1、分子前驱体衍生的氧化镍电极催化水氧化性能研究（英文） 李斐 李华 朱勇 杜健 王勇 孙立成 大连理工大学精细化工国家重点实验室 瑞典皇家工学院化学系 摘 要： 太阳能分解水制氢是解决当前能源和环境危机的潜在手段之一.其中由于水氧化半反应涉及 4 个电子和 4 个质子的转移, 因此是全分解水反应的瓶颈所在.为了发展高效的水氧化催化剂, 降低水氧化过电位, 人们付出了巨大的努力.目前活性最高的水氧化催化剂都是基于钌和铱的贵金属催化剂, 高昂的成本阻碍了这些催化剂的规模化应用, 因此人们尝试利用各种方法制备基于廉价金属的水氧化催化剂.2008 年, Nocera 课题组利用电沉积法从磷酸溶液中得到了
2、高活性氧化钴催化剂, 之后该法逐渐得到推广.最近, Spiccia 和 Allen 课题组利用分子前驱体通过电沉积法制备了氧化镍催化剂, 但其催化活性和稳定性有待进一步提高.本文将一个简单的镍配合物Ni (en) 3Cl2 (en=1, 2-乙二胺) 作为前驱体溶解到磷酸缓冲溶液中, 在 FTO 基底上电沉积得到具有高催化活性的氧化镍水氧化催化剂.在 pH=11 的磷酸缓冲溶液中, 由分子前驱体沉积所得到的 NiOx的催化电流达到 1 mA/cm2时的过电位为 375 mV, 且可稳定工作 10 h 以上.其催化过程中的 Tafel 斜率为 46 mV/decade, 表现出优异的动力学特性.
3、该电极和之前文献中催化活性最高的从分子前驱体衍生得到的 NiOx相比展现出较大的优势.比如在 1.3 V (相对于 NHE) 电压下, Ni (en) 3Cl2 衍生的 NiOx催化电流密度可以达到 8.5 mA/cm2, 法拉第效率为 98%.而 Ni-氨基乙酸衍生的 NiOx在相同条件下催化电流密度为 4 mA/cm2, 法拉第效率仅为 60%.该工作充分证明以分子配合物作为前驱体是制备高效高稳定性多相水氧化催化剂的简便途径.有机配体和金属螯合的分子前驱体在结构上具有灵活可调的特性, 从而有助于构建活性和效率更高的催化体系.关键词： 电催化; 氧化镍; 水氧化; 分子前驱体; 分解水; 作
4、者简介：李斐 电话: (0411) 84986247;传真: (0411) 84986245;电子信箱:收稿日期：30 August 2017基金：supported by the National Basic Research Program of China (973 program, 2014CB239402) Electrocatalytic water oxidation by a nickel oxide film derived from a molecular precursorFei Li Hua Li Yong Zhu Jian Du Yong Wang Licheng S
5、un State Key Laboratory of Fine Chemicals, DUT-KTH Joint Education and Research Center on Molecular Devices, Dalian University of Technology (DUT) ; Abstract： In this study, we fabricated a NiOx film by electrodeposition of an ethanediamine nickel complex precursor (pH = 11) on a fluorine-doped tin
6、oxide substrate. The resulting film is robust and exhibits high catalytic activity for electrochemical water oxidation. Water oxidation is initiated with an overpotential of 375 mV (1 m A/cm2) and a steady current density of 8.5 m A/cm2 is maintained for at least 10 h at 1.3 V versus the normal hydr
7、ogen electrode. Kinetic analysis reveals that there is a 2 e-/3H+ pre-equilibrium process before the chemical rate-determining step. The low-cost preparation, robustness, and longevity make this catalyst competitive for applications in solar energy conversion and storage.Keyword： Electrolysis; Nicke
8、l oxide; Water oxidation; Molecular precursor; Water splitting; Received： 30 August 20171. IntroductionThe massive consumption of fossil fuel is creating worldwide interest in searching for renewable energy sources1, 2.Splitting water into oxygen and hydrogen by sunlight is a promising approach to a
9、chieve this goal3, 4.However, water oxidation involves the release of 4e-and 4H+, and it requires a high thermodynamic overpotential (2H2OO 2+4H+4e-, 1.23 V vs.the normal hydrogen electrode (NHE) at p H=0) 5.To efficiently drive this reaction, water oxidation catalysts (WOCs) with low overpotentials
10、 are crucial.Inspired by nature, great progress has been made in developing WOCs based on transition metals614.Although the activities of WOCs based on noble metals, such as ruthenium and iridium, are impressive, their practical application is hampered by their scarcity and high cost.Consequently, d
11、evelopment of efficient WOCs based on earth-abundant elements is urgently required in the field of solar energy conversion.Great effort has been made to develop WOCs derived from first row transition metals.In 2008, Nocera and co-workers reported that electrodeposition of cobalt phosphate (Co-Pi) on
12、 conductive substrates gave highly efficient electrocatalysts for water oxidation15.This method has been extended to other metals, such as iron and nickel1623.For example, electrochemical deposition of an oxide film from molecular complexes is a promising approach to fabricate active Ni-based WOCs17
13、, 24, 25.Recently, Spiccia and co-workers reported a facile protocol for electrodeposition of a Ni Ox film from a nickel amine complex in borate buffer24.The film exhibited a steady current density of 1.8 m A/cm2 at 1.3 V vs.NHE in borate buffer.More recently, Allen and co-workers reported that the
14、combination of Ni2+and glycine could act as a precursor for an efficient nickel oxide electrocatalyst in phosphate buffer at p H=1125.However, the activity and durability of these catalysts needs to be improved.Here, inspired by this progress, we report a simpleNi (en) 3Cl2 (en=ethanediamine) comple
15、x for facile electrodeposition of a nickel oxide film in phosphate buffer solution.The resulting film is inert to corrosion and exhibits superior catalytic activity to other molecular complex-derived catalysts.A current density of 8.5 m A/cm2 is maintained for at least 10 h at a constant applied pot
16、ential of1.3 V.2. Experimental2.1. Materials and methodsAll of the chemicals were purchased from Aladdin Chemical Company and used without further purification.Deionized water (18.2 M/cm) obtained from a Milli-Q system (Millipore, Direct-Q 3 UV) was used throughout.The phosphate buffer solution (0.2
17、5 mol/L, p H=11) was prepared by dissolving appropriate amounts of Na2HPO412H2O and Na3PO412H2O in deionized water.TheNi (en) 3Cl2 complex was prepared according to a reported procedure26.The fluorine-doped tin oxide (FTO) substrates were purchased from Dalian Heptachroma Solar Tech Co., Ltd. (thick
18、ness2.2 mm, transmittance90%, resistance8 m) .Scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) analysis were performed with a Nova Nano SEM 450 scanning electron microscopy.The SEM images and EDX spectra were obtained with acceleration voltages of 3 and 20 k V, respectively.The c
19、atalyst content on the surface of the working electrode (FTO) was determined by inductively coupled plasma mass spectrometry (ICP-MS, (Optima 2000DV, America Perkin Elmer Co.) .The sample used for ICP-MS was prepared by depositing the Ni Ox film on FTO with an applied bias of 1.2 V (vs.NHE) .The sam
20、ple was then gently rinsed with deionized water and dissolved in concentrated HNO3.The sample used for the measurement was diluted with water.X-ray photoelectron spectroscopy (XPS) was performed with a Thermo Scientific ESCALAB250 X-ray photoelectron spectrometer using 200 W K radiation.The electroc
21、hemical measurements were recorded with a CHI 630D electrochemical potentiostat.The counter electrode was platinum wire.The reference electrode was an aqueous Ag/Ag Cl (3 mol/L Na Cl) electrode.A glassy carbon electrode (diameter 3 mm) or FTO film was used as the working electrode.All of the potenti
22、als were measured against a Ag/Ag Cl reference and converted to the NHE by addition of 0.197 V to the measured potentials.2.2. Fabrication of the catalyst filmThe FTO substrates were ultrasonically cleaned in deionized water, ethanol, and acetone (30 min each) , and then air-dried.The Ni Ox films fo
23、r the water oxidation experiments were deposited by constant potential electrolysis (CPE) at 1.2 V (vs.NHE) using a solution ofNi (en) 3Cl2 (1 mmol/L) in phosphate buffer solution (0.25 mol/L) .The films were then gently rinsed with deionized water and transferred to fresh 0.25 M phosphate buffer so
24、lution for anodization (1.3 V vs.NHE bias was applied for about 1.5 h) .2.3. Currentpotential measurements for the Tafel plotCurrentpotential data were obtained by performing controlled potential electrolysis in 0.25 mol/L phosphate buffer solution at p H=11 with a variety of applied potentials.A FT
25、O electrode (1 cm2) coated with a Ni Ox film (after anodization) was used as the working electrode.Ag/Ag Cl and Pt wire were used as the reference and counter electrodes, respectively.Before data collection, the solution resistance (35) was measured with a clean FTO working electrode using the poten
26、tial loss for resistance (i R) test function to correct the Tafel plot for the i R drop.A catalyst film (1 cm2) was prepared by electrodeposition.Preconditioning the film by subjecting it to bulk electrolysis for several hours is necessary to obtain a reproducible Tafel slope value.A current density
27、 of 1.1 m A/cm2 was applied for6 h before collecting data for the Tafel plot.The steady-state currents were measured at a variety of applied potentials while the solution was stirred.The Tafel plot measurements were performed in 10 m V steps between 0.88 and0.97 V.In a typical experiment, the curren
28、t reached a steady state at a particular potential in 3 min and the current values were recorded after 5 min.All of the measurements were performed twice.The obtained current values ranged from 22A/cm2 to 1.84 m A/cm2 in the applied potential range.The variation in the steady-state current of two ru
29、ns at a particular potential was5%.According to the currentpotential data, the Tafel slope is 43 m V/decade.2.4. Currentp H measurementsThe currentp H data were collected by performing electrolysis at a fixed applied potential of 0.91 V (vs.NHE) in 0.25mol/L phosphate buffer solution with a variety
30、of p H values.A FTO electrode (1 cm2) coated with a Ni Ox film (after anodization) was used as the working electrode.Ag/Ag Cl and Pt wire were used as the reference and counter electrodes, respectively.The solution had an initial p H value of 10.4.The p H value gradually increased by adding small am
31、ounts of Na OH solution, and the current density was recorded after 5 min electrolysis at each p H value point.All of the data were collected with i R correction using a solution resistance value measured before each electrolysis.The p H values of the solution ranged from 10.4 to11.6 and the measure
32、d currents ranged from 14A/cm2 to1.56 m A/cm2.2.5. Determination of the faradaic efficiencyThe faradaic efficiency measurements for oxygen evolution were performed in a gas-tight electrochemical cell.The cell was equipped with a FTO electrode coated with a Ni Ox film.Ag/Ag Cl and Pt wire were used a
33、s the reference and counter electrodes, respectively.Before the measurements, the solution was degassed by bubbling Ar for 2 h with vigorous stirring.Electrolysis was initiated at 1.3 V without i R correction.During bulk electrolysis, the amount of evolved oxygen in the headspace was determined by g
34、as chromatography.Fig.1.Continuous CV scans of 1 mmol/LNi (en) 3Cl2 in 0.25 mol/L phosphate buffer (p H=11) recorded at a glassy carbon electrode with a scan rate of 100 m V/s.The black trace is the scan of phosphate buffer solution containing 1 mmol/L Ni2+.The blue trace is the scan of phos-phate b
35、uffer without a nickel source.The insert shows the 1st (black) and 10th (red) CV scans. 下载原图3. Results and discussionFig.1 shows the cyclic voltammograms (CVs) of a glassy carbon electrode immersed in 0.25 mol/L phosphate buffer solution (p H=11) containing 1 mmol/LNi (en) 3Cl2.In the first anodic s
36、can, an anodic peak occurs at 0.89 V (all of the reported potentials are versus NHE) , which is followed by a steep catalytic current arising from water oxidation.In the cathodic return scan, there is a broad peak centered at 0.85 V, which is attributed to reduction of the surface deposit formed in
37、the initial anodic sweep.Repeated CV scans show the onset potential of water oxidation cathodically shifts while the cathodic peak anodically shifts.The catalytic current and cathodic peak increase in amplitude with scanning, suggesting growth of the material deposited on the surface.The intensity o
38、f the anodic peak decreases for the first three cycles and then remains stable.When Ni2+is used instead ofNi (en) 3Cl2, the CV trace is relatively featureless and the current is below 10A, essentially overlapping the background.This is probably because of the low solubility of Ni2+in the phosphate e
39、lectrolyte solution inhibiting surface deposition.Fig.2.SEM images of the Ni Ox film before (a) and after (b) electrolysis. 下载原图Based on the CV experiments, a thin catalyst film was fabricated by anodic deposition on a FTO substrate at a constant potential of 1.2 V in 0.25 mol/L phosphate buffer sol
40、ution (p H=11) containing 1 mmol/LNi (en) 3 (Cl) 2.With 0.5 C/cm2 charge, about 0.84mol/cm2 of Ni was deposited on the surface of FTO, as determined by ICP-MS measurement.The as-prepared film was transferred into a fresh phosphate buffer solution.Upon applying a constant potential of 1.3 V, the curr
41、ent density further increased until reaching a plateau.According to previous studies, this phenomenon might be associated with an anodization process17, 27, 28.The SEM image in Fig.2 (a) show that the obtained film consists of compact packed nodules with sizes in the range200400 nm.XRD showed no cha
42、racteristic diffraction peaks belonged to nickel species, indicating the amorphous character of the catalyst.EDX analysis was directly performed on the film after thorough rinsing with distilled water to remove any surface adsorbed components.The results showed that the film contained Ni, C, O, N, a
43、nd P, as well as Sn and Si from the FTO substrate.The surface composition on the catalyst film was further investigated by XPS.The survey spectrum indicated the presence of Ni 2p, P 2p, and O 1s.In the Ni 2p spectrum (Fig.3 (a) ) , the peaks at 855.4 and 873.2 e V and the two satellite peaks at 860.
44、7 and 879.1 e V are ascribed to typical Ni (II) species.In the P 2p spectrum (Fig.3 (b) ) , the characteristic peak of P at 132.7 e V indicates the presence of a very small amount of PO43-.In the O 1s spectrum (Fig.3 (c) ) , the peak at 531.2 e V can be attributed to oxide and hydroxide on the surfa
45、ce of the film.According to the XPS results, the main component of the catalytic film is Ni Ox.Fig.3.Ni 2p (a) , P 2p (b) , and O 1s (c) XPS spectra of the electrodeposited film on the FTO electrode. 下载原图Fig.4.Current density trace obtained by controlled potential electroly-sis of the FTO electrode
46、coated with a Ni Ox film in stirred 0.25 mol/L phosphate buffer solution (p H=11) (reference electrode Ag/Ag Cl, counter electrode Pt wire, applied potential 1.3 V, stirring rate 500r/min, no i R correction) . 下载原图Long-term electrolysis was performed at a constant potential of 1.3 V.Fig.4 shows that
47、 a steady current of 8.5 m A/cm2 is maintained for at least 10 h, confirming the robustness of the catalyst film for water oxidation.The SEM image of the electrode shows no morphology change after electrolysis (Fig.2 (b) ) .During this process, the faradaic efficiency was determined to be 98%.The hi
48、gh activity and long-term stability show the potential of using Ni complexes as precursors for preparation of efficient WOCs.The Tafel plot of log (j) against the overpotential () has a slope of 46 m V/decade for current densities ranging from 22A/cm2 to 1.8 m A/cm2 (Fig.5 (a) ) .It should be noted
49、that preconditioning the film by bulk electrolysis for several hours is necessary to obtain a reproducible Tafel slope value.The slope of the Tafel plot ofNi (en) 32+-derived Ni Ox is comparable with that of a glycinenickel derived WOC25and significantly lower than those of other Ni oxides reported in the literature15, 22, 27, 28, indicating favorable kinetics for water oxidation.According to the Tafel plot, water oxidation starts at=330m V (reaching a current density of 10-4 A/cm2) and an ov
