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

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 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.This 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 aVolmer–Heyrovsky mechanism, and the rates of the discharge step and desorption step 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 nanorods 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Volmer–Heyrovsky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，

1

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 solving the global energy crisis and current environmental issues [1–3]. 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 catalyst, 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 demonstrable 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 reported, including molybdenum sulfide [6,7], molybdenum carbide [8–10], 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) [18–20], molybdenum phosphide [21,22], 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

2

hydride acceptor and proton-acceptor in [NiFe] hydrogenase and its analogues ([Ni(PS3n)(CO)]1and [Ni(PNP)2]2+) [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 performance 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 current density of 20 mA/cm2 (η20) is as small as 167 mV in acidic solution and 171 mV in basic solution. The η20 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 η20 values better than that of the Co2P nanorods were obtained 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 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 catalyst, [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

3

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（η20）是在酸性溶液中167和171 mV在碱性溶液中为小。电子η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η20值优于电子纳米棒复合材料的催化剂得到纳米导电支持，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

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 added 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. The 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.

4

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）混合。烧瓶抽真空在120℃30min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, 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 copper 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 Kα radiation (λ=1.54178?) . The X-ray photoelectron spectroscopy (XPS) experiments were carried out on an ESCALAB250Xi System (ThermoFisher) equipped with a monochromatic Al Kα (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（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 Kα辐射波长

5

（λ= 1.54178?）。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Kα（1486 6 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 μL) was dropped onto a clean Tifoil (0.5cm2) and dried naturally. The Ti foil was polished 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（17μL）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纳米棒在450 ℃5% H2／N2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 onto Ti foil) as a working electrode ,a graphite rod(6mm

diameter) as a counter electrode ,and a mercury/ mercurous sulfate electrode(MSE) or mercury/mercury oxide electrode (MMO) as a reference electrode .The samples were

assembled into a homemade electrochemical cell ,with only a defined area( 0.07 cm2) of the front surface of the

sample exposed to solution during the measurements .The

6

counter electrode was separated from the working chamber by 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)was used for electrochemical measurements. The MSE was used as the reference electrode in H2SO4 solution, and the MMO was used in KOH solution. The solutions were purged with high purity H2 (99.999%) for 30min prior to electrochemical measurements. The reversible hydrogen evolution potential(RHE) was determined by the open circuit potential of a clean Pt electrode in the solution of interest bubbledwithH2 (99.999%), being 0.694 V vs. MSE for the 0.5M H2SO4 solution and 0.876 V vs. MMO for the 1 M KOH solution. A potential measured with respect to the MSE electrode was therefore referenced to the RHE by adding a value of 0.694V for the measurements in the 0.5M H2SO4 solution, and a potential measured with respect to the MMO electrode in the KOH solution was referenced to the 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

7

中。在电化学测量前30min该溶液被高纯度H2（99.999%）净化。可逆的析氢电位（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 (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 polarization curve by fitting

experimental data to the equation η=a+blogj, where η 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 frequency 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η= A + blogj导出，在这里η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,

8

and -0.330 and0.080V vs. RHE in the 1 M KOH solution, without 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 electrolysis experiment was monitored by water displacement in a configuration shown in Figure S1 (Electronic supplementary information). In this experiment , the backside of the Ti foil was connected to a Cu wire with Ag paste. The Cu wire was threaded to a glass tube(6mmdiameter),and the backside and front side of the sample electrode were then sealed with epoxy resin with the exception of an exposed area ( 0.5 cm2). A Free scale MPXV7002DP differential pressure

transducer was employed to monitor pressure variation in the gas gathering tube, and then the volume of generated H2 was computed from pressure variation in the gas gathering tube(see details in Electronic supplementary information). The current and charge passing the Co2P nanorods

were measured with the electrochemical workstation and the voltage change of the differential pressure transducer was monitored with a digital multi meter (41/2digits).Prior to experiment, the relationship between the volume of gathered gas and the variation of the output voltage of the differential pressure transducer(i.e., pressure variation in

9

the gas gathering tube)was calibrated by injecting known amounts of air into the gas gathering tube and recording the variation of the output voltage of the differential pressure transducer.

恒电位电解实验中氢气的体积是由图S1所示配置水位移监测（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（即气体收集管中的压力变化）进行了标定。

Results and discussion

The HER performance of the Co2P nanorods was evaluated by polarization curve measurements(j–V plots).Prior to

measurements ,the Co2P nanorods were applied to Ti foils (loading amount ca.1mg/cm2) by drop coating, and

Annealed at 450 ℃ in a 5%H2/N2 atmosphere for 30 min to remove surface ligand [17]. The measurements were carried out in a 0.5M H2SO4 solution with a three-electrode

configuration(see details in the Materials and methods section).

10

Figure 4a shows the j–V plots of the Co2P nanorods ,bare Ti foil, commercial Pt/C(Johnson Matthey, Hispec3000,20wt%) loaded on a glassy carbon electrode (GCE),and a bare GCE. It was verified that cobalt phosphate can be dissolved in the 0.5 M H2SO4 solution (Figure S6, Electronic supplementary information); therefore ,the cobalt phosphate present on the surface of the Co2P nanorods would be dissolved and should not affect the catalytic activity of the Co2P nanorods during the electrochemistry measurements. It can be seen that the bare Ti foil exhibits negligible current flow in the potential range of 250 to 0mV vs. RHE, suggesting that the current flow of the Co2P nanorods supported on the Ti foil sample in this potential range is induced by the Co2P nanorods, but not the Ti foil. The onset of current is found at ca.- 70 mV vs. RHE for the Co2P nanorods. The η20 is 167 mV for the Co2P nanorods, and the over potential required for a current density of 10mA/cm2 (η10) is 134 mV. The η20 and η10 values of different catalysts are usually compared in order to evaluate their efficiencies, because in photoelectrochemical applications a photo-

cathode produces a current flux of 10–20 mA /cm 2 under 1 sun of AM1.5 G illumination [3]. The performance of representative HER catalysts is summarized in Table1. It can be seen that the η20 and η10 values of the Co2P nanorods in our experiments are larger than those of CoP nanoparti- cles [19], Ni2P nanoparticles [17], Ni–Mo nanopowders [29],

11

MoP nanoparticles [21,22], CoP/CC [20], CoP/CNT [18], Mo1Soy/rGO [10] and MoS2/MGF) [24], and smaller than other listed catalysts. Ni–Mo nanopowders degrade rapidly in acidic condition, rendering their exploitation problematic [29]. 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. This

comparison demonstrates that the HER performance of the Co2P nanorods in our experiments compares favorably with most 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（J–V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% H2／N2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显示电子纳米棒的J–V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η20是167 mV，和在一个10mA/cm2电流密度所需的过电位（η10）为134 mV。不同催化剂的η20和η10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10–20毫安/厘米2的电流密度在

12

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η20和η10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，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 0.100 Vvs.RHE in the 0.5M H2SO4 solution. The corresponding j–V curves of the CV sweeps(without iR correction)are shown in Figure4b. After 1000CV sweeps the η20 increases from181 to 194mV,the increase of the η20 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,

implying that the current density at 0V vs. RHE in the CV sweeps is associated with the capacitive process on the surface of the catalyst. In addition, current density exhibits only a ca.10%

decrease after 12h potentiostatic electrolysis(FigureS7, Electronic supplementary information).The CV sweeps and potentiostatic electrolysis experiment are consistent with HER stability of the Co2P nanorods in acidic solution.

高耐久性是重要的对于一个很好的催化剂而言。采用循环伏安法测定Co2P纳米棒的稳定性（CV），在0.5M H2SO4溶液中扫描RHE在-0.240 至0.100 之间。该相应的J–V曲线扫描（无红外校正）显示在图4b。后来η20的1000cv扫描从181增加到194mv，η20的

13

增加小于15mV。在0 V vs.RHE CV扫描的电流密度（2毫安/平方厘米，图4b）大于图4a（-0 mA/cm2）。很明显的，更大的电流密度和更快的扫描速度相关（50mV／s）CV扫描，这意味着在CV扫描0V vs.RHE电流密度与催化剂表面上的电容过程相关。此外，电流密度仅具有一个ca 10%的降低在恒电位电解12h后（图7，电子补充资料），CV扫描和恒电位电解实验与在酸性溶液中的Co2P纳米棒的HER稳定性一致。

Thus far ,only a few catalysts (e.g. Pt and Mo2C) [8] have been shown to perform reliably in both acidic and basic solutions. For this reason, the HER performance of the Co2P nanorods in basic solution (KOH,1M)was also evaluated. The corresponding results are shown in Figure S8 (Electronic supplementary

information),with a η20 value of 171mV(iR corrected). After 1000CV sweeps between -0.330 and 0.080 Vvs.RHE, the η20 increases from 217 to 229mV (without iR correction). The

relatively small in crease of the η20 (12 mV)and a potentiostatic electrolysis experiment (Figure S8c, Electronic supplementary information)show the stability of the Co2P nanorods in the basic solution. These experiments demonstrate that the Co2P nanorods can function efficiently and stably under basic conditions as well. It has been reported that the η20 of Ni2P nanoparticles in basic solution(KOH,1M)is 205mV,and the HER performance declined rapidly on cycling [17]. The reason for the different stabilities of the Co2P nanorods and the Ni2P nanoparticles in basic solution remains unknown, but the different crystal

structures(orthorhombic Co2P and hexagonal Ni2P) are likely to be an important factor.

到目前为止，只有少数的催化剂（如铂、Mo2C）[ 8 ]已被证明在酸性和碱性溶液中表现可靠。为此，在碱性溶液（KOH，1m）对 Co2P纳米棒的HER性能进行了评价。相应的结果如图S8所示（S8电子补充资料），一个η20价值171mv（IR校正）。在经过RHE 0.330 至0.080 v vs的 1000cv扫描之后，η20从217增加到

14

229mv（无红外校正）。在η20增加相对较小（12 mV）和恒电位电解实验（图s8c，电子补充资料）在碱性溶液中显示Co2P纳米棒的稳定性。这些实验结果表明，Co2P纳米棒也可以在碱性条件下功能高效和稳定。据报道，Ni2P纳米颗粒在碱性溶液中η20（KOH，1m）是205mV，以及HER的性能迅速循环下降 [ 17 ]。对于Co2P纳米棒，以及在碱性溶液中磷化镍纳米颗粒的稳定性不同的原因仍然未知，但不同的晶体结构（正交Co2P、六Ni2P）可能是一个重要的因素。

The faradaic efficiency of the Co2P nanorods during H2 evolution was probed by comparing the volume of generated gas and quantity of charges passing the Co2P nanorods while a

potentiostatic electrolysis measurement was carried out. The volume of generated gas was monitored by the water

displacement method. Figure 5 shows the comparison of the

theoretical volume of hydrogen and the experimentally measured volume of hydrogen.10C of charges pass through the cathode( -0.5 cm2) in 1310s.1.25mL of H2 should be produced according to the theoretical computation. In the experiment, the volume of generated H2 was measured to be 1.18mL,very close to the

theoretical value. In addition, the faradaic yield of H2 production is nearly 100% over the time scale of the measurement(Figure 5). 在恒电位电解法进行测量时Co2P纳米棒产H2的法拉第效率是通过比较Co2P纳米棒产生的气体体积和通过的电荷数量探讨的。产生的气体的体积用水置换法监测。图5显示了氢的理论体积和实验测得的氢气的体积的比较。10C的电荷通过阴极（0.5平方厘米）在1310秒内，根据理论计算应产生1.25ml H2。在实验中，测得1.18ml是生成的H2的体积，非常接近理论值。此外，在测量的时间尺度内产H2的法拉第效率接近100%（图5）。

To obtain insight into the HER process and mechanism with the Co2P nanorods ,electrochemical impedance spectroscopy (EIS) investigations were carried out. The EIS experiments were carried out at different applied potentials, the results being shown in

15

Nyquist plots in Figure 6. The EIS spectra display two semicircles. The semicircles at high frequencies(shown more clearly in Figure 6b) can be related to the contact between the catalyst(Co2P) and the catalyst support(Ti foil),while those at low frequencies

correspond to the kinetics of the HER process on the surface of the catalyst.

为了洞察HER过程与获得Co2P纳米棒机制，电化学阻抗光谱（EIS）调查被进行了。EIS实验是在不同的应用潜力下进行，其结果被展示在如图6所示的Nyquist部分。电化学阻抗光谱显示为两个半圆。在高频率的半圆（更清晰地展示在图6b）可能和催化剂（Co2P）和催化剂载体（Ti箔）之间的联系相关，而在较低的频率则对应催化剂表面的HER过程的反应动力学。

The diameters of the semicircles at low frequencies are potential-dependent, decreasing with increasing applied potential. This is qualitatively in accordance with faster HER kinetics occurring at higher overpotential. The kinetics of electrochemical reaction at an electrode’s surface is usually assessed by charge transfer

resistance(Rct), with a smaller Rct value corresponding to faster kinetics. Rct can be deduced from EIS spectra by data fitting, in the present case using the equivalent circuit shown in FigureS9 (Electronic supplementary information).The results are listed in TableS1 (Electronic supplementary information).The high frequency constant phase element(CPE1) and resistance

element(R1) are nearly potential-independent(Figs.S10a and S10b, Electronic Supplementary information).In contrast, Rct is potential-dependent(Figure S10d, Electronic supplementary

information)whereas the low frequency constant phase

element(CPEdl) is nearly potential-independent (FigureS10c, Electronic supplementary information).The invariant CPEdl value suggests that the active surface area of the catalyst is maintained at different applied potentials, consistent with a high durability of the catalyst.

在较低频率的半圆的直径是潜在的依赖性，随着施加电位的增加而降低。较快的HER动力学于较高的过电位在质量上一致。在电极表面的电化学反应动力学通常是由电荷转移电阻（RCT）进行评估的，

16

以较小的RCT值对应较快的动力学反应。RCT可以通过数据拟合推导出EIS谱的范围，在本案例中使用等效电路的方式展示在图S9中（电子补充资料）。结果列在表S1中（电子补充资料）。高频率恒定相位元件（CPE1）和电阻元件（R1）几乎是潜在独立的（图S10a和S10b，电子补充信息）。与此相反，RCT是潜在依赖的（图S10d，电子补充资料）而低频率恒定相位元件（CPEdl）几乎是潜在的独立（图S10c，电子补充资料），不变的CPEdl值表明，催化剂的活性表面积在不同的应用潜力，与具有高耐久性的催化剂保持一致。

In general, a classic two-electron-reaction model suggests that the HER process proceeds in two steps: a discharge step (Volmer reaction:H3O+ +e _-Hads+H2O) followed by a desorption step(Heyrovsky reaction: Hads+H3O+ +e _-H2+H2O), or a discharge step followed by a recombination step(Tafel

reaction:Hads+Hads-H2), where H ads represents an H atom absorbed at the active site of the catalyst. The rate-limiting step can be identified by the Tafel analysis of the catalyst, with a Tafel slope of 116, 38, or 29mV/decade assigning the rate-determining step in the HER process to Volmer, Heyrovsky, or Tafel reactions, respectively. The polarization curve(Figure 4a) is presented in a Tafel plot in Figure 7a, and the apparent Tafel slope obtained by fitting the linear portion of the Tafel plot to the Tafel equation. The apparent Tafel slope of Co2P fitted from the polarization curve is 71.0mV/decade.The apparent Tafel slope extracted from the polarization curve will be higher than the true value, if electron transport in the catalyst is sufficiently slow. For example, it has been shown that different loading amounts of MoS x on GCE result in different Tafel slopes [43], and the introduction of reduced

graphene oxide to MoS2 can markedly reduce the Tafel slope [25]. Hu and Vrubel et al. demonstrated that the contribution of the electrontran sport process in the catalyst to the Tafel slope can be eliminated if the Tafel slope is derived from EIS data [43]. The log(1/Rct)–η plot of the Co2P nanorods is shown in Figure 7b, showing a Tafel slope of 51.7 mV/decade.

17

一般来说，一个经典的两电子反应模型表明，HER的过程分为两步：放电步骤（Volmer反应：H3O + E _ - HADS + H2O）随后解吸步骤（Heyrovsky反应：H3O+ + E _ - H2 + H2O），或一个放电步骤，然后跟着一个重组步骤（塔菲尔反应：HADS + hads-h2），其中H ads代表一个H原子在催化剂活性部位的吸收。限速步骤可以通过催化剂的塔菲尔分析确定，分别分析116、38或29mv /十组的塔菲尔斜率在HER的过程分配的沃玛，海罗夫，塔菲尔反应步骤的确定率。极化曲线（图4a）被展现在图7a的塔菲尔图上，以及明显的塔菲尔斜率通过塔菲尔方程的拟合塔菲尔曲线的线性部分获得。Co2P的塔菲尔斜率明显是从71.0mv/decade的极化曲线提取的。如果催化剂的电子传输足够缓慢，明显的从极化曲线上提取的塔菲尔斜率将高于实际值。例如，已被证明MOS X在玻碳电极上不同的装载量产生不同的塔菲尔斜率[ 43 ]，并且减少氧化石墨烯对MoS2的引入可以显著降低塔菲尔斜率[ 25 ]。胡和弗鲁贝尔等人表明，催化剂的电子运动过程对塔菲尔斜率的作用可以被消除，如果塔菲尔坡是来自EIS数据[ 43 ]。Co2P纳米棒log（1 / RCT）–η图由图7b所示，表现出51.7毫伏/十个一组的塔菲尔斜率。

The Tafel slope of 51.7mV/decadelies between 38 and 116 mV/decade, suggesting that a Volmer–Heyrovsky mechanism might be responsible for the HER process [10], and that the rates of the discharge step and the desorption step might be

comparable during the HER process [16]. The Tafel slope fitted from the polarization curve is larger than that obtained from Rct, suggesting that electron transport resistance in the catalyst may well be comparable to the charge transfer resistance at the catalyst/electrolyte interface.

位于38和116 mV/decade之间的51.7mv/decade塔菲尔斜率，表明由Volmer–Heyrovsky机制可能是负责HER的过程[ 10 ]，并且在HER的过程中[ 16 ]放电步骤和解吸步骤的速率可能具有可比性。塔

18

菲尔极化曲线拟合的斜率大于从RCT获得的，表明催化剂的电子传输电阻可以比拟在催化剂/电解质界面的电荷转移电阻。 The apparent Tafel slope would thereby be reduced and, accordingly, the HER performance could be further improved by reducing the electron transport resistancein the catalyst, for example via the introduction of conductive graphene [25]. The turnover frequencies (TOFs) of the HER were estimated according to the number active sites determined from the CV sweep [7]. In this approach the total number of active sites is the upper limit value. Figure S11 (Electronic supplementary information) shows the plot of TOFs versus the applied potentials in the 0.5M H2SO4 solution. The Co2P nanorods show a TOF of 0.725S1 at 143 mV vs. RHE .This value is larger than that of CoP/CC (~-75 mV vs. RHE) [20], and smaller than that of MoS3 deposited on GCE(~-220 mV vs. RHE) [7].

明显的塔菲尔斜率将因此减少，因此，可能她的表现是降低电子传输能力，催化剂的进一步改进，例如通过导电的石墨烯[ 25 ]介绍。周转频率（Tofs）的她是根据数量的活性位点的CV扫描[ 7 ]确定的估计。在这种方法中，活动站点的总数是上限值。图S11（电子补充资料）显示的Tofs与在0.5M H2SO4溶液的电位图。电子纳米棒显示TOF 0.725s1在143 mV vs RHE。这个值大于警察/ CC（~ - 75 mV vs RHE）[ 20 ]，小于沉积在玻碳电极上生成（~ - 220 mV vs RHE）[ 7 ]。

明显的塔菲尔斜率将因此减少，于是，催化剂的HER性能可能进一步改良通过降低电子传电阻，例如通过导电的石墨烯[ 25 ]介绍。

19

HER的周转频率（Tofs）是根据CV扫描的活性位点的数量[ 7 ]确定的估计。在这种方法中，活动站点的总数是上限值。图S11（电子补充资料）显示的Tofs与在0.5M H2SO4溶液的电位图。电子纳米棒显示TOF 0.725s1在143 mV vs RHE。这个值大于警察/ CC（~ - 75 mV vs RHE）[ 20 ]，小于沉积在玻碳电极上生成（~ - 220 mV vs RHE）[ 7 ]。

20

21

本文来源：https://www.bwwdw.com/article/5xi7.html