氢氧化镍对碱性介质中甲醇氧化的电催化作用

【作者】 张黎；

【导师】 周益明；

【作者基本信息】 南京师范大学 ， 物理化学， 2008， 硕士

【摘要】 甲醇等有机小分子的电化学氧化是燃料电池研究中广为涉及的课题,当前对其在铂、钯等贵金属电极上的电氧化反应研究较多,但因易中毒及成本问题而受到困扰。金属镍及其相关化合物来源广泛,价格便宜,且对甲醇等醇类分子的氧化具有一定的催化活性,自20世纪70年代以来,人们开展了碱性介质中用各种形式的镍电极电催化氧化小分子醇类化合物如甲醇、乙醇的研究。研究发现,在碱性介质中,大块存在的Ni对甲醇氧化并没有催化作用,采用电沉积法,使Ni负载在石墨电极上,这时表现出了良好的催化活性,同时还发现,Ni/C电极对甲醇电氧化催化活性的大小与石墨电极上沉积的Ni的数量有关。甲醇氧化的电催化反应中,真正起催化作用的是Ni³⁺,因此我们直接合成出Ni（OH）₂,避免了从单质Ni到Ni²⁺的过程。本论文采用低热固相反应的方法,合成出了不掺杂的纯Ni（OH）2和掺杂不同元素的Ni（OH）₂,采用XRD对合成的Ni（OH）₂进行了分析表征,并就电极涂布物的比例的选择,甲醇浓度、Ni（OH）₂晶型结构、掺杂元素的种类及含量等方面因素对碱性介质中甲醇的催化氧化的影响进行了研究。实验结果表明:1.通过NiSO₄·6H₂O和NaOH的直接低热固相反应,合成得到不掺杂的纯Ni（OH）₂,XRD测试结果证实为β型Ni（OH）₂;采用类似的方法,可以得到Al、Zn、Mn、Co单掺杂的Ni（OH）₂,XRD测试结果表明除20%Al掺杂的为仅型Ni（OH）₂外,其余均为β型Ni（OH）₂。2.通过对电极涂布物不同比例下的电极的可逆性进行研究,发现活性物质:碳粉:PTFE按55:40:5的质量比是比较合适的比例,因为在此比例下电极显示出良好的可逆性。3.在研究甲醇浓度的影响时发现,随着甲醇浓度从0.1mol/L逐渐增加到0.5mol/L时,稳态电流呈现逐渐增加的趋势,但是增大的幅度逐渐减小。因此最终选择O.1mol/L CH₃OH+1mol/L KOH溶液为电解液。4.通过低热固相反应法合成的Ni（OH）₂,无论是何种晶型,对甲醇氧化都具有催化作用。但从循环伏安实验和计时电流的实验数据看出,β型对甲醇的催化作用高于α型,这可能是由β型Ni（OH）₂的氧化电位低于α型引起的。5.为了寻找合适的β型的Ni（OH）₂作为甲醇氧化的催化剂,我们对合成的单掺杂不同量的Zn、Co、Mn元素的样品进行了系统的循环伏安、计时电流实验,经过分析比较发现:它们与不掺杂的Ni（OH）₂电极比较,催化性能有一定的改善:掺Zn15%、掺Co5%、掺Mn10%是各自掺杂元素中性能最好的:三者中又以掺Mn10%为最佳。这说明掺杂金属离子能有效提高Ni（OH）₂对甲醇氧化的催化效率。更多还原

【Abstract】 The electrochemical oxidation of methanol and other small organic molecules is widely involved in fuel cell research.At present,extensive investigations are being carried out on Pt,Pd and other related electrode materials,which are commonly used as catalyst in the electrochemical oxidation of methanol.However,the application is restricted in the lab scale due to its poisonous characteristics and high cost.Nickel and its related compounds have many advantages,such as low cost and wide sources. Besides,most of them have some catalytic activity to the oxidation of methanol and other alcohol molecules.Since 1970s,oxidation of small organic molecule compounds such as methanol and ethanol at various forms of nickel electrode in alkaline medium has drawn considerable attention.The research found that only Ni dispersed on graphite showed a catalytic activity on methanol oxidation while massive Ni did not.It also suggested that the catalytic activity of the Ni/C electrodes towards methanol oxidation varied with the amount of electro-deposited Ni.In the methanol oxidation catalytic reaction,Ni³⁺was the only effective catalyst.So we synthesized Ni（OH）₂ directly,avoiding the transition process from single nickel to Ni²⁺.Pure Ni（OH）₂ and Ni（OH）₂ doped with different additives were synthesized by solid-state reaction at room temperature.The crystal structures of samples were tested by XRD.The electro-catalysis to the methanol oxidation was investigated. Also the effects of coating proportion,concentration of methanol,crystal structure of Ni（OH）₂,species and content of additives were studied.The following results were obtained:1.Pure Ni（OH）₂ without any additives were synthesized by the solid state reaction of NiSO₄·6H₂O and NaOH at room temperature.The XRD results revealed that the pure Ni（OH）₂ wasβtype.The substituted Ni（OH）₂ samples such as aluminum,zinc,cobalt,manganese substituted were also prepared by solid-state reaction at room temperature.The XRD results revealed that Ni（OH）₂ substituted with 20%aluminum wasαform,while other samples were allβform.2.The reversibility of the electrode with different coated proportion was investigated.The experiments showed that the best reversibility was obtained at the mass ratio of 55:40:5（active substance:carbon powder:PTFE）.3.The effect of the methanol concentration was also investigated.As the methanol concentration raised from 0.1mol/L to 0.5mol/L,the steady-state current trends showed a gradual increase with a gradually decreased rate.Finally,a solution with 0.1mol/L CH₃OH +1 mol/L KOH was selected as the electrolyte solution for all the experiments.4.Regardless of their crystal structure,Ni（OH）₂ had some catalytic effect towards the oxidation of methanol in alkaline media.According to the results of cyclic voltammetry and chronoamperometric experiments,the catalytic effect ofβ-Ni（OH）₂ was better thanα-Ni（OH）₂,the reason might be the oxidation potential ofβ-Ni（OH）₂ was relatively lower.5.In order to find out a more suitableβ-Ni（OH）₂ as methanol oxidation catalyst,we synthesized samples with different proportion and different elements such as zinc,cobalt,manganese.The tests of cyclic voltammetry and chronoamperometric revealed the electro-catalysis performances of different electrodes.In comparison with pure Ni（OH）₂,the catalytic effect of substituted samples showed certain improvement.Among all samples,15%zine,5%cobalt and 10%manganese showed the best catalytic effect respectively,while the sample with 10%manganese was the best among all.This demonstrated that the doping metal ions could increase the catalyst efficiency of nickel hydroxide.更多还原