Atomically dispersed Fe sites on hierarchically porous carbon nanoplates for oxygen reduction reaction

Developing cost-effective,robust and stable non-precious metal catalysts for oxygen reduction reaction(ORR) is of paramount importance for electrochemical energy conversion devices such as fuel cells and metal-air batteries.Although Fe-N-C single atom catalysts(SACs) have been hailed as the most promising candidate due to the optimal binding strength of ORR intermediates on the Fe-N_(4) sites,they suffer from serious mass transport limitations as microporous templates/substrates,i.e.,zeolitic imidazolate frameworks(ZIFs),are usually employed to host the active sites.Motivated by this challenge,we herein develop a hydrogen-bonded organic framework(HOF)-assisted pyrolysis strategy to construct hierarchical micro/mesoporous carbon nanoplates for the deposition of atomically dispersed Fe-N_(4) sites.Such a design is accomplished by employing HOF nanoplates assembled from 2-aminoterephthalic acid(NH_(2)-BDC) and p-phenylenediamine(PDA) as both soft templates and C,N precursors.Benefitting from the structural merits inherited from HOF templates,the optimized catalyst(denoted as Fe-N-C SAC-950) displays outstanding ORR activity with a high half-wave potential of 0.895 V(vs.reversible hydrogen electrode(RHE)) and a small overpotential of 356 mV at 10 mA cm^(-2) for the oxygen evolution reaction(OER).More excitingly,its application potential is further verified by delivering superb rechargeability and cycling stability with a nearly unfading charge-discharge gap of 0.72 V after 160 h.Molecular dynamics(MD) simulations reveal that micro/mesoporous structure is conducive to the rapid mass transfer of O_(2),thus enhancing the ORR performance.In situ Raman results further indicate that the conversion of O_(2) to~*O_(2)-the rate-determining step(RDS) for Fe-N-C SAC-950.This work will provide a versatile strategy to construct single atom catalysts with desirable catalytic properties.

质子交换膜燃料电池阴极非贵金属M-N_(x)/C型氧还原催化剂研究进展

氧还原反应(ORR)是燃料电池能量转换的关键步骤,开发高性能低成本的催化剂以替代铂族贵金属是推动燃料电池商业化的重要途径。本文综述了质子交换膜燃料电池非贵金属M-N_(x)/C型催化剂的最新研究进展,概括了氧还原反应的基础理论,系统展示了先进表征技术对活性位点鉴定和反应机理研究的作用,总结了M-N_(x)/C型催化剂近年来的代表工作和活性突破,阐述了稳定性问题的根源及对应的方案策略,我们认为M-N_(x)/C型催化剂未来的发展方向是理性设计具高位点密度和高稳定性的催化剂。

重塑位于火山曲线右支的弱吸附金属单原子位点的配位环境及电子结构

金属单原子位点独特的电子结构使其在燃料电池阴极氧还原催化反应(ORR)的应用引起广泛兴趣.金属单原子中心作为催化剂的活性位点遵循Sabatier原则,即位于火山型曲线右侧的金属中心与ORR中间产物的结合能力弱,使中间产物难以被位点活化,与之相反,左侧的位点则与中间产物的结合力太强,使中间产物难以脱附.因此,构建中间产物结合强度适中的位点可以提高催化剂活性.研究表明,通过改变金属中心的配位环境可以调控金属的电子结构,进而调节对ORR中间产物的吸附能力,提高催化剂活性.例如,对于位于火山曲线左侧的FeN4位点,通过在配位原子中引入高电负性的基团,如OH,S,Cl,减弱其对ORR中间产物的结合强度,使其活性向右侧活性顶点移动.然而,目前还没有提出有效的策略来改善火山曲线右侧位点的弱吸附能力.例如,Pd和Pt的纳米催化剂具有较高的ORR活性,而其单原子MN4位点却对ORR呈惰性,原因为高电负性的N使M具有较高的电荷密度,使其对中间产物的吸附能力太弱而位于火山曲线的最右边,具有较差的本征活性.为了缓解此类金属与配位N之间的不匹配的问题,我们提出通过利用电负性低的C(χp=2.55)代替N(χp=3.04)配位,以提升弱吸附位点的本征活性.本文制备了Ir-N-C,Pd-N-C,Pt-N-C,Ir-C,Pd-C和Pt-C六种催化剂;X射线衍射、透射电极和球差电镜结果表明,催化剂不含金属颗粒;同步辐射结果表明,催化剂呈M-N/C配位形式存在,且价态为氧化态.飞行时间二次离子质谱法(ToF-SIMS)测试可以检测到MNXCY和MCX的位点结构,结合同步辐射结果,认为活性位点分别是IrNXC((4-X)),IrC4,PdN_(2)CX和PdN3CX,PdC4,PtN3C1和PtN4,PtC4.电化学结果表明,相比N配位的催化剂,C配位的催化剂具有更高的半波电位、起始电位及本征活性,表明C配位更适合配位金属Ir,Pd和Pt,证明本文策略的可行性.为了进一步研究催化活性提升的原因,本文采用密度泛函理论计算探究催化机理,根据实验结果(X射线吸附光谱和ToF-SIMS)模拟了催化剂的活性位点结构,并计算反应中间产物的吸附能ΔGOH*.将ΔGOH*作为横坐标,反应的起始电位即催化活性作为纵坐标,绘制图谱得到火山型曲线.结果表明,相比N配位的金属位点,C配位的金属位点向强吸附端移动并靠近活性顶点,与实验结果一致,说明催化活性的提升是因为配位环境的改变改善了位点吸附能力.随后,通过计算活性位点的Bader电荷密度探究配位环境对电子结构的调控.结果显示,低电负性的C由于吸电子能力弱于高电负性的N,使MCX位点具有较低的Bader电荷密度,从而具有较强的中间产物吸附能力,进而提高催化活性.综上,本文提出了一个普遍性的原则,通过利用低电负性的元素配位调节具有弱结合能的金属(不仅是Ir,Pd,Pt)位点活性,为不同性质的金属匹配合适的配位环境,以实现高催化活性.

The decisive role of adsorbed OH^(*)in low‐potential CO electro‐oxidation on single‐atom catalytic sites

CO impurity-induced catalyst deactivation has long been one of the biggest challenges in proton-exchange membrane fuel cells,with the poisoning phenomenon mainly attributed to the overly strong adsorption on the catalytic site.Here,we present a mechanistic study that overturns this understanding by using Rh-based single-atom catalysis centers as model catalysts.We precisely modulated the chelation structure of the Rh catalyst by coordinating Rh with C or N atoms,and probed the reaction mechanism by surface-enhanced Raman spectroscopy.Direct spectroscopic evidence for intermediates indicates that the reactivity of adsorbed OH^(*),rather than the adsorption strength of CO^(*),dictates the CO electrocatalytic oxidation behavior.The RhN_(4)sites,which adsorb the OH^(*)intermediate more weakly than RhC4 sites,showed prominent CO oxidation activity that not only far exceeded the traditional Pt/C but also the RhC4 sites with similar CO adsorption strength.From this study,it is clear that a paradigm shift in future research should be considered to rationally design high-performance CO electro-oxidation reaction catalysts by sufficiently considering the water-related reaction intermediate during catalysis.

Design strategy and comprehensive performance assessment towards Zn anode for alkaline rechargeable batteries

Alkaline Zn-based primary batteries have been commercialized in the past decades.However,their success has not been extended to secondary batteries due to the poor cycle reversibility of Zn anodes.Although some research has been conducted on alkaline Zn anodes,their performance is still far from commercial requirements.A variety of degradation mechanisms,including passivation,dendrites,morphological changes,and hydrogen precipitation,are claimed responsible for the failure of alkaline Zn metal anodes.What’s worse,these constraints always interact with each other,which leads to a single strategy being unable to suppress all the issues.Therefore,a comprehensive evaluation of the positive and negative effects of various strategies on performance is important to promote the commercialization of alkaline Zn batteries.Herein,the recent progress and performance of improvement strategies for Zn anode in alkaline conditions are reviewed systematically.First,the principles and challenges of alkaline Zn anodes are briefly analyzed.Then,various design strategies for alkaline Zn anodes from the perspectives of ion and electron regulation are highlighted.Last,through a comprehensive summary of various performance parameters,the advantages and disadvantages of different strategies are compared and evaluated.On the basis of this assessment,we aim to provide more insights into the anode design of high-performance alkaline rechargeable Zn batteries.

采用薄层氮化碳促进的高性能钯基催化剂用于甲酸分解制氢

自第一次工业革命以来,传统的化石能源(煤炭,石油等)一直是能源消费的主体。但是,随着社会的进步和技术的发展,能耗不断增加。但是化石能源不仅储量有限,而且还会引起严重的环境问题(环境污染和温室效应)。因此,清洁和可持续能源的研究与开发尤为重要,氢能是研究的重点之一。由于氢具有高能量密度、清洁和可持续性的特点,因而成为了最有前景的能源载体。然而,氢气的储存和运输困难严重限制了其在质子交换膜燃料电池中的实际应用。作为液态氢存储材料之一,甲酸在催化剂存在下于室温下即可分解。另外,甲酸分解制氢的反应中不会释放有毒有害气体,对环境友好。用于甲酸分解(FAD)的高效催化剂是制氢的关键材料。本文制备了由薄层氮化碳促进的高性能钯(Pd)基催化剂,用于甲酸分解。首先,通过一步法直接煅烧三聚硫氰酸,以获得氮化碳(C_(3)N_(4)-S),然后制备以C_(3)N_(4)-S为载体的Pd基FAD催化剂(Pd/C_(3)N_(4)-S)。在三聚硫氰酸的热解过程中,―SH基团的溢出具有剥离作用,因此形成的C_(3)N_(4)为破碎的薄层,具有较大的比表面积和孔体积。由于改善的比表面积和孔体积以及大量的缺陷附着位点,C_(3)N_(4)-S载体可以有效地分散Pd纳米颗粒。此外,由于载体和金属之间的电子效应,该载体可以有效地调节催化剂表面上的Pd^(2+)含量。因此,Pd/C_(3)N_(4)-S表现优异的FAD性能。在30°C下,该催化剂可将甲酸有效分解为CO_(2)和H_(2),转换频率(TOF值)和质量比活性分别达到了2083 h−1和19.52 mol·g−1·h−1。并且气相色谱测试结果表明,气体产物中不含CO,表明Pd/C_(3)N_(4)-S催化剂具有优异的选择性。另外,Pd/C3N4-S催化剂也具有良好的稳定性。经过4次循环测试,催化性能仅下降了不到10%。该研究为研究高性价比、制备方法简单的甲酸制氢催化剂提供了一定的指导作用。

Recent development of methanol electrooxidation catalysts for directmethanol fuel cell

Direct methanol fuel cells(DMFCs) are very promising power source for stationary and portable miniature electric appliances due to its high efficiency and low emissions of pollutants. As the key material, catalysts 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 methanol electrooxidation on Pt and Pt-based catalysts in acidic medium have been introduced. The influences of size effect and morphology on electrocatalytic activity are discussed though whether there is a size effect in MOR catalyst is under debate. Besides, the non Pt catalysts are also listed to emphasize though Pt is still deemed as the indispensable element in anode catalyst of DMFCs in acidic medium. Different catalyst systems are compared to illustrate the level of research at present. Some debates need to be verified with experimental evidences.

Correlating Fe source with Fe-N-C active site construction： Guidancefor rational design of high-performance ORR catalyst

Pyrolyzed Fe-N_X/C materials derived from Fe-doped ZIF-8 are recently emerged as promising alternatives to noble metal platinum-based catalysts towards oxygen reduction reaction(ORR) and elucidating the dependacne of Fe source on the active site structure and final ORR performance is highly desirbale for further development of these materials. Here, we designed and synthesized a series of Fe-N-C catalysts using ZIF-8 and various iron salts(Fe(acac)_3, FeCl_3, Fe(NO_3)_3) as precusors. We found that the iron precursors,mainly the molecular size, hydrolysis extent, do play a major role in determining the final morphology of Fe, namely forming the Fe-Nx coordination or Fe_3C nanoparticles, as well as the site density, therefore,significantly affecting the ORR activity. Among the three iron sources, Fe(acac)_3 is most advantageous to the preferential formation of single-atom Fe-Nx active sites and the derived catalyst demonstrated best ORR performance.

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.

Hydrogen etching induced hierarchical meso/micro-pore structure with increased active density to boost ORR performance of Fe-N-C catalyst

Rational regulation on pore structure and active site density plays critical roles in enhancing the performance of Fe-N-C catalysts. As the microporous structure of the carbon substrate is generally regarded as the active site hosts, its hostility to electron/mass transfer could lead to the incomplete fulfillment of the catalytic activity. Besides, the formation of inactive metallic Fe particles during the conventional catalyst synthesis could also decrease the active site density and complicate the identification of real active site. Herein, we developed a facial hydrogen etching methodology to yield single site Fe-N-C catalysts featured with micro/mesoporous hierarchical structure. The hydrogen concentration in pyrolysis process was designated to effectively regulate the pore structure and active site density of the resulted catalysts.The optimized sample achieves excellent ORR catalytic performance with an ultralow H2O2 yield(1%)and superb stability over 10,000 cycles. Our finding provides new thoughts for the rational design of hierarchically porous carbon-based materials and highly promising non-precious metal ORR catalysts.

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℃.

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'.

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].

A new pathway for formic acid electro-oxidation:The electro-chemically decomposed hydrogen as a reaction intermediate

Formic acid electro-oxidation reaction(FAOR)is generally believed that follows a two-pathway mechanism.Herein,we resorted to in situ electrochemical mass spectrometry and successfully captured the trace of H_(2),as the new intermediate species,during the process of FAOR on both Pt based catalyst and two single atom catalysts(Rh-N-C and Ir-N-C).Inspired by this,we proposed a new reaction path named hydrogen oxidation pathway:at the oxidation potential,formic acid will break the C–H bond and combine with the protons in the solution to form H_(2) species,then hydrogen oxidation reaction(HOR)will occur to generate two protons.This process is accompanied by electron transfer and contributes currently to the whole reaction.

Stabilizing high-efficiency iridium single atoms via lattice confinement for acidic oxygen evolution

Stable and efficient single atom catalysts(SACs)are highly desirable yet challenging in catalyzing acidic oxygen evolution reaction(OER).Herein,we report a novel iridium single atom catalyst structure,with atomic Ir doped in tetragonal PdO matrix(IrSAs-PdO)via a lattice-confined strategy.The optimized IrSAs-PdO-0.10 exhibited remarkable OER activity with an overpotential of 277 mV at 10 mA·cm^(-2) and long-term stability of 1000 h in 0.5 M H_(2)SO_(4).Furthermore,the turnover frequency attains 1.6 s^(-1) at an overpotential of 300 mV with a 24-fold increase in the intrinsic activity.The high activity originates from isolated iridium sites with low valence states and decreased Ir–O bonding covalency,and the excellent stability is a result of the effective confinement of iridium sites by Ir–O–Pd motifs.Moreover,we demonstrated for the first time that SACs have great potential in realizing ultralow loading of iridium(as low as microgram per square center meter level)in a practical water electrolyzer.

CO耐受的单位点/纳米颗粒协同型质子交换膜燃料电池阳极催化剂

Nanosized Pt catalysts are the catalyst-of-choice for proton exchange membrane fuel cell(PEMFC)anode,but are limited by their extreme sensitivity to CO in parts per million(ppm)level,thereby making the use of ultrapure H_(2)a prerequisite to ensure acceptable performance.Herein,we confront the CO poisoning issue by bringing the Ir/Rh single atom sites to synergistically working with their metallic counterparts.In presence of 1000 ppm CO,the catalyst represents not only undisturbed H_(2)oxidation reaction(HOR)catalytic behavior in electrochemical cell,but also unparalleled peak power density at 643 mW cm^(-2)in single cell,27-fold in mass activity of the best PtRu/C catalysts available.Pre-poisoning experiments and surface-enhanced Raman scattering spectroscopy(SERS)and calculation results in combine suggest the presence of adjacent Ir/Rh single atom sites(SASs)to the nanoparticles(NPs)as the origin for this prominent catalytic behavior.The single sites not only exhibit superb CO oxidation performance by themselves,but can also scavenge the CO adsorbed on approximated NPs via supplying reactive OH*species.We open up a new route here to conquer the formidable CO poisoning issue through single atom and nanoparticle synergistic catalysis,and pave the way towards a more robust PEMFC future.

Suppressing the lattice oxygen diffusion via high-entropy oxide construction towards stabilized acidic water oxidation

The scale-up deployment of ruthenium(Ru)-based oxygen evolution reaction(OER)electrocatalysts in proton exchange membrane water electrolysis(PEMWE)is greatly restricted by the poor stability.As the lattice-oxygen-mediated mechanism(LOM)has been identified as the major contributor to the fast performance degradation,impeding lattice oxygen diffusion to inhibit lattice oxygen participation is imperative,yet remains challenging due to the lack of efficient approaches.Herein,we strategically regulate the bonding nature of Ru–O towards suppressed LOM via Ru-based high-entropy oxide(HEO)construction.The lattice disorder in HEOs is believed to increase migration energy barrier of lattice oxygen.As a result,the screened Ti_(23)Nb_(9)Hf_(13)W_(12)Ru_(43)O_(x) exhibits 11.7 times slower lattice oxygen diffusion rate,84%reduction in LOM ratio,and 29 times lifespan extension compared with the state-of-the-art RuO_(2) catalyst.Our work opens up a feasible avenue to constructing stabilized Ru-based OER catalysts towards scalable application.

Identification of active sites and synergistic effect in multicomponent carbon-based Ru catalysts during electrocatalytic hydrogen evolution

Single atom catalysts(SACs)were reported to demonstrate exciting catalytic features for a number of reactions,including hydrogen evolution reaction(HER).However,the true role of these single atom sites in catalysts remains elusive,particularly for those prepared via pyrolysis,where the formation of active nanoparticle counterparts is often unavoidable.Here we report a Ru based catalyst(Ru embedded in N doped carbon spheres(Ru/NPCS))comprising of both Ru nanoclusters and Ru single sites,who demonstrates activity exceeding Pt catalyst and mass activity among the best of the Ru based catalysts under acidic conditions.The integration of proton exchange membrane water electrolysis with Ru/NPCS as a cathode exhibited an excellent hydrogen generation activity and extraordinary stability(during 120 h of electrolysis)with a 1/48 Ru loading(16.5μgRu·cm^(−2))of a commercial 20%Pt/C catalyst.Through precisely tailoring the dispersion status of the catalysts,we reveal that while ruthenium nanoclusters actively catalyze HER via Volmer–Tafel mechanism,the Ru SACs barely catalyze HER,with H*adsorption difficult to occur.Moreover,no synergy between Ru SACs and Ru cluster is revealed,meaning the Ru SACs act as a spectator rather than active species during H2 evolution.

Polymer-chelation approach to high-performance Fe-Nx-C catalyst towards oxygen reduction reaction

Pyrolyzed Fe-Nx-C with atomically dispersed Fe-Nxsites are hailed as the most promising alternative to the noble metal Pt-based catalysts towards oxygen reduction reaction(ORR). However, the conventional micropore-confinement synthetic approach usually causes the insufficient utilization of active sites and mass transport resistance as the sites are located inside the micropore. We herein report a polymerchelation strategy to directly disperse the Fe-Nxactive sites onto the carbon surface. The N-rich monomer was in-situ polymerized on the carbon support and then chelated with Fe. The strong Fe-N chelating interaction is crucial to suppress Fe aggregation when undergoing the high-temperature pyrolysis. Due to the enriched surface sites, hierarchically porous structure and excellent conductivity of carbon support,the optimal catalyst(denoted as Fe-Nx-C@C-900) exhibits impressive ORR activity of onset and half-wave potential of 1.02 and 0.87 V, respectively, superior to the Pt/C benchmark.

Formic acid electro-oxidation:Mechanism and electrocatalysts design

As a model reaction for the electrooxidation of many small organic molecules,formic acid electrooxidation(FAEO)has aroused wide concern.The promises of direct formic acid fuel cells(DFAFC)in application further strengthen people’s attention to the related research.However,despite decades of study,the FAEO mechanism is still under debate due to the multi-electron and multi-pathway nature of the catalytic process.In this review,the progresses towards understanding the FAEO mechanism along with the developed methodology(electrochemistry,in-situ spectroscopy,and theoretical calculation and simulation)are summarized.We especially focused on the construction of anti-poisoning catalysts system based on understanding of the catalytic mechanism,with anti-poisoning catalyst design being systemically summarized.Finally,we provide a brief summarization for current challenges and future prospects towards FAEO study.