Showing papers on "Overpotential published in 2019"

Atomically dispersed Fe3+ sites catalyze efficient CO2 electroreduction to CO.

Abstract: Currently, the most active electrocatalysts for the conversion of CO2 to CO are gold-based nanomaterials, whereas non-precious metal catalysts have shown low to modest activity. Here, we report a catalyst of dispersed single-atom iron sites that produces CO at an overpotential as low as 80 millivolts. Partial current density reaches 94 milliamperes per square centimeter at an overpotential of 340 millivolts. Operando x-ray absorption spectroscopy revealed the active sites to be discrete Fe3+ ions, coordinated to pyrrolic nitrogen (N) atoms of the N-doped carbon support, that maintain their +3 oxidation state during electrocatalysis, probably through electronic coupling to the conductive carbon support. Electrochemical data suggest that the Fe3+ sites derive their superior activity from faster CO2 adsorption and weaker CO absorption than that of conventional Fe2+ sites.

Defect-rich and ultrathin N doped carbon nanosheets as advanced trifunctional metal-free electrocatalysts for the ORR, OER and HER

Abstract: Rational design and facile preparation of non-noble trifunctional electrocatalysts with high performance, low cost and strong durability for the oxygen reduction reaction (ORR), oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) are highly demanded, but remain as a big challenge. Herein, we report a spontaneous gas-foaming method to prepare nitrogen doped ultrathin carbon nanosheets (NCNs) by simply pyrolysing a mixture of citric acid and NH4Cl. Under the optimized pyrolysis temperature (carbonized at 1000 °C) and mass ratio of precursors (1 : 1), the synthesized NCN-1000-5 sample possesses an ultrathin sheet structure, an ultrahigh specific surface area (1793 m2 g−1), and rich edge defects, and exhibits low overpotential and robust stability for the ORR, OER and HER. By means of density functional theory (DFT) computations, we revealed that the intrinsic active sites for the ORR, OER and HER are the carbon atoms located at the armchair edge and adjacent to the graphitic N dopants. When practically used as a catalyst in rechargeable Zn–air batteries, a high energy density (806 W h kg−1), a low charge/discharge voltage gap (0.77 V) and an ultralong cycle life (over 330 h) were obtained at 10 mA cm−2 for NCN-1000-5. This work not only presents a versatile strategy to develop advanced carbon materials with ultrahigh specific surface area and abundant edge defects, but also provides useful guidance for designing and developing multifunctional metal-free catalysts for various energy-related electrocatalytic reactions.

Dendrite‐Free Zinc Deposition Induced by Multifunctional CNT Frameworks for Stable Flexible Zn‐Ion Batteries

Abstract: The current boom of safe and renewable energy storage systems is driving the recent renaissance of Zn-ion batteries. However, the notorious tip-induced dendrite growth on the Zn anode restricts their further application. Herein, the first demonstration of constructing a flexible 3D carbon nanotube (CNT) framework as a Zn plating/stripping scaffold is constituted to achieve a dendrite-free robust Zn anode. Compared with the pristine deposited Zn electrode, the as-fabricated Zn/CNT anode affords lower Zn nucleation overpotential and more homogeneously distributed electric field, thus being more favorable for highly reversible Zn plating/stripping with satisfactory Coulombic efficiency rather than the formation of Zn dendrites or other byproducts. As a consequence, a highly flexible symmetric cell based on the Zn/CNT anode presents appreciably low voltage hysteresis (27 mV) and superior cycling stability (200 h) with dendrite-free morphology at 2 mA cm-2 , accompanied by a high depth of discharge (DOD) of 28%. Such distinct performance overmatches most of recently reported Zn-based anodes. Additionally, this efficient rechargeability of the Zn/CNT anode also enables a substantially stable Zn//MnO2 battery with 88.7% capacity retention after 1000 cycles and remarkable mechanical flexibility.

Atomically dispersed platinum supported on curved carbon supports for efficient electrocatalytic hydrogen evolution

Abstract: Dispersing catalytically active metals as single atoms on supports represents the ultimate in metal utilization efficiency and is increasingly being used as a strategy to design hydrogen evolution reaction (HER) electrocatalysts. Although platinum (Pt) is highly active for HER, given its high cost it is desirable to find ways to improve performance further while minimizing the Pt loading. Here, we use onion-like nanospheres of carbon (OLC) to anchor stable atomically dispersed Pt to act as a catalyst (Pt1/OLC) for the HER. In acidic media, the performance of the Pt1/OLC catalyst (0.27 wt% Pt) in terms of a low overpotential (38 mV at 10 mA cm−2) and high turnover frequencies (40.78 H2 s−1 at 100 mV) is better than that of a graphene-supported single-atom catalyst with a similar Pt loading, and comparable to a commercial Pt/C catalyst with 20 wt% Pt. First-principle calculations suggest that a tip-enhanced local electric field at the Pt site on the curved support promotes the reaction kinetics for hydrogen evolution. Isolating metal atoms on supports is becoming an increasingly studied approach to design water splitting electrocatalysts. Here, the authors prepare a hydrogen evolution catalyst comprising atomically dispersed Pt atoms on curved carbon supports, which outperform similar catalysts where the support is flat.

Robust noble metal-based electrocatalysts for oxygen evolution reaction

Abstract: The oxygen evolution reaction (OER) is a kinetically sluggish anodic reaction and requires a large overpotential to deliver appreciable current. Despite the fact that non-precious metal-based alkaline water electrocatalysts are receiving increased attention, noble metal-based electrocatalysts (NMEs) applied in proton exchange membrane water electrolyzers still have advantageous features of larger current and power densities with lower stack cost. Engineering NMEs for OER catalysis with high efficiency, durability and utilization rate is of vital importance in promoting the development of cost-effective renewable energy production and conversion devices. In this tutorial review, we covered the recent progress in the composition and structure optimization of NMEs for OER including Ir- and Ru-based oxides and alloys, and noble-metals beyond Ir and Ru with a variety of morphologies. To shed light on the fundamental science and mechanisms behind composition/structure–performance relationships and activity–stability relationships, integrated experimental and theoretical studies were pursued for illuminating the metal–support interaction, size effect, heteroatom doping effect, phase transformation, degradation processes and single-atom catalysis. Finally, the challenges and outlook are provided for guiding the rational engineering of OER electrocatalysts for applications in renewable energy-related devices.

Engineering the electronic structure of single atom Ru sites via compressive strain boosts acidic water oxidation electrocatalysis

Abstract: Single-atom precious metal catalysts hold the promise of perfect atom utilization, yet control of their activity and stability remains challenging. Here we show that engineering the electronic structure of atomically dispersed Ru1 on metal supports via compressive strain boosts the kinetically sluggish electrocatalytic oxygen evolution reaction (OER), and mitigates the degradation of Ru-based electrocatalysts in an acidic electrolyte. We construct a series of alloy-supported Ru1 using different PtCu alloys through sequential acid etching and electrochemical leaching, and find a volcano relation between OER activity and the lattice constant of the PtCu alloys. Our best catalyst, Ru1–Pt3Cu, delivers 90 mV lower overpotential to reach a current density of 10 mA cm−2, and an order of magnitude longer lifetime over that of commercial RuO2. Density functional theory investigations reveal that the compressive strain of the Ptskin shell engineers the electronic structure of the Ru1, allowing optimized binding of oxygen species and better resistance to over-oxidation and dissolution. While Ru-based electrocatalysts are among the most active for acidic water oxidation, they suffer from severe deactivation. Now, Yuen Wu, Wei-Xue Li and co-workers report a core–shell Ru1–Pt3Cu catalyst with surface-dispersed Ru atoms for a highly active and stable oxygen evolution reaction in acid electrolyte.

Lattice-strained metal–organic-framework arrays for bifunctional oxygen electrocatalysis

Abstract: Oxygen electrocatalysis is central to technologies such as fuel cells and electrolysers, but challenges remain due to the lack of effective earth-abundant electrocatalysts and insufficient understanding of catalytic mechanisms. Here we demonstrate that robust bifunctional oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) activity can be achieved by inducing lattice strain in noble-metal-free metal–organic frameworks (MOFs). Lattice-strained NiFe MOFs exhibit mass activities of 500 A gmetal−1 at a half-wave potential of 0.83 V for the ORR and 2,000 A gmetal−1 at an overpotential of 0.30 V for the OER, which are 50–100 times that of pristine NiFe metal–organic frameworks. The catalyst maintains ~97% of its initial activity after 200 h of continuous ORR/OER reaction at a high current density of 100–200 mA cm−2. Using operando synchrotron spectroscopies, we observed a key superoxide *OOH intermediate emerging on Ni4+ sites during both the ORR and OER processes, which suggests a four-electron mechanistic pathway. Metal–organic frameworks (MOFs) are increasingly being explored as electrocatalysts for the oxygen evolution and reduction reactions, which are important processes in electrolysers and fuel cells. Here, the authors increase the activity of MOFs for these reactions by introducing strain into the lattice using UV light illumination.

The Three-Dimensional Dendrite-Free Zinc Anode on a Copper Mesh with a Zinc-Oriented Polyacrylamide Electrolyte Additive

Abstract: Rechargeable aqueous zinc-ion batteries have been considered as a promising candidate for next-generation batteries. However, the formation of zinc dendrites are the most severe problems limiting their practical applications. To develop stable zinc metal anodes, a synergistic method is presented that combines the Cu-Zn solid solution interface on a copper mesh skeleton with good zinc affinity and a polyacrylamide electrolyte additive to modify the zinc anode, which can greatly reduce the overpotential of the zinc nucleation and increase the stability of zinc deposition. The as-prepared zinc anodes show a dendrite-free plating/stripping behavior over a wide range of current densities. The symmetric cell using this dendrite-free anode can be cycled for more than 280 h with a very low voltage hysteresis (93.1 mV) at a discharge depth of 80 %. The high capacity retention and low polarization are also realized in Zn/MnO2 full cells.

Atomically Dispersed Binary Co-Ni Sites in Nitrogen-Doped Hollow Carbon Nanocubes for Reversible Oxygen Reduction and Evolution.

Abstract: With the inspiration of developing bifunctional electrode materials for reversible oxygen electrocatalysis, one strategy of heteroatom doping is proposed to fabricate dual metal single-atom catalysts. However, the identification and mechanism functions of polynary single-atom structures remain elusive. Atomically dispersed binary Co-Ni sites embedded in N-doped hollow carbon nanocubes (denoted as CoNi-SAs/NC) are synthesized via proposed pyrolysis of dopamine-coated metal-organic frameworks. The atomically isolated bimetallic configuration in CoNi-SAs/NC is identified by combining microscopic and spectroscopic techniques. When employing as oxygen electrocatalysts in alkaline medium, the resultant CoNi-SAs/NC hybrid manifests outstanding catalytic performance for bifunctional oxygen reduction/evolution reactions, boosting the realistic rechargeable zinc-air batteries with high efficiency, low overpotential, and robust reversibility, superior to other counterparts and state-of-the-art precious-metal catalysts. Theoretical computations based on density functional theory demonstrate that the homogenously dispersed single atoms and the synergistic effect of neighboring Co-Ni dual metal center can optimize the adsorption/desorption features and decrease the overall reaction barriers, eventually promoting the reversible oxygen electrocatalysis. This work not only sheds light on the controlled synthesis of atomically isolated advanced materials, but also provides deeper understanding on the structure-performance relationships of nanocatalysts with multiple active sites for various catalytic applications.

Highly crystalline Ni-doped FeP/carbon hollow nanorods as all-pH efficient and durable hydrogen evolving electrocatalysts

Abstract: Herein, we report the synthesis of uniform hollow nanorods of Ni-doped FeP nanocrystals hybridized with carbon as electrocataysts for the electrocatalytic hydrogen evolution reaction (HER). These hollow nanorods are prepared based on the etching and coordination reaction between metal-organic frameworks and phytic acid, followed by a pyrolysis process. Benefiting from the abundant active sites, the improved mass and charge transport capability, the optimized Ni-doped FeP/C hollow nanorods exhibit excellent HER activities for achieving a current density of 10 mA cm−2 at an overpotential of 72, 117, and 95 mV in acidic, neutral, and alkaline media, respectively, as well as superior stability. X-ray photoelectron spectroscopy and basic density functional theory calculations suggest that the improved HER activity originates from the synergistic modulation of the active components, structural and electronic properties. This protocol provides a general and friendly strategy to construct hollow phosphides for energy-related applications.

CoP-Doped MOF-Based Electrocatalyst for pH-Universal Hydrogen Evolution Reaction

Abstract: Although electrocatalysts based on transition metal phosphides (TMPs) with cationic/anionic doping have been widely studied for hydrogen evolution reaction (HER), the origin of performance enhancement still remains elusive mainly due to the random dispersion of dopants. Herein, we report a controllable partial phosphorization strategy to generate CoP species within the Co-based metal-organic framework (Co-MOF). Density functional theory calculations and experimental results reveal that the electron transfer from CoP to Co-MOF through N-P/N-Co bonds could lead to the optimized adsorption energy of H2 O (ΔG H 2 O * ) and hydrogen (ΔGH* ), which, together with the unique porous structure of Co-MOF, contributes to the remarkable HER performance with an overpotential of 49 mV at a current density of 10 mA cm-2 in 1 m phosphate buffer solution (PBS, pH 7.0). The excellent catalytic performance exceeds almost all the documented TMP-based and non-noble-metal-based electrocatalysts. In addition, the CoP/Co-MOF hybrid also displays Pt-like performance in 0.5 m H2 SO4 and 1 m KOH, with the overpotentials of 27 and 34 mV, respectively, at a current density of 10 mA cm-2 .

Anion-Modulated HER and OER Activities of 3D Ni-V-Based Interstitial Compound Heterojunctions for High-Efficiency and Stable Overall Water Splitting.

Abstract: Overall water splitting driven by a low voltage is crucial for practical H2 evolution, but it is challenging. Herein, anion-modulation of 3D Ni-V-based transition metal interstitial compound (TMIC) heterojunctions supported on nickel foam (Ni3 N-VN/NF and Ni2 P-VP2 /NF) as coupled hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) catalysts for efficient overall water splitting is demonstrated. The heterointerface in Ni3 N-VN has a suitable H* absorption energy, being favorable for enhancing HER activity with onset overpotential (ηonset ) of zero and Tafel slope of 37 mV dec-1 in 1 m KOH (close to that of Pt/C/NF). For the OER, the synergy of Ni2 P-VP2 with oxide species can give enhanced activity with ηonset of 220 mV and Tafel slope of 49 mV dec-1 . The good activity is ascribed to heterointerface for activating the intermediates, good conductivity of TMICs for electron-transfer, and porous structure facilitation of mass-transport. Additionally, the minimal mutual influence of Ni3 N-VN/NF and Ni2 P-VP2 /NF allows easy coupling for efficient overall water splitting with a low driving voltage (≥1.43 V), a voltage of 1.51 V at 10 mA cm-2 , and remarkable durability for 100 h. It can be driven by a solar cell (1.5 V), indicating its potential to store intermittent energy.

An Isolated Zinc–Cobalt Atomic Pair for Highly Active and Durable Oxygen Reduction

Abstract: A competitive complexation strategy has been developed to construct a novel electrocatalyst with Zn-Co atomic pairs coordinated on N doped carbon support (Zn/CoN-C). Such architecture offers enhanced binding ability of O2 , significantly elongates the O-O length (from 1.23 A to 1.42 A), and thus facilitates the cleavage of O-O bond, showing a theoretical overpotential of 0.335 V during ORR process. As a result, the Zn/CoN-C catalyst exhibits outstanding ORR performance in both alkaline and acid conditions with a half-wave potential of 0.861 and 0.796 V respectively. The in situ XANES analysis suggests Co as the active center during the ORR. The assembled zinc-air battery with Zn/CoN-C as cathode catalyst presents a maximum power density of 230 mW cm-2 along with excellent operation durability. The excellent catalytic activity in acid is also verified by H2 /O2 fuel cell tests (peak power density of 705 mW cm-2 ).

Boosting oxygen evolution of single-atomic ruthenium through electronic coupling with cobalt-iron layered double hydroxides

Abstract: Single atom catalyst, which contains isolated metal atoms singly dispersed on supports, has great potential for achieving high activity and selectivity in hetero-catalysis and electrocatalysis. However, the activity and stability of single atoms and their interaction with support still remains a mystery. Here we show a stable single atomic ruthenium catalyst anchoring on the surface of cobalt iron layered double hydroxides, which possesses a strong electronic coupling between ruthenium and layered double hydroxides. With 0.45 wt.% ruthenium loading, the catalyst exhibits outstanding activity with overpotential 198 mV at the current density of 10 mA cm−2 and a small Tafel slope of 39 mV dec−1 for oxygen evolution reaction. By using operando X-ray absorption spectroscopy, it is disclosed that the isolated single atom ruthenium was kept under the oxidation states of 4+ even at high overpotential due to synergetic electron coupling, which endow exceptional electrocatalytic activity and stability simultaneously. While water splitting offers a carbon-neutral means to store energy, water oxidation is sluggish and corrosive over earth-abundant electrocatalysts. Here, authors show single ruthenium atoms over cobalt-iron layered double hydroxides to be effective and stable oxygen evolution electrocatalysts.

Multi-site electrocatalysts for hydrogen evolution in neutral media by destabilization of water molecules

Abstract: High-performance hydrogen evolution reaction (HER) catalysts are compelling for the conversion of renewable electricity to fuels and feedstocks. The best HER catalysts rely on the use of platinum and show the highest performance in acidic media. Efficient HER catalysts based on inexpensive and Earth-abundant elements that operate in neutral (hence biocompatible) media could enable low-cost direct seawater splitting and the realization of bio-upgraded chemical fuels. In the challenging neutral-pH environment, water splitting is a multistep reaction. Here we present a HER catalyst comprising Ni and CrOx sites doped onto a Cu surface that operates efficiently in neutral media. The Ni and CrOx sites have strong binding energies for hydrogen and hydroxyl groups, respectively, which accelerates water dissociation, whereas the Cu has a weak hydrogen binding energy, promoting hydride coupling. The resulting catalyst exhibits a 48 mV overpotential at a current density of 10 mA cm−2 in a pH 7 buffer electrolyte. These findings suggest design principles for inexpensive, efficient and biocompatible catalytic systems. Integrating electrocatalytic H2 production with biological H2-fed systems for CO2 upgrading requires H2 generation to occur in biocompatible media—typically with neutral pH. Here, the authors design multi-site H2 evolution catalysts that minimize the water dissociation barrier and promote hydride coupling in neutral media.

Ruthenium atomically dispersed in carbon outperforms platinum toward hydrogen evolution in alkaline media.

Abstract: Hydrogen evolution reaction is an important process in electrochemical energy technologies. Herein, ruthenium and nitrogen codoped carbon nanowires are prepared as effective hydrogen evolution catalysts. The catalytic performance is markedly better than that of commercial platinum catalyst, with an overpotential of only -12 mV to reach the current density of 10 mV cm-2 in 1 M KOH and -47 mV in 0.1 M KOH. Comparisons with control experiments suggest that the remarkable activity is mainly ascribed to individual ruthenium atoms embedded within the carbon matrix, with minimal contributions from ruthenium nanoparticles. Consistent results are obtained in first-principles calculations, where RuCxNy moieties are found to show a much lower hydrogen binding energy than ruthenium nanoparticles, and a lower kinetic barrier for water dissociation than platinum. Among these, RuC2N2 stands out as the most active catalytic center, where both ruthenium and adjacent carbon atoms are the possible active sites.

Stable Hierarchical Bimetal-Organic Nanostructures as HighPerformance Electrocatalysts for the Oxygen Evolution Reaction.

Abstract: The integration of heterometallic units and nanostructures into metal-organic frameworks (MOFs) used for the oxygen evolution reaction (OER) can enhance the electrocatalytic performance and help elucidate underlying mechanisms. We have synthesized a series of stable MOFs (CTGU-10a1-d1) based on trinuclear metal carboxylate clusters and a hexadentate carboxylate ligand with a (6,6)-connected nia net. We also present a strategy to synthesize hierarchical bimetallic MOF nanostructures (CTGU-10a2-d2). Among these, CTGU-10c2 is the best material for the OER, with an overpotential of 240 mV at a current density of 10 mA cm-2 and a Tafel slope of 58 mV dec-1 . This is superior to RuO2 and confirms CTGU-10c2 as one of the few known high-performing pure-phase MOF-OER electrocatalysts. Notably, bimetallic CTGU-10b2 and c2 show an improved OER activity over monometallic CTGU-10a2 and d2. Both DFT and experiments show that the remarkable OER performance of CTGU-10c2 is due to the presence of unsaturated metal sites, a hierarchical nanobelt architecture, and the Ni-Co coupling effect.

Se-Doping Activates FeOOH for Cost-Effective and Efficient Electrochemical Water Oxidation.

Abstract: Ni or Co is commonly required in efficient electrocatalysts for oxygen evolution reaction (OER). Although Fe is much more abundant and cheaper, full-Fe or Fe-rich catalysts suffer from insufficient activity. Herein, we discover that Se-doping can drastically promote OER on FeOOH and develop a facile on-site electrochemical activation strategy for achieving such a Se-doped FeOOH electrode via an FeSe precatalyst. Theoretical analysis and systematic experiments prove that Se-doping enables FeOOH as an efficient and low-cost OER electrocatalyst. By optimizing the electrode structure, an industrial-level OER current output of 500 mA cm-2 is secured at a low overpotential of 348 mV. The application of such an Fe-rich OER electrode in a practical solar-driven water splitting system demonstrates a high and stable solar-to-hydrogen efficiency of 18.55%, making the strategy promising for exploring new cost-effective and highly active electrocatalysts for clean hydrogen production.

Efficient alkaline hydrogen evolution on atomically dispersed Ni–Nx Species anchored porous carbon with embedded Ni nanoparticles by accelerating water dissociation kinetics

Abstract: Developing inexpensive and efficient electrocatalysts for hydrogen evolution reaction (HER) during alkaline water electrolysis is crucial for renewable and sustainable energy harvesting. Herein, we report a novel hybrid electrocatalyst comprising atomically dispersed Ni–Nx species anchored porous carbon (Ni–N–C) matrix with embedded Ni nanoparticles for HER. This new catalyst is synthesized via pyrolysis of hydrothermally prepared supermolecular composite of dicyandiamide and Ni ions followed by an acid etching treatment. The achieved hybrid exhibits superior catalytic performance toward HER with a small overpotential of 147 mV at 10 mA cm−2 and a low Tafel slope of 114 mV dec−1, comparable to those of state-of-the-art heteroatom-doped nanocarbon catalysts and even outperforming other reported transition-metal-based compounds in basic media. Experimental observations and theoretical calculations reveal that the presence of Ni nanoparticles can optimize surface states of Ni−Nx active centers and reduce energy barriers of dissociated water molecules, which synergistically improve OH− adsorption and promote HER kinetics. When served as electrodes for both cathode and anode, an alkaline water electrolyzer could afford a current density of 10 mA cm−2 at a low cell voltage of 1.58 V, rivalling the sufficiently high overpotentials of integrated Pt/C–Ir/C benchmark electrodes.

Atomically dispersed nickel-nitrogen-sulfur species anchored on porous carbon nanosheets for efficient water oxidation.

Abstract: Developing low-cost electrocatalysts to replace precious Ir-based materials is key for oxygen evolution reaction (OER). Here, we report atomically dispersed nickel coordinated with nitrogen and sulfur species in porous carbon nanosheets as an electrocatalyst exhibiting excellent activity and durability for OER with a low overpotential of 1.51 V at 10 mA cm−2 and a small Tafel slope of 45 mV dec−1 in alkaline media. Such electrocatalyst represents the best among all reported transition metal- and/or heteroatom-doped carbon electrocatalysts and is even superior to benchmark Ir/C. Theoretical and experimental results demonstrate that the well-dispersed molecular S|NiNx species act as active sites for catalyzing OER. The atomic structure of S|NiNx centers in the carbon matrix is clearly disclosed by aberration-corrected scanning transmission electron microscopy and synchrotron radiation X-ray absorption spectroscopy together with computational simulations. An integrated photoanode of nanocarbon on a Fe2O3 nanosheet array enables highly active solar-driven oxygen production. Water oxidation is considered the bottleneck reaction for light-driven water splitting due to the sluggish kinetics and poor stability. Here, authors show metal- and heteroatom-doped carbons as effective catalysts for both electrochemical and photoelectrochemical water splitting.

Direct observation of active catalyst surface phases and the effect of dynamic self-optimization in NiFe-layered double hydroxides for alkaline water splitting

Abstract: Earth-abundant transition metal-based compounds are of high interest as catalysts for sustainable hydrogen fuel generation. The realization of effective electrolysis of water, however, is still limited by the requirement of a high sustainable driving potential above thermodynamic requirements. Here, we report dynamically self-optimized (DSO) NiFe layered double hydroxide (LDH) nanosheets with promising bi-functional performance. Compared with pristine NiFe LDH, DSO NiFe LDH exhibits much lower overpotential for the hydrogen evolution reaction (HER), even outperforming platinum. Under 1 M KOH aqueous electrolyte, the bi-functional DSO catalysts show an overpotential of 184 and −59 mV without iR compensation for oxygen evolution reaction (OER) and HER at 10 mA cm−2. The material system operates at 1.48 V and 1.29 V to reach 10 and 1 mA cm−2 in two-electrode measurements, corresponding to 83% and 95% electricity-to-fuel conversion efficiency with respect to the lower heating value of hydrogen. The material is seen to dynamically reform the active phase of the surface layer during HER and OER, where the pristine and activated catalysts are analyzed with ex situ XPS, SAED and EELS as well as with in situ Raman spectro-electrochemistry. The results show transformation into different active interfacial species during OER and HER, revealing a synergistic interplay between iron and nickel in facilitating water electrolysis.

A Graphene‑Supported Single‑Atom FeN 5 Catalytic Site for Efficient Electrochemical CO 2 Reduction

Abstract: Electrochemical conversion of CO2 into valued products is one of the most important issues but remains a great challenge in chemistry. Herein, we report a novel synthetic approach involving prolonged thermal pyrolysis of hemin and melamine molecules on graphene for the fabrication of a robust and efficient single-iron-atom electrocatalyst for electrochemical CO2 reduction. The single-atom catalyst exhibits high Faradaic efficiency (ca. 97.0 %) for CO production at a low overpotential of 0.35 V, outperforming all Fe-N-C-based catalysts. The remarkable performance for CO2 -to-CO conversion can be attributed to the presence of highly efficient singly dispersed FeN5 active sites supported on N-doped graphene with an additional axial ligand coordinated to FeN4 . DFT calculations revealed that the axial pyrrolic nitrogen ligand of the FeN5 site further depletes the electron density of Fe 3d orbitals and thus reduces the Fe-CO π back-donation, thus enabling the rapid desorption of CO and high selectivity for CO production.

Metal Organic Framework-Templated Synthesis of Bimetallic Selenides with Rich Phase Boundaries for Sodium-Ion Storage and Oxygen Evolution Reaction.

Abstract: Two-phase or multiphase compounds have been evidenced to exhibit good electrochemical performance for energy applications; however, the mechanism insights into these materials, especially the performance improvement by engineering the high-active phase boundaries in bimetallic compounds, remain to be seen. Here, we report a bimetallic selenide heterostructure (CoSe2/ZnSe) and the fundamental mechanism behind their superior electrochemical performance. The charge redistribution at the phase boundaries of CoSe2/ZnSe was experimentally and theoretically proven. Benefiting from the abundant phase boundaries, CoSe2/ZnSe exerts low Na+ adsorption energy and fast diffusion kinetics for sodium-ion batteries and high activity for oxygen evolution reaction. As expected, excellent sodium storage capability, specifically a superb cyclic stability of up to 800 cycles for the Na3V2(PO4)3∥CoZn-Se full cell, and efficient water oxidation with a small overpotential of 320 mV to reach 10 mA cm–2 were obtained. This work de...

Chromium-ruthenium oxide solid solution electrocatalyst for highly efficient oxygen evolution reaction in acidic media

Abstract: The development of active, acid-stable and low-cost electrocatalysts for oxygen evolution reaction is urgent and challenging. Herein we report an Iridium-free and low ruthenium-content oxide material (Cr0.6Ru0.4O2) derived from metal-organic framework with remarkable oxygen evolution reaction performance in acidic condition. It shows a record low overpotential of 178 mV at 10 mA cm−2 and maintains the excellent performance throughout the 10 h chronopotentiometry test at a constant current of 10 mA cm−2 in 0.5 M H2SO4 solution. Density functional theory calculations further revealed the intrinsic mechanism for the exceptional oxygen evolution reaction performance, highlighting the influence of chromium promoter on the enhancement in both activity and stability. H2 generated from water electrolysis is a promising, carbon-neutral fuel, but sluggish oxidation kinetics and poor catalyst stability in acid limits the chemistry. Here, authors prepare solid-solution catalysts based on ruthenium-chromium oxides that are stable and active for acidic water oxidation.

Atomically Transition Metals on Self-Supported Porous Carbon Flake Arrays as Binder-Free Air Cathode for Wearable Zinc-Air Batteries.

Abstract: Metal single-atom materials with their high atom utilization efficiency and unique electronic structures usually show remarkable catalytic performances in many crucial chemical reactions. Herein, a facile and easily scalable "impregnation-carbonization-acidification" strategy for fabricating a class of single-atom-anchored (including cobalt and nickel single atoms) monolith as superior binder-free electrocatalysts for developing high-performance wearable Zn-air batteries is reported. The as-prepared single atoms, supported by N-doped carbon flake arrays grown on carbon nanofibers assembly (M SA@NCF/CNF), demonstrate the dual characteristics of excellent catalytic activity (reversible oxygen overpotential of 0.75 V) and high stability, owing to the greatly improved active sites' accessibility and optimized single-sites/pore-structures correlations. Furthermore, wearable Zn-air battery based on Co SA@NCF/CNF air electrode displays superior stability under deformation, satisfactory energy storage capacity, and good practicality to be utilized as an integrated battery system. Theoretical calculations reveal a mechanism for the promotion of the catalytic performances on single atomic sites by lowering the overall oxygen reduction/evolution reaction barriers comparing to metal cluster co-existing configuration. These findings provide a facile strategy for constructing free-standing single-atom materials as well as the engineering of high-performance binder-free catalytic electrodes.

Large-Scale, Bottom-Up Synthesis of Binary Metal-Organic Framework Nanosheets for Efficient Water Oxidation.

Abstract: Ultrathin metal-organic framework (MOF) nanosheets (NSs) offer potential for many applications, but the synthetic strategies are largely limited to top-down, low-yield exfoliation methods. Herein, Ni-M-MOF (M=Fe, Al, Co, Mn, Zn, and Cd) NSs are reported with a thickness of only several atomic layers, prepared by a large-scale, bottom-up solvothermal method. The solvent mixture of N,N-dimethylacetamide and water plays key role in controlling the formation of these two-dimensional MOF NSs. The MOF NSs can be directly used as efficient electrocatalysts for the oxygen evolution reaction, in which the Ni-Fe-MOF NSs deliver a current density of 10 mA cm-2 at a low overpotential of 221 mV with a small Tafel slope of 56.0 mV dec-1 , and exhibit excellent stability for at least 20 h without obvious activity decay. Density functional theory calculations on the energy barriers for OER occurring at different metal sites confirm that Fe is the active site for OER at Ni-Fe-MOF NSs.

Scaled-Up Synthesis of Amorphous NiFeMo Oxides and Their Rapid Surface Reconstruction for Superior Oxygen Evolution Catalysis.

Abstract: The anode oxygen evolution reaction (OER) is known to largely limit the efficiency of electrolyzers owing to its sluggish kinetics. While crystalline metal oxides are promising as OER catalysts, their amorphous phases also show high activities. Efforts to produce amorphous metal oxides have progressed slowly, and how an amorphous structure benefits the catalytic performances remains elusive. Now the first scalable synthesis of amorphous NiFeMo oxide (up to 515 g in one batch) is presented with homogeneous elemental distribution via a facile supersaturated co-precipitation method. In contrast to its crystalline counterpart, amorphous NiFeMo oxide undergoes a faster surface self-reconstruction process during OER, forming a metal oxy(hydroxide) active layer with rich oxygen vacancies, leading to superior OER activity (280 mV overpotential at 10 mA cm-2 in 0.1 m KOH). This opens up the potential of fast, facile, and scale-up production of amorphous metal oxides for high-performance OER catalysts.

Interface engineering of (Ni, Fe)S2@MoS2 heterostructures for synergetic electrochemical water splitting

Abstract: Water splitting to generate hydrogen is a promising and sustainable process, whereas is limited by the large overpotential of electrode materials for cathode hydrogen evolution reaction (HER) and anode oxygen evolution reaction (OER). Here we present an interface engineering strategy to construct efficient bifunctional electrocatalyst based on (Ni, Fe)S2@MoS2 heterostructrues for water-splitting process. The as-prepared (Ni, Fe)S2@MoS2 catalyst gives remarkable electrochemical activity and durability under alkaline environments, with a low overpotential of 130 mV for HER and 270 mV for OER to deliver the current density of 10 mA cm−2, respectively. In combination with in-situ Raman spectra, we demonstrate that the constructed interfacial active sites are favorable to the formation of S-Hads, which synergistically lower the chemisorption energy of the intermediates of HER and OER, thereby facilitating the electrocatalytic overall water splitting. Furthermore, we regulate the interface of (Ni, Fe)S2@MoS2 and electrolyte through changing the composition of electrolyte to achieve much longer electrochemical stability of this hybrid sulfide electrocatalyst.

Dynamic oxygen adsorption on single-atomic Ruthenium catalyst with high performance for acidic oxygen evolution reaction.

Abstract: Achieving active and stable oxygen evolution reaction (OER) in acid media based on single-atom catalysts is highly promising for cost-effective and sustainable energy supply in proton electrolyte membrane electrolyzers. Here, we report an atomically dispersed Ru1-N4 site anchored on nitrogen-carbon support (Ru-N-C) as an efficient and durable electrocatalyst for acidic OER. The single-atom Ru-N-C catalyst delivers an exceptionally intrinsic activity, reaching a mass activity as high as 3571 A gmetal−1 and turnover frequency of 3348 O2 h−1 with a low overpotential of 267 mV at a current density of 10 mA cm−2. The catalyst shows no evident deactivation or decomposition after 30-hour operation in acidic environment. Operando synchrotron radiation X-ray absorption spectroscopy and infrared spectroscopy identify the dynamic adsorption of single oxygen atom on Ru site under working potentials, and theoretical calculations demonstrate that the O-Ru1-N4 site is responsible for the high OER activity and stability. Monitoring catalyst structural changes under working conditions is crucial for understanding how catalysts operate. Here, authors examine single-atom Ru electrocatalyst by operando synchrotron spectroscopies to identify the catalytic mechanism during the acidic oxygen evolution reaction.