Carbon-supported Pt^Ag nanostructures as cathode catalysts for oxygen reduction reaction

Pt(m)^Ag nanostructures (m being the atomic Pt/Ag ratio, m = 0.1-0.6) were prepared by reflux citrate reduction of PtCl(6)(2-) ions in aqueous solution containing colloidal Ag (6.3 ± 3.9 nm). A distinct alloying of Pt with Ag was detected due to an involvement of the galvanic replacement reaction between PtCl(6)(2-) and metallic Ag colloids. The nanostructure transformed from a structure with an Ag-core and an alloyed PtAg-shell to a hollow PtAg alloy structure with the increase in m. Compared to a commercial E-TEK Pt/C catalyst, the catalytic performance of Pt in the Pt(m)^Ag/C samples for the cathode oxygen reduction reaction (ORR) strongly correlated with the electronic structure of Pt, as a consequence of varied Pt dispersion and Pt-Ag interaction. With either H(2)SO(4) or KOH as an electrolyte, Pt in the Pt(m)^Ag nanostructures with a relatively high m (≥0.4) showed significantly enhanced intrinsic activity whereas Pt in those catalysts with low m (≤0.2) appeared less active than the Pt/C catalyst. These data are used to discuss the role of electronic structure and geometric effects of Pt toward ORR.     

Chalcogen cathode and its conversion electrochemistry in rechargeable Li/Na batteries

Chalcogen elements, such as sulfur(S), selenium(Se), tellurium(Te) and the interchalcogen compounds, have been studied extensively as cathode materials for the next-generation rechargeable lithium/sodium(Li/Na) batteries. The high energy output of the Li/Na-chalcogen battery originates from the two-electron conversion reaction between chalcogen cathode and alkali metal anode, through which both electrodes are able to deliver high theoretical capacities. The reaction also leads to parasitic reactions that deteriorate the chemical environment in the battery, and different cathode-anode combinations show their own features. In this article, we intend to discuss the fundamental conversion electrochemistry between chalcogen elements and alkali metals and its potential influence, either positive or negative, on the performance of batteries. The strategies to improve the conversion electrochemistry of chalcogen cathode are also reviewed to offer insights into the reasonable design of rechargeable Li/Nachalcogen batteries.     

Noble metal-free catalysts for oxygen reduction reaction

Developing noble metal-free catalysts with low cost, high performance and stability for oxygen reduction reaction(ORR) in fuel cells is of great interest to promote sustainable energy devices. In this review, we summarized noble metal-free catalysts for ORR,including non-noble metal-based and heteroatom-doped carbon nanomaterials. Mesoporous structure, homogeneous distribution of nanocrystals and synergistic effect of carbon base and nanocrystals/doped heteroatoms have great effect on the ORR property.The noble metal-free nanomaterials showed comparable catalytic property, better stability and methanol tolerance than commercial platinum(Pt)-based catalysts, showing great potential as substitutes for noble metal-based catalysts. In addition, the challenges and chances of developing noble metal-free ORR catalysts are also discussed.     

Hydrogen peroxide electrochemistry on platinum: towards understanding the oxygen reduction reaction mechanism

Understanding the hydrogen peroxide electrochemistry on platinum can provide information about the oxygen reduction reaction mechanism, whether H(2)O(2) participates as an intermediate or not. The H(2)O(2) oxidation and reduction reaction on polycrystalline platinum is a diffusion-limited reaction in 0.1 M HClO(4). The applied potential determines the Pt surface state, which is then decisive for the direction of the reaction: when H(2)O(2) interacts with reduced surface sites it decomposes producing adsorbed OH species; when it interacts with oxidized Pt sites then H(2)O(2) is oxidized to O(2) by reducing the surface. Electronic structure calculations indicate that the activation energies of both processes are low at room temperature. The H(2)O(2) reduction and oxidation reactions can therefore be utilized for monitoring the potential-dependent oxidation of the platinum surface. In particular, the potential at which the hydrogen peroxide reduction and oxidation reactions are equally likely to occur reflects the intrinsic affinity of the platinum surface for oxygenated species. This potential can be experimentally determined as the crossing-point of linear potential sweeps in the positive direction for different rotation rates, hereby defined as the "ORR-corrected mixed potential" (c-MP).     

Design of oxygen reduction bimetallic catalysts: ab-initio-derived thermodynamic guidelines

The Gibbs free energies of key elementary steps for the electrocatalytic oxygen reduction reaction (ORR) are calculated with B3LYP type of density functional theory: O2 + M + H+ + e- (0 eV) --> HOO-M (deltaG1), HOO-M + M --> HO-M + O-M (deltaG2), O2 + 2M + H+ + e- (0 eV) --> O-M + HO-M (deltaG3), and HO-M + O-M + 3H+ + 3e- (0 eV) --> 2H2O + 2M (deltaG4), where H+ is modeled as H3(+)O(H2O)3 and M stands for the adsorption site of a metal catalyst modeled by a single metal atom as well as by an M3 cluster. Taking Pt as a reference, deltaG4 is plotted against deltaG1 for 17 metals from groups V to XII. It is found that no single metal has both deltaG1 and deltaG4 more negative than Pt, although some of them have either more negative deltaG1 or more negative deltaG4. This enables us to explain thermodynamically why no other single metal catalyzes the ORR as effectively as Pt does. Moreover, a thermodynamic analysis reveals that the signs of delta deltaG (the difference between deltaG of other metals and deltaG of Pt) strongly correlate with the valence electronic structure of metals, i.e., delta deltaG1 < 0 and delta deltaG4 > 0 for metals M with vacant valence d orbitals, whereas delta deltaG1 > 0 and delta deltaG4 < 0 for metals M' with fully occupied valence d orbitals. Thus, a simple thermodynamic rule for the design of bimetallic catalysts for the ORR is proposed: couple a metal M (delta deltaG1 < 0) with a second metal M' (delta deltaG4 < 0) to form an alloy catalyst MM'3. The rationale behind this selection is based on M being more efficient for the rate-determining step, i.e., for the formation of the adsorbed species M-OOH, while M' can enhance the reductions of O and OH in the last three electron-transfer steps.     

A sodium vanadium three-fluorophosphate cathode for rechargeable batteries synthesized by carbothermal reduction

Na₃V₂(PO₄)₂F₃, which could be used as cathode in Na-ion or Li-ion rechargeable batteries, was synthesized by carbothermal reduction (CTR) method at 700 °C with coralline structure by SEM analysis. The Na₃V₂(PO₄)₂F₃ cathode in Li-ion battery displayed an appreciable capacity of 188 mAh/g with a discharge plateau at 3.7 V under close-to-equilibrium galvanostatic conditions illuminating the feasible extraction of three Na ions per unit, and 130 mAh/g with retention of 96.8% after 37 cycles at 0.1 C. The NASICON-type structure could produce a large diffusion coefficient due to the three-dimensional framework, but cause a low conductivity and OCV around 3 V. The cathode could reach 188 mAh/g at 0.1 C, with a more 20 mAh/g at 3.7 V after washing by distilled water.     

Rotating ring-disk electrode theory and method to correct quasi-four-electron oxygen reduction over Fe/N/C and N/C cathode catalysts

Understanding the mechanism of the oxygen reduction reaction over nonprecious metal catalysts is important for green and sustainable electrochemical energy conversion. This article presents our recent progress in kinetic studies using rotating ring-disk electrodes. We have established a mathematically and experimentally modified rotating ring-disk electrode approach to calculate the corresponding kinetic rate constants of the 4-e reduction of O₂ to H₂O (k₁), the 2-e reduction of O₂ to H₂O₂ (k₂), and the 2-e reduction of H₂O₂ to H₂O (k₃). Furthermore, the overestimation of the 4-e reduction process, which derives from the (2 + 2)-e pathway in the catalyst layer matrix, was corrected by studying the effect of catalyst loading density. The established method has been successfully applied to the oxygen reduction reaction over Fe/N/C and N/C catalysts in acidic and alkaline media.     

RuxCoySez纳米簇合物对阴极氧还原反应的催化性能

以Ru3（CO）12、Co4（CO）12和Se为原料,采用低温回流技术在1,6-己二醇中合成了RuxCoySez纳米簇合物.利用SEM和XRD测试表征了催化剂的微观形貌和相结构,催化剂粉末以六方结构的Rux簇为主相,同时形成无定形相,呈现高度均匀聚集的纳米颗粒.利用旋转圆盘电极研究了催化剂对氧还原反应电催化活性.在0.5molL-1H2SO4溶液中,RuxCoySez催化剂对氧还原的催化活性和电化学稳定性明显高于RuxSey,开路电位达到0.91V（vs.NHE）.     

Single-atom catalysts for CO oxidation,CO2 reduction,and O2 electrochemistry

CO_x(x=1,2)and O₂ chemistry play key roles in tackling global severe environmental challenges and energy issues.To date,the efficient selective electrocatalytic transformations of COx-carbon chemicals,and O₂-hydrogenated products are still huge challenges.Single-atom catalysts(SACs)as atomic-scale novel catalysts in which only isolated metal atoms are dispersed on supports shed new insights in overcome these obstacles in CO_x and O₂ chemistry,including CO oxidation,CO₂ reduction reaction(CO₂RR),oxygen reduction reaction(ORR),and oxygen evolution reaction(OER).In this review,the unique features and advanced synthesis strategies of SACs from a viewpoint of fundamental synthesis design are first highlighted to guide future strategy design for controllable SAC synthesis.Then,the to-date reported CO₂RR,CO oxidation,OER,and ORR mechanism are included and summarized.More importantly,the design principles and design strategies of improving the intrinsic activity,selectivity,and stability are extensively discussed and the engineering strategy is classified as neighbor coordination engineering,metal-atom engineering,and substrate engineering.Via the comprehensive review and summary of state-of-the-art SACs,the synthesis–structure–property–mechanism–design principle relation can be revealed to shed lights into the structural construction of SACs.Finally,we present an outlook on current challenges and future directions for SACs in CO_x and O₂ chemistry.     

Application of pyrolysed iron(II) phthalocyanine and CoTMPP based oxygen reduction catalysts as cathode materials in microbial fuel cells

The performance of iron(II) phthalocyanine (FePc) and cobalt tetramethoxyphenylporphyrin (CoTMPP) based oxygen reduction catalysts was studied in view of the application as cathode materials in microbial fuel cells. Galvanostatic and potentiostatic experiments were performed in order to compare the proposed materials to platinum and hexacyanoferrate(III) based systems. Additionally, two-chamber microbial fuel cell experiments were carried out to demonstrate that the transition metal based materials are well suitable to fully substitute the traditional cathode materials in microbial fuel cells.     

Facilitating oxygen reduction by silver nanoparticles on lanthanum strontium ferrite cathode

Journal of Solid State Electrochemistry - Single-phase silver (Ag)-doped La0.85-xSr0.15AgxFeO3-δ (x = 0–0.05) materials (LSAF) were synthesized by wet synthesis route and...     

Catalytic activity trends of oxygen reduction reaction for nonaqueous Li-air batteries

We report the intrinsic oxygen reduction reaction (ORR) activity of polycrystalline palladium, platinum, ruthenium, gold, and glassy carbon surfaces in 0.1 M LiClO(4) 1,2-dimethoxyethane via rotating disk electrode measurements. The nonaqueous Li(+)-ORR activity of these surfaces primarily correlates to oxygen adsorption energy, forming a "volcano-type" trend. The activity trend found on the polycrystalline surfaces was in good agreement with the trend in the discharge voltage of Li-O(2) cells catalyzed by nanoparticle catalysts. Our findings provide insights into Li(+)-ORR mechanisms in nonaqueous media and design of efficient air electrodes for Li-air battery applications.     

基于氮掺杂碳载铁复合物的锌空电池氧阴极催化剂

迫在眉睫的环境和能源问题推动人类探索可行、可靠和可再生的能源技术.锌-空气电池和氢氧燃料电池等器件显示出高能量转换效率,但是仍有许多难题有待克服,例如阴极侧上缓慢的氧还原反应(ORR),以及高昂的成本极大地限制了铂基催化剂在商业上的广泛应用.因此,开发高性能的廉价ORR催化剂具有重要意义.过渡金属碳氮化合物(M-N-C, M=Co, Fe等)成为最有希望替代铂基催化剂的一类材料, M-N-C催化剂可以通过直接热解含有过渡金属、氮和碳物种的前驱体合成.然而热解时金属原子易团聚,多孔结构不能被有效地控制,导致相对较差的催化活性.目前, MOF衍生的催化剂在能源转化和储存技术中得到了广泛的关注,其具有丰富的氮含量、高比表面积和可调的孔道结构等特点.本文报道了一种简便可靠可控的合成铁氮共掺杂碳十二面体纳米结构催化剂的方法,并作为阴极电催化剂用于锌空气电池中,测试结果证实,合成的铁氮共掺杂的纳米碳具有与铂基材料相当的活性和更加优异的稳定性.表面吸附了的邻菲罗啉铁的ZIF-8在碳化过程中,氮基团能够结合铁形成Fe Nx结构单元,因此可得到铁氮共掺杂的电催化剂.粉末X射线衍射,扫描电镜证实ZIF-8的成功合成.经过热解得到的催化剂中Fe Nx或Fe Cx衍射峰较弱,表明样品中铁含量较低,存在部分无定型铁.通过拉曼光谱分析发现,引入的邻菲罗啉在热解过程中诱导了缺陷的形成,所以Fe-NCDNA-0的ID/IG比值明显高于NC.同时ID/IG随着铁含量的增加而减少,这是因为铁可以诱导石墨化,诱导效应随着铁含量的增加而增加.分析氮气吸附-脱附等温线得出,引入邻菲罗啉之后,比表面积增加;而铁的引入因其占据了微孔结构,导致比表面积下降.同时电镜证实Fe-NCDNA-2具有较大的形貌扭曲,使得该材料具有较大的比表面积.系统的电化学研究表明,氮掺杂有利于增强ORR活性,在引入铁之后形成高效的活性中心会进一步提高催化性能.因此, Fe-NCDNA-2在碱性条件下表现出优异的ORR性能.线性扫描伏安法曲线表明,铁氮共掺杂的材料表现出与Pt/C相似的性能,其中Fe-NCDNA-2的半波电位(E1/2)为0.863 V,比商业Pt/C的电位更正(E1/2=0.841 V).同时, Fe-NCDNA-2具有更加优异的稳定性,测试30000 s后的电流保持率为80%(Pt/C:64%).在中性介质中,合成的材料也展示了较高的ORR活性.Fe-NCDNA-2的E1/2=0.715 V,催化30000 s后电流保持率77%,均优于商业Pt/C催化剂.组装的锌空气电池进一步验证其作为氧还原催化剂实际应用的可行性.相比于以Pt/C为催化剂做空气阴极的电池,以Fe-NCDNA-2组装的电池表现出更高的开路电压,更高的功率密度(184 m Wcm^-2),以及更加优异的充放电循环稳定性.该工作也有利于启发研究人员探索类似的氮掺杂过渡金属碳材料在各种催化上的应用.     

S-doped carbon aerogels/GO composites as oxygen reduction catalysts

Composites of carbon aerogel and graphite oxide(GO) were synthesized using a self-assembly method based on dispersive forces. Their surface was modified by treatment in hydrogen sulfide at 650 and800 ℃. The samples obtained were characterized by adsorption of nitrogen, TA-MS, XPS, potentiometric titration, and HRTEM and tested as catalysts for oxygen reduction reactions(ORR) in an alkaline medium.The synergistic effect of the composite(electrical conductivity, porosity and surface chemistry) leads to a good ORR catalytic activity. The onset potential for the composite of carbon aerogel heated at 800 ℃ is shifted to a more positive value and the number of electron transfer was 2e-at the potential 0.68 V versus RHE and it increased to 4e-with an increase in the negative values of the potential. An excellent tolerance to methanol crossover was also recorded.     

Transition metal-doped carbon sphere as enhanced catalysts for oxygen reduction

Herein, carbon sphere (CS-T) were successfully prepared by pyrolyzation melamine formaldehyde resin. And then different transition metals (Fe, Co, Ni) were doped on carbon sphere (CS-M-900). The scanning electron microscopes and elemental mappings prove that the transition metal particles are uniformly doped on the carbon sphere. Meanwhile, the X-ray photoelectron spectrum prove that the transition metals are zero-valence. Furthermore, the electrochemical testings showed the CS-Co-900 had more positive onset potential (0.93 V), half-wave potential (0.84 V), and peak potential (0.81 V). Furthermore, the CS-Co-900 had lower charge transfer resistance (44 Ω) and smaller Tafel slope (65 mV/dec). Most importantly, the oxygen reduction reaction on the CS-Co-900 turned out to be a four electron procedure with excellent methanol tolerance. The improved electrochemical properties towards oxygen reduction reactions of carbon sphere via cobalt doping suggested a design strategy towards future high-performance electrochemical devices.     

Transition metal-nitrogen-carbon catalysts for oxygen reduction reaction in neutral electrolyte

Platinum group metal-free (PGM-free) catalysts based on M-N-C types of materials with M as Mn, Fe, Co and Ni and aminoantipyrine (AAPyr) as N-C precursors were synthesized using sacrificial support method. Catalysts kinetics of oxygen reduction reaction (ORR) was studied using rotating ring disk electrode (RRDE) in neutral pH. Results showed that performances were distributed among the catalysts as: Fe-AAPyr > Co-AAPyr > Mn-AAPyr > Ni-AAPyr. Fe-AAPyr had the highest onset potential and half-wave potential. All the materials showed similar limiting current. Fe-AAPyr had an electron transfer involving 4e⁻ with peroxide formed lower than 5%. Considering H₂O₂ produced, it seems that Co-AAPyr, Mn-AAPyr and Ni-AAPyr follow a 2 × 2e⁻ mechanism with peroxide formed during the intermediate step. Durability test was done on Fe-AAPyr for 10,000 cycles. Decrease of activity was observed only after 10,000 cycles.     

Effect of oxygen on NO reduction over nickel chromite catalysts

Reduction rates of NO_x in HO_x+CO(H₂)+O₂ mixtures over a pure nickel chromite catalyst and samples supported on -Al₂O₃ and faience are high. At a space velocity of 10,000 h^–1, the complete reduction of nitrogen oxides by hydrogen and by carbon monoxide is achieved at 400–450°C and 450–500°C, respectively. Hence these catalysts can be recommended as a basis to develop commercial catalysts for NO_x removal from oxygen-containing exhaust gases.

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-Al₂O₃ NO_x NO_x–CO (H₂)–O₂ . 10 ·^–1 400–450°C, — 450–500°C. .

     

Dendritic PtCo alloy nanoparticles as high performance oxygen reduction catalysts

We studied the physical properties and the concentration profile of benzene+water+caprolactam mixtures near the fluid-fluid interface using self-consistent field (SCF) theory. This yields the interfacial tension which plays an important role in describing the stability of transient liquid droplets of one phase in the other. The studies were performed at a fixed temperature of 313K. Flory-Huggins binary interaction parameters and the compound lattice segment numbers are input parameters for the applied SCF theory. These parameters were derived from activity coefficient relations, which are used to describe experimental liquid-liquid and vapor-liquid phase equilibrium measurements. Using first principles, the benzene-water interface was studied and the resulting interfacial tension was found to be in agreement with experimental values. This study illustrates that caprolactam accumulates at the benzene-water interface, acting as a weak surfactant. The interfacial tension is also demonstrated to be affected by the caprolactam concentration and the SCF results are in fair agreement with the experimental observations.