Tip-like Fe-N4 Sites Induced Surface Microenvironments Regulation Boosts the Oxygen Reduction Reaction.

Single atom catalysts with defined local structures and favorable surface microenvironments are significant for overcoming slow kinetics and accelerating O2 electroreduction. Here, enriched tip-like FeN4 sites (T-Fe SAC) on spherical carbon surfaces were developed to investigate the change in surface microenvironments and catalysis behavior. Finite element method (FEM) simulations, together with experiments, indicate the strong local electric field of the tip-like FeN4 and the more denser interfacial water layer, thereby enhancing the kinetics of the proton-coupled electron transfer process. In situ spectroelectrochemical studies and the density functional theory (DFT) calculation results indicate the pathway transition on the tip-like FeN4 sites, promoting the dissociation of O-O bond via side-on adsorption model. The adsorbed OH* can be facilely released on the curved surface and accelerate the oxygen reduction reaction (ORR) kinetics. The obtained T-Fe SAC nanoreactor exhibits excellent ORR activities (E1/2 =0.91 V vs. RHE) and remarkable stability, exceeding those of flat FeN4 and Pt/C. This work clarified the in-depth insights into the origin of catalytic activity of tip-like FeN4 sites and held great promise in industrial catalysis, electrochemical energy storage, and many other fields.

Attenuating metal-substrate conjugation in atomically dispersed nickel catalysts for electroreduction of CO2 to CO.

Atomically dispersed transition metals on carbon-based aromatic substrates are an emerging class of electrocatalysts for the electroreduction of CO2. However, electron delocalization of the metal site with the carbon support via d-π conjugation strongly hinders CO2 activation at the active metal centers. Herein, we introduce a strategy to attenuate the d-π conjugation at single Ni atomic sites by functionalizing the support with cyano moieties. In situ attenuated total reflection infrared spectroscopy and theoretical calculations demonstrate that this strategy increases the electron density around the metal centers and facilitates CO2 activation. As a result, for the electroreduction of CO2 to CO in aqueous KHCO3 electrolyte, the cyano-modified catalyst exhibits a turnover frequency of ~22,000 per hour at -1.178 V versus the reversible hydrogen electrode (RHE) and maintains a Faradaic efficiency (FE) above 90% even with a CO2 concentration of only 30% in an H-type cell. In a flow cell under pure CO2 at -0.93 V versus RHE the cyano-modified catalyst enables a current density of -300 mA/cm2 with a FE above 90%.

Accelerating CO2 Electroreduction to Multicarbon Products via Synergistic Electric-Thermal Field on Copper Nanoneedles.

Electrochemical CO2 reduction is a promising way to mitigate CO2 emissions and close the anthropogenic carbon cycle. Among products from CO2RR, multicarbon chemicals, such as ethylene and ethanol with high energy density, are more valuable. However, the selectivity and reaction rate of C2 production are unsatisfactory due to the sluggish thermodynamics and kinetics of C-C coupling. The electric field and thermal field have been studied and utilized to promote catalytic reactions, as they can regulate the thermodynamic and kinetic barriers of reactions. Either raising the potential or heating the electrolyte can enhance C-C coupling, but these come at the cost of increasing side reactions, such as the hydrogen evolution reaction. Here, we present a generic strategy to enhance the local electric field and temperature simultaneously and dramatically improve the electric-thermal synergy desired in electrocatalysis. A conformal coating of ∼5 nm of polytetrafluoroethylene significantly improves the catalytic ability of copper nanoneedles (∼7-fold electric field and ∼40 K temperature enhancement at the tips compared with bare copper nanoneedles experimentally), resulting in an improved C2 Faradaic efficiency of over 86% at a partial current density of more than 250 mA cm-2 and a record-high C2 turnover frequency of 11.5 ± 0.3 s-1 Cu site-1. Combined with its low cost and scalability, the electric-thermal strategy for a state-of-the-art catalyst not only offers new insight into improving activity and selectivity of value-added C2 products as we demonstrated but also inspires advances in efficiency and/or selectivity of other valuable electro-/photocatalysis such as hydrogen evolution, nitrogen reduction, and hydrogen peroxide electrosynthesis.

Optimizing Hydrogen Binding on Ru Sites with RuCo Alloy Nanosheets for Efficient Alkaline Hydrogen Evolution.

Ruthenium (Ru)-based catalysts, with considerable performance and desirable cost, are becoming highly interesting candidates to replace platinum (Pt) in the alkaline hydrogen evolution reaction (HER). The hydrogen binding at Ru sites (Ru-H) is an important factor limiting the HER activity. Herein, density functional theory (DFT) simulations show that the essence of Ru-H binding energy is the strong interaction between the 4 d z 2 orbital of Ru and the 1s orbital of H. The charge transfer between Ru sites and substrates (Co and Ni) causes the appropriate downward shift of the 4 d z 2 -band center of Ru, which results in a Gibbs free energy of 0.022 eV for H* in the RuCo system, much lower than the 0.133 eV in the pure Ru system. This theoretical prediction has been experimentally confirmed using RuCo alloy-nanosheets (RuCo ANSs). They were prepared via a fast co-precipitation method followed with a mild electrochemical reduction. Structure characterizations reveal that the Ru atoms are embedded into the Co substrate as isolated active sites with a planar symmetric and Z-direction asymmetric coordination structure, obtaining an optimal 4 d z 2 modulated electronic structure. Hydrogen sensor and temperature program desorption (TPD) tests demonstrate the enhanced Ru-H interactions in RuCo ANSs compared to those in pure Ru nanoparticles. As a result, the RuCo ANSs reach an ultra-low overpotential of 10 mV at 10 mA cm-2 and a Tafel slope of 20.6 mV dec-1 in 1 M KOH, outperforming that of the commercial Pt/C. This holistic work provides a new insight to promote alkaline HER by optimizing the metal-H binding energy of active sites.

Atomically Dispersed s-Block Magnesium Sites for Electroreduction of CO2 to CO.

Atomically dispersed transition metal sites have been extensively studied for CO2 electroreduction reaction (CO2 RR) to CO due to their robust CO2 activation ability. However, the strong hybridization between directionally localized d orbits and CO vastly limits CO desorption and thus the activities of atomically dispersed transition metal sites. In contrast, s-block metal sites possess nondirectionally delocalized 3s orbits and hence weak CO adsorption ability, providing a promising way to solve the suffered CO desorption issue. Herein, we constructed atomically dispersed magnesium atoms embedded in graphitic carbon nitride (Mg-C3 N4 ) through a facile heat treatment for CO2 RR. Theoretical calculations show that the CO desorption on Mg sites is easier than that on Fe and Co sites. This theoretical prediction is demonstrated by experimental CO temperature program desorption and in situ attenuated total reflection infrared spectroscopy. As a result, Mg-C3 N4 exhibits a high turnover frequency of ≈18 000 per hour in H-cell and a large current density of -300 mA cm-2 in flow cell, under a high CO Faradaic efficiency ≥90 % in KHCO3 electrolyte. This work sheds a new light on s-block metal sites for efficient CO2 RR to CO.

CoS2 needle arrays induced a local pseudo-acidic environment for alkaline hydrogen evolution.

The alkaline electrocatalytic hydrogen evolution reaction (HER) is a potential way to realize industrial hydrogen production. However, the sluggish process of H2O dissociation, as well as the accumulation of OH- around the active sites, seriously limit the alkaline HER performance. In this work, we developed a unique CoS2 needle array grown on a carbon cloth (NAs@C) electrode as an alkaline HER catalyst. Finite-element simulations revealed that CoS2 needle arrays (NAs) induce stronger local electric field (LEF) than CoS2 disordered needles (DNs). This LEF can greatly repel the local OH- around the active sites, and then promote the forward H2O dissociation process. The local pH changes of the electrode surface confirmed the lower OH- concentration and stronger local pseudo-acidic environment of NAs@C compared to those of DNs@C. As a result, the NAs@C catalyst exhibited a low HER overpotential of 121 mV at a current density of 10 mA cm-2 in 1 M KOH, with the Tafel slope of 59.87 mV dec-1. This work provides a new insight into nanoneedle arrays for the alkaline HER by electric field-promoted H2O dissociation.

Chemical Identification of Catalytically Active Sites on Oxygen-doped Carbon Nanosheet to Decipher the High Activity for Electro-synthesis Hydrogen Peroxide.

Electrochemical production of hydrogen peroxide (H2 O2 ) through two-electron (2 e- ) oxygen reduction reaction (ORR) is an on-site and clean route. Oxygen-doped carbon materials with high ORR activity and H2 O2 selectivity have been considered as the promising catalysts, however, there is still a lack of direct experimental evidence to identify true active sites at the complex carbon surface. Herein, we propose a chemical titration strategy to decipher the oxygen-doped carbon nanosheet (OCNS900 ) catalyst for 2 e- ORR. The OCNS900 exhibits outstanding 2 e- ORR performances with onset potential of 0.825 V (vs. RHE), mass activity of 14.5 A g-1 at 0.75 V (vs. RHE) and H2 O2 production rate of 770 mmol g-1 h-1 in flow cell, surpassing most reported carbon catalysts. Through selective chemical titration of C=O, C-OH, and COOH groups, we found that C=O species contributed to the most electrocatalytic activity and were the most active sites for 2 e- ORR, which were corroborated by theoretical calculations.

Iron phthalocyanine with coordination induced electronic localization to boost oxygen reduction reaction.

Iron phthalocyanine (FePc) is a promising non-precious catalyst for the oxygen reduction reaction (ORR). Unfortunately, FePc with plane-symmetric FeN4 site usually exhibits an unsatisfactory ORR activity due to its poor O2 adsorption and activation. Here, we report an axial Fe-O coordination induced electronic localization strategy to improve its O2 adsorption, activation and thus the ORR performance. Theoretical calculations indicate that the Fe-O coordination evokes the electronic localization among the axial direction of O-FeN4 sites to enhance O2 adsorption and activation. To realize this speculation, FePc is coordinated with an oxidized carbon. Synchrotron X-ray absorption and Mössbauer spectra validate Fe-O coordination between FePc and carbon. The obtained catalyst exhibits fast kinetics for O2 adsorption and activation with an ultralow Tafel slope of 27.5 mV dec-1 and a remarkable half-wave potential of 0.90 V. This work offers a new strategy to regulate catalytic sites for better performance.

Graphitic Carbon Nitride with Dopant Induced Charge Localization for Enhanced Photoreduction of CO2 to CH4.

The photoreduction of CO2 to hydrocarbon products has attracted much attention because it provides an avenue to directly synthesize value-added carbon-based fuels and feedstocks using solar energy. Among various photocatalysts, graphitic carbon nitride (g-C3N4) has emerged as an attractive metal-free visible-light photocatalyst due to its advantages of earth-abundance, nontoxicity, and stability. Unfortunately, its photocatalytic efficiency is seriously limited by charge carriers' ready recombination and their low reaction dynamics. Modifying the local electronic structure of g-C3N4 is predicted to be an efficient way to improve the charge transfer and reaction efficiency. Here, boron (B) is doped into the large cavity between adjacent tri-s-triazine units via coordination with two-coordinated N atoms. Theoretical calculations prove that the new electron excitation from N (2p x , 2p y ) to B (2p x , 2p y ) with the same orbital direction in B-doped g-C3N4 is much easier than N (2p x , 2p y ) to C 2p z in pure g-C3N4, and improves the charge transfer and localization, and thus the reaction dynamics. Moreover, B atoms doping changes the adsorption of CO (intermediate), and can act as active sites for CH4 production. As a result, the optimal sample of 1%B/g-C3N4 exhibits better selectivity for CH4 with ≈32 times higher yield than that of pure g-C3N4.

Untying thioether bond structures enabled by "voltage-scissors" for stable room temperature sodium-sulfur batteries.

A steady sulfur cathode is central to realizing stable room temperature sodium-sulfur (RT Na-S) batteries. Here, sulfur atoms are incorporated into carbon to form thioether bond functionalized carbon (SC) as a cathode material for sodium storage, which is capable of the complete elimination or great suppression of the shuttle effect of soluble poly-sulfides by the generation of insoluble sulfides in RT Na-S batteries using an ordinary carbonate electrolyte. This thioether bond structure (C-S-C) in sulfur-doped carbon can be disassembled by the assistance of "voltage-scissors" in the low voltage range of 0.01-0.50 V vs. Na/Na+. The small sulfur species derived from the thioether bond structure react with the sodium anode to form insoluble sulfides. Moreover, the insoluble sulfides Na2S2 and Na2S are confined in the carbon defects induced by the cleaved sulfur to prevent them from degrading the long-chain poly-sulfides. The distinctive electrochemical performance of SC enriches our understanding of the sulfur cathode, and could provide novel insights for constructing stable sulfur cathode materials for specific energy storage devices.