct tie-line to metallic lithium on the equilibrium phase diagram), so this Coble creep mechanism can effectively relieve stress, maintain electronic and ionic contacts, eliminate solid-electrolyte interphase debris, and allow the reversible deposition/stripping of lithium across a distance of 10 micrometres for 100 cycles. A centimetre-wide full cell—consisting of approximately 1010 MIEC cylinders/solid electrolyte/LiFePO4—shows a high capacity of about 164 milliampere hours per gram of LiFePO4, and almost no degradation for over 50 cycles, starting with a 1× excess of Li. Modelling shows that the design is insensitive to MIEC material choice with channels about 100 nanometres wide and 10–100 micrometres deep. The behaviour of lithium metal within the MIEC channels suggests that the chemical and mechanical stability issues with the metal–electrolyte interface in solid-state lithium metal batteries can be overcome using this architecture. By containing lithium metal within oriented tubes of a mixed ionic-electronic conductor, a 3D anode for lithium metal batteries is produced that overcomes chemomechanical stability issues at the electrolyte interface.

Lattice oxygen activation enabled by high-valence metal sites for enhanced water oxidation

Abstract: Anodic oxygen evolution reaction (OER) is recognized as kinetic bottleneck in water electrolysis. Transition metal sites with high valence states can accelerate the reaction kinetics to offer highly intrinsic activity, but suffer from thermodynamic formation barrier. Here, we show subtle engineering of highly oxidized Ni4+ species in surface reconstructed (oxy)hydroxides on multicomponent FeCoCrNi alloy film through interatomically electronic interplay. Our spectroscopic investigations with theoretical studies uncover that Fe component enables the formation of Ni4+ species, which is energetically favored by the multistep evolution of Ni2+→Ni3+→Ni4+. The dynamically constructed Ni4+ species drives holes into oxygen ligands to facilitate intramolecular oxygen coupling, triggering lattice oxygen activation to form Fe-Ni dual-sites as ultimate catalytic center with highly intrinsic activity. As a result, the surface reconstructed FeCoCrNi OER catalyst delivers outstanding mass activity and turnover frequency of 3601 A gmetal−1 and 0.483 s−1 at an overpotential of 300 mV in alkaline electrolyte, respectively. Electrocatalytic water oxidation is facilitated by high valence states, but these are challenging to achieve at low applied potentials. Here, authors report a multicomponent FeCoCrNi alloy with dynamically formed Ni4+ species to offer high catalytic activity via lattice oxygen activation mechanism.

Iridium Single Atoms Coupling with Oxygen Vacancies Boosts Oxygen Evolution Reaction in Acid Media

Abstract: Simultaneous realization of improved activity, enhanced stability, and reduced cost remains a desirable yet challenging goal in the search of electrocatalysis oxygen evolution reaction (OER) in acid. Herein, we report a novel strategy to prepare iridium single-atoms (Ir-SAs) on ultrathin NiCo2O4 porous nanosheets (Ir-NiCo2O4 NSs) by the co-electrodeposition method. The surface-exposed Ir-SAs couplings with oxygen vacancies (VO) exhibit boosting the catalysts OER activity and stability in acid media. They display superior OER performance with an ultralow overpotential of 240 mV at j = 10 mA cm-2 and long-term stability of 70 h in acid media. The TOFs of 1.13 and 6.70 s-1 at an overpotential of 300 and 370 mV also confirm their remarkable performance. Density functional theory (DFT) calculations reveal that the prominent OER performance arises from the surface electronic exchange-and-transfer activities contributed by atomic Ir incorporation on the intrinsic VO existed NiCo2O4 surface. The atomic Ir sites substantially elevate the electronic activity of surface lower coordinated Co sites nearby VO, which facilitate the surface electronic exchange-and-transfer capabilities. With this trend, the preferred H2O activation and stabilized *O have been reached toward competitively lower overpotential. This is a generalized key for optimally boosting OER performance.

Achieving Efficient Alkaline Hydrogen Evolution Reaction over a Ni 5 P 4 Catalyst Incorporating Single-Atomic Ru Sites

Abstract: Developing efficient electrocatalysts for alkaline water electrolysis is central to substantial progress of alkaline hydrogen production. Herein, a Ni5 P4 electrocatalyst incorporating single-atom Ru (Ni5 P4 -Ru) is synthesized through the filling of Ru3+ species into the metal vacancies of nickel hydroxides and subsequent phosphorization treatment. Electron paramagnetic resonance spectroscopy, X-ray-based measurements, and electron microscopy observations confirm the strong interaction between the nickel-vacancy defect and Ru cation, resulting in more than 3.83 wt% single-atom Ru incorporation in the obtained Ni5 P4 -Ru. The Ni5 P4 -Ru as an alkaline hydrogen evolution reaction catalyst achieves low onset potential of 17 mV and an overpotential of 54 mV at a current density of 10 mA cm-2 together with a small Tafel slope of 52.0 mV decade-1 and long-term stability. Further spectroscopy analyses combined with density functional theory calculations reveal that the doped Ru sites can cause localized structure polarization, which brings the low energy barrier for water dissociation on Ru site and the optimized hydrogen adsorption free energy on the interstitial site, well rationalizing the experimental reactivity.

Rational Catalyst and Electrolyte Design for Co2 Electroreduction Towards Multicarbon Products

Abstract: CO2 electroreduction reaction (CO2RR) to fuels and feedstocks is an attractive route to close the anthropogenic carbon cycle and store renewable energy. The generation of more reduced chemicals, especially multicarbon oxygenate and hydrocarbon products (C2+) with higher energy density is highly desirable for industrial applications. However, selective conversion of CO2 to C2+ suffers from high overpotential, low reaction rate and low selectivity, and the process is extremely sensitive to the catalyst structure and electrolyte. Here we discuss strategies to achieve high C2+ selectivity through rational design of the catalyst and electrolyte. Current state-of-the-art catalysts, including Cu and Cu-bimetallic catalysts as well as alternative materials are considered. The importance of taking into consideration the dynamic evolution of the catalyst structure and composition are highlighted, focusing on findings extracted from in situ and operando characterizations. Additional theoretical insight into the reaction mechanisms underlying the improved C2+ selectivity of specific catalyst geometries/compositions in synergy with a well-chosen electrolyte are also provided.

Highly stable Zn metal anodes enabled by atomic layer deposited Al 2 O 3 coating for aqueous zinc-ion batteries

Abstract: Rechargeable aqueous zinc-ion batteries (ZIBs) have attracted increasing attention as an energy storage technology for large-scale applications, due to their high capacity (820 mA h g−1 and 5854 A h L−1), inherently high safety, and their low cost. However, the overall performance of ZIBs has been seriously hindered by the poor rechargeability of Zn anodes, because of the dendrite growth, passivation, and hydrogen evolution problems associated with Zn anodes. Herein, Al2O3 coating by an atomic layer deposition (ALD) technique was developed to address the aforementioned problems and improve the rechargeability of Zn anodes for ZIBs. By coating the Zn plate with an ultrathin Al2O3 layer, the wettability of Zn was improved and corrosion was inhibited. As a result, the formation of Zn dendrites was effectively suppressed, with a significantly improved lifetime in the Zn–Zn symmetric cells. With the optimized coating thickness of 100 cycles, 100Al2O3@Zn symmetric cells showed a reduced overpotential (36.5 mV) and a prolonged life span (over 500 h) at 1 mA cm−2. In addition, the 100Al2O3@Zn has been verified in Zn–MnO2 batteries using layered δ-MnO2 as the cathode and consequently exhibits superior electrochemical performance with a high capacity retention of 89.4% after over 1000 cycles at a current density of 1 mA cm−2 (3.33C for MnO2) was demonstrated. It is expected that the novel design of Al2O3 modified Zn anodes may pave the way towards high-performance aqueous ZIBs and shed light on the development of other metal anode-based battery systems.

High-purity pyrrole-type FeN4 sites as a superior oxygen reduction electrocatalyst

Abstract: Atomically dispersed iron–nitrogen (FeN4) catalysts have emerged as the most promising alternative to costly Pt-based counterparts in proton exchange membrane fuel cells (PEMFCs), but often they suffer from high overpotential and poor stability due to the diverse iron–nitrogen coordination structure. Herein, we demonstrate high-purity pyrrole-type FeN4 sites for the first time, as a superior ORR electrocatalyst for PEMFCs. The high-purity pyrrole-type FeN4 catalyst exhibited extremely outstanding ORR activity with an ultra-high active area current density of 6.89 mA m−2 in acid medium, which exceeds that of most reported metal–nitrogen coordination catalysts. Experimental and theoretical analyses reveal that high-purity pyrrole-type coordination significantly modifies the atomic and electronic structures of FeN4 sites, bringing with it high intrinsic catalytic activity, preferable O2 adsorption energy and full four-electron reaction selectivity for ORR catalysis. Therefore, PEMFCs built with this high-purity FeN4 catalyst achieve a high open-circuit voltage (1.01 V) and a large peak power density (over 700 mW cm−2). High-purity iron–nitrogen coordination would give new insights into highly efficient electrocatalysts for PEMFCs.

Bismuthene for highly efficient carbon dioxide electroreduction reaction

Abstract: Bismuth (Bi) has been known as a highly efficient electrocatalyst for CO2 reduction reaction. Stable free-standing two-dimensional Bi monolayer (Bismuthene) structures have been predicted theoretically, but never realized experimentally. Here, we show the first simple large-scale synthesis of free-standing Bismuthene, to our knowledge, and demonstrate its high electrocatalytic efficiency for formate (HCOO−) formation from CO2 reduction reaction. The catalytic performance is evident by the high Faradaic efficiency (99% at −580 mV vs. Reversible Hydrogen Electrode (RHE)), small onset overpotential (<90 mV) and high durability (no performance decay after 75 h and annealing at 400 °C). Density functional theory calculations show the structure-sensitivity of the CO2 reduction reaction over Bismuthene and thicker nanosheets, suggesting that selective formation of HCOO− indeed can proceed easily on Bismuthene (111) facet due to the unique compressive strain. This work paves the way for the extensive experimental investigation of Bismuthene in many different fields. Stable free-standing two-dimensional Bi monolayer (Bismuthene) structures have been predicted theoretically, but never realized experimentally. Here, the authors show a large-scale synthesis of free-standing Bismuthene and its electrocatalytic activity for CO2 reduction to formate.

Nitrogen-Doped carbon coupled FeNi3 intermetallic compound as advanced bifunctional electrocatalyst for OER, ORR and zn-air batteries

Abstract: Developing OER&ORR bifunctional electrocatalysts has great significance for promoting the green energy conversion and storage technologies. Herein, nitrogen doped carbon coupled FeNi3 intermetallic compound (FeNi3@NC) is designed and constructed by a super facile route. Rich defects, multiple active centers and strong synergistic effect allow FeNi3@NC with superior OER&ORR performance in alkaline environments. It exhibits an ultralow overpotential (Eover) of 277 mV at 10 mA/cm2 for OER, and a high half-wave potential (E1/2) of 0.86 V for ORR which outperform commercial Pt/C electrocatalysts. As a result, the ΔE (ΔE = Eover,OER – E1/2,ORR) value of 0.65 V largely surpasses Pt/C-IrO2 noble metal benchmarks (ΔE = 0.73 V). Furthermore, density function theory calculations are performed to probe its electrocatalytic mechanism. As expected, the peak power density and energy density of FeNi3@NC based rechargeable Zn-air battery reaches 139 mW/cm2 at 234 mA/cm2 and 915 Wh kg−1 at 10 mA/cm2, respectively, with excellent stability. Our exploration provides a guideline for synthesis of advanced bifunctional electrocatalyst.

Dendrite-Free Zinc Deposition Induced by Tin-Modified Multifunctional 3D Host for Stable Zinc-Based Flow Battery

Abstract: Zinc (Zn) plating/stripping is a promising anodic electrochemical reaction for aqueous batteries because of its high safety, low cost, two electron transfer, and rapid reaction kinetics. However, the notorious dendrite growth of Zn has become one of the biggest obstacles hindering its further commercialization. A multifunctional binder-free tin (Sn)-modified 3D carbon felt anodic host (SH) is constructed for aqueous zinc-based flow batteries (ZFB) via a facile and scalable strategy. Compared with the pristine carbon felt host (PH), the as-fabricated SH affords more robust Zn nucleation sites, lower hydrogen evolution reaction potential and lower nucleation overpotential of Zn and thus better induces uniform Zn plating/stripping with very high Coulombic efficiency (CE). Based on such an SH, a symmetrical flow battery exhibits superior CE (290 cycles with average CE of 99.4%) and a zinc-bromine flow battery demonstrates a longer cycle life (142 cycles with average CE of 97.2%), much better than pristine PH. This is a simple, novel, and effective way to suppress Zn dendrites and improve the performance of ZFBs.

Identifying Dense NiSe2 /CoSe2 Heterointerfaces Coupled with Surface High-Valence Bimetallic Sites for Synergistically Enhanced Oxygen Electrocatalysis.

Abstract: Constructing heterostructures with abundant interfaces is essential for integrating the multiple functionalities in single entities. Herein, the synthesis of NiSe2 /CoSe2 heterostructures with different interfacial densities via an innovative strategy of successive ion injection is reported. The resulting hybrid electrocatalyst with dense heterointerfaces exhibits superior electrocatalytic properties in an alkaline electrolyte, superior to other benchmarks and precious metal catalysts. Advanced synchrotron techniques, post structural characterizations, and density functional theory (DFT) simulations reveal that the introduction of atomic-level interfaces can lower the oxidation overpotential of bimetallic Ni and Co active sites (whereas Ni2+ can be more easily activated than Co2+ ) and induce the electronic interaction between the core selenides and surface in situ generated oxides/hydroxides, which play a critical role in synergistically reducing energetic barriers and accelerating reaction kinetics for catalyzing the oxygen evolution. Hence, the heterointerface structure facilitates the catalytic performance enhancement via increasing the intrinsic reactivity of metallic atoms and enhancing the synergistic effect between the inner selenides and surface oxidation species. This work not only complements the understanding on the origins of the activity of electrocatalysts based on metal selenides, but also sheds light on further surface and interfacial engineering of advanced hybrid materials.

Fast site-to-site electron transfer of high-entropy alloy nanocatalyst driving redox electrocatalysis

Abstract: Designing electrocatalysts with high-performance for both reduction and oxidation reactions faces severe challenges. Here, the uniform and ultrasmall (~3.4 nm) high-entropy alloys (HEAs) Pt18Ni26Fe15Co14Cu27 nanoparticles are synthesized by a simple low-temperature oil phase strategy at atmospheric pressure. The Pt18Ni26Fe15Co14Cu27/C catalyst exhibits excellent electrocatalytic performance for hydrogen evolution reaction (HER) and methanol oxidation reaction (MOR). The catalyst shows ultrasmall overpotential of 11 mV at the current density of 10 mA cm−2, excellent activity (10.96 A mg−1Pt at −0.07 V vs. reversible hydrogen electrode) and stability in the alkaline medium. Furthermore, it is also the efficient catalyst (15.04 A mg−1Pt) ever reported for MOR in alkaline solution. Periodic DFT calculations confirm the multi-active sites for both HER and MOR on the HEA surface as the key factor for both proton and intermediate transformation. Meanwhile, the construction of HEA surfaces supplies the fast site-to-site electron transfer for both reduction and oxidation processes. The design of nanostructured catalysts plays a key role in the electrocatalytic redox reaction performances. Here, authors prepared uniform and small-sized high-entropy alloy PtNiFeCoCu nanoparticles that showed improved activities for H2 evolution methanol oxidation reactions.

Engineering Isolated Mn-N2C2 Atomic Interface Sites for Efficient Bifunctional Oxygen Reduction and Evolution Reaction

Abstract: Oxygen-involved electrochemical reactions are crucial for plenty of energy conversion techniques. Herein, we rationally designed a carbon-based Mn-N2C2 bifunctional electrocatalyst. It exhibits a half-wave potential of 0.915 V versus reversible hydrogen electrode for oxygen reduction reaction (ORR), and the overpotential is 350 mV at 10 mA cm-2 during oxygen evolution reaction (OER) in alkaline condition. Furthermore, by means of operando X-ray absorption fine structure measurements, we reveal that the bond-length-extended Mn2+-N2C2 atomic interface sites act as active centers during the ORR process, while the bond-length-shortened high-valence Mn4+-N2C2 moieties serve as the catalytic sites for OER, which is consistent with the density functional theory results. The atomic and electronic synergistic effects for the isolated Mn sites and the carbon support play a critical role to promote the oxygen-involved catalytic performance, by regulating the reaction free energy of intermediate adsorption. Our results give an atomic interface strategy for nonprecious bifunctional single-atom electrocatalysts.

Activating the hydrogen evolution and overall water splitting performance of NiFe LDH by cation doping and plasma reduction

Abstract: NiFe layered double hydroxides (LDHs) have been intensively investigated as promising electrocatalysts for oxygen evolution reaction. However, their hydrogen evolution reaction (HER) and overall water splitting performance are not satisfactory. Herein, we report a facile cation doping combined with plasma reduction strategy to generate heterostructured Ni nanoparticles/V-doped NiFe LDH nanosheet array with multiple vacancies, exhibiting excellent HER performance with a small overpotential of 19 mV at 10 mA cm−2. Moreover, when evaluated as bi-functional electrocatalyst for overall water splitting, a small cell voltage (1.43 V at 10 mA cm−2) and ultralong stability (over 1000 h) are achieved. Density functional theory (DFT) calculations reveal that V-doping, oxygen vacancy (Ov), Ni vacancy (Niv), and Ni nanoparticles can effectively improve the conductivity and optimize the hydrogen adsorption, Ov, Niv, and Ni nanoparticles help to facilitate H2O adsorption and dissociation progress in HER, the V-doping and Ov can efficiently reduce the energy barrier of O* in OER, and Ni/NiFe LDH heterostructure ameliorates the electronic structure and tunes electron transfer route.

In Situ Phosphatizing of Triphenylphosphine Encapsulated within Metal-Organic Frameworks to Design Atomic Co1-P1N3 Interfacial Structure for Promoting Catalytic Performance.

Abstract: The engineering coordination environment offers great opportunity in performance tunability of isolated metal single-atom catalysts. For the most popular metal-Nx (MNx) structure, the replacement of N atoms by some other atoms with relatively weak electronegativity has been regarded as a promising strategy for optimizing the coordination environment of an active metal center and promoting its catalytic performance, which is still a challenge. Herein, we proposed a new synthetic strategy of an in situ phosphatizing of triphenylphosphine encapsulated within metal-organic frameworks for designing atomic Co1-P1N3 interfacial structure, where a cobalt single atom is costabilized by one P atom and three N atoms (denoted as Co-SA/P-in situ). In the acidic media, the Co-SA/P-in situ catalyst with Co1-P1N3 interfacial structure exhibits excellent activity and durability for the hydrogen evolution reaction (HER) with a low overpotential of 98 mV at 10 mA cm-2 and a small Tafel slope of 47 mV dec-1, which are greatly superior to those of catalyst with Co1-N4 interfacial structure. We discover that the bond-length-extended high-valence Co1-P1N3 atomic interface structure plays a crucial role in boosting the HER performance, which is supported by in situ X-ray absorption fine structure (XAFS) measurements and density functional theory (DFT) calculation. We hope this work will promote the development of high performance metal single-atom catalysts.

Ru Single Atoms on N-Doped Carbon by Spatial Confinement and Ionic Substitution Strategies for High-Performance Li–O2 Batteries

Abstract: Nonaqueous rechargeable lithium-oxygen batteries (LOBs) are one of the most promising candidates for future electric vehicles and wearable/flexible electronics. However, their development is severely hindered by the sluggish kinetics of the ORR and OER during the discharge and charge processes. Here, we employ MOF-assisted spatial confinement and ionic substitution strategies to synthesize Ru single atoms riveted with nitrogen-doped porous carbon (Ru SAs-NC) as the electrocatalytic material. By using the optimized Ru0.3 SAs-NC as electrocatalyst in the oxygen-breathing electrodes, the developed LOB can deliver the lowest overpotential of only 0.55 V at 0.02 mA cm-2. Moreover, in-situ DEMS results quantify that the e-/O2 ratio of LOBs in a full cycle is only 2.14, indicating a superior electrocatalytic performance in LOB applications. Theoretical calculations reveal that the Ru-N4 serves as the driving force center, and the amount of this configuration can significantly affect the internal affinity of intermediate species. The rate-limiting step of the ORR on the catalyst surface is the occurrence of 2e- reactions to generate Li2O2, while that of the OER pathway is the oxidation of Li2O2. This work broadens the field of vision for the design of single-site high-efficiency catalysts with maximum atomic utilization efficiency for LOBs.

Confined local oxygen gas promotes electrochemical water oxidation to hydrogen peroxide

Abstract: Electrochemical two-electron water oxidation is a promising route for renewable and on-site H2O2 generation as an alternative to the anthraquinone process. However, it is currently restricted by low selectivity due to strong competition from the traditional four-electron oxygen evolution reaction, as well as large overpotential and low production rates. Here we report an interfacial engineering approach, where by coating the catalyst with hydrophobic polymers we confine in situ produced O2 gas to tune the water oxidation reaction pathway. Using carbon catalysts as a model system, we show a significant increase of the intrinsic H2O-to-H2O2 selectivity and activity compared to that of the pristine catalyst. The maximal H2O2 Faradaic efficiency was enhanced by sixfold to 66% with an overpotential of 640 mV, under which a H2O2 production rate of 23.4 µmol min−1 cm−2 (75.2 mA cm−2 partial current) was achieved. This approach was successfully extended to nickel metal, demonstrating the wide applicability of our local gas confinement concept. Electrochemical 2e− water oxidation is a promising route for renewable H2O2 production but it suffers from low selectivity due to the competing 4e− process. Here the authors demonstrate an interfacial engineering approach where the catalyst is coated with a hydrophobic polymer to confine in situ produced O2 and promote the 2e− pathway.

Mechanical rolling formation of interpenetrated lithium metal/lithium tin alloy foil for ultrahigh-rate battery anode.

Abstract: To achieve good rate capability of lithium metal anodes for high-energy-density batteries, one fundamental challenge is the slow lithium diffusion at the interface. Here we report an interpenetrated, three-dimensional lithium metal/lithium tin alloy nanocomposite foil realized by a simple calendering and folding process of lithium and tin foils, and spontaneous alloying reactions. The strong affinity between the metallic lithium and lithium tin alloy as mixed electronic and ionic conducting networks, and their abundant interfaces enable ultrafast charger diffusion across the entire electrode. We demonstrate that a lithium/lithium tin alloy foil electrode sustains stable lithium stripping/plating under 30 mA cm−2 and 5 mAh cm−2 with a very low overpotential of 20 mV for 200 cycles in a commercial carbonate electrolyte. Cycled under 6 C (6.6 mA cm−2), a 1.0 mAh cm−2 LiNi0.6Co0.2Mn0.2O2 electrode maintains a substantial 74% of its capacity by pairing with such anode. Sluggish lithium diffusion on the surface of Li metal anodes poses a fundamental challenge. Here the authors report a Li/Li22Sn5 alloy design to address this issue. The composite anode sustains stable Li stripping/plating cycling with a low overpotential of 20 mV under 30 mA cm−2 in a commercial carbonate electrolyte.

A Mn-N3 single-atom catalyst embedded in graphitic carbon nitride for efficient CO2 electroreduction.

Abstract: Developing effective catalysts based on earth abundant elements is critical for CO2 electroreduction. However, simultaneously achieving a high Faradaic efficiency (FE) and high current density of CO (jCO) remains a challenge. Herein, we prepare a Mn single-atom catalyst (SAC) with a Mn-N3 site embedded in graphitic carbon nitride. The prepared catalyst exhibits a 98.8% CO FE with a jCO of 14.0 mA cm−2 at a low overpotential of 0.44 V in aqueous electrolyte, outperforming all reported Mn SACs. Moreover, a higher jCO of 29.7 mA cm−2 is obtained in an ionic liquid electrolyte at 0.62 V overpotential. In situ X-ray absorption spectra and density functional theory calculations demonstrate that the remarkable performance of the catalyst is attributed to the Mn-N3 site, which facilitates the formation of the key intermediate COOH* through a lowered free energy barrier. Developing effective catalysts based on earth abundant elements is critical for electrochemical CO2 reduction. Here, the authors prepare a manganese single atom catalyst with high CO2 reduction performance, which can be attributed to the Mn-N3 site embedded in graphitic carbon nitride.

High-Entropy Alloys as Catalysts for the CO2 and CO Reduction Reactions: Experimental Realization

Abstract: Conversion of carbon dioxide into selective hydrocarbon using a stable catalyst remains a holy grail in the catalysis community. The high overpotential, stability, and selectivity in the use of a s...