The Design of High-performance Ruthenium Oxide Catalyst for Electrocatalytic Oxygen Evolution Reaction
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摘要: 采用高温固相法结合离子交换法制备了一种HRu4O8微米棒；利用X射线衍射仪、透射电子显微镜、扫描电子显微镜、X射线光电子能谱等对材料进行形貌和物相表征；采用线性扫描伏安、循环伏安、塔菲尔、计时电位等电化学方法研究了HRu4O8微米棒电解水氧析出反应的催化活性。结果表明：以RuO2纳米颗粒为前驱体制备HRu4O8，其电化学活性比表面积显著增大，展现出优异的氧析出催化反应活性，在10 mA/cm2电流密度下的过电位(仅为208 mV)低于RuO2纳米颗粒(276 mV)。另外，HRu4O8微米棒具有出色的稳定性，该研究为设计高活性的电解水催化剂提供了新思路。Abstract: One kind of HRu4O8 microrods were fabricated with solid-state calcination and the proton exchange method. The morphology and crystal structure of HRu4O8 microrods were characterized with X-ray diffraction, transmission electron microscopy, scanning electron microscopy, X-ray photoelectron spectroscopy and so on. The catalytic activity of HRu4O8 was evaluated with the linear scanning voltammetry, cyclic voltammetry, Tafel plot, chronopotentiometry and other electrochemical methods. The results showed that the preparation of HRu4O8 with the RuO2 nanoparticle precursor resulted in a significant increase of the electrochemical active surface area. HRu4O8 exhibited excellent electrocatalytic activity towards OER with an overpotential of only 208 mV at 10 mA/cm2, lower than that of RuO2 nanoparticles(276 mV). Moreover, HRu4O8 also presented outstanding stability for 10 h without apparent degradation. Such work sheds light on new perspectives for designing highly active electrocatalyst of water splitting.
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Keywords:
- catalyst /
- oxygen evolution reaction /
- ruthenium oxide
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电催化氧析出反应(OER)在电化学能源存储与转化的众多系统中扮演着重要角色，包括金属空气电池[1-3]、水分解[4-6]等，其中，涉及阴极析氢反应和阳极析氧反应的电解水过程被认为是可再生能源生产、储存和利用的有效途径。OER通常需要经历一个四电子和四质子的转移过程，造成缓慢的电化学反应惰性动力学以及复杂的反应过程，需要较高的能量克服动力学能垒，从而导致反应过电位较高[7-9]。近年来，国内外科研工作者致力于开发高效的OER电催化剂以降低反应能垒，包括：过渡金属及其氧化物[10-12]、金属硫化物[13]、金属氮化物[8]、尖晶石、层状双氢氧化物[14-15]、钙钛矿[16-17]、碳基复合材料[18]等。在众多的电催化剂中，Ru基材料由于其高本征活性和相对较低的成本(相比其他贵金属)等优点而被广泛研究[19-22]。尤其是在酸性介质中，氧化钌显示出非常优异的电催化活性。然而，在高电位下氧化钌不稳定，容易被氧化而生成RuO4[23]，并且氧化钌在强酸性环境下易发生溶解行为[24-26]，造成催化活性和稳定性的下降，限制了它的进一步应用。HODNIK等[27]通过实验证实氧化钌和钌在酸性环境下会溶解生成可溶性的H2RuO5和H2RuO3且其OER活性与溶解度呈负相关。针对这些问题，国内外科研工作者对氧化钌开展了多途径研究，包括形貌设计、结构调控、缺陷工程、异质结工程等[28-32]。
本研究采用高温固相法制备了氧化钌纳米棒，解决了传统纳米颗粒易团聚的问题，并通过调控晶格氧显著提高其OER电催化活性，过电位降低至208 mV，优于商业RuO2纳米催化剂，并能稳定循环10 h而无明显衰减。研究结果可为设计高活性电解水催化剂提供重要参考。
1. 研究方法
1.1 试剂与仪器
主要试剂：碳酸钾、二氧化钌、Nafion溶液、盐酸、硫酸等均为市售，实验用水为去离子水。
主要仪器：X射线衍射仪(XRD，Empyrean Mal-vern PANalytical)、X射线光电子能谱(XPS，K-Alpha，Thermo Scientific)、扫描电子显微镜(SEM，Zeiss Sigma 300，15 kV)、透射电子显微镜(TEM，FEI Tecnai G2 F30，300 kV)、电化学工作站(CHI760e，上海辰华)。
1.2 催化剂的制备
采用高温固相法结合离子交换法制备催化剂：(1)称取碳酸钾和二氧化钌粉末(按物质的量之比5∶8)在研钵中研磨均匀，压制成片；(2)将其转移至管式炉，采用高温固相烧结法在氩气气氛中950 ℃煅烧12 h，并冷却至室温；(3)取出样品用去离子水润洗去除可溶性杂质并抽滤；(4)将抽滤后的样品与1 mol/L盐酸混合，在60 ℃下搅拌3 d，每天进行1次离子交换离心后更换1次盐酸，进行离子交换；(5)将离子交换后的样品进行抽滤，并用去离子水润洗数次，采用真空烘箱干燥。
1.3 表征方法
采用XRD对样品进行物相表征。利用SEM和TEM对样品进行形貌和结构表征。通过XPS对样品进行表面电子结构分析。
1.4 电化学性能测试
取1 mg粉末样品与1 mL 0.25% Nafion/乙醇溶液混合，超声分散成均匀浆料。用移液器取5 μL上述溶液滴涂至玻碳电极表面，在60 ℃烘干备用。商业RuO2粉末样品电极采用相同方法制备。电化学性能测试采用三电极体系，其中，涂有催化剂材料的玻碳电极作为工作电极，石墨棒为对电极，银/氯化银电极(Ag/AgCl)为参比电极，以0.5 mol/L H2SO4溶液作为电解液。电势用标准氢电极(RHE)标定，电压根据能斯特方程换算：
ERHE=EAg/AgCl+0.059pH+0.222。 2. 结果与讨论
2.1 物相与形貌结构
以碳酸钾和二氧化钌为前驱体，采用高温煅烧法制备体心四方结构的KRu4O8(a=b=0.988 5 nm，c=0.312 7 nm)，并通过酸交换(以H+取代K+)获得质子化的具有体心四方结构的HRu4O8(图 1)。二者的RuO6八面体在c轴方向上表现出一维隧道型结构，可容纳不同的阳离子[33-34]。利用XRD对KRu4O8和HRu4O8进行物相表征。KRu4O8的XRD图谱与标准卡片(PDF No.70-0724)对应，无杂相，进一步表明高温固相烧结法生成了纯相KRu4O8。在2θ=12.6°、17.9°、25.5°、28.5°、35.1°、36.3°等位置出现较强的衍射峰，分别对应于体心四方结构KRu4O8的(110)、(200)、(220)、(130)、(211)和(400)晶面。经过酸处理之后，HRu4O8衍射峰的位置与KRu4O8基本保持一致，只是峰的强度有所减弱，该结果表明：当质子取代钾离子的位置后，材料的基本结构保持不变。
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图 1 KRu4O8和HRu4O8的晶体结构及XRD图谱Figure 1. The crystal structures and XRD patterns of KRu4O8 and HRu4O8采用扫描电子显微镜(SEM)观察KRu4O8的形貌(图 2A(a))，KRu4O8呈现一维棒状结构，长度在1 μm以上，直径范围约为0.3~4 μm。采用透射电子显微镜(TEM)对单根KRu4O8微米棒进行形貌和结构分析(图 2A(b~c))，选区电子衍射(SAED，图 2A(c))的表征结果显示：KRu4O8微米棒为单晶结构，衍射斑点标定结果与KRu4O8相符，无其他杂相。由HRu4O8的SEM图(图 2B(a))可看出，经过酸处理之后HRu4O8仍然保持着微米棒的结构，其成分由Ru和O组成，2种元素均匀分布在微米棒中(图 2B(a~d))。EDS谱也显示：微米棒由Ru和O组成(Si来自基底，Pt来自喷金)，K的谱峰强度非常弱，表明K+基本被H+取代(图 2B(e))。
利用X射线光电子能谱(XPS)对HRu4O8的组成和表面电子结构进行分析。XPS全谱检测到Ru和O元素(图 3A)，Ru 3p峰的拟合曲线与测试结果基本吻合(图 3B)，该结果表明HRu4O8微米棒中Ru元素只存在1种价态(结合能为463.2 eV)，对应于Ru元素的+4价[35-36]。对O 1s曲线进行分峰拟合可知：HRu4O8中存在3种氧组分，其结合能分别为529.2、530.8、532.4 eV(图 3C), 其中结合能为529.2、530.8 eV的峰对应于HRu4O8中Ru—O键和H—O键，结合能为532.4 eV的峰归属于材料表面吸附水中氧的孤对电子与质子空轨道形成的H—O配位键。
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图 3 HRu4O8的XPS谱以及Ru 3p与O 1s峰的拟合曲线Figure 3. The XPS spectrum of HRu4O8 and the fitting curves of Ru 3p and O 1s peaks2.2 电化学性能
利用线性扫描伏安(LSV)和塔菲尔(Tafel)曲线对HRu4O8和RuO2进行OER电化学性能评价。图 4A显示了HRu4O8、RuO2电极在电流密度j=10 mV/s扫速下的线性扫描伏安曲线，起始氧化电位分别为1.38、1.42 V。HRu4O8和RuO2电极在10 mA/cm2下的过电位分别为208、276 mV。结果表明：与纳米RuO2颗粒的形貌相比，纳米棒形貌可显著增强HRu4O8电分解水的催化活性。
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图 4 HRu4O8和RuO2的线性扫描伏安和Tafel斜率曲线Figure 4. The LSV and Tafel profiles of HRu4O8 and RuO2Tafel斜率是判别电极材料催化活性的另一重要依据。由图 4B可知，HRu4O8、RuO2的Tafel斜率分别为95.3、126.1 mV/dec，说明HRu4O8具有更优异的电极反应动力学性质，能快速增大氧析出反应电流。
在不同扫描下测试循环伏安(CV)曲线，分析材料的电容性能(图 5)，曲线呈现准矩形，表现出双电层电容的特性，即使扫速增大到200 mV/s，也没有出现明显的畸变。随着扫描速率的增大，其电流密度也逐步增大。取固定电位下的电流密度对扫描速率作图并进行线性拟合，可得到HRu4O8电极的比电容为10.15 mF/cm2。根据平整表面的比电容(20~60 μF/cm2[37-39])可得电化学活性比表面积(Electrochemical Active Specific Surface Area, ECSA)约为253 cm2。对RuO2纳米颗粒采用相同的方法研究，结果表明：比电容为0.54 mF/cm2，ECSA约13.5 cm2，说明通过结构调控将纳米颗粒转变为棒状结构可显著增加电化学活性比表面积，有效避免纳米颗粒的团聚问题，从而为OER提供更多的催化活性位点，最终增强了HRu4O8微米棒电极的电催化活性。
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图 5 HRu4O8和RuO2电极在不同扫速下的CV曲线及其拟合曲线Figure 5. The CV and fitting curves of HRu4O8 and RuO2 electrodes at different scan rates采用计时电位技术测试两种电极材料(HRu4O8和RuO2)的电化学稳定性(图 6)。在恒电流密度(10 mA/cm2)下，HRu4O8的电势保持在1.48 V左右，持续稳定10 h没有明显衰减。相反，在相同电流密度下，RuO2的电压在前期持续上升，后期保持在1.67 V，高于HRu4O8微米棒电极。
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图 6 HRu4O8和RuO2在10 mA/cm2电流密度下的计时电位曲线Figure 6. The chronopotentiometry profiles of HRu4O8 and RuO2 at a current density of 10 mA/cm2以上电化学测试结果表明：HRu4O8微米棒具有优异的OER电催化性能。这归因于以下几个因素：首先，通过形貌结构调控将纳米颗粒转变为微米棒，有效解决了纳米粒子极易团聚的问题，保持较大的活性比表面积，为电催化反应持续提供较多的活性位点；其次，通过酸交换处理将微米棒转变为质子化结构，有效解决了氧化钌在酸性溶液中的溶解问题，维持了结构的稳定性。另外，质子化的HRu4O8可以利用H+的空轨道与O2的孤对电子进行配位，增强了氧的吸附能力和电极反应动力学性质。
3. 结论
采用高温固相烧结法结合质子交换法制取了HRu4O8微米棒，利用XRD、SEM、TEM和XPS等手段对材料进行了形貌结构表征和表面电子价态分析，研究了HRu4O8微米棒在酸性介质中的电解水OER性能。电化学性能测试结果表明：所制备的HRu4O8微米棒具有较高的催化活性，在10 mA/cm2电流密度下的过电位仅为208 mV。计时电位测试表明：该材料在10 mA/cm2电流密度下可以稳定保持10 h，为设计高活性和高稳定性的水分解催化剂提供了新思路。
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图 1 KRu4O8和HRu4O8的晶体结构及XRD图谱
Figure 1. The crystal structures and XRD patterns of KRu4O8 and HRu4O8
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图 3 HRu4O8的XPS谱以及Ru 3p与O 1s峰的拟合曲线
Figure 3. The XPS spectrum of HRu4O8 and the fitting curves of Ru 3p and O 1s peaks
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图 4 HRu4O8和RuO2的线性扫描伏安和Tafel斜率曲线
Figure 4. The LSV and Tafel profiles of HRu4O8 and RuO2
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图 5 HRu4O8和RuO2电极在不同扫速下的CV曲线及其拟合曲线
Figure 5. The CV and fitting curves of HRu4O8 and RuO2 electrodes at different scan rates
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