CN109216039B - 一种Na2Mn5O10纳米棒的制备方法及应用 - Google Patents
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Abstract
本发明公开了一种Na2Mn5O10纳米棒的制备方法及应用,按照物质的量之比Mn:C6H8O7=1:1,Na:Mn=0.2,0.3,0.4,0.44,0.5的比例,分别称取Mn(Ac)2·4H2O、C6H8O7·H2O和无水NaAc于烧杯中混合,加入蒸馏水,于水浴下磁力搅拌至混合物完全溶解;将生成的溶胶置于恒温鼓风干燥箱中,于100–150℃下干燥8 h,得到白色干凝胶;将此干凝胶倒入玛瑙研钵中捣碎,研磨成细粉后,转入方形瓷舟中,将瓷舟放入带有智能温控仪的马弗炉中,于400–800℃下煅烧,得到棕黑或黑色产物即可。本发明实现了Na2Mn5O10纳米棒的精确化制备,且得到的产品性能优越,制备方法简单,便于产业化实施和应用,市场前景广阔。
Description
技术领域
本发明属于新材料领域,具体涉及一种Na2Mn5O10纳米棒的制备方法及应用。
背景技术
当前,超级电容器研究领域面临的最大挑战是如何显著提高现有超级电容器的能量密度。针对双电层电容器由于在充放电过程中存在电解质离子的分离导致浓度降低而使得其能量密度受限的问题,采用以高嵌锂电位的嵌锂化合物为正极、多孔活性炭为负极,以Li2SO4水溶液为电解液组装成“摇椅式”超级电容器被认为是一种提升超级电容器能量密度的有效策略。由于嵌锂化合物是该策略得以实施的关键,因此,许多工作正致力于嵌锂化合物作为超级电容器电极材料的研究。
近年来,随着钠离子电池(Sodium-ion batteray,SIB)的兴起和嵌钠化合物作为储能材料研究的深入,嵌钠化合物作为超级电容器电极材料开始进入人们的视线。其中,锰基嵌钠化合物因其原料资源丰富、成本低且环境友好而尤为引人注目,有关的研究相继得以开展。目前,作为超级电容器电极材料的锰基嵌钠化合物已经报道的有NaMnO2,Na4Mn9O18(Na0.44MnO2),Na0.7MnO2.05,Na0.95MnO2,Na0.35MnO2等。最近,纳米结构Na0.21MnO2作为水系钠离子混合超级电容器电极材料的研究也得到了报道。
在众多的锰基嵌钠化合物中,Na2Mn5O10(Na0.40MnO2)是一种具有单斜晶相和2×3隧道结构的人工合成化合物,该化合物最早由J. Parant 等人于1971年以高温固相法合成得到,并确定了其晶体结构参数。此后,在很长时间内,有关该化合物的研究并没有引起人们的重视。直到2004年,才有人报道了利用Na0.40MnO2作为前驱物制备Li离子二次电池正极材料用MnO2的电化学性能。最近,Y. Cui课题组报道了用Na2Mn5O10纳米棒电极组装成“混合熵”电池用于提取海水或废水中的盐差能量和除盐电池用于海水净化。S. Liu报道了Na2Mn5O10的低温合成及其超级电容器应用,表明Na2Mn5O10有作为超级电容器电极材料的应用前景。不过,由于S. Liu的工作仅报道了Na2Mn5O10单电极的性能,且尚无其它有关Na2Mn5O10作为超级电容器电极材料应用的文献报道,因而尚有进一步深入研究的必要。
另外,在Na2Mn5O10的获取途径方面,目前可供选择的制备方法并不多见。除了J.Parant的高温固相法以外,F. Hu报道的用作制备Li离子二次电池正极材料MnO2的前驱物Na0.40MnO2系将NaNO3及含47.5%Mn(NO3)2的HNO3溶液与甘氨酸混合后滴在加热的金属烧杯上的产物经600 °C下煅烧4h后获得;Y. Cui课题组报道的用做“混合熵”电池和除盐电池电极的Na2Mn5O10纳米棒系将在NaNO3和Mn(NO3)2溶液中浸泡后的棉花拧干,置箱式炉中于空气气氛下加热至700 °C并保持24h后获得,且所得产物中尚有少许Na2Mn3O7和Mn2O3物相存在;S.Liu报道的无定形和纳米棒Na2Mn5O10则是将配合物 [Mn12O12(CH3COO)16(H2O)4]经碱水解后的产物于200–600°C下煅烧5h后得到,其制备过程较为复杂。由此可见,发展新的纳米结构Na2Mn5O10的制备方法十分必要。
发明内容
为克服现有技术的不足,本发明以NaAc为Na源、Mn(AC)2为Mn源,柠檬酸为配位剂,通过溶胶–凝胶合成技术路线制备纳米棒Na2Mn5O10,运用XRD、FTIR、SEM/EDS、等分析手段对产物进行表征,并运用循环伏安、交流阻抗以及恒电流充放电等电化学测试方法评价纳米棒Na2Mn5O10在0.5 mol L–1的Na2SO4水溶液中的电化学性能。
为获得纳米棒Na2Mn5O10,本发明采取以下工艺步骤:
按照物质的量之比Mn:C6H8O7(柠檬酸) = 1:1,Na:Mn=0.2,0.3,0.4,0.44,0.5的比例,分别称取Mn(Ac)2·4H2O 4.6540 g,C6H8O7·H2O 3.991g,无水NaAc 0.3113,0.4675,0.6201,0.6822,0.7789 g 于烧杯中混合,加入100 mL 蒸馏水,于60–70 °C水浴下磁力搅拌至混合物完全溶解(此时以激光笔照射,可明显看到有丁道尔现象,证明生成了溶胶)。将生成的溶胶置于恒温鼓风干燥箱中,于100–150℃下干燥8 h,得到白色干凝胶。将此干凝胶倒入玛瑙研钵中捣碎,研磨成细粉后,转入方形瓷舟中,将瓷舟放入带有智能温控仪的马弗炉中,于400–800℃下煅烧,得到棕黑或黑色产物。
采用XRD, FTIR, SEM/EDS等分析手段对产物进行表征。用X–射线衍射仪(RigakuD/max-2500)测试产物的物相,测试条件为:管电流250 mA,电压30 kV,CuKα(λ= 1.54056Å), 2θ角扫描范围10–80°;用红外光谱仪(Thermo scientific, Nicolet iS10)测试产物的红外光谱,波数扫描范围400–4000 cm–1,使用KBr做盐窗。用场发射扫描电镜(SIGMA HD,ZEISS, German)观察产物的形貌,并用仪器附带的EDX测试产物的元素组成。
与现有技术相比,本发明的研究结论和有益效果如下:
(1)以NaAc、Mn(AC)2和柠檬酸为原料,通过溶胶–凝胶合成技术路线,可以成功获得2×3隧道结构的Na2Mn5O10纳米棒。研究结果表明,当控制原料中物料比为Na : Mn= 0.4:1,于700 °C下煅烧凝胶6 h时,所得产物具有Na2Mn5O10的XRD和FTIR特征,化学组成符合Na:Mn=2:5的计量关系,微观形貌为直径在60–200 nm,长0.8–6μm的棒状纳米晶体。
(2)煅烧温度和煅烧时间对获得Na2Mn5O10纳米棒起决定性作用。过短的煅烧时间和过低的煅烧温度均不利于获得Na2Mn5O10纳米棒。
(3)物料中的钠锰比对获得Na2Mn5O10产物起关键作用。为获得单一物相的Na2Mn5O10,合适的钠锰比为0.4:1,低于此值,得到的产物中有Mn2O3存在,当钠锰比超过0.4:1时不能得到Na2Mn5O10产物。
(4)Na2Mn5O10纳米棒电极在0.5 mol L–1的Na2SO4水溶液中的电化学行为具有嵌入赝电容的特征,在100 mA g–1的电流密度下,Na2Mn5O10纳米棒单电极的放电比电容可达186F g–1。
综上所述,本发明实现了Na2Mn5O10纳米棒的精确化制备,且得到的产品性能优越,制备方法简单,便于产业化实施和应用,市场前景广阔。
具体实施方式
以下对Na2Mn5O10纳米棒的制备方法及性能进行详细说明。
按照物质的量之比Mn:C6H8O7(柠檬酸) = 1:1,Na:Mn=0.2,0.3,0.4,0.44,0.5的比例,分别称取Mn(Ac)2·4H2O 4.6540 g,C6H8O7·H2O 3.991g,无水NaAc 0.3113,0.4675,0.6201,0.6822,0.7789 g 于烧杯中混合,加入100 mL 蒸馏水,于60–70 °C水浴下磁力搅拌至混合物完全溶解(此时以激光笔照射,可明显看到有丁道尔现象,证明生成了溶胶)。将生成的溶胶置于恒温鼓风干燥箱中,于100–150℃下干燥8 h,得到白色干凝胶。将此干凝胶倒入玛瑙研钵中捣碎,研磨成细粉后,转入方形瓷舟中,将瓷舟放入带有智能温控仪的马弗炉中,于400–800℃下煅烧,得到棕黑或黑色产物。
首先对所得产物的物相进行分析。Na:Mn=0.4:1在马弗炉中于700 ℃下煅烧6 h后所得产物的XRD图谱可见衍射峰较为尖锐,说明产物结晶度较高,这符合高温下煅烧产物的一般特征。用MDI Jade 6.5 软件检索标准卡片库(PDF-2004),并进行指标化计算,发现该产物的衍射峰与Na2Mn5O10(PDF# 27-0749,2θ角范围为5–40 °)的衍射峰位置一致,并且晶面指标也与Na2Mn5O10晶体的(001),(200),(201),(002),(–102),(–301),(–202),(301),(400),(302),(–402),(010),(110),(–111),(211),(–502),(012),(112)相一致,说明700°C下煅烧6 h后所得产物的XRD图谱符合单斜相2×3 隧道结构Na2Mn5O10晶体的XRD特征。这里需要说明的是,对所得产物在2θ角40 –80°范围的衍射峰没有进行分析,这是因为Na2Mn5O10的标准卡PDF# 27-0749本身没有2θ角40 –80°范围的标准化信息所致。另外,从Y.Cui和S. Liu报道的Na2Mn5O10纳米棒的XRD结果中可知,两者所获得的Na2Mn5O10产物中均有其它物相存在,而从本工作得到的XRD结果来看,除代表Na2Mn5O10物相的衍射峰外,并无其它物相的衍射峰存在,说明本工作提出的制备方法更有利于获得纯相的Na2Mn5O10产物。
与MnO2相类似,Na x MnO2也具有红外活性,且其红外相应与其晶体结构中原子排列的有序性密切相关。一般地,有序程度越低,引起晶格中Mn–O伸缩振动所需的能量越低,红外吸收对应的波数也越低。层状结构的Na x MnO2结晶性差,晶体结构中原子排列的有序程度低,其红外吸收带一般处于较低的400–650 cm–1波数范围;而隧道结构的Na x MnO2结晶性好,晶体结构中原子排列高度有序,引起晶格中Mn–O伸缩振动所需的能量高,红外吸收带发生紫移,因此除在低波数400–650 cm–1范围内有吸收外,在高波数范围650–800 cm–1内亦有吸收。比如,2 × 4 隧道结构的Na-todorokite,其红外光谱除在453, 522, 588 cm–1处有吸收外,还在743 cm–1处有吸收峰;而层状结构的Na-birnessite则仅在461–564 cm–1处有吸收。因此,高波数650–800 cm–1范围内是否有红外吸收被认为是区别层状结构和隧道结构Na x MnO2的重要依据。
之前的XRD分析结果表明,于700 °C下煅烧6 h后所得的产物具有2×3 隧道结构特征,那么该产物也应当具备隧道结构晶体的红外光谱特征。为进一步确定所得产物的晶体结构,测试了其FTIR光谱。图中可见,所得产物的红外光谱在400–650 cm–1范围内有两个吸收带,分别位于415–560 cm–1和682–770 cm–1处,表明所得产物同样具有隧道结构Na x MnO2的红外光谱特征,这就很好地印证了之前XRD的分析结果。此外,由于目前有关2×3隧道结构Na2Mn5O10的红外光谱十分少见,本工作的结果或可为丰富Na2Mn5O10的结构信息提供依据。
为进一步确认700 °C下煅烧6 h后所得产物的微观形貌和化学组成,对所得产物进行了SEM和EDS测试。放大5000倍时获得的SEM照片,可见产物的微观形貌为一系列直径大小和长短不一的棒状颗粒,这与Y. Cui和S. Liu报道的Na2Mn5O10纳米棒的微观形貌非常相像,他们所获得的Na2Mn5O10纳米棒也是一系列直径大小和长短不一的棒状颗粒。另外,根据图中所给出的标尺,本工作所得产物颗粒的长度在0.8–6 μm,稍长于文献报道的1–3 μm,这可能与制备方法不同有关。放大50000倍时获得的SEM照片可清晰地看到产物颗粒的棒状形貌,同时可以清楚地看到颗粒形状较为规则,说明结晶完好,这符合隧道结构Na x MnO2结晶度较高的特点。根据标尺,可知产物颗粒的直径在60–200 nm之间,而Y. Cui报道的Na2Mn5O10纳米棒的平均大小为300 nm,因此,可以认为通过本工作所提出的制备方法,成功获得了Na2Mn5O10纳米棒。所得产物的X–射线能谱清楚地表明,所得产物中仅含Na、Mn、O三种元素,除此以外,并无其它元素存在,说明产物的纯度是较高的,这与XRD的结果是相互印证的。元素含量测试结果,很容易计算得到产物中Na : Mn : O = 0.4 : 1: 2.1,这非常接近于Na2Mn5O10的化学计量比,扣除仪器误差,可以认为所得的产物即为Na2Mn5O10。
因此,通过本工作提出的溶胶–凝胶法,成功地获得了直径为60–200 nm,长约0.8–6 μm的Na2Mn5O10纳米棒,由于本工作提出的方法获得的Na2Mn5O10纳米棒物相单一,因而较文献报道的方法更具优势。
制备电化学测试所需的Na2Mn5O10电极和活性炭(AC)电极,Na2Mn5O10电极中活性物质、粘接剂和导电剂三者的质量比为Na2Mn5O10 : PTFE: 乙炔黑 = 80 : 5 : 15,活性炭电极中三者的质量比为AC: PTFE: 乙炔黑 = 80 : 10 : 10。用作单电极测试时的电极片面积为1 cm2,每片活性物质量为8–10 mg;用作混合式超级电容器测试时的电极片面积为2.25 cm2,每片活性物质量为15–20 mg。所有电极均采用316L不锈钢丝网作为集流体。采用循环伏安(CV)、计时电位(CP)、交流阻抗(EIS)以及恒电流充放电的测试结果评价单电极和所组装的超级电容器的电化学性能。电化学性能测试时所用的电解液均为0.5 mol L–1的Na2SO4水溶液。
S. Liu首次报道了Na2Mn5O10纳米棒在0.5 mol L–1Na2SO4水溶液中的循环伏安行为,测试条件是电势扫描速率为2 mV s–1和电势窗口为0–0.8 V(vs.AgCl/Ag);并测得该电极在电流密度在0.1 A g–1时的比电容值为175 Fg–1。除此以外,目前尚无其它关于Na2Mn5O10纳米棒电化学性能的报道。
为评价所制备Na2Mn5O10纳米棒的电化学性能,将Na2Mn5O10纳米棒制作成电极,并采用三电极测试装置对其进行了循环伏安(CV)、计时电位(CP)和交流阻抗(EIS)测试。为比较起见,循环伏安测试时的电势扫描速率和计时电位测试时的电流密度均与S. Liu相同。
Na2Mn5O10纳米棒电极的电化学测试结果表明:电势扫描速率为2 mV s–1时,电极经历了六次循环的CV曲线,可以看到,稳定后的CV曲线基本重合,表明所制备电极的循环伏安行为具有很好的重现性。CV曲线的形状与S. Liu的报道极为相似,表明所制备的Na2Mn5O10纳米棒在0.5 mol L–1Na2SO4水溶液中的循环伏安行为符合所报道的Na+的电化学“脱–嵌”行为特征。另外,可以看到Na2Mn5O10纳米棒电极的CV曲线关于零坐标轴基本呈“镜像”对称,可定性判断Na2Mn5O10纳米棒电极上发生的氧化–还原反应是可逆的。
在0–1.0 V(vs.SCE)的电势范围内,当电势正向扫描时,CV曲线上依次出现了3个氧化峰,分别位于0.251 V、0.538V和0.860 V处,表明Na+的电化学脱出分三步进行;而当电势负向扫描时,对应地在CV曲线上看到出现了3个还原峰,分别位于0.820 V、0.430 V和0.183 V, 表明Na+的电化学嵌入同样分三步进行,同时,成对的氧化–还原峰(0.860 V/0.820 V、0.538 V/0.430 V、0.251 V/0.183 V)的峰电位之差低于0.2 V,说明Na2Mn5O10纳米棒的循环伏安行为符合赝电容行为的CV特点。
CP法获得的单电极充放电曲线(CP曲线)和EIS法测得的Nyquist曲线,可以看到,Na2Mn5O10纳米棒的CP曲线尽管略有一些弯曲,但整体上呈斜线,Nyquist曲线在低频时表现为一条斜线而非理想电容器的垂线,这是完全符合V. Augustyn及B. Dunn.所归纳的嵌入赝电容的特征的,因此,综上所述,可以认为,Na2Mn5O10纳米棒的电化学行为表现为赝电容性质,而其赝电容行为是Na+的电化学“脱–嵌”所致。
为评价Na2Mn5O10纳米棒电极的倍率性能,分别在0.1 A g–1,0.25 A g–1,0.4 A g–1,0.6 A g–1,0.8 A g–1,1 A g–1的电流密度下以CP法测试了单电极的比电容,结果表明:比电容随电流密度的增大而下降,1 A g–1电流密度时的比电容较0.1 A g–1电流密度时的比电容下降了30%,这与S. Liu的报道是一致的,不过,本工作测得的0.1 A g–1电流密度时的比电容为186 F g–1,略高于S. Liu报道的175 F g–1。
上述实施例并非对本发明作任何形式上的限制,任何熟悉本领域的技术人员,在不脱离本发明技术方案范围的情况下,都可利用上述揭示的技术内容对本发明技术方案做出许多可能的变动和修饰,或修改为等同变化的等效实施例。因此,凡是未脱离本发明技术方案的内容,依据本发明技术实质对以上实施例所做的任何简单修改、等同变化及修饰,均应落在本发明技术方案保护的范围内。
Claims (2)
1.一种Na2Mn5O10纳米棒的制备方法,其特征在于包含以下几个步骤:
按照物质的量之比Mn:C6H8O7(柠檬酸) = 1:1,Na:Mn= 0.4的比例,分别称取Mn(Ac)2·4H2O、C6H8O7·H2O和无水NaAc于烧杯中混合,加入蒸馏水,于60–70℃水浴下磁力搅拌至混合物完全溶解,并用激光笔照射看到有丁道尔现象;将生成的溶胶置于恒温鼓风干燥箱中,于100–150℃下干燥8 h,得到白色干凝胶;将此干凝胶倒入玛瑙研钵中捣碎,研磨成细粉后,转入方形瓷舟中,将瓷舟放入带有智能温控仪的马弗炉中,于700℃下煅烧7小时,得到棕黑或黑色产物即可。
2.根据权利要求1所述Na2Mn5O10纳米棒的制备方法,其特征在于:所述Na2Mn5O10纳米棒的电极在0.5 mol L–1的Na2SO4水溶液中的电化学行为具有嵌入赝电容的特征,在100 mA g–1的电流密度下,Na2Mn5O10纳米棒单电极的放电比电容达186 F g–1。
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