Isolated Metal Centers Activate Small Molecule Electrooxidation: Mechanisms and Applications

Electrochemical oxidation of small molecules shows great promise to substitute oxygen evolution reaction (OER) or hydrogen oxidation reaction (HOR) to enhance reaction kinetics and reduce energy consumption, as well as produce high-valued chemicals or serve as fuels. For these oxidation reactions, high-valence metal sites generated at oxidative potentials are typically considered as active sites to trigger the oxidation process of small molecules. Isolated atom site catalysts (IASCs) have been developed as an ideal system to precisely regulate the oxidation state and coordination environment of single-metal centers, and thus optimize their catalytic property. The isolated metal sites in IASCs inherently possess a positive oxidation state, and can be more readily produce homogeneous high-valence active sites under oxidative potentials than their nanoparticle counterparts. Meanwhile, IASCs merely possess the isolated metal centers but lack ensemble metal sites, which can alter the adsorption configurations of small molecules as compared with nanoparticle counterparts, and thus induce various reaction pathways and mechanisms to change product selectivity. More importantly, the construction of isolated metal centers is discovered to limit metal d-electron back donation to CO 2p* orbital and reduce the overly strong adsorption of CO on ensemble metal sites, which resolve the CO poisoning problems in most small molecules electro-oxidation reactions and thus improve catalytic stability. Based on these advantages of IASCs in the fields of electrochemical oxidation of small molecules, this review summarizes recent developments and advancements in IASCs in small molecules electro-oxidation reactions, focusing on anodic HOR in fuel cells and OER in electrolytic cells as well as their alternative reactions, such as formic acid/methanol/ethanol/glycerol/urea/5-hydroxymethylfurfural (HMF) oxidation reactions as key reactions. The catalytic merits of different oxidation reactions and the decoding of structure-activity relationships are specifically discussed to guide the precise design and structural regulation of IASCs from the perspective of a comprehensive reaction mechanism. Finally, future prospects and challenges are put forward, aiming to motivate more application possibilities for diverse functional IASCs.

Ultralow Catalytic Loading for Optimised Electrocatalytic Performance of AuPt Nanoparticles to Produce Hydrogen and Ammonia

The hydrogen evolution and nitrite reduction reactions are key to producing green hydrogen and ammonia. Antenna-reactor nanoparticles hold promise to improve the performances of these transformations under visible-light excitation, by combining plasmonic and catalytic materials. However, current materials involve compromising either on the catalytic activity or the plasmonic enhancement and also lack control of reaction selectivity. Here, we demonstrate that ultralow loadings and non-uniform surface segregation of the catalytic component optimize catalytic activity and selectivity under visible-light irradiation. Taking Pt-Au as an example we find that fine-tuning the Pt content produces a 6-fold increase in the hydrogen evolution compared to commercial Pt/C as well as a 6.5-fold increase in the nitrite reduction and a 2.5-fold increase in the selectivity for producing ammonia under visible light excitation relative to dark conditions. Density functional theory suggests that the catalytic reactions are accelerated by the intimate contact between nanoscale Pt-rich and Au-rich regions at the surface, which facilitates the formation of electron-rich hot-carrier puddles associated with the Pt-based active sites. The results provide exciting opportunities to design new materials with improved photocatalytic performance for sustainable energy applications.

Electrochemical CO2 Reduction on Cu-Based Monatomic Alloys: A DFT Study

In recent years, single-atom alloy catalysts (SAAs) have received much attention due to the combination of structural features of both single-atom and alloy catalysts, as well as their efficient catalytic activity, high selectivity, and high stability in various chemical reactions. In this work, we designed a series of Cu-based SAAs by doping isolated 3d transition metal (TM1) atoms on the surface of Cu(111) (TM1 = Fe, Co, Ru, Rh, Os and Ir), in which Ir1/Cu(111) SAAs are considered to be the most stable among 3d-series SAAs due to their optimal binding energy (Eb). The density of states of SAAs have been systematically investigated to further discuss structural properties. Based on density functional theory calculations, the activity and selectivity of Ir1/Cu(111) SAAs are investigated for electrocatalytic CO2 reduction reaction (CO2RR). The initial hydrogenation of CO2 on Ir1/Cu(111) SAAs can form *CO intermediates, which will be further to CH4 production by the pathway of *CO → *CHO → *CHOH → *CH2OH → *CH2 → *CH3 → CH4. This study provides theoretical insights for the rational design of selective Cu-based monatomic alloy catalysts.

Enhancing the catalytic activity of the MnNC catalyst by regulating the coordination environment

The catalytic activity is largely determined by the coordination environment of the active sites. The catalytic activity of MnNC was much enhanced by the regulation of the coordination environment. The introduction of optimal epoxy to the vicinity of the Mn centers improved its half-wave potential by ∼300 mV.

MoS2 nanoflower decorated bio-derived chitosan nanocomposites for sustainable energy storage: Structural, optical and electrochemical studies

Bio-derived chitosan-molybdenum di sulfide (Cs-MoS2) nanocomposites are prepared by a simple and economical aqueous casting method with varying concentrations of MoS2. The structural, surface morphological, optical, and electrochemical properties of the nanocomposites were studied. FTIR analysis reveals the strong interaction between Cs and MoS2. FESEM micrograph showed an increment of the surface roughness due to the incorporation of MoS2 layers into Cs. The surface wettability of the nanocomposites was found to be decreased from 73° to 33° due to the incorporation of MoS2 into the chitosan. UV-vis spectroscopy study demonstrates a reduction of optical bandgap from 4.29 to 3.44 eV as the nanofiller, MoS2, introduces localized states within the forbidden energy bandgap. The incorporation of MoS2 was found to increase the specific capacitance of Cs from 421 mFg-1 to 1589 mFg-1 at a current density of 100 μAg-1. The EIS analysis revealed an increase in the pseudo-capacitance from 0.09 μF to 4.13 μF and a reduction of charge transfer resistance that comes from the nanofiller contribution. MoS2 nanoflower introduces more active sites and expands the electroactive zone, thus improving the charge storage property of Cs. The Cs-MoS2 may offer a new route for the synthesis of eco-friendly, biodegradable, and electrical storage devices.

Morphological and Coordination Modulations in Iridium Electrocatalyst for Robust and Stable Acidic OER Catalysis

Proton exchange membrane water splitting (PEMWS) technology has high-level current density, high operating pressure, small electrolyzer-size, integrity, flexibility, and has good adaptability to the volatility of wind power and photovoltaics, but the development of both active and high stability of the anode electrocatalyst in acidic environment is still a huge challenge, which seriously hinders the promotion and application of PEMWS. In recent years, researchers have made tremendous attempts in the development of high-quality active anode electrocatalyst, and we summarize some of the research progress made by our group in the design and synthesis of PEMWS anode electrocatalysts with different nanostructures, and makes full use of electrocatalytic activity points to increase the inherent activity of Iridium (Ir) sites, and provides optimization strategies for the long-term non-decay of catalysts under high anode potential in acidic environments. At this stage, these research advances are expected to facilitate the research and technological progress of PEMWS, and providing some research ideas and references for future research on efficient and inexpensive PEMWS anode electrocatalysts.

Cooperative Ni(Co)-Ru-P Sites Activate Dehydrogenation for Hydrazine Oxidation Assisting Self-powered H2 Production

Water electrolysis for H2 production is restricted by the sluggish oxygen evolution reaction (OER). Using the thermodynamically more favorable hydrazine oxidation reaction (HzOR) to replace OER has attracted ever-growing attention. Herein, we report a twisted NiCoP nanowire array immobilized with Ru single atoms (Ru1 -NiCoP) as superior bifunctional electrocatalyst toward both HzOR and hydrogen evolution reaction (HER), realizing an ultralow working potential of -60 mV and overpotential of 32 mV for a current density of 10 mA cm-2 , respectively. Inspiringly, two-electrode electrolyzer based on overall hydrazine splitting (OHzS) demonstrates outstanding activity with a record-high current density of 522 mA cm-2 at cell voltage of 0.3 V. DFT calculations elucidate the cooperative Ni(Co)-Ru-P sites in Ru1 -NiCoP optimize H* adsorption, and enhance adsorption of *N2 H2 to significantly lower the energy barrier for hydrazine dehydrogenation. Moreover, a self-powered H2 production system utilizing OHzS device driven by direct hydrazine fuel cell (DHzFC) achieve a satisfactory rate of 24.0 mol h-1  m-2 .

Hybrid Nanostructured Compounds of Mo2C on Vertical Graphene Nanoflakes for a Highly Efficient Hydrogen Evolution Reaction

Organizing a post-fossil fuel economy requires the development of sustainable energy carriers. Hydrogen is expected to play a significant role as an alternative fuel as it is among the most efficient energy carriers. Therefore, nowadays, the demand for hydrogen production is increasing. Green hydrogen produced by water splitting produces zero carbon emissions but requires the use of expensive catalysts. Therefore, the demand for efficient and economical catalysts is constantly growing. Transition-metal carbides, and especially Mo2C, have attracted great attention from the scientific community since they are abundantly available and hold great promises for efficient performance toward the hydrogen evolution reaction (HER). This study presents a bottom-up approach for depositing Mo carbide nanostructures on vertical graphene nanowall templates via chemical vapor deposition, magnetron sputtering, and thermal annealing processes. Electrochemical results highlight the importance of adequate loading of graphene templates with the optimum amount of Mo carbides, controlled by both deposition and annealing time, to enrich the available active sites. The resulting compounds exhibit exceptional activities toward the HER in acidic media, requiring overpotentials of 82 mV at -10 mA/cm2 and demonstrating a Tafel slope of 56 mV/dec. The high double-layer capacitance and low charge transfer resistance of these Mo2C on GNW hybrid compounds are the main causes of the enhanced HER activity. This study is expected to pave the way for the design of hybrid nanostructures based on nanocatalyst deposition on three-dimensional graphene templates.

Three-Dimensional Au(NiMo)/Ti Catalysts for Efficient Hydrogen Evolution Reaction

In this study, NiMo catalysts that have different metal loadings in the range of ca. 28-106 µg cm^-2 were electrodeposited on the Ti substrate followed by their decoration with a very low amount of Au-crystallites in the range of ca. 1-5 µg cm^-2 using the galvanic displacement method. The catalytic performance for hydrogen evolution reaction (HER) was evaluated on the NiMo/Ti and Au(NiMo)/Ti catalysts in an alkaline medium. It was found that among the investigated NiMo/Ti and Au(NiMo)/Ti catalysts, the Au(NiMo)/Ti-3 catalyst with the Au loading of 5.2 µg cm^-2 gives the lowest overpotential of 252 mV for the HER to reach a current density of 10 mA·cm^-2. The current densities for HER increase ca. 1.1-2.7 and ca. 1.1-2.2 times on the NiMo/Ti and Au(NiMo)/Ti catalysts, respectively, at -0.424 V, with an increase in temperature from 25 °C to 75 °C.

Tuning Reconstruction Level of Precatalysts to Design Advanced Oxygen Evolution Electrocatalysts

Surface reconstruction engineering is an effective strategy to promote the catalytic activities of electrocatalysts, especially for water oxidation. Taking advantage of the physicochemical properties of precatalysts by manipulating their structural self-reconstruction levels provide a promising methodology for achieving suitable catalysts. In this review, we focus on recent advances in research related to the rational control of the process and level of surface transformation ultimately to design advanced oxygen evolution electrocatalysts. We start by discussing the original contributions to surface changes during electrochemical reactions and related factors that can influence the electrocatalytic properties of materials. We then present an overview of current developments and a summary of recently proposed strategies to boost electrochemical performance outcomes by the controlling structural self-reconstruction process. By conveying these insights, processes, general trends, and challenges, this review will further our understanding of surface reconstruction processes and facilitate the development of high-performance electrocatalysts beyond water oxidation.

Alloyed AuPt nanoframes loaded on h-BN nanosheets as an ingenious ultrasensitive near-infrared photoelectrochemical biosensor for accurate monitoring glucose in human tears

Photo-electro-chemical (PEC) glucose biosensor has recently attracted extensive attention due to the double advantages of both photocatalysis via photon energy utilization and electrocatalytic oxidation through extra electric field. Compared with previous shorter wavelength (violet-visible) light-induced PEC reaction, the anticipated near infrared (NIR, >~700 nm) excited PEC biosensor with multiple fascinating features should be more suitable for clinical diagnostic biology. Herein, we report an ingenious NIR-PEC biosensor by loading alloyed Au₅Pt₉ nanoframes on two dimensional (2D) hexagonal boron nitride (h-BN) nanosheets. The obtained h-BN/Au₅Pt₉ nanoframes exhibit a remarkable higher NIR-PEC activity in comparison with other as-prepared h-BN/AuPt references. The improved PEC performance is attributed to the enhanced synergetic coupling effect between Au₅Pt₉ nanoalloys and constitutionally stable h-BN that gives rise to a stronger absorbance capacity and pronounced localized surface plasmon resonance (LSPR) in visible-NIR region as well as high free-electron mobility of framework-like Au/Pt. Interestingly, the obtained h-BN/Au₅Pt₉ nanoframes excited by 808 nm NIR light provide superior PEC accuracy and sensitivity as compared to visible or other NIR light irradiation. Then, the novel 808 nm NIR-PEC biosensor was used for precise glucose monitoring in human tears with a detectable concentration of 0.03~100 μM and a low detection limit of 0.406 nM. Undoubtedly, the proposed h-BN/Au₅Pt₉ nanoframes as an appealing NIR-PEC glucose biosensor can possess greater potential values for practical glucose monitoring in biomedicine.

Atomic PdAu Interlayer Sandwiched into Pd/Pt Core/Shell Nanowires Achieves Superstable Oxygen Reduction Catalysis

Rationally designing the core/shell architecture of Pt-based electrocatalysts has been demonstrated as an effective way to induce a surface strain effect for promoting the sluggish kinetics of the oxygen reduction reaction (ORR) at the cathode of fuel cells. However, unstable core dissolution and structural collapse usually occur in Pt-based core/shell catalysts during the long-term cycling operation, greatly impacting actual fuel cell applications. Impeding the dissolution of cores beneath the Pt shells is the key to enhancing the catalytic stability of materials. Herein, a method for sandwiching atomic PdAu interlayers into one-dimensional (1D) Pd/Pt core/shell nanowires (NWs) is developed to greatly boost the catalytic stability of subnanometer Pt shells for ORR. The Pd/PdAu/Pt core/shell/shell NWs display only 7.80% degradation of ORR mass activity over 80 000 potential cycles with no dissolution of Pd cores and good preservation of the holistic sandwich core/shell nanostructures. This is a significant improvement of electrocatalytic stability compared with the Pd/Pt core/shell NWs, which deformed and inactivated over 80 000 potential cycles. The density functional theory (DFT) calculations further demonstrate that the electron-transfer bridge Pd and electron reservoir Au, serving in the PdAu atomic interlayer, both guarantee the preservation of the high electroactivity of surface Pt sites during the long-term ORR stability test. In addition, the Pd/PdAu/Pt NWs show a 1.7-fold higher mass activity (MA) for ORR than the conventional Pd/Pt NWs. The enhanced activity can be attributed to the strong interaction between PdAu interlayers and subnanometer-Pt shells, which suppresses the competitive Pd-4d bands and boosts the surface Pt-5d bands toward the Fermi level for higher electroactivity, proved from DFT.

Single-Atom Catalysts for Electrochemical Hydrogen Evolution Reaction: Recent Advances and Future Perspectives

Hydrogen, a renewable and outstanding energy carrier with zero carbon dioxide emission, is regarded as the best alternative to fossil fuels. The most preferred route to large-scale production of hydrogen is by water electrolysis from the intermittent sources (e.g., wind, solar, hydro, and tidal energy). However, the efficiency of water electrolysis is very much dependent on the activity of electrocatalysts. Thus, designing high-effective, stable, and cheap materials for hydrogen evolution reaction (HER) could have a substantial impact on renewable energy technologies. Recently, single-atom catalysts (SACs) have emerged as a new frontier in catalysis science, because SACs have maximum atom-utilization efficiency and excellent catalytic reaction activity. Various synthesis methods and analytical techniques have been adopted to prepare and characterize these SACs. In this review, we discuss recent progress on SACs synthesis, characterization methods, and their catalytic applications. Particularly, we highlight their unique electrochemical characteristics toward HER. Finally, the current key challenges in SACs for HER are pointed out and some potential directions are proposed as well.

Atomically Dispersed Pt on Screw-like Pd/Au Core-shell Nanowires for Enhanced Electrocatalysis

Engineering noble metal nanostructures at the atomic level can significantly optimize their electrocatalytic performance and remarkably reduce their usage. We report the synthesis of atomically dispersed Pt on screw-like Pd/Au nanowires by using ultrafine Pd nanowires as seeds. Au can selectively grow on the surface of Pd nanowires by an island growth pattern to fabricate surface defect sites to load atomically dispersed Pt, which can be confirmed by X-ray absorption fine structure measurements and aberration corrected HRTEM images. The nanowires with 2.74 at % Pt exhibit superior HER properties in acidic solution with an overpotential of 20.6 mV at 10 mA cm^-2 and enhanced alkaline ORR performance with a mass activity over 15 times greater than the commercial platinum/carbon (Pt/C) catalysts.

Nonmetal Doping as a Robust Route for Boosting the Hydrogen Evolution of Metal-Based Electrocatalysts

Recently, nonmetal doping has exhibited its great potential for boosting the hydrogen evolution reaction (HER) of transition-metal (TM)-based electrocatalysts. To this end, this work overviews the recent achievements made on the design and development of the nonmetal-doped TM-based electrocatalysts and their performance for the HER. It is also shown that by rationally doping nonmetal elements, the electronic structures of TM-based electrocatalysts can be effectively tuned and in turn the Gibbs free energy of the TM for adsorption of H* intermediates (ΔG_H* ) optimized, consequently enhancing the intrinsic activity of TM-based electrocatalysts. Notably, we highlight that concurrently doping two nonmetal elements can continuously and precisely regulate the electronic structures of the TM, thereby maximizing the activity for HER. Moreover, nonmetal doping also accounts for enhancing the physical properties of the TM (i.e. surface area). Therefore, nonmetal doping is a robust strategy for simultaneous regulation of the chemical and physical features of the TM.