磷化钴纳米棒作为一种高效电化学催化剂在析氢反应过程中的应用

1、Cobalt phosphide nanorods as an efficient electrocatalyst for the hydrogen evolution reaction磷化钴纳米棒作为一种高效电化学催化剂在析氢反应过程中的应用Zhipeng Huanga,n, Zhongzhong Chena, Zhibo Chena, Cuncai Lva, Mark G. Humphreyb, Chi Zhanga,n摘要（Abstract）Cobalt phosphide (Co2P) nanorods are found to exhibit efficient catalytic

2、activity for the hydrogen evolution reaction (HER), with the overpotential required for the current density of 20 mA/cm2 as small as 167 mV in acidic solution and 171 mV in basic solution. In addition, the Co2P nanorods can work stably in both acidic and basic solution during hydrogen production.Thi

3、s performance can be favorably compared to typical high efficient non-precious catalysts, and suggests the promising application potential of Co2P nanorods in the field of hydrogen production. The HER process follows aVolmerHeyrovsky mechanism, and the rates of the discharge step and desorption step

4、 appear to be comparable during the HER process. The similarity of charged natures of Co and P in the Co2P nanorods to those of the hydride-acceptor and proton-acceptor in highly efficient Ni2P catalysts, NiFe hydrogenase, and its analogues implies that the HER catalytic activity of the Co2P nanorod

5、s might be correlated with the charged natures of Co and P. & 2014 Elsevier Ltd. All rights reserved.磷化钴纳米棒（Co2P）被发现具有高效的催化性能在析氢反应过程（HER），该过程所需的过电位在20 mA/cm2电流密度的情况下，在酸性溶液中尽可能小于167mV以及在碱性溶液中尽可能小于171 mV。此外，电子纳米棒可以稳定工作在酸性和碱性溶液中在整个析氢过程。这种性能可媲美的典型的高效的非贵金属催化剂，并表明了Co2P纳米棒在产氢领域中的广阔应用前景。析氢反应过程遵循VolmerHe

6、yrovsky机制，并且放电步骤和解吸步骤的速率出现在析氢过程中具有可比性。Co2P纳米棒的Co和P的带电性质与高效Ni2P催化剂,镍铁氢化酶以及它的相似物的氢受体和质子受体类似，包括Co2P纳米棒的析氢催化过程都与Co和P的带电性质相关联。2014 Elsevier公司，保留所有权利相关。介绍（Introduction）The solar-driven splitting of water into molecular hydrogen and oxygen is one of the most promising possibilities forsimultaneously solvin

7、g the global energy crisis and current environmental issues 13. Because of the intrinsically slow hydrogen evolution reaction (HER) kinetics of semiconductors, photocathodes must be decorated with HER catalysts for efficient hydrogen production. Though platinum remains the most effective HER catalys

8、t, having been shown to significantly enhance the hydrogen production capability of photocathodes several decades ago 4,5, it is a limited resource and expensive, and so its widespread practicalapplication in the field of solar-driven hydrogen production may be limited. There is therefore a demonstr

9、able need for non-precious HER catalysts.太阳能驱动的水分子分解成分子氢和氧是同时解决全球能源危机和当前环境问题的最有前途的可能性 1 - 3 。由于半导体固有的析氢反应（HER）动力学缓慢，所以光电阴极必须用HER催化剂进行高效制氢。虽然铂仍是最有效的HER的催化剂，光阴电极在几十年前已经显示出显著的高效的产氢性能4,5，但是它是一种有限的资源，价格昂贵，所以在太阳能制氢领域的广泛应用可能是有限的。因此，明显需要一种非昂贵的HER催化剂。Recently, a variety of new HER catalysts have been reporte

10、d, including molybdenum sulfide 6,7, molybdenum carbide 810, molybdenum nitride 10, molybdenum boride 8, tungsten carbide 11,12, tungsten carbonitride 13, first-row transition-metal dichalcogenides 14,15, nickel selenide 16, nickel phosphide 17, cobalt phosphide (CoP) 1820, molybdenum phosphide 21,2

11、2, etc. The first-row transition-metal dichalcogenides have similar coordination structure as the active centers in efficient hydrogenase 15, and the charged natures of metal and P in metal phosphides are similar to those of the hydride acceptor and proton-acceptor in NiFe hydrogenase and its analog

12、ues (Ni(PS3n)(CO)1and Ni(PNP)22+) 23.近年来，各种新的HER催化剂已被报道，包括硫化钼 6,7 ，碳化钼8- 10 ，氮化钼 10 ，硼化钼 8 ，碳化钨11,12，碳氮化钨 13 ，第一行过渡金属硫化物14,15，硒化镍 16 ，磷化镍 17 ，钴磷化物（COP） 18-20 ，磷化钼21,22等等。第一行过渡金属硫化物在有效的氢化酶活性中心都具有相似的配位结构 15 ，并且金属磷化物的金属元素和磷元素的带电性质和NiFe氢化酶及其类似物（Ni（PS3N）（CO）和镍（PNP）2 2 +）的氢受体和质子受体相似。 23 。Here the HER perf

13、ormance of cobalt phosphide (Co2P) nanorods is described. Although their structure and composition are different from all heretofore reported HER catalysts, the Co2P nanorods exhibit efficient and stable HER catalytic activity in both acidic and basic solutions. The overpotential required for a curr

14、ent density of 20 mA/cm2 (20) is as small as 167 mV in acidic solution and 171 mV in basic solution. The 20 of the Co2P nanorods lies in the top 10 of the reported values of non-precious HER catalysts. It is worth noting that the four reported 20 values better than that of the Co2P nanorods were obt

15、ained from composites of catalysts and nanostructured conductive supports, including Mo1Soy particles loaded on reduced graphene oxide (Mo1Soy/rGO) 10, CoP nanoparticles loading on carbon nanotube (CoP/CNT) 18, CoP nanowires loaded on carbon cloth (CoP/CC) 20, and MoS2 loaded on mesoporous graphene

16、foam (MoS2/MGF) 24. The nanostructured conductive supports are well known to improve electron transport among catalysts and therefore the performance of catalysts 25. The charged natures of Co and P in the Co2P nanorods are similar to those of the hydride-acceptor and proton-acceptor in Ni2P catalys

17、t, NiFe hydrogenase, and its analogues, which may explain the efficient catalytic activity of the Co2P nanorods.这里磷化钴纳米棒（Co2P）的HER催化表现被发现。虽然它们的结构和组成不同于所有以前报道HER催化剂、磷化钴纳米棒在酸性和碱性溶液中都展现出高效、稳定的HER催化活性。为20 mA/cm2的电流密度所需的过电位（20）是在酸性溶液中167和171 mV在碱性溶液中为小。电子的纳米棒20在10大报告值的非贵她催化剂。值得注意的是，四日报道20值优于电子纳米棒复合材料的催化剂

18、得到纳米导电支持，包括mo1soy颗粒负载在石墨烯（mo1soy / RGO） 10 ，在碳纳米管负载系数（COP /碳纳米管纳米） 18 ，警察线加载对碳纤维布（警察/ CC） 20 ，和二硫化钼装载在介孔石墨烯泡沫（MoS2 / MGF） 24 。纳米结构的导电载体是众所周知的，以提高催化剂的电子传递，因此，催化剂的性能 25 。带电性质的有限和P电子纳米棒是类似的氢受体和质子受体Ni2P催化剂，镍铁氢化酶，和它的类似物，这可以解释的电子纳米棒的高效催化活性。Materials and methods 实验材料与方法Synthesis of Co2P nanorods对Co2P纳米棒的合成

19、Cobalt acetate tetrahydrate(0.50g,2mmol) was mixed with oleylamine (12g,45mmol) in a 100mL round-bottom flask. The flask was heated via a heating mantle. A dispersion was obtained by stirring the mixture at 70 1C. The dispersion was then heated to 120 1C and triphenylphosphine (5.246g,20mmol) was ad

20、ded to the mixture. The flask was pumped under vacuum at 120 1C for30min to remove water, and then refilled with N2. The temperature of the heating mantle was increased to 370 1C and maintained at this value for 10 min. The flask was removed from the heating mantle and cooled to room temperature. Th

21、e black product was isolated and washed by repeated centrifugation/ultrasonication, with hexane as good solvent and ethanol as non-solvent. Finally, the product was dried under vacuum at room temperature.四水合醋酸钴（0.50g，2mmol）混合油胺（12g，45mmol）在100mL圆底瓶中。烧瓶通过加热罩加热。搅拌，在70得到的分散。 分散加热至120 1C和三苯基膦（5.246g，20）

22、混合。烧瓶抽真空在12030min至去除水分，然后再充满N2。加热罩的温度 上升到370 1C，并且维持在这个温度10 min。烧瓶被调离加热罩和冷却到室温。 黑色产品隔离以及重复离心/超声波清洗，用正己烷作为溶剂和非溶剂提取好。最后，该产品是在室温下进行真空干燥。Characterization表征The morphology of the Co2P nanorods was assessed by transmission electron microscopy (TEM,200kV,JEM2100,JEOL) and scanning electron microscopy (SEM,

23、7001F,JEOL). The energy-dispersive X-ray spectroscopy (EDX) spectrum was recorded using a GENESIS 2000 XM 30T (EDXA) on a JEM 2100. For the TEM investigation, the Co2P nanorods were dispersed in hexane by ultrasonication. The dispersion was dropped onto a carbon-coated copper grid (300-mesh). The co

24、pper grid was then dried at 100 1C for 5min before the TEM characterization. Powder X-ray diffraction (XRD) patterns were collected using a D8 Advance diffractometer with graphite-monochromated Cu K radiation (=1.54178Å) . The X-ray photoelectron spectroscopy (XPS) experiments were carried out

25、on an ESCALAB250Xi System (ThermoFisher) equipped with a monochromatic Al K (1486. 6 eV) source and a concentric hemispherical energy analyzer. The binding energy C1 speak from surface adventitious carbon (284.8 eV) was adopted as a reference for the binding energy measurements.Co2P的纳米棒的形态通过透射电子显微镜（

26、TEM、200kV，评估jem2100，JEOL）和扫描电子显微镜（SEM，7001f，JEOL）确定。能量色散X射线光谱仪（EDX）频谱被记录一个GENESIS 2000 XM 30T (EDXA) 在一个JEM 2100。对于TEM 调查，Co2P纳米棒分散在正己烷中超声分散。分散投到碳包覆铜网（300目）。铜网格然后100条件下被干燥 5min在做TEM表征之前。粉末X射线衍射（XRD）图案使用D8 高级衍射与石墨单色器收集Cu K辐射波长（= 1.54178Å）。X射线光电子能谱 （XPS）的实验是在一个escalab250xi系统进行 （招聘）配备了单色铝K（1486 6

27、eV）源和同心半球能量分析仪。 结合能从C1讲表面不定碳 （284.8 eV）的结合能作为参考 测量。Electrochemical performance电化学性能Co2P nanorods (15mg) were dispersed in hexane(0.5mL) with the aid of an ultrasonic horn(2mmdiameter,130W, 60 min).The dispersion(17 L) was dropped onto a clean Tifoil (0.5cm2) and dried naturally. The Ti foil was poli

28、shed by sandpaper (7000 mesh),and then cleaned by acetone, ethanol, and de-ionized water(15 min each) prior to the drop-coating of the Co2P nanorods.Co2P（15mg）纳米棒被分散在正己烷（0.5ml）中在超声变幅杆（直径2毫米，130W，60分钟）帮助下。分散物（17L）滴到一个干净的钛箔（0.5cm2），并且自然晾干。钛箔是用砂纸打磨（7000目），然后用丙酮、乙醇和去离子水（每15分钟）清洗，在Co2P纳米棒的滴涂之前。装上钛箔的Co2P纳

29、米棒在450 5% H2N2中退火30分钟，以除去表面配体。All electrochemical measurements were carried out with an electrochemical workstation(CHI 614D,CH Instrument)in a three-electrode configuration, with Co2P (loaded ontoTi foil) as a working electrode ,a graphite rod(6mmdiameter) as a counter electrode ,and a mercury/ mer

30、curous sulfate electrode(MSE) or mercury/mercury oxide electrode (MMO) as a reference electrode .The samples wereassembled into a homemade electrochemical cell ,with onlya defined area(0.07 cm2) of the front surface of thesample exposed to solution during the measurements .Thecounter electrode was s

31、eparated from the working chamberby a porous glass frit.所有的电化学测量都在电化学工作站(CHI 614D,CH Instrument)进行三电极结构，用Co2P（装载在Ti箔）作为工作电极，石墨棒（6mm直径）作为反电极，和汞/汞硫酸电极（MSE）或汞/氧化汞电极（MMO）作为参考电极。样品组装成一个自制的电化学电池，只有一个定义的区域（0.07平方厘米）在测试过程中暴露于溶液样品的前表面。反电极与工作腔通过多孔玻璃粉分离。H2SO4 aqueous solution(0.5M)or KOH aqueous solution(1 M)w

32、as used for electrochemical measurements. The MSEwas used as the reference electrode in H2SO4 solution, andthe MMO was used in KOH solution. The solutions werepurged with high purity H2 (99.999%) for 30min prior toelectrochemical measurements. The reversible hydrogenevolution potential(RHE) was dete

33、rmined by the opencircuit potential of a clean Pt electrode in the solution ofinterest bubbledwithH2 (99.999%), being 0.694 V vs. MSEfor the 0.5M H2SO4 solution and 0.876 V vs. MMO for the1 M KOH solution. A potential measured with respect to theMSE electrode was therefore referenced to the RHE byad

34、ding a value of 0.694V for the measurements in the 0.5MH2SO4 solution, and a potential measured with respect tothe MMO electrode in the KOH solution was referenced tothe RHE by adding a value of 0.876V.硫酸水溶液（0.5M）或氢氧化钾水溶液（1M）被用于电化学测量。MSE在硫酸溶液中作为参考电极，以及MMO被用在氢氧化钾溶液中。在电化学测量前30min该溶液被高纯度H2（99.999%）净化。可

35、逆的析氢电位（RHE）是由冒出H2（99.999%）气泡的溶液中测定的一个清洁的Pt电极的开路电位，一般是0.694 V vs. MSE为0.5M H2SO4溶液和0.876 V vs. MMO为1 M KOH溶液。因此，对于可逆析氢电位在0.5M H2SO4溶液中MSE电极的测量电位参考方面应增加0.694V的测量值，并且在KOH溶液中MMO电极应增加0.876V的测量值。The polarization curves of Co2P samples were measured at a sweep rate of 5mV/s in rigorously stirred solution (

36、1600 rpm).The uncompensated cell resistance (R) was determined by the current-interrupt method, and the experimental potential was corrected by subtracting ohmic drop (iR), where i is the current corresponding to the experimental potential. The apparent Tafel slope was derived from the iR-corrected

37、polarization curve by fitting experimental data to the equation =a+blogj, where is the iR-corrected potential, a is the Tafel constant, b is the Tafel slope, and j is the current density. Electrochemical impedance spectroscopy(EIS)measurements were carried out at different potentials in the frequenc

38、y range of 10 -2 to 10-6 Hz with 10mV sinusoidal perturbations and 12 steps per decade in0.5M H2SO4 solution.在严格的搅拌溶液中用5mV/s扫描速率测量Co2P样品的极化曲线（1600转）。未补偿电阻（R）用电流中断法确定，并且实验电位的校正是通过对应的实验电位减去电阻电压降（IR），i是对应的实验电流。显然塔菲尔斜率由iR校正极化曲线通过实验数据拟合方程= A + blogj导出，在这里是iR校正电位，a是塔菲尔常数，b是塔菲尔斜率，和j是电流密度。电化学阻抗谱（EIS）进行了测量在不

39、同电位的10 2至10-6赫兹的频率范围，10mV的正弦扰动和在0.5M H2SO4溶液每十年12步骤。For accelerated degradation investigations , cyclic vol-tammetric (CV) measurements were carried out with a 50 mV/s sweep rate between -0.240 and0.100V vs. RHE in the 0.5M H2SO4 solution, and -0.330 and0.080V vs. RHE in the 1 M KOH solution, withou

40、t accounting for uncompen sated resistance.为加速退化的调查，循环体积（CV）测量进行50 mV/s速率的扫描在-0.240 和0.100v vs之间.在0.5M硫酸溶液之间进行的RHE，并在0.330 和0.080v vs之间.在1M 氢氧化钾溶液的RHE，没有记录 uncompen满足电阻。The volume of H2 during a potentiostatic electrolysisexperiment was monitored by water displacement in aconfiguration shown in Figu

41、re S1 (Electronic supplementaryinformation). In this experiment , the backside of the Ti foilwas connected to a Cu wire with Ag paste. The Cu wire wasthreaded to a glass tube(6mmdiameter),and the backsideand front side of the sample electrode were then sealedwith epoxy resin with the exception of an

42、 exposed area(0.5 cm2). A Free scale MPXV7002DP differential pressuretransducer was employed to monitor pressure variation inthe gas gathering tube, and then the volume of generatedH2 was computed from pressure variation in the gas gathering tube(see details in Electronic supplementary information).

43、 The current and charge passing the Co2P nanorodswere measured with the electrochemical workstation andthe voltage change of the differential pressure transducerwas monitored with a digital multi meter (41/2digits).Priorto experiment, the relationship between the volume ofgathered gas and the variat

44、ion of the output voltage of thedifferential pressure transducer(i.e., pressure variation inthe gas gathering tube)was calibrated by injecting knownamounts of air into the gas gathering tube and recording thevariation of the output voltage of the differential pressuretransducer.恒电位电解实验中氢气的体积是由图S1所示配

45、置水位移监测（电子补充资料）。在这个实验中，对钛箔的背面是连接到一个与银浆铜导线。铜导线穿过玻璃管（直径6mm），并且样品电极的背面和正面除暴露的区域外全部用环氧树脂密封（0.5平方厘米）。一个自由的尺度MPXV7002DP差压传感器来监测在集气管压力的变化，然后由气体收集管的压力变化计算出生成的H2的体积（电子辅助信息查看详情）。Co2P纳米棒的电流和电荷传递用电化学工作站测量并且差压压力传感器的电压变化是由数字多用表监测（41 / 2digits）。实验前，收集到的气体的体积和输出电压的变化之间的关系通过将已知量的空气注入到气体收集管中，并记录差压压力传感器输出电压的变化，对差压压力传感器

46、（即气体收集管中的压力变化）进行了标定。Results and discussionThe HER performance of the Co2P nanorods was evaluated by polarization curve measurements(jV plots).Prior to measurements ,the Co2P nanorods were applied to Ti foils (loading amount ca.1mg/cm2) by drop coating, andAnnealed at 450 in a 5%H2/N2 atmosphere for

47、30 min toremove surface ligand 17. The measurements were carriedout in a 0.5M H2SO4 solution with a three-electrode configuration(see details in the Materials and methods section).Figure 4a shows the jV plots of the Co2P nanorods ,bare Tifoil, commercial Pt/C(Johnson Matthey, Hispec3000,20wt%)loaded

48、 on a glassy carbon electrode (GCE),and a bare GCE.It was verified that cobalt phosphate can be dissolved in the0.5 M H2SO4 solution (Figure S6, Electronic supplementaryinformation); therefore ,the cobalt phosphate present onthe surface of the Co2P nanorods would be dissolved andshould not affect th

49、e catalytic activity of the Co2P nanorodsduring the electrochemistry measurements. It can be seenthat the bare Ti foil exhibits negligible current flow in thepotential range of 250 to 0mV vs. RHE, suggesting thatthe current flow of the Co2P nanorods supported on the Tifoil sample in this potential r

50、ange is induced by the Co2Pnanorods, but not the Ti foil. The onset of current is foundat ca.- 70 mV vs. RHE for the Co2P nanorods. The 20 is167 mV for the Co2P nanorods, and the over potentialrequired for a current density of 10mA/cm2 (10) is134 mV. The 20 and 10 values of different catalysts areus

51、ually compared in order to evaluate their efficiencies, because in photoelectrochemical applications a photo-cathode produces a current flux of 1020 mA /cm 2 under1 sun of AM1.5 G illumination 3. The performance ofrepresentative HER catalysts is summarized in Table1. Itcan be seen that the 20 and 10

52、 values of the Co2P nanorodsin our experiments are larger than those of CoP nanoparti-cles 19, Ni2P nanoparticles 17, NiMo nanopowders 29,MoP nanoparticles 21,22, CoP/CC 20, CoP/CNT 18,Mo1Soy/rGO 10 and MoS2/MGF) 24, and smaller thanother listed catalysts. NiMo nanopowders degrade rapidlyin acidic c

53、ondition, rendering their exploitation problematic29. The conductive CC, CNT, rGO , or MGF in CoP/CC,CoP/CNT,Mo1Soy/rGO orMoS2/MGF enhance the HER performance of these composites, in contrast to the Co2P nanorods in our experiments which are loaded on the Ti foil. Thiscomparison demonstrates that th

54、e HER performance of theCo2P nanorods in our experiments compares favorably withmost values of recently reported high efficiency non-precious electrocatalysts.通过极化曲线测量评估了HER的电子纳米棒的性能（JV图）。测量前，Co2P纳米棒用于钛箔（装载量约1mg/cm2）滴涂，退火在450在5% H2N2气氛30分钟去除表面配体 17 。测量是在一个有一三个电极配置0.5M H2SO4溶液中进行（在部分的材料和方法查看详情）。图4a显示

55、电子纳米棒的JV图，纯钛箔、商业Pt/C（Johnson Matthey，hispec3000,20wt %）装在玻碳电极（GCE），与裸玻碳电极上。结果表明，磷酸钴可以溶解在0.5 M H2SO4溶液（图S6中，电子补充资料）；因此，在电化学测量过程中Co2P纳米棒表面的磷酸钴会溶解，不影响Co2P纳米棒的催化活性。可以看出，裸Ti箔在250至0mV电位范围电流可以忽略不计。首先，这表明支撑Co2P纳米棒在Ti箔样品在该电位范围的电流是由Co2P纳米棒的诱导，而不是钛箔。目前被发现Co2P纳米棒的RHE的开始电流大约是在70 mV vs。Co2P纳米棒的20是167 mV，和在一个10mA/

56、cm2电流密度所需的过电位（10）为134 mV。不同催化剂的20和10值通常比较，以评估他们的效率，因为在光电应用中一个光电阴极产生1020毫安/厘米2的电流密度在一个AM1.5 G的阳光照明之下。HER催化剂的典型性能被总结在表1中。可以看出，在我们的实验中，Co2P纳米棒的20和10值大于CoP纳米粒 19 ， Ni2P纳米粒 17 ，镍钼纳米粉末 29 ，MoP纳米粒21,22，CoP/CC 20 ，CoP / CNT 18 ，Mo1Soy/rGO 10 MoS2/MGF 24 ，小于其他上市的催化剂。镍钼粉末在酸性条件下迅速降解，补偿他们的利用问题 29 。导电的CC，CNT，RGO

57、，或MGF在CoP / CC，CoP /碳纳米管，mo1soy / RGO ormos2 / MGF提高这些复合材料的HER性能，相比之下，在我们实验中是负载于钛箔的Co2P纳米棒。比较表明，HER在我们的实验中，Co2P纳米棒性能可以媲美最近报道的价值最高的高效催化剂。High durability is of importance for a good electrocatalyst. The stability of the Co2P nanorods was evaluated by cyclic voltammetry (CV)sweeps between -0.240 and 0.

58、100 Vvs.RHE in the 0.5M H2SO4 solution. The corresponding jV curves of the CV sweeps(without iR correction)are shown in Figure4b. After 1000CV sweeps the 20 increases from181 to 194mV,the increase of the 20 being smaller than 15mV.The current density at 0 V vs.RHE in CV sweeps (2mA/ cm2, Figure4b) is larger than that in Figure 4a ( -0 mA/cm2). It is apparent that the larger current density is correlated with the faster scan rate(50mV/s)in CV sweeps, impl