Low-temperature synthesis of nitrogen doped carbon nanotubes as promising catalyst support for methanol oxidation

The electrochemical methanol oxidation reaction(MOR) is of paramount importance for direct methanol fuel cell(DMFC) application, where efficient catalysts are required to facilitate the complicated multiple charge transfer process. The catalyst support not only determines the dispersion status of the catalysts particles, but also exerts great influence on the electronic structure of the catalysts, thereby altering its intrinsic activity. Herein, we demonstrated that nitrogen atoms, assisted by the pre-treatment of carbon matrix with oxidants, can be easily doped into carbon nanotubes at low temperature. The obtained nitrogen-doped carbon nanotubes can effectively improve the dispersion of the supported platinum nanoparticles and facilitate the MOR by modifying the electronic structure of platinum atoms,through catalyst-support interaction.

Tuning strategies and structure effects of electrocatalysts for carbon dioxide reduction reaction

Carbon dioxide emissions have increased due to the consumption of fossil fuels,making the neutralization and utilization of CO_(2) a pressing issue.As a clean and efficient energy conversion process,electrocatalytic reduction can reduce carbon dioxide into a series of alcohols and acidic organic molecules,which can effectively realize the utilization and transformation of carbon dioxide.This review focuses on the tuning strategies and structure effects of catalysts for the electrocatalytic CO_(2) reduction reaction(CO_(2)RR).The tuning strategies for the active sites of catalysts have been reviewed from intrinsic and external perspectives.The structure effects for the CO_(2)RR catalysts have also been discussed,such as tandem catalysis,synergistic effects and confinement catalysis.We expect that this review about tuning strategies and structure effects can provide guidance for designing highly efficient CO_(2)RR electrocatalysts.

Building Fe atom–cluster composite sites using a site occupation strategy to boost electrochemical oxygen reduction

The high-temperature pyrolysis process for preparing M–N–C single-atom catalyst usually results in high heterogeneity in product structure concurrently contains multiscale metal phases from single atoms(SAs),atomic clusters to nanoparticles.Therefore,understanding the interactions among these components,especially the synergistic effects between single atomic sites and cluster sites,is crucial for improving the oxygen reduction reaction(ORR)activity of M–N–C catalysts.Accordingly,herein,we constructed a model catalyst composed of both atomically dispersed FeN4 SA sites and adjacent Fe clusters through a site occupation strategy.We found that the Fe clusters can optimize the adsorption strength of oxygen reduction intermediates on FeN4 SA sites by introducing electron-withdrawing–OH ligands and decreasing the d-band center of the Fe center.The as-developed catalyst exhibits encouraging ORR activity with halfwave potentials(E1/2)of 0.831 and 0.905 V in acidic and alkaline media,respectively.Moreover,the catalyst also represents excellent durability exceeding that of Fe–N–C SA catalyst.The practical application of Fe(Cd)–CNx catalyst is further validated by its superior activity and stability in a metalair battery device.Our work exhibits the great potential of synergistic effects between multiphase metal species for improvements of singleatom site catalysts.

Recent development of methanol electrooxidation catalysts for directmethanol fuel cell

Direct methanol fuel cells （DMFCs） are very promising power source for stationary and portable miniatureelectric appliances due to its high efficiency and low emissions of pollutants. As the key material, cata-lysts for both cathode and anode face several problems which hinder the commercialization of DMFCs.In this review, we mainly focus on anode catalysts of DMFCs. The process and mechanism of methanolelectrooxidation on Pt and Pt-based catalysts in acidic medium have been introduced. The influences ofsize effect and morphology on electrocatalytic activity are discussed though whether there is a size effectin MOP, catalyst is under debate. Besides, the non Pt catalysts are also listed to emphasize though Pt isstill deemed as the indispensable element in anode catalyst of DMFCs in acidic medium. Different cata-lyst systems are compared to illustrate the level of research at present. ome debates need to be verifiedwith experimental evidences.

Engineering the HER catalytic behavior of heteroatom-doped molybdenum disulfide via versatile partial cation exchange

Water electrolysis is regarded as an environmental friendly and effective technique for large-scale hydrogen(H2)production[1,2].To date,Pt-based electrocatalysts are still the most efficient HER catalysts[3].However,the prohibitive cost and scarcity of precious metal catalysts have restricted its large-scale applications.Thus,finding an earth-abundant and effective alternative electrocatalysts is crucial to the development of‘hydrogen economy'.

Meta-analysis of commercial Pt/C measurements for oxygen reduction reactions via data mining

The rotating disk electrode technique is commonly used for screening and characterizing the performance of electrocatalysts for the oxygen reduction reaction(ORR).However,a reliable performance comparison of different electrocatalysts from different labs remains a challenge because of the inconsistency in the measurement of commercial Pt/C.Commercial Pt/C has been adopted extensively as a reference for evaluating the ORR performance of a new electrocatalyst.However,the reported ORR performances of commercial Pt/C from different labs could be significantly different because of multiple factors.Herein,we conducted a meta‐analysis of the ORR performance of commercial Pt/C via data mining of the literature.This revealed the optimal testing conditions for the most repeatable ORR performance,with commercial Pt/C in both acid and alkaline electrolytes;the optimal Pt loading was 20μg/cm^(2) on a 4 mm glassy carbon working electrode.The value of 0.84±0.03 V was suggested as the“Golden reference”of the commercial Pt/C(with Pt 20 wt%)ORR half‐wave potential for the performance evaluation of other ORR catalysts in both acid and alkaline electrolytes.The conclusion obtained through the meta‐analysis was confirmed by experiments.This study provides general guidance for a reliable measurement of the ORR performance of commercial Pt/C as a reference.

Key role of electron accessibility at the noble metal-free catalytic interface in hydrogen evolution reaction

The reactant concentration at the catalytic interface holds the key to the activity of electrocatalytic hydrogen evolution reaction(HER),mainly referring to the capacity of adsorbing hydrogen and electron accessibility.With hydrogen adsorption free energy(ΔGH)as a reactivity descriptor,the volcano curve based on Sabatier principle is established to evaluate the hydrogen evolution activity of catalysts.However,the role of electron as reactant received insufficient attention,especially for noble metal-free compound catalysts with poor conductivity,leading to cognitive gap between electronic conductivity and apparent catalytic activity.Herein we successfully construct a series of catalyst models with gradient conductivities by regulating molybdenum disulfide(MoS_(2))electronic bandgap via a simple solvothermal method.We demonstrate that the conductivity of catalysts greatly affects the overall catalytic activity.We further elucidate the key role of intrinsic conductivity of catalyst towards water electrolysis,mainly concentrating on the electron transport from electrode to catalyst,the electron accumulation process at the catalyst layer,and the charge transfer progress from catalyst to reactant.Theoretical and experimental evidence demonstrates that,with the enhancement in electron accessibility at the catalytic interface,the dominant parameter governing overall HER activity gradually converts from electron accessibility to combination of electron accessibility and hydrogen adsorbing energy.Our results provide the insight from various perspective for developing noble metal-free catalysts in electrocatalysis beyond HER.

Heterostructured Pt-Ni_(3)Mo_(3)N formed via ammonia-containing polyoxometalates for highly efficient electrocatalytic hydrogen evolution in acid medium

Constructing heterostructured nanohybrid is considered as a prominent route to fabricate alternative electrocatalysts to commercial Pt/C for hydrogen evolution reaction(HER).In this work,(NH_(4))_(4)[NiH_(6)Mo_(6)O_(4)]·5H_(2)O polyoxometalates(NiMo_(6))are adopted as the cluster precursors for simple fabrication of heterostructured Pt-Ni_(3)Mo_(3)N nanohybrids supported by carbon black(Pt-Ni_(3)Mo_(3)N/C)without using additional N sources.The improved porosity and enhanced electronic interaction of Pt-Ni_(3)Mo_(3)N/C should be attributed to the integration of Pt with NiMo_(6),which favors the mass transport,promotes the formation of exposed catalytic sites,and benefits the regulation of intrinsic activity.Thus,the as-obtained Pt-Ni_(3)Mo_(3)N/C exhibits impressive and durable HER performance as indicated by the low overpotential of 13.7 mV at the current density of 10 mA cm^(-2) and the stable overpotential during continuous working at 100 mA cm^(-2) for 100 h.This work provides significant insights for the synthesis of new highly active heterostructured electrocatalysts for renewable energy devices.

Molecule aging induced by electron attacking

Here we propose a new concept of"molecule aging":with some special treatment,a molecule could be"aged"by losing some unknown tiny particles or pieces from atoms in the molecule,Such"aging"or loss of unknown tiny particles does not change apparently its molecular structure or chemical composition,but some physicochemical properties could be changed irreversibly.We further confirm such"molecule aging"via a long-term electron attacking to age water(H_(2)O)molecules.The IR spectra show no structural difference between the fresh water and the aged one,while the NMR spectra show that the electron attacking can decrease the size of water clusters.Such facts indicate that the electron attacking indeed can"affect"the structure of water molecule slightly but without damaging to its basic molecule frame.Further exploration reveals that the hydrogen evolution reaction(HER)activity of the aged water molecule is lower than the fresh water on the same Pt/C electrocatalyst.The density functional theory calculations indicate that the shortened O-H bond in H_(2)O indeed can present lower HER activity,so the observed size decrease of water clusters from NMR probably could be attributed to the shortening of O-H bond in water molecules.Such results indicate significantly that the molecule aging can produce materials with new functions for new possible applications.

Surface interaction between Pd and nitrogen derived from hyperbranched polyamide towards highly effective formic acid dehydrogenation

Hydrogen production from formic acid decomposition(FAD)is a promising means of hydrogen energy storage and utilization in fuel cells.Development of efficient catalysts for dehydrogenation of formic acid is a challenging topic.The surface chemical and electronic structure of the active catalysis components is important in formic acid decomposition at room-temperature.Here,the pyrdinic-nitrogen doped catalysts from hyperbranched polyamide were prepared via in situ polymerization reaction process by using activated carbon as a support.Because of the introduction of the polymer,the particles of the catalysts were stabilized,and the average particle diameter was only 1.64 nm.Under mild conditions,the catalysts activities were evaluated for FAD.The optimized Pd-N30/C catalyst exhibited high performance achieving almost full conversion,with a turnover frequency of 3481 h^-1 at 30℃.

Highly active PtAu alloy surface towards selective formic acid electrooxidation

The electrochemical oxidation of formic acid has been attracting significant attention in the past few years due to the great potential prospect of direct formic acid fuel cell (DFAFC) in applications, including high theoretical open circuit potential (1.48 V), low fuel crossover, high practical power densities at low temperature, facilitating of proton transport in catalyst layers and low toxicity [1-5].

Metal−support interaction in single-atom electrocatalysts:A perspective of metal oxide supports

The discovery of single-atom catalysts(SACs)represents a groundbreaking advancement in the field of catalysis over the past decades.With the in-depth exploration of relevant structure-activity relationships,the metal−support interaction(MSI)is widely adopted to elucidate variations in electronic structure and coordination configuration of atomic active sites on various kinds of supports.Herein,we briefly summarize the metal oxide supports for SACs fabrication,including the distinctive characteristics of metal oxide supports,enlightening advancements in metal oxide support-based SACs(MO-SACs),feasible preparation methods for MO-SACs and effective regulation strategies of MSI effect in MO-SACs.In addition,we present our viewpoints and outlook in this field to stimulate rational design and construction of novel MO-SACs applied in diverse renewable energy devices,while some universal suggestions are sincerely given to provoke thoughtful considerations during the research process.

Insights into the Dynamic Surface Reconstruction of Electrocatalysts in Oxygen Evolution Reaction

Electrocatalytic water splitting,which is recognized as an ideal technology to tackle escalating energy demands and related environmental problems,has attracted growing interest.The sluggish dynamics of the oxygen evolution reaction(OER)has posed an intractable problem in this regard,hindering the practical commercial application of hydrogen production via water splitting.Therefore,the development of active and stable electrocatalysts is a prerequisite for accelerating OER kinetics,which greatly relies on the mechanistic understanding of the structural-property relationship.Owing to the harsh anodic oxidation conditions,most of the catalysts undergo surface reconstruction during the OER process,which means the authentic active sites are the in-situ reconstructed species rather than the freshly prepared one.In this regard,fully comprehending the surface reconstruction process will help us to determine the active sites on the catalyst surface and gain insights into the design principles for more efficient OER catalysts.In this review,we will first give a summary of surface reconstruction of OER electrocatalysts.Then we will focus on the factors that affect surface reconstruction,in-situ/operando characterization technologies,and the strategies to govern surface reconstruction.In addition,we outline existing challenges and the outlook for the development of OER catalysts by tuning surface reconstruction.

Recent advances in active sites identification and regulation of M-N/C electro-catalysts towards ORR

Transition metal and nitrogen co-doped carbon(M–N/C) catalysts are recognized as the most prospective alternatives for platinum-based electro-catalysts towards oxygen reduction reaction(ORR) in polymer electrolyte fuel cells. Recently, significant progress has been achieved in the identification and regulation of active sites of this kind of catalysts. In this mini review,we summarize the techniques and strategies to identify active sites in M–N/C catalysts, the main debates on active sites types, the measurement method for active site density, the reactivity descriptors for M–N/C catalysts, and directions to the design of ORR M–N/C catalysts.

Direct self-assembly of CTAB-capped Au nanotriangles

Densely packed and ordered ＂suprastructures＂ are new types of nanomaterials exhibiting broad applications. The direct self-assembly of cetyltrimethylammonium bromide （CTAB）-capped gold nanotriangles to form ＂suprastructures＂ was systematically investigated by varying the temperature and tilt angle of the silicon wafer used in the assembly process. Under optimal conditions, nanotriangles form into regular patterns, maintain their integrity, and form edge-to-edge, point-to-point, and face-to-face connections to form ordered ＂suprastructures＂ within an area of hundreds of square microns, achieving a high level of regularity. The formation of the ＂suprastructures＂ under optimal conditions could be mainly attributed to the complex balance between multiple temperature-dependent factors, including the atom diffusion rate, solvent evaporation rate, self-assembly rate, and the time for which the nanoparticle stays in the wet medium.

Single-Molecule Fluorescence Imaging of Nanocatalysis

Single-molecule fluorescence microscopy(SMFM)has been considered as a powerful tool to study nanocatalysis of single nanoparticles,due to its single-molecule sensitivity and high spatiotemporal resolution.In this review,we discuss recent progresses on investigating nanocatalysis at single-mol-ecule/particle level by using SMFM.The discussion focuses on the applications of single-molecule methods in probing the chemocatalysis,electrocatalysis,photocatalysis and photoelectrocatalysis.Finally,we provide our opinions on limitations and prospects of the single-molecule fluorescence approach for investigating nanocatalysis.

Oxygen-vacancy-rich TiO_(2)enables highly active and durable water electrolysis of urchin-like RuO_(2)catalyst

The material innovation is prerequisite to accelerating sluggish oxygen evolution reaction(OER)kinetics,thus promoting the realization of hydrogen energy community.Herein,we develop an oxygen-vacancy-rich TiO_(2)supported RuO_(2)catalyst(RuO_(2)@r-TiO_(2))towards improved OER activity and stability.The oxygen vacancy on TiO_(2)not only supplies electrons to produce lower valence Ru,but also provides sufficient anchoring site for the deposition of RuO_(2)nanocrystal.Beyond that,it can generate strong electronic interaction between TiO_(2)and supported RuO_(2),and thereby tailors the intermediates’adsorption energy on the RuO_(2)surface.As a result,the derived RuO_(2)@r-TiO_(2)catalyst exhibits superior OER activity and stability with the overpotential of 211 mV at a current density of 10 mA cm^(−2)and negligible activity degradation after 6 h operation,outperforming the non-oxygen-vacancy counterpart(223.3 mV,12.75%activity loss)and RuO_(2)catalyst(234.6 mV,42.86%activity loss).

High-performance oxygen reduction catalysts in both alkaline and acidic fuel cells based on pre-treating carbon material and iron precursor

We demonstrate a new and simple method for pre-treating the carbon material and iron precursor to prepare oxygen reduction reaction(ORR) catalysts, which can produce super-high performance and stability in alkaline solution, with high performance in acid solution. This strategy using cheap materials is simply controllable. Moreover, it has achieved smaller uniform nanoparticles to exhibit high stability, and the synergetic effect of Fe and N offered much higher performance in ORR than commercial Pt/C, with high maximum power density in alkaline and acid fuel cell test. So it can make this kind of catalysts be the most promising alternatives of Pt-based catalysts with best performance/price.

Fe,Cu-codoped metal-nitrogen-carbon catalysts with high selectivity and stability for the oxygen reduction reaction

Metal-nitrogen-carbon materials(M-N-C) are non-noble-metal-based alternatives to platinum-based catalysts and have attracted tremendous attention due to their low-cost,high abundance,and efficient catalytic performance towards the oxygen reduction reaction(ORR).Among them,Fe-based materials show remarkable ORR activity,but they are limited by low selectivity and low stability.To address these issues,herein,we have synthesized FeCu-based M-N-C catalysts,inspired by the bimetal center of cytochrome c oxidase(CcO).In acidic media,the selectivity was notably improved compared with Febased materials,with peroxide yields less than 1.2%(<1/3 of the hydrogen peroxide yields of Fe-N-C catalysts).In addition to Cu-N-C catalysts which can catalytically reduce hydrogen peroxide,the reduction current of hydrogen peroxide using FeCu-N-C-20 exceeded that of Fe-N-C by about 6% when the potential was greater than 0.4 V.Furthermore,FeCu-based M-N-C catalysts suffered from only a15 mV attenuation in their half-wave potentials after 10,000 cycles of accelerated degradation tests(ADT),while there was a 30 mV negative shift for Fe-N-C.Therefore,we propose that the H_(2)O_(2) released from Fe-Nx sites or N-doped carbon sites would be reduced by adjacent Cu-Nx sites,re sulting in low H_(2)O_(2) yields and high stability.

Revealing the true origin of size-dependent Pd/C catalytic behavior towards formic acid decomposition

Formic acid decomposition(FAD)is considered a promising hydrogen production route to facilitate the ambient storage and on demand release of hydrogen energy.To optimize the catalysts for FAD,efforts have been paid to explore the underlying reason for the varied catalytic activity among catalysts with similar composition but differed structure.However,such endeavors are highly challenging due to the deeply intermingled effects of electronic structure,particle size,and facets,etc.Herein,to separately evaluate the respective effects of these factors,a series of catalysts with the same surface electronic structure and different particle size was prepared by cation dipole adjustment method.The performance and characterization results showed that the catalysts with different sizes and facets exhibited similar intrinsic activity with deviation of less than 5%.However,they showed 252%deviation of site stability,indicating that only the optimized electronic structure could enhance the intrinsic activity and a smaller particle size could extend the catalyst’s life.