碳模板诱导生长Fe-N_x活性位点（英文）

能源危机和环境恶化是当今社会面临的巨大挑战.燃料电池作为一种高效、清洁的发电装置,受到了社会各界特别是新能源行业的高度关注.尤其是,日本丰田推出Mirai燃料电池汽车量产上市计划,把燃料电池及其关键技术发展推向了一个新的发展纪元.然而,制约燃料电池走向大规模商业化的核心问题依然是其综合性能不具竞争力.其中,氧电极的缓慢动力学以及贵金属Pt的有限资源、高昂成本等是关键所在,因此,亟待实现高性能非贵金属催化剂的突破.近年来,大量研究表明,Fe-N_x掺杂的碳催化剂具有极大的代Pt潜力,研究者们尝试各种手段进行开发,如:调控Fe化合物及N前驱体的类型与添加量,改变温度、压力等合成条件,采用轴向配位体连接、共价接枝、球磨等非热解路线,构建核壳、有序介孔碳、阵列、类石墨烯薄片、多孔碳等碳纳米结构,制备石墨烯/碳纳米管、石墨烯/碳黑、碳纳米带/碳纳米管、碳纳米颗粒/碳纤维、碳球/碳纳米管/石墨烯等复合材料,进行酸洗、造孔、二次加热等后处理,调控不同类型Fe物种相生成等.此外,EXAFS及M?sbauer等谱学技术已经证实Fe-N_x特别是Fe-N_4为强活性位点.因此,有待提出合理策略以促进非贵金属碳催化剂中Fe-N_x强活性位点的高密度掺杂.本文提出了一种碳模板诱导Fe-N_x活性位点生长的方法即通过高温热解含有Fe盐的三聚氰胺前驱体混合物,成功制备了Fe-N_x掺杂的碳催化剂,并结合多种表征技术证实了碳模板对制备碳催化剂结构组成及电化学性能的影响.形貌结果说明,碳模板的引入有利于Fe、N化合物的均匀吸附以至于Fe基纳米颗粒的均一成核,促使竹状碳纳米管在碳模板表面以及中间均一生长;氮气吸脱附及孔径分布曲线显示,引入碳模板形成的复合材料较单一的碳纳米管和碳黑材料具有提高的比表面积和总孔体积,说明复合材料中存在两种单体的有效协同;M?sbauer、XPS及XRD测试数据证实,碳模板可以调控Fe、N两种元素的耦合方式,能够抑制金属Fe和Fe碳化物等非活性Fe物种的生成、诱导Fe-N4和其它Fe氮化物等强活性Fe-N_x物种的生长.电化学测试数据表明,复合材料具有提升的面积活性和质量活性,且TOF值明显提高,说明碳模板的引入增强了Fe-N_x位点的本征活性;此外,复合材料的氧还原过程为高效的4e–途径,且较商业Pt/C催化剂表现出了优异的循环稳定性和甲醇耐受性     

Fe-N共掺杂的碳纳米管串联空心球对氧还原反应的电催化

以ZIF-8为模板,通过表面包覆聚多巴胺、同时刻蚀ZIF-8中的Zn²⁺,形成空心球,在与三氯化铁络合后,经高温碳化和氨气热处理,得到了高比表面积的Fe-N共掺杂的碳纳米管串联的碳纳米空心球催化剂. 氨气不仅刻蚀碳基底提高比表面积,还可还原铁元素形成Fe₄N纳米粒子,提升了催化剂对氧还原反应的电催化活性,其氧还原半波电位达0.79 V,仅比商业Pt/C低60 mV,而且其稳定性和耐甲醇性更优于商业Pt/C,展示出良好的燃料电池应用潜力.     

碳基非贵金属氧还原催化剂活性位点的研究进展

铂基催化剂是目前氢氧燃料电池中实际应用的阴极氧还原催化剂,由于铂昂贵的价格以及稀缺性,开发非贵金属氧还原催化剂对于氢氧燃料电池的规模化应用非常必要.碳基非贵金属氧还原催化剂,包括金属-氮掺杂碳(M–N–C)材料和非金属杂原子掺杂碳材料,是目前最重要也是研究最广泛的两类非贵金属氧还原催化剂.对其活性位点的认知是研究热点之一,也是明显提高性能和宏量制备的关键所在.对于金属-氮掺杂碳催化剂,目前受到广泛认可的活性位点包括:M–N_x/C(x=1,2,3,4)、Nx–C、包覆的纳米金属粒子活化的碳层等.对于非金属杂原子掺杂碳材料(如氮掺杂碳材料),氮原子毗邻的碳原子一般被认为是活性位点.但由于原料本身、制备过程等因素,可能引入痕量的金属元素,严格意义上的非金属杂原子掺杂碳材料难以制备,使得明确其活性位点非常困难.结合本研究组在该领域的工作,本文介绍了当前上述两类催化剂在研究方面的进展,总结分析了几种对活性位点探索和确认的主流认识,以期有助于碳基非贵金属氧还原催化剂的进一步研究.     

铁、氮掺杂石墨烯/碳黑复合材料的制备及氧还原电催化性能

以高含氮量的2-氨基咪唑为氮源,三氯化铁为铁源,高比表面积的KJ600碳黑为载体,通过水热法制得氨基咪唑聚合物前驱体,再经二次高温热处理,制得石墨烯/碳黑复合材料. 透射电镜表征显示该材料为石墨烯纳米片与碳黑颗粒的复合结构. BET表征表明这是一种多孔结构,具有很高的比表面积（882 m²•g^-1）,这有利于暴露更多活性位点,并促进传质. XRD证实催化剂中存在石墨烯,且石墨烯结构是在第一次热处理过程中形成的. 电化学测试表明,该催化剂在酸性和碱性介质中都具有很高的氧还原电催化活性和低H₂O₂产率,并且在碱性介质中对甲醇小分子的抗毒化性能明显优于商业Pt/C催化剂,展示出在实际燃料电池系统中的应用潜力.     

单源MOF衍生具有三维有序大孔结构的氮掺杂碳包覆铁-氮合金复合材料用于高效氧还原

氧还原反应在一些能源转换系统如金属-空气电池中起着至关重要的作用.目前贵金属基材料(Pt/C)被认为是最有效的氧还原电催化剂,然而价格昂贵和储量有限等因素限制了它的商业化应用,因此探索高效的非贵金属氧还原电催化剂具有重要的意义.近年来,负载过渡金属铁的多孔碳催化剂由于独特的结构和优异的氧还原催化活性成为替代铂基催化剂最有潜力的候选者.该类材料的合成通常采用直接煅烧含有氮源、碳源和铁盐的混合前驱体的制备方法,但是热解时材料的多孔结构以及活性位点的均匀分布很难得到有效的控制.近年来,金属有机框架(MOFs)由于其多孔结构和组成可控等优点而经常被用作自牺牲模板来制备负载铁基纳米材料的多孔碳催化剂,并表现出优异的电催化活性.目前以MOF为前驱体制备高活性的载铁氮掺杂碳复合材料通常需要引入额外的氮源或铁源,因此选择氮含量丰富的铁基MOF材料作为单源前驱体制备载铁氮掺杂多孔碳复合材料具有重要的意义.除此之外,具有多级孔隙率的催化剂可以改善反应时的传质过程,同时有序交联的网络结构能够提供连续的电子传输.本文报道了一种简单可控的制备具有三维有序大孔结构的载铁氮掺杂多孔碳复合催化剂的合成方法,该材料表现出优异的电催化氧气还原性能和优异的催化稳定性.首先,以氮含量丰富的双氰胺和吡嗪配体所构筑的Fe-MOF作为前驱体,利用具有均一尺寸的聚苯乙烯微球作为造孔剂,合成得到了具有三维有序大孔结构的Fe-MOF前驱体,然后通过高温煅烧该单源前驱体制备得到具有三维有序大孔结构的氮掺杂多孔碳包覆铁-氮合金的复合型催化剂(3DOM Fe/Fe-NA@NC).扫描电镜和透射电镜结果表明,材料内形成了有序交联的大孔结构;氮气吸附测试表明,刻蚀之后材料的比表面积明显增加,结合分级多孔特性可以共同促进催化反应的传质过程.粉末X射线衍射结果证实了多孔碳材料中铁和铁-氮合金物种的成功合成.电化学测试结果表明,在0.1 M KOH电解液中,3DOM Fe/Fe-NA@NC-800催化剂表现出优于Pt/C的氧还原活性,其半波电位(E_1/2)为0.88 V,大于商业Pt/C的半波电位(E_1/2=0.85 V).同时,3DOM Fe/Fe-NA@NC-800表现出更加优异的稳定性,经过20000 s测试后,其电流保持率为94%,而Pt/C只保持了78%.关于活性位点探究的对比实验证明在所制备的复合材料中,铁物种作为高效的活性位点参与了电催化氧还原反应,与氮掺杂多孔碳之间的协同作用共同主导了3DOM Fe/Fe-NA@NC优异的氧还原活性.得益于其优异的氧还原活性,将其作为阴极活性材料组装为锌-空气电池进一步探究了其在实际应用中的可行性.本结果拓宽了高效的铁基催化剂的类型,同时也为制备封装非贵金属的多孔碳基催化剂提供了实验指导和理论依据.     

碱性条件下Pt-M(Ni, Fe, Mo)/C电催化氧化甲醇的性能对比研究

采用溶胶法制备了碳载Pt-M(M为Ni, Fe, Mo)电催化剂, 并用TEM和XRD技术表征活性物微观结构, 实验结果表明, Pt基合金微粒在碳黑表面分布均匀, 粒径约为2~4 nm. 用循环伏安法测定催化剂在不同碱性条件下的活性, 研究结果表明, 不同掺杂元素催化剂的活性大小顺序为Pt75Ni25/C>Pt75Fe25/C>Pt50Mo50/C, 掺杂Ni可明显地促进纳米Pt的催化活性, Pt75Ni25/C在1.0 mol/L NaOH+1.0 mol/L CH3OH溶液中的峰电流密度可以达到726.9 mA/mg.     

导电共聚物衍化新策略制备硫、氮共掺杂碳纳米管及其对改善PtCu纳米晶分散性与甲醇电催化氧化的促进作用

有效调控碳纳米材料的几何和电子结构的协同效应和缺陷是获得优良电化学性能的关键.然而,如何设计一种具有优势结构的杂化材料及对其电催化机理的认识尚不清楚.本文提出了一种聚(3,4-乙撑二氧噻吩)/聚苯胺导电共聚物热解策略来制备S和N共掺杂多壁碳纳米管(MWCNTs),发现改变前驱体溶液中两种单体的比例可以调控掺杂MWCNTs中S和N原子的含量与表面活性位结构.S和N的共掺杂明显增大了碳纳米管表面的缺陷程度并暴露出更丰富的活性位点,从而有利于超细Pt和PtCu纳米颗粒的均匀分布和沉积.透射电镜和扫描透射电镜结果表明,所制备S和N共掺杂MWCNTs(SN-MWCNTs)负载的催化剂中Pt和PtCu纳米颗粒以及掺杂的S和N原子都均匀地分布在MWCNTs上,且沉积的Pt和PtCu纳米颗粒的平均尺寸仅分别为2.30和2.87 nm.X射线光电子能谱结果表明,S和N共掺杂MWCNTs与负载的Pt基纳米颗粒之间存在强烈的电荷转移相互作用,明显改变了贵金属Pt的表面电子结构.电化学测试结果表明,与Pt/SN-MWCNTs,Pt/N-MWCNTs,Pt/S-MWCNTs和商业Pt/C催化剂相比,Pt₁Cu₂/SN-MWCNTs表现出更大的电化学活性表面积(148.85 m² g^?1),更高的甲醇氧化质量活性(1589.9 mA mg_Pt^?1)、电化学稳定性和抗CO毒化能力.密度泛函理论研究表明,S和N共掺杂导致碳纳米管极大地变形,同时极化和激活了相邻的C原子.因此,增强了Pt₁Cu₂纳米颗粒在SN-MWCNTs上的吸附以及随后甲醇分子的吸附.此外,Pt₁Cu₂/SN-MWCNTs对甲醇氧化的电催化活性均在热力学和动力学上优于相应的CNTs和N-CNTs基材料.本文提供了一种新颖的在碳基材料上构建高度分散且稳定的Pt基纳米颗粒高性能燃料电池电催化剂的方法.     

Ya‐Ru Lv,Xue‐Jing Zhai,Shan Wang,Hong Xu,Rui Wang #, Shuang‐Quan Zang

氧还原反应在一些能源转换系统如金属-空气电池中起着至关重要的作用.目前贵金属基材料(Pt/C)被认为是最有效的氧还原电催化剂,然而价格昂贵和储量有限等因素限制了它的商业化应用,因此探索高效的非贵金属氧还原电催化剂具有重要的意义.近年来,负载过渡金属铁的多孔碳催化剂由于独特的结构和优异的氧还原催化活性成为替代铂基催化剂最有潜力的候选者.该类材料的合成通常采用直接煅烧含有氮源、碳源和铁盐的混合前驱体的制备方法,但是热解时材料的多孔结构以及活性位点的均匀分布很难得到有效的控制.近年来,金属有机框架(MOFs)由于其多孔结构和组成可控等优点而经常被用作自牺牲模板来制备负载铁基纳米材料的多孔碳催化剂,并表现出优异的电催化活性.目前以MOF为前驱体制备高活性的载铁氮掺杂碳复合材料通常需要引入额外的氮源或铁源,因此选择氮含量丰富的铁基MOF材料作为单源前驱体制备载铁氮掺杂多孔碳复合材料具有重要的意义.除此之外,具有多级孔隙率的催化剂可以改善反应时的传质过程,同时有序交联的网络结构能够提供连续的电子传输.本文报道了一种简单可控的制备具有三维有序大孔结构的载铁氮掺杂多孔碳复合催化剂的合成方法,该材料表现出优异的电催化氧气还原性能和优异的催化稳定性.首先,以氮含量丰富的双氰胺和吡嗪配体所构筑的Fe-MOF作为前驱体,利用具有均一尺寸的聚苯乙烯微球作为造孔剂,合成得到了具有三维有序大孔结构的Fe-MOF前驱体,然后通过高温煅烧该单源前驱体制备得到具有三维有序大孔结构的氮掺杂多孔碳包覆铁-氮合金的复合型催化剂(3DOM Fe/Fe-NA@NC).扫描电镜和透射电镜结果表明,材料内形成了有序交联的大孔结构;氮气吸附测试表明,刻蚀之后材料的比表面积明显增加,结合分级多孔特性可以共同促进催化反应的传质过程.粉末X射线衍射结果证实了多孔碳材料中铁和铁-氮合金物种的成功合成.电化学测试结果表明,在0.1 MKOH电解液中, 3DOMFe/Fe-NA@NC-800催化剂表现出优于Pt/C的氧还原活性,其半波电位(E1/2)为0.88 V,大于商业Pt/C的半波电位(E1/2=0.85 V).同时, 3DOM Fe/Fe-NA@NC-800表现出更加优异的稳定性,经过20000 s测试后,其电流保持率为94%,而Pt/C只保持了78%.关于活性位点探究的对比实验证明在所制备的复合材料中,铁物种作为高效的活性位点参与了电催化氧还原反应,与氮掺杂多孔碳之间的协同作用共同主导了3DOM Fe/Fe-NA@NC优异的氧还原活性.得益于其优异的氧还原活性,将其作为阴极活性材料组装为锌-空气电池进一步探究了其在实际应用中的可行性.本结果拓宽了高效的铁基催化剂的类型,同时也为制备封装非贵金属的多孔碳基催化剂提供了实验指导和理论依据.     

以CaCl2为模板合成的高活性和高稳定性铁、氮、硫共掺杂多子孔碳氧还原电催化剂

燃料电池是一种可将化学能通过电催化反应直接转化成电能的装置,具有能量密度高和清洁无污染等优点.燃料电池阴极氧还原反应(ORR)的动力学较迟缓,是电池能量效率损失的主要原因.目前ORR催化活性最高的是铂基催化剂,但由于贵金属铂价格昂贵,储量稀少,且对燃料小分子渗透的抗性较差,严重制约了燃料电池的大规模应用.因此,高性能、低成本的非贵金属催化剂成为燃料电池领域的研究热点.本文选用含氮量高达45％的三聚氰胺-甲醛树脂为碳源和氮源,Fe(SCN)3为铁源和硫源,以CaCl2为模板,在高温和铁的催化作用下将树脂碳化,经酸洗和二次热处理工艺,制备出铁、氮、硫共掺杂的多孔碳(FeNS-PC).干燥后的CaCl2颗粒可防止树脂在高温下交联形成块状碳颗粒,同时起到造孔模板的作用.CaCl2颗粒在温和条件下即可除去,无需强腐蚀性条件,因此不会对催化活性中心造成破坏.在Fe/N/C催化剂中掺杂S可进一步提高催化活性,不添加碳载体可避免低活性的碳载体降低质量活性,多孔结构可促进传质,充分利用活性位点.我们优化了热处理温度,并对催化剂的结构、组分及催化性能等进行了表征分析.结果表明,热处理温度为900℃时,可将树脂完全转化成多孔碳,并获得较高的杂原子掺杂量,可达到最优活性.CaCl2为模板剂可避免使用强腐蚀性试剂去除模板,有利于保留活性位,并得到多孔结构.FeNS-PC-900的比表面积可达775 m2/g.得益于原位掺杂的合成工艺,各掺杂元素在多孔碳表面均匀分布.在酸性介质中,FeNS-PC-900的半波电位可达到0.811V,仅比商业Pt/C催化剂低78 mV;在0.8V电位下的质量活性为10.2 A/g,表现出优异的催化活性.经过10000圈加速衰减测试后,其半波电位仅下降了20 mV,在0.75V电位下持续放电10000s后,其ORR电流仍保持初始电流的84.4％,具有比Pt/C更加优异的稳定性.以FeNS-PC-900为阴极催化剂的质子交换膜燃料电池的最大功率密度可达到0.49 W/cm2,并在0.6V电压下持续放电10h后,其电流仍可保持初始电流的65％,表现出良好的应用潜力.FeNS-PC-900具有高掺杂含量、高比表面积和多孔结构,并且杂原子在催化剂表面均匀分散,在半电池和燃料电池测试中都表现出优异的催化活性和稳定性,表明其是一种非常有潜力应用于燃料电池的非贵金属氧还原催化剂.     

氮掺杂碳材料负载Pd纳米催化剂在有机反应中的最新研究进展

氮掺杂碳材料负载Pd纳米催化剂因其具有反应活性高、反应完成后便于分离和重复使用等优点,在催化领域引起了极为广泛的关注.简要综述了基于氮掺杂多孔/介孔碳NC、氮掺杂石墨烯NG、氮掺杂碳纳米管NCNT和氮掺杂碳纳米片NCNS等不同类型碳材料载体制备的负载型Pd纳米催化剂的合成与应用的最新研究进展,同时对氮掺杂碳材料负载Pd纳米催化剂的发展方向进行了展望.     

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

Pyrolyzed Fe-N_x-C with atomically dispersed Fe-N_x sites 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 polymer-chelation strategy to directly disperse the Fe-N_x active 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-N_x-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.     

Ordered mesoporous carbon fiber bundles with high-density and accessible Fe-NX active sites as efficient ORR catalysts for Zn-air batteries

Fe-N_X/C electrocatalysts have aroused extensive interest in accelerating sluggish oxygen reduction reaction (ORR) kinetics as potential alternatives to platinum catalysts in rechargeable Zn-air batteries (ZABs). However, the low density and poor accessibility of Fe-N_X sites have severely restricted the electrocatalytic performance of Fe-N_X/C. Herein, Fe, N co-doped ordered mesoporous carbon fiber bundles are prepared through a ligand-assisted strategy with nitrogen-rich 1,10-phenanthroline as space isolation agent. 1,10-Phenanthroline reveals a six-membered heterocyclic structure containing abundant nitrogen species to tightly coordinate with Fe ions, which is conducive to achieving high-density Fe-N_X sites. Meanwhile, the adoption of SBA-15 as hard-templates enables the catalysts with highly ordered channels and large specific surface areas, improving the accessibility of Fe-N_X sites. The optimal catalyst (PDA-Fe-900) demonstrates a positive half-wave potential of 0.84 V (vs. RHE) in alkaline solution, outperforming the commercial Pt/C (0.83 V). In addition, PDA-Fe-900 delivers comparable ORR performance to commercial Pt/C in acidic electrolyte. Impressively, when PDA-Fe-900 is employed as an air cathode, it achieves large power densities of 163.0 mW/cm² in liquid-state ZAB and 116.6 mW/cm² in the flexible solid-state ZAB. This work provides an efficient ligand-assisted pathway for fabricating catalysts with dense and accessible Fe-N_X sites as high-performance ORR electrocatalysts for ZABs.     

A Reusable CNT-Supported Single-Atom Iron Catalyst for the Highly Efficient Synthesis of C−N Bonds

C−N bond formation is regarded as a very useful and fundamental reaction for the synthesis of nitrogen-containing molecules in both organic and pharmaceutical chemistry. Noble-metal and homogeneous catalysts have frequently been used for C−N bond formation, however, these catalysts have a number of disadvantages, such as high cost, toxicity, and low atom economy. In this work, a low-toxic and cheap iron complex (iron ethylene-1,2-diamine) has been loaded onto carbon nanotubes (CNTs) to prepare a heterogeneous single-atom catalyst (SAC) named Fe-N_x/CNTs. We employed this SAC in the synthesis of C−N bonds for the first time. It was found that Fe-N_x/CNTs is an efficient catalyst for the synthesis of C−N bonds starting from aromatic amines and ketones. Its catalytic performance was excellent, giving yields of up to 96 %, six-fold higher than the yields obtained with noble-metal catalysts, such as AuCl₃/CNTs and RhCl₃/CNTs. The catalyst showed efficacy in the reactions of thirteen aromatic amine substrates, without the need for additives, and seventeen enaminones were obtained. High-angle annular dark-field scanning transmission electron microscopy in combination with X-ray absorption spectroscopy revealed that the iron species were well dispersed in the Fe-N_x/CNTs catalyst as single atoms and that Fe-N_x might be the catalytic active species. This Fe-N_x/CNTs catalyst has potential industrial applications as it could be cycled seven times without any significant loss of activity.     

Insight into Iron Leaching from an Ascorbate-Oxidase-like Fe−N−C Nanozyme and Oxygen Reduction Selectivity**

Ascorbate (H₂A) is a well-known antioxidant to protect cellular components from free radical damage and has also emerged as a pro-oxidant in cancer therapies. However, such “contradictory” mechanisms underlying H₂A oxidation are not well understood. Herein, we report Fe leaching during catalytic H₂A oxidation using an Fe−N−C nanozyme as a ferritin mimic and its influence on the selectivity of the oxygen reduction reaction (ORR). Owing to the heterogeneity, the Fe-N_x sites in Fe−N−C primarily catalyzed H₂A oxidation and 4 e⁻ ORR via an iron-oxo intermediate. Nonetheless, trace O₂⋅⁻ produced by marginal N−C sites through 2 e⁻ ORR accumulated and attacked Fe-N_x sites, leading to the linear leakage of unstable Fe ions up to 420 ppb when the H₂A concentration increased to 2 mM. As a result, a substantial fraction (ca. 40 %) of the N−C sites on Fe−N−C were activated, and a new 2+2 e⁻ ORR path was finally enabled, along with Fenton-type H₂A oxidation. Consequently, after Fe ions diffused into the bulk solution, the ORR at the N−C sites stopped at H₂O₂ production, which was the origin of the pro-oxidant effect of H₂A.     

Constructing Fe-N4 Sites through Anion Exchange-mediated Transformation of Fe Coordination Environments in Hierarchical Carbon Support for Efficient Oxygen Reduction

Metal single atoms (SAs) anchored in carbon support via coordinating with N atoms are efficient active sites to oxygen reduction reaction (ORR). However, rational design of single atom catalysts with highly exposed active sites is challenging and urgently desirable. Herein, an anion exchange strategy is presented to fabricate Fe-N₄ moieties anchored in hierarchical carbon nanoplates composed of hollow carbon spheres (Fe-SA/N-HCS). With the coordinating O atoms are substituted by N atoms, Fe SAs with Fe-O₄ configuration are transformed into the ones with Fe-N₄ configuration during the thermal activation process. Insights into the evolution of central atoms demonstrate that the SAs with specific coordination environment can be obtained by modulating in situ anion exchange process. The strategy produces a large quantity of electrochemical accessible site and high utilization rate of Fe-N₄. Fe-SA/N-HCS shows excellent ORR electrocatalytic performance with half-wave potential of 0.91 V (vs. RHE) in 0.1 M KOH, and outstanding performance when used in rechargeable aqueous and flexible Zn-air batteries. The evolution pathway for SAs demonstrated in this work offers a novel strategy to design SACs with various coordination environment and enhanced electrocatalytic activity.     

Cation distribution and related properties of MnxZn1−xFe2O4 spinel nanoparticles

Mn_xZn_1−xFe₂O₄ (x = 0.05…0.9) nanoparticles prepared via sol–gel hydrothermal process were investigated by X-ray powder diffractometry (XRPD), transmission electron microscopy (TEM), ⁵⁷Fe Mössbauer spectroscopy (MS), electron paramagnetic resonance spectroscopy (EPR), X-ray absorption near edge structure spectroscopy (XANES) and magnetic hysteresis measurements. XRPD measurements revealed a non-monotonic dependence of the cubic lattice parameter on the Mn concentration, which is interpreted as being the result of a corresponding variation in the inversion degree (concentration of Fe ions on the occupied tetrahedral lattice sites) of the studied spinels. XANES measurements indicated that the average oxidation state of Mn ions decreases with the applied Mn concentration, in contrast with Fe ions that were found to be exclusively in the 3+ oxidation state by MS measurements. EPR spectra recorded as a function of temperature enabled the determination of the characteristic anisotropy energy barrier of the superparamagnetic particles, and contributed to the clarification of peculiarities of the corresponding ⁵⁷Fe Mössbauer spectra. On the basis of the observed results the interdependences among the sample stoichiometry, the cubic cell parameter, the particle size, the inversion degree, the magnetic ordering temperature and the effective magnetic anisotropy are discussed.     

Local structure analysis of Bi2(Ga1-xFex)4O9 mullite-type solid solutions synthesized via combination of ball milling and thermal treatment

Mullite-type Bi₂(Ga_1-xFe_x)₄O₉ solid solutions, with 0.1 ≤ x ≤ 0.9, have been synthesized by a combination of mechanical and thermal treatments of a Bi₂O₃/Ga₂O₃/α-Fe₂O₃ stoichiometric mixture. The microstructure of the as-prepared materials on the long-range and local atomic scales was investigated by X-ray diffraction and ⁵⁷Fe Mössbauer spectroscopy, respectively. The XRD data analysis revealed in all cases linear dependence of the lattice parameters related on x. Due to the ability of the applied Mössbauer spectroscopy to probe the local environment of Fe cations, the local structural disorder in investigated solid solutions is provided. It was shown that the presence of Fe³⁺ cations in octahedral sites of the orthorhombic structure causes a local distortion of polyhedra in the material. The preferential occupation of Fe in octahedral site was revealed. Detailed quantitative information on both the cation distribution and the bond lengths provided is discussed in relation to the derived hyperfine parameters.     

Highly exposed NiFeOx nanoclusters supported on boron doped carbon nanotubes for electrocatalytic oxygen evolution reaction

The development of efficient and cost-effective electrocatalysts for oxygen evolution reaction (OER) is crucial for the overall water splitting. Herein, we prepared a highly exposed NiFeO_x ultra-small nanoclusters supported on boron-doped carbon nonotubes catalyst, which achieves a 10 mA/cm² anodic current density at a low overpotential of 213 mV and the Tafel slope of 52 mV/dec in 1.0 mol/L KOH, superior to the pristine NiFeO_x-CNTs and other state-of-the-art OER catalysts in alkaline media. A combination study (XPS, sXAS and XAFS) verifies that the local atomic structure of Ni and Fe atoms in the nanoclusters are similar to NiO and Fe₂O₃, respectively, and the B atoms which are doped into the crystal lattice of CNTs leads to the optimization of Ni 3d e_g orbitals. Furthermore, in-situ X-ray absorption spectroscopies reveal that the high valence state of Ni atoms are served as the real active sites. This work highlights that the precise control of highly exposed multicomponent nanocluster catalysts paves a new way for designing highly efficient catalysts at the atomic scale.     

One-Step Preparation of Fe/N/C Single-Atom Catalysts Containing Fe−N4 Sites from an Iron Complex Precursor with 5,6,7,8-Tetraphenyl-1,12-Diazatriphenylene Ligands

Fe/N/C single-atom catalysts containing Fe−N_x sites prepared by pyrolysis are promising cathode materials for fuel cells and metal-air batteries due to their high oxygen reduction reaction (ORR) activities. We have developed iron complexes containing N2- or N3-chelating coordination structures with preorganized aromatic rings in a 1,12-diazatriphenylene framework tethering bromo substituents as precursors to precisely construct Fe−N₄ sites in an Fe/N/C catalyst. One-step pyrolysis of the iron complex with carbon black forms atomically dispersed Fe−N₄ sites without iron aggregates. X-ray absorption spectroscopy (XAS) and electrochemical measurements revealed that the iron complex with N3-coordination is more effectively converted to Fe−N₄ sites catalyzing ORR with a TOF value of 0.21 e site⁻¹ s⁻¹ at 0.8 V vs. RHE. This indicates that the formation of Fe−N₄ sites is controlled by precise tuning of the chemical structure of the iron complex precursor.