馬 昌, 史景利, 李亞娟, 宋 燕, 劉 朗

富氮多孔納米炭纖維的制備及其用作超級電容器電極材料

馬 昌1, 史景利1, 李亞娟1, 宋 燕2, 劉 朗2

(1.天津工業(yè)大學(xué)材料科學(xué)與工程學(xué)院,天津300387; 2.中國科學(xué)院山西煤炭化學(xué)研究所中國科學(xué)院炭材料重點(diǎn)實(shí)驗(yàn)室,山西太原030001)

以商業(yè)聚酰亞胺樹脂為前驅(qū)體,經(jīng)過靜電紡絲和一步炭化制備出富含氮原子的納米炭纖維,采用掃描電鏡、低溫氮吸附和XPS等手段對納米炭纖維的結(jié)構(gòu)進(jìn)行表征,考察不同炭化溫度下納米炭纖維的孔結(jié)構(gòu)與表面含氮官能團(tuán)的演變。結(jié)果顯示,所得聚酰亞胺纖維經(jīng)過一步高溫處理便可得到微孔發(fā)達(dá)且富含氮原子的納米炭纖維。隨著炭化溫度的升高,納米炭纖維的比表面積與氮含量均逐漸降低。700℃炭化得到的納米炭纖維的比表面積達(dá)到447 m2/g、纖維平均直徑為234 nm、表面氮含量達(dá)到4.1%。將所得納米炭纖維直接用作超級電容器電極,采用循環(huán)伏安法、恒流充放電和交流阻抗對其電化學(xué)性能進(jìn)行考察。所得富氮納米炭纖維表現(xiàn)出優(yōu)異的電容量和表面電化學(xué)活性,其比電容達(dá)到214 F/g,單位比表面的電容量達(dá)到0.57 F/m2。

富氮;電容器;納米炭纖維;多孔炭

1 前言

多孔納米炭纖維作為新型炭材料家族中重要的一員,不僅具有傳統(tǒng)多孔炭的固有特性,例如優(yōu)異的電子導(dǎo)電性、高的比表面積、良好的化學(xué)穩(wěn)定性和溫度穩(wěn)定性,還具有大比表面效應(yīng)和量子尺寸效應(yīng)等納米材料的特性,同時(shí)它還具有一維材料的高機(jī)械強(qiáng)度和較好的成型性,因此,它在開發(fā)新的納米復(fù)合材料、吸附材料、催化材料和電極材料等研究領(lǐng)域中扮演著重要的角色[1]。

近年來,人們發(fā)現(xiàn)將氮原子引入納米多孔炭會(huì)帶來一些新的特性,譬如,表面含氮官能團(tuán)能夠增加材料表面潤濕性,體內(nèi)氮原子則能提高材料的電導(dǎo)率,某些具有給電子能力的表面氮?jiǎng)t能增加炭表面的電化學(xué)活性[2]。因此,含氮多孔炭在電容器、燃料電池和鋰離子電池等應(yīng)用領(lǐng)域顯現(xiàn)出出色的電化學(xué)性能[3]。將摻氮原子帶來的電化學(xué)特性和多孔納米炭纖維特有的結(jié)構(gòu)特性進(jìn)行結(jié)合,制備富氮的納米炭纖維具有重要的研究意義與應(yīng)用價(jià)值。

目前,制備多孔納米炭纖維的方法主要有: CVD法、模板法和靜電紡絲法[2,4]。其中,CVD法得到納米炭纖維比表面積低,而模板法則必須經(jīng)繁復(fù)的模板制備與洗滌工序。相比之下,靜電紡絲法更易得到高比表面積的納米炭纖維。目前,采用靜電紡絲法制備多孔納米炭纖維的報(bào)道不少,但關(guān)于富氮納米炭纖維的制備以及氮原子與電化學(xué)性能的關(guān)聯(lián)研究卻并不多。Ra等[5]以聚丙烯腈為前驅(qū)體,以靜電紡絲法制備了聚丙烯腈納米纖維,然后經(jīng)炭化和活化制備富氮的多孔納米炭纖維,考察了其作為超級電容器的電化學(xué)性能。Ma等[6]以三聚氰胺樹脂為前驅(qū)體,采用靜電紡絲工藝制備了富氮多孔的納米炭纖維。Kim等[7]以聚酰胺酸為前驅(qū)體制備多孔炭纖維,卻并沒有考察氮原子對電化學(xué)性能的影響。因此,富氮納米炭纖維的制備以及氮原子與電化學(xué)性能的關(guān)聯(lián)研究還有待進(jìn)一步開展。

聚酰亞胺樹脂是一類含氮芳雜環(huán)高分子,其通過簡單加熱便能實(shí)現(xiàn)固化,且殘?zhí)柯士捎^。筆者直接以商業(yè)化聚酰亞胺樹脂作為碳/氮前驅(qū)體,采用靜電紡絲和一步炭化便制備富含氮原子的多孔納米炭纖維,主要考察納米炭纖維的結(jié)構(gòu),例如表面氮氧官能團(tuán)和孔結(jié)構(gòu),對其作為超級電容器電極材料的電化學(xué)性能的影響。

2 實(shí)驗(yàn)與表征

2.1納米炭纖維的制備

將商業(yè)聚酰亞胺樹脂溶液PI-G2525(美國杜邦。溶劑為N,N-二甲基乙酰胺,固含量為20%)以相同的溶劑稀釋到15%制得紡絲原液。靜電紡絲時(shí),正負(fù)極電壓為25 kV,距離為18 cm,噴頭為直徑0.62 mm的不銹鋼針頭。將初紡纖維在空氣中于200℃固化1 h后升至300℃固化1 h,然后在氮?dú)鈿夥罩幸?℃/min的升溫速率加熱到不同的溫度下炭化1 h。將炭化纖維記為PICF-T(T代表炭化溫度700,800,900℃)。

2.2表征

炭纖維的表面形貌采用掃描電子顯微鏡(SEM,Hitachi S-4800)進(jìn)行觀察,纖維的直徑采用ImageJ軟件進(jìn)行測量,直徑分布是基于50根隨機(jī)纖維的直徑統(tǒng)計(jì)。纖維的表面化學(xué)狀態(tài)采用X射線光電子能譜儀(XPS,Thermo ESCALAB 250, USA)進(jìn)行表征;表面積與孔結(jié)構(gòu)是采用ASAP 2020型物理吸附儀(Micromeritics,USA)進(jìn)行表征,吸附介質(zhì)為N2,測試溫度為77 K。測試前樣品均在300℃下脫氣10 h。

電化學(xué)性能:采用三電極體系進(jìn)行電化學(xué)性能表征,將所得炭纖維薄膜直接壓在兩片泡沫鎳中間作為工作電極,以鉑片電極和Hg/HgO電極分別作為對電極和參比電極,以6 mol/L KOH作為電解液。測試前,先將工作電極在電解液中浸漬24 h以保證電解液在電極材料中的充分浸潤。以電化學(xué)工作站(CHI660C,上海辰華儀器有限公司)進(jìn)行電極的恒流充放電、循環(huán)伏安和交流阻抗測試。其中,交流阻抗測試頻率范圍為10 kHz至10 mHz,施加的交流信號振幅為5 mV;恒流充放電的電流范圍為100 mA/g到10 A/g;循環(huán)伏安的掃描速率為10 mV/s。

3 結(jié)果與討論

3.1結(jié)構(gòu)特征

圖1(a),(b),(c)是PICFs在不同倍率下的電鏡照片?？梢钥吹嚼w維的形態(tài)良好,但纖維薄膜上有少量顆粒,這是未牽伸的液滴形成的。圖1(d)是纖維的直徑分布圖,纖維直徑為50-400 nm,平均直徑為234 nm。纖維之間并沒有明顯的粘并現(xiàn)象,說明紡絲過程中溶劑能夠從纖維中順利脫除。

圖2(a)為PICFs的脫吸附曲線。所有樣品在低壓區(qū)吸附就達(dá)到飽和,顯示出典型的微孔特征。吸附等溫線在高壓區(qū)吸附量呈現(xiàn)輕微的下降,這一現(xiàn)象也出現(xiàn)在其他的純微孔材料中[8-10],這可能是吸附過程微孔結(jié)構(gòu)發(fā)生形變或坍塌所致。圖2(b)為不同炭化溫度下PICFs的孔徑分布圖,所有樣品的孔均集中在0.9 nm和1.1 nm附近,顯示典型的微孔分布,沒有中孔或大孔。隨著溫度的變化,孔的尺寸和分布基本沒有變化,表明到700℃時(shí)碳骨架基本定型。

圖1 (a,b,c)PICFs的掃描電鏡照片與(d)直徑分布直方圖Fig.1 (a,b,c)SEM images of PICFs;(d)histogram displays the corresponding fiber diameter distributions.

圖2 (a)PICFs的脫吸附和(b)孔徑分布曲線Fig.2 (a)N2adsorption/desorption isotherms and(b)pore size distributions for PICFs.

表1列出了PICFs的孔結(jié)構(gòu)參數(shù)。PICF-700的表面積最大,為447 m2/g,大于目前研究最多的PAN基納米炭纖維。隨著溫度的升高,比表面積略下降,碳骨架在更高的溫度下出現(xiàn)一定的塌陷會(huì)導(dǎo)致一些微孔閉合或孔道融合都可能是表面積下降的原因。表面積的下降幅度并不顯著,900℃處理的炭纖維仍具有376 m2/g的比表面積。具有納米直徑的聚酰亞胺炭纖維顯示出明顯大于樹脂基顆粒炭的比表面積[11],這可能與材料的尺寸有關(guān),小尺寸更利于分解氣的逸出而形成孔道,大顆粒則易形成閉孔。

表1 PICFs的孔結(jié)構(gòu)參數(shù)Table 1 Texture properties of PICFs.

表2 XPS表面元素組成Table 2 Surface elemental composition derived from XPS.

表2是XPS測試的樣品表面元素含量。由表可見所得炭纖維含有豐富的氮原子和氧原子,其中PICF-700含有4.1%的氮和10.57%的氧,可見聚酰亞胺樹脂中的氮原子經(jīng)高溫處理得到較好的保留。隨炭化溫度上升,氮含量下降,氧含量變化不大,表明在高溫下氧原子比氮原子具有更高的穩(wěn)定性。

為了考察表面氮和氧的化學(xué)狀態(tài),進(jìn)一步對N1s峰和O1s峰進(jìn)行擬合。圖3為樣品N1s峰和O1s峰的擬合譜圖,各種官能團(tuán)含量見表3。

圖3 PICFs的XPS譜圖:(a)N1s和(b)O1sFig.3 XPS spectra for PICFs:(a)N1s and(b)O1s.

結(jié)果顯示,表面氮原子有3種存在狀態(tài),分別是吡啶型氮(N6,取代邊緣六元芳香環(huán)中裸露炭的氮原子,(398.7±0.3)eV)、吡咯型氮(N5,取代邊緣五元環(huán)中裸露炭的氮原子,(400.3±0.3)eV)和四價(jià)氮(NQ,取代內(nèi)部芳香環(huán)炭的氮原子,(401.4± 0.3)eV)[12-14]。其中N6和N5占主導(dǎo)。隨著炭化溫度的升高,N6和N5均不斷下降,這是因?yàn)樗鼈兙哂休^低的結(jié)合能,高溫穩(wěn)定性差。由于具有相對較高的結(jié)合能和熱穩(wěn)定性,NQ的原子百分?jǐn)?shù)在高溫下得到了提高。表面氧原子則有2種存在狀態(tài),即((531.9±0.2)eV和(533.3±0.2)eV),分別對應(yīng)＝-C O(O I)和C-OH/C-O-C(OⅡ)[14,15]。隨著炭化溫度的升高,兩類含氧官能團(tuán)的含量變化均不大,顯示出較高的溫度穩(wěn)定性。研究顯示,暴露于表面的官能團(tuán),包括N6、N5、O I和OⅡ,因?yàn)榕c水分子具有更高的親和力而能夠潤濕表面,加速電解質(zhì)離子的遷移速度。而具有一定給電子能力的官能團(tuán),包括N6、N5和O I,具有一定的電化學(xué)反應(yīng)活性,對于多孔炭電極而言,它們能夠提供非?？捎^的贗電容。研究表明,氮原子具有比氧原子更高的電化學(xué)活性[16],尤其在堿性電解質(zhì)體系中,氮原子能夠提供更多的贗電容。從官能團(tuán)組成看,所得聚酰亞胺基納米炭纖維所含有的表面官能團(tuán)大部分是具有電化學(xué)活性的,可以期待它們在堿性電解質(zhì)體系中會(huì)有較高的單位表面電容。

表3 PICFs的XPS表面官能團(tuán)組成Table 3 Surface functional compositions of PICFs derived from XPS.

3.2電化學(xué)性能

圖4為樣品在100 mA/g下的質(zhì)量比電容和比表面電容。3個(gè)樣品雖然比表面積并不高,但具有較高的比電容,分別達(dá)到:195 F/g(PICF-700), 196 F/g(PICF-800)和214 F/g(PICF-900)。計(jì)算得到3個(gè)樣品的比表面電容分別為:0.43 F/m2(PICF-700)、0.50 F/m2(PICF-800)和0.57 F/m2(PICF-900),均高于一般的多孔炭[17,18],這主要?dú)w因于所得材料高含量的表面活性官能團(tuán)和特定尺寸的微孔。從XPS結(jié)果可知,3個(gè)樣品均含有豐富的活性官能團(tuán),其含量分別達(dá)到:3.76%N和4.75% O(PICF-700)、3.27%N和4.76%O(PICF-800)和2.66%N,和4.22%O(PICF-900)。這些活性官能團(tuán)顯著提高了材料的表面電容。另外,最近的研究顯示[19],在特殊尺寸的微孔中,離子以去溶劑化的狀態(tài)進(jìn)出,不僅使得單位表面的電荷密度更高,還能縮短兩極間距,因而能夠存儲(chǔ)更多的電能。

從孔徑分布上看,3個(gè)樣品的孔徑分布基本一致,另外,由于3個(gè)樣品均來自于相同前驅(qū)體,處理溫度也相差不大,可以認(rèn)為它們的孔形狀、結(jié)構(gòu)以及深度上相差無異。也就是說,孔徑與孔結(jié)構(gòu)對表面電容的貢獻(xiàn)基本相同。另外,電化學(xué)活性基團(tuán)對表面電容起著積極作用,即相同條件下,活性官能團(tuán)越多,單位表面的電容越大。然而,對于所制得的聚酰亞胺樹脂基炭纖維,反而是活性基團(tuán)低的樣品比表面電容更高,這顯然不合常理。因此,筆者推測表面活性官能團(tuán),尤其是納米孔道表面的官能團(tuán),提供的贗電容需要較高的電導(dǎo)率來保證,處理溫度越高,基體材料電子傳導(dǎo)能力越強(qiáng),孔道表面的官能團(tuán)對電容的貢獻(xiàn)才能得到最大程度的發(fā)揮。圖5為3個(gè)樣品的放電曲線?？梢钥闯?PICF-900的電壓降最小,其次是PICF-800,然后是PICF-700。可以看出,更高的處理溫度對于電極內(nèi)阻的降低以及容量的最大化有著顯著的影響。對于只提供雙電層電容的多孔炭而言,放電曲線為直線。3個(gè)樣品的放電曲線呈現(xiàn)出不同的彎曲程度。這是表面活性官能團(tuán)發(fā)生氧化還原反應(yīng)所致。就彎曲程度而言,PICF-700最大,其次是PICF-800,PICF-900最小,體現(xiàn)了表面活性官能團(tuán)在含量上的差異。表面官能團(tuán)含量越高,發(fā)生的氧化還原反應(yīng)越多,放電曲線的彎曲程度越明顯,這與文獻(xiàn)報(bào)道結(jié)果一致[20]。

圖4 PICFs的比電容和比表面電容Fig.4 Specific capacitance and specific surface capacitance for PICFs.

圖5 100 mA/g下PICFs的放電曲線Fig.5 Discharging curves of PICFs at100 mA/g.

圖6 為比電容保持率與電流密度的關(guān)系曲線。由圖可知當(dāng)電流密度增大100倍,即從100 mA/g升為10 A/g時(shí),3個(gè)樣品的電容保持率分別為: 37%(PICF-700)、43%(PICF-800)和57%(PICF-900)。隨著炭化溫度升高,樣品電容保持率增加。一般來講,大電流下的電容保持率主要由離子擴(kuò)散和電子傳導(dǎo)決定,3個(gè)樣品的孔徑基本一致,離子的傳輸環(huán)境相差不大,因此可以認(rèn)為孔道離子擴(kuò)散速率基本一致。高溫處理的樣品具有高的電導(dǎo)率,因而,電導(dǎo)率在倍率性能起到了決定性的作用。這一點(diǎn)從循環(huán)伏安曲線上也能明顯地體現(xiàn)(圖7),在電壓拐點(diǎn),PICF-900的電流響應(yīng)十分迅速,PICF-800和PICF-700則較為緩慢。

圖6 PICFs隨電流密度增加時(shí)的電容保持率Fig.6 Capacitance retention of PICFs at current densities.

圖7 PICFs在10 mV/s下的循環(huán)伏安曲線Fig.7 CV curves of PICFs at10 mV/s.

為了更清晰地考察電極的阻抗行為,測試了各個(gè)電極的交流阻抗圖(圖8)。高頻區(qū)的半圓弧反映電極的內(nèi)阻,包括電極的界面電阻、接觸電阻和纖維內(nèi)部電阻。半圓越大,說明電極內(nèi)阻越大。由圖可見:各個(gè)電極的內(nèi)阻存在較大的差異。相同的制備工藝和測試條件下,電極的界面電阻和接觸電阻是相同的,因此半圓的大小主要反映了炭纖維電阻的差異。PICF-900具有最小的電阻,PICF-700和PICF-800次之。圖8(b)為電極的Bode曲線,45°相角對應(yīng)的頻率(f0)反映了電極的離子響應(yīng)頻率,在此特征頻率,電容器的電阻和電容達(dá)到平衡。由圖可得,3個(gè)電極的響應(yīng)頻率分別為:0.08 Hz(PICF-700),0.12 Hz(PICF-800)和0.89 Hz(PICF-900),根據(jù)特征頻率計(jì)算了電極的時(shí)間常數(shù)(τ0=1/f0),分別為:12.5 s(PICF-700),8.3 s(PICF-800)和1.1 s(PICF-900)?？梢?PICF-900具有最小的時(shí)間常數(shù),表明其具有最快的離子響應(yīng)速度,因此在大電流下,其表面的利用率最高,電容保持率也最高。

圖8 PICFs在6 mol/L KOH中的交流阻抗譜圖:(a)Nyquist譜圖和(b)Bode譜圖Fig.8 AC impedance spectra of PICFs in 6 mol/L KOH:(a)Nyquist plot and(b)Bode plot.

4 結(jié)論

以聚酰亞胺樹脂為前驅(qū)體通過電紡法制備出富含氮原子的多孔納米炭纖維,其氮含量達(dá)3.01%～4.1%,氧含量達(dá)9.56%～10.56%。該富氮納米炭纖維為典型的微孔炭,顯示出較高的比表面積376～447 m2/g和優(yōu)異的比電容(195～214 F/g),并表現(xiàn)出優(yōu)于一般多孔炭的比表面電容(0.43～0.57 F/m2),這主要?dú)w因于其豐富的表面活性官能團(tuán)和特殊的微孔結(jié)構(gòu)。表面官能團(tuán)能夠提供可觀的贗電容,特殊尺寸的微孔提供更高的雙電層電容,而高的電導(dǎo)率為表面電容的提升創(chuàng)造了條件。所得富含氮多孔納米炭纖維展示出良好的電化學(xué)活性,是一類具有潛力的電容器電極材料。

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[1] Mordkovich V Z,Baxendale M,Yoshimura S,et al.Intercalation into nanotubes.Carbon,1996,34(10):1301-1303.

[2] Lovell D R.Carbon and High-Performance Fibers Directory.5th ed.,London:Chapman&Hall,1991:66.

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[6] Jones L E.The Effect of Boron on Carbon Fiber Microstructure and Reactivity.Ph.D.Thesis.Penn State University,University Partk, PA 1987.

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Preparation of nitrogen-enriched porous carbon nanofibers and their electrochemical performance as electrode materials of supercapacitors

MA Chang1, SHI Jing-li1, LI Ya-juan1, SONG Yan2, LIU Lang2
(1.SchoolofMaterialsScienceandEngineering,TianjinPolytechnicUniversity,Tianjin300387,China;2.KeyLaboratoryofCarbonMaterials,InstituteofCoalChemistry,ChineseAcademyofSciences,Taiyuan030001,China)

Nitrogen-enriched porous carbon nanofibers were prepared from a commercial polyimide resin by electrospinning,followed by carbonization.The products were characterized by scanning electron microscopy,nitrogen sorption and X-ray photoelectron spectroscopy.As-prepared carbon nanofibers were directly used as a supercapacitor electrode,and their electrochemical performance was investigated by cyclic voltammetry,charge-discharge tests and electrochemical impedance spectroscopy.The evolution of the porous structure and the surface nitrogen-containing functionality of the carbon nanofibers with carbonization temperature was also investigated.Results showed the carbon nanofibers with developed micropores and enriched with nitrogen were obtained by carbonization of polyimide nanofibers.Both the specific surface area and surface nitrogen content decreased gradually with the carbonization temperature.The carbon nanofibers obtained at700℃had the highest specific surface area of 447 m2/g,a fiber diameter of 234 nm and a nitrogen content of 4.1%.They also exhibited a specific capacity of 214 F/g or 0.57 F/m2.

Nitrogen-enriched;Supercapacitor;Carbon nanofibers;Porous carbon

SHI Jing-li.E-mail:shijingli1963@163.com;SONG Yan.E-mail:songyan1026@126.com

TQ127.1+1

A

2014-12-30;

:2015-07-05

山西省自然科學(xué)基金(2012011219-3).

史景利.E-mail:shijingli1963@163.com;宋 燕.E-mail:songyan1026@126.com

馬 昌,博士,講師.E-mail:fdoy_lt54@163.com

1007-8827(2015)04-0295-07

Foundation item:National Natural Science Foundation of Shanxi Province,China(2012011219-3).

Author introduction:MA Chang,Ph.D.,Lecturer.E-mail:fdoy_lt54@163.com