金屬-空氣電池陰極雙功能催化劑研究進(jìn)展

王 亞，來慶學(xué)，朱軍杰，梁彥瑜

(南京航空航天大學(xué) 材料科學(xué)與技術(shù)學(xué)院，江蘇省能量轉(zhuǎn)換材料與技術(shù)重點(diǎn)實(shí)驗(yàn)室，江蘇 南京 211106)

金屬-空氣電池陰極雙功能催化劑研究進(jìn)展

王 亞，來慶學(xué)，朱軍杰，梁彥瑜*

(南京航空航天大學(xué) 材料科學(xué)與技術(shù)學(xué)院，江蘇省能量轉(zhuǎn)換材料與技術(shù)重點(diǎn)實(shí)驗(yàn)室，江蘇 南京 211106)

可充電金屬-空氣電池因具有超高的能量密度被認(rèn)為是最有發(fā)展前景的能源存儲(chǔ)與轉(zhuǎn)換裝置之一.陰極電化學(xué)氧還原/生成反應(yīng)緩慢動(dòng)力學(xué)是影響金屬-空氣電池性能的關(guān)鍵因素，因此其充放電過程需要雙功能催化劑進(jìn)行催化.在此我們?cè)敿?xì)論述了近年來開發(fā)的新型雙功能催化劑，包括貴金屬、碳材料、過渡金屬氧化物和復(fù)合材料.其中，強(qiáng)耦合過渡金屬氧化物/納米碳復(fù)合物成為新一代具有高催化活性的氧催化材料.最后，基于目前所存在的問題提出了幾個(gè)未來可能的研究方向.

金屬-空氣電池；空氣陰極；雙功能催化劑；氧還原/生成反應(yīng)

能源和環(huán)境是現(xiàn)代社會(huì)面臨的兩大問題[1-3]，可再生能源轉(zhuǎn)換是顯著降低化石燃料依賴的有效解決方案.目前，各國(guó)致力于提高各式電化學(xué)能源存儲(chǔ)與轉(zhuǎn)換裝置的容量和穩(wěn)定性，可充電鋰離子電池由于循環(huán)壽命長(zhǎng)(>5 000圈)和能量轉(zhuǎn)換效率高(>90%)等優(yōu)點(diǎn)被認(rèn)為是最有前途的存儲(chǔ)技術(shù)，但是能量密度低(理論值約400 Wh·kg-1)限制其長(zhǎng)期應(yīng)用[4-5]，因此，開發(fā)高能量密度的存儲(chǔ)技術(shù)迫在眉睫.金屬-空氣電池由于能量密度高、成本低和環(huán)境友好受到廣泛關(guān)注.金屬-空氣電池(圖1)具有開放的電池結(jié)構(gòu)[6]，能持續(xù)供應(yīng)陰極活性材料(氧氣)，因此具備超高的理論比能量密度(11 700 Wh·kg-1)，是鋰離子電池的幾十倍，甚至能與汽油能源系統(tǒng)(13 000 Wh·kg-1)媲美[7-8]，金屬-空氣電池是最有發(fā)展前景的能源存儲(chǔ)裝置之一.

圖1 可充電金屬-空氣電池示意圖[6]Fig.1 Schematic illustrations of a typical regenerative metal-air battery[6]

盡管金屬-空氣電池的研究已取得較大進(jìn)展，仍面臨很多挑戰(zhàn)，主要包括陽(yáng)極的利用率低和陰極的動(dòng)力學(xué)過程緩慢、過電勢(shì)高和可逆性差，導(dǎo)致實(shí)際能量密度低[9]，而且陰極反應(yīng)機(jī)理仍不是很明晰.毫無疑問，空氣電極是目前影響金屬-空氣電池性能的關(guān)鍵因素.圖2所示是典型的金屬-空氣電池充放電循環(huán)示意圖[10]，氧還原反應(yīng)(ORR)和氧生成反應(yīng)(OER)的過電勢(shì)嚴(yán)重降低了金屬-空氣電池的輸出功率和循環(huán)效率.因此，想要發(fā)揮金屬-空氣電池的全部潛能，首先要闡明基礎(chǔ)的氧化學(xué)過程，其次發(fā)展高性能、低成本的雙效氧電極降低ORR和OER過電勢(shì).為此，研究者們致力于探究陰極反應(yīng)機(jī)理，發(fā)展高效的雙功能催化劑材料和設(shè)計(jì)合理的電極結(jié)構(gòu).

本文主要介紹金屬-空氣電池的原理以及近年來取得的重大進(jìn)展，重點(diǎn)論述雙功能催化劑材料的種類和發(fā)展.內(nèi)容如下：1)陰極電化學(xué)反應(yīng)的基本原理，2)雙功能催化劑的種類和發(fā)展，3)未來的挑戰(zhàn)和發(fā)展.本文的目的是對(duì)極具發(fā)展前景的金屬-空氣電池進(jìn)行深入的理解.

圖2 金屬-空氣電池充放電循環(huán)示意圖[10]Fig.2 Discharge-charge loop for a metal-air battery[10]

1 電池構(gòu)造和工作原理

金屬-空氣電池由金屬陽(yáng)極、空氣陰極和電解液組成.不同種類的金屬-空氣電池涉及不同的電化學(xué)反應(yīng)和產(chǎn)物，取決于選擇的金屬、電解質(zhì)和催化材料[11-12].本節(jié)內(nèi)容中，金屬-空氣電池基礎(chǔ)的氧電化學(xué)反應(yīng)機(jī)理分為兩類：水系電解液體系和非水系電解液體系.

1.1 水系電解液體系

在酸性電解液中，金屬(例如Li、Mg、Zn)會(huì)發(fā)生劇烈反應(yīng)，產(chǎn)生氫氣，同時(shí)釋放大量的熱，導(dǎo)致嚴(yán)重的陽(yáng)極腐蝕和復(fù)雜的熱管理.而且，有些電催化材料在強(qiáng)酸環(huán)境下不穩(wěn)定，因此，金屬-空氣電池通常使用堿性電解液.在堿性電解液中，放電反應(yīng)如下：

氧的電化學(xué)過程相當(dāng)復(fù)雜，涉及一系列多步電子轉(zhuǎn)移的電化學(xué)反應(yīng)[13-16].目前普遍認(rèn)為，金屬表面進(jìn)行的ORR一般有兩種途徑：四電子轉(zhuǎn)移和二電子轉(zhuǎn)移.當(dāng)ORR按二電子轉(zhuǎn)移途徑反應(yīng)時(shí)，產(chǎn)物中除了有氫氧根離子外，還產(chǎn)生大量過氧化物，降低ORR效率，而且過氧化物具有強(qiáng)氧化作用，損壞電池隔膜而影響循環(huán)壽命，因此對(duì)于水系電解液而言，四電子轉(zhuǎn)移是實(shí)現(xiàn)高效ORR的主要途徑.此外， ORR路徑和機(jī)制會(huì)隨使用的催化材料發(fā)生改變.金屬-空氣電池充電時(shí)空氣電極發(fā)生的OER是ORR的逆過程，反應(yīng)機(jī)制也取決于電極材料.典型的OER催化劑二氧化釕(RuO2)和二氧化銥(IrO2)具有相當(dāng)高的活性，低的氧化電勢(shì)(約1.39和1.35 Vvs.RHE)[17]和高的導(dǎo)電性.一些過渡金屬氧化物也是高效的OER催化劑，尤其是尖晶石型氧化物(例如NiCo2O4)，具體電化學(xué)活性將在第2部分討論.

1.2 非水系電解液體系

水系電解液體系具有廉價(jià)、來源廣泛和高離子電導(dǎo)率等優(yōu)點(diǎn).但是，在水介質(zhì)中發(fā)生的電化學(xué)過程受析氫和析氧的限制具有較窄的電位區(qū)間，導(dǎo)致金屬-空氣電池實(shí)際電壓遠(yuǎn)低于高的理論電壓.而且金屬和水之間的反應(yīng)很危險(xiǎn)[18-22].因此，非水系電解液金屬-空氣電池成為研究熱點(diǎn).發(fā)生反應(yīng)如下(以鋰-空為例)：

和水介質(zhì)中發(fā)生的ORR一樣，非質(zhì)子電解液里發(fā)生的陰極反應(yīng)也涉及多步反應(yīng).根據(jù)電化學(xué)分析，LAOIRE等提出以下可能的陰極反應(yīng)[23-24]：

除了上述機(jī)理外，催化劑表面性質(zhì)也對(duì)陰極反應(yīng)路徑產(chǎn)生很大的影響.通過比較有機(jī)電解液中玻碳電極、多晶Au和Pt電極的循環(huán)伏安曲線，LU等[25]表明還原機(jī)理和放電產(chǎn)物可能與催化劑有關(guān)，認(rèn)為氧分子首先接受一個(gè)電子形成超氧自由基，該自由基與Li+結(jié)合生成表面吸附的LiO2.當(dāng)催化劑表面的吸附氧鍵能較弱時(shí)(例如碳)，LiO2能快速得到電子還原為L(zhǎng)i2O2；而當(dāng)吸附氧鍵能較強(qiáng)時(shí)(例如Pt)，電子轉(zhuǎn)移受到阻礙，LiO2更易還原為L(zhǎng)i2O.因此，催化劑的選擇對(duì)反應(yīng)歷程起著非常重要的作用.催化劑依賴的ORR活性[26-28]和路徑表明復(fù)合催化材料電極機(jī)制的研究可能比較復(fù)雜，催化劑的篩選應(yīng)予以更多關(guān)注.

金屬-空氣電池的氧電化學(xué)是極其復(fù)雜的過程，ORR和OER反應(yīng)受電解液、電極材料等影響.深入了解反應(yīng)機(jī)理、機(jī)理/活性與催化劑間的關(guān)系是理解金屬-空氣電池充放電起源的關(guān)鍵，對(duì)發(fā)展高活性空氣電極也具有重要的指導(dǎo)意義.

2 雙功能催化劑

眾所周知，雙功能催化劑可以降低充放電反應(yīng)的過電勢(shì)，從而提高循環(huán)性能.雙功能催化劑通過兩種方式催化ORR和OER，一種是組合多功能組分，通過協(xié)同作用降低過電勢(shì)；另一種是對(duì)ORR和OER起到雙催化作用的單組分催化劑[29].近年來，發(fā)展雙功能催化劑已取得重大成就，大致分為以下4種：1)貴金屬及其合金，2)碳材料，3)過渡金屬氧化物，4)復(fù)合材料.

2.1 貴金屬及其合金

貴金屬催化劑Pt、Au、Ag、Pd等應(yīng)用的較多.貴金屬的d電子軌道通常都未填滿，表面易吸附反應(yīng)物且強(qiáng)度適中，因而具有很高的催化活性，同時(shí)還具有抗氧化、耐腐蝕和耐高溫等優(yōu)良特性，被認(rèn)為是最好的化學(xué)反應(yīng)催化劑.但貴金屬成本高且資源短缺，阻礙金屬-空氣電池產(chǎn)業(yè)化，這就要求在降低貴金屬用量的同時(shí)保證高活性和穩(wěn)定性，因此國(guó)內(nèi)外在貴金屬合金方面進(jìn)行了大量研究.

YANG課題組系統(tǒng)研究貴金屬在非水系鋰-空電池的應(yīng)用，發(fā)現(xiàn)Au/C能有效催化ORR而Pt/C能有效催化OER(圖3).有趣的是，PtAu/C展現(xiàn)出優(yōu)異的雙功能活性，以及在可充電鋰-氧電池中具有高的循環(huán)性能[30].KO等[31]合成各種碳支撐的金屬及其合金(Pt、Pd、Ir、Ru、Pt-Pd、Pd-Ir和Pt-Ru)，并將其作為鋰-空電池陰極催化劑.Pt-Ru、Pt-Pd和Pd-Ir電流密度為0.2 mA·cm-2時(shí)的初始放電容量分別是346、153和135 mAh·g-1，因此，他們研究表明不同金屬合金催化劑產(chǎn)生與純金屬催化劑不同的行為特征.為了降低貴金屬成本，還可以用便宜的過渡金屬修飾Pt.例如：KIM等[32]合成碳支撐的Pt3Co合金納米材料，電流密度為100 mA·g-1時(shí)，Pt3Co/KB、Pt/KB和 KB的過電勢(shì)分別為135、635和1 085 mV.作者認(rèn)為提升的性能與Pt催化位點(diǎn)最外層LiO2減小的吸附能有關(guān)，同時(shí)，合金催化劑趨于產(chǎn)生無定型Li2O2，在充電過程中更易分解.LEE等[33]基于密度泛函理論研究Pt-Cu合金催化劑的電化學(xué)性能，與Pt (111)和Pt3Cu (111)相比，PtCu (111)顯著降低鋰氧電池ORR/OER過電勢(shì)，由于PtCu (111)表面帶較多的負(fù)電荷，與Li-O中間體的結(jié)合力較弱.這是首次表明充電過電勢(shì)受合金催化劑表面電荷特征影響，為設(shè)計(jì)高效的鋰氧電池催化劑提供新思路.

盡管貴金屬及其合金在電化學(xué)活性和穩(wěn)定性方面都是最高效的催化劑，成本高和資源稀缺阻礙其商業(yè)化.因此，需要探索和研究具有低成本、高活性和穩(wěn)定性的電催化材料.為此提出兩種不同的方案：通過增加貴金屬的利用率減少催化劑載量；發(fā)展非貴金屬電催化劑.與貴金屬催化劑相比，非貴金屬催化劑由于高豐度和低成本廣受關(guān)注.

圖3 (a) C和PtAu/C在0.04 mA·cm-2下第三圈充放電曲線，(b) C (電流密度85 mA·g-1) 和Au/C、Pt/C和PtAu/C (電流密度100 mA·g-1) 首次充放電曲線[30]Fig.3 (a) Charge/discharge profiles of carbon and PtAu/C in the third cycle at 0.04 mA·cm-2,(b) First charge/discharge profiles of carbon at 85 mA·g-1 and of Au/C,Pt/C,and PtAu/C at 100 mA·g-1 [30]

2.2 碳材料

碳材料具有高導(dǎo)電性和大比表面積，已廣泛應(yīng)用在金屬-空氣電池，通常作為電極材料構(gòu)造有孔的電極，也可以作為催化ORR/OER雙功能催化劑[34].與ORR不同，碳表面OER具體的反應(yīng)機(jī)理尚不明確，而且碳材料在OER高壓下電化學(xué)性能不穩(wěn)定，但碳基催化劑的發(fā)展持續(xù)順利推進(jìn).碳材料可以劃分為以下3類：商業(yè)碳材料、功能化碳材料和摻雜的碳，下面將給予詳細(xì)的討論.

2.2.1 商業(yè)碳材料

商業(yè)碳材料(例如導(dǎo)電炭黑、科琴黑KB、Vulcan XC-72和BP 2000等)已被研究為非水系金屬-空氣電池的陰極材料[35-37].MEINI等[38]報(bào)道了碳材料的表面積與放電容量有著密切聯(lián)系.例如：碳材料Vulcan XC72 和 BP 2000的表面積依次為240 和1 509 m2·g-1，對(duì)應(yīng)的放電容量分別是183和 517 mAh·g-1.

商業(yè)碳材料作為金屬-空氣電池的陰極具有可行性，但在投入實(shí)際應(yīng)用前也面臨許多問題，例如：低的放電電壓、高的充電電壓、差的倍率性能和循環(huán)性能[39-41].因此，商業(yè)碳材料通常作為非水系金屬-空氣電池陰極導(dǎo)電劑或者催化劑載體，而不是反應(yīng)位點(diǎn)[42-44].

2.2.2 功能化碳材料

不同于商業(yè)碳材料高OER電位下發(fā)生碳腐蝕導(dǎo)致催化活性衰減，功能化碳材料在非水系金屬-空氣電池陰極反應(yīng)中展現(xiàn)出優(yōu)異的性能，由于其獨(dú)特的結(jié)構(gòu)和大量的缺陷/空位.功能化碳材料包括一維(1D)納米管、2D石墨和石墨烯、3D納米多孔結(jié)構(gòu)碳.

碳納米管(CNTs)包括單壁碳納米管[45]和多壁碳納米管[46]，已被研究為非水系金屬-空氣電池陰極材料，具有高化學(xué)和熱穩(wěn)定性、高強(qiáng)度和高導(dǎo)電性等優(yōu)點(diǎn).TIAN等[47]報(bào)道CNT@NCNT作為高效的無金屬納米碳電催化劑，碳納米管由于高比表面作為全面暴露表面活性位的平臺(tái)，褶皺的摻氮碳層外延生長(zhǎng)在圓柱形CNT外表面，此獨(dú)特的結(jié)構(gòu)使其表面聚集活性位，降低ORR/OER間過電勢(shì)，成為有前景的雙功能電催化劑.

石墨烯是由碳原子構(gòu)成的單層二維蜂窩狀晶格結(jié)構(gòu)的一種新型碳材料，具有大表面積(理論上單層是2 630 m2·g-1)、高導(dǎo)電性以及熱和化學(xué)穩(wěn)定性等優(yōu)點(diǎn)[48-50]，近年來石墨烯由于放電容量高和循環(huán)效率高已成為有前景的金屬-空氣電池陰極材料.YOO和ZHOU[51]以無金屬石墨烯納米片(GNS)作為混合可充電鋰-氧電池的催化劑，其充放電間過電勢(shì)只有0.56 V，表明GNSs能有效降低ORR和OER過電勢(shì)，此高性能來源于邊緣和表面缺陷位引起的sp3雜化，這些邊緣和表面缺陷利于空氣中的氧氣分解為氧原子，然后遷移到GNS表面，與H2O分子結(jié)合形成OH-.

對(duì)于非水系鋰-空電池而言，不溶性放電產(chǎn)物L(fēng)i2O2堆積在空氣電極活性位處會(huì)堵塞孔，從而降低電池性能.因此，研究者們致力于優(yōu)化非水系鋰-空電池空氣電極的微觀結(jié)構(gòu)[52].SAKAUSHI等[53]用SiO2模板法制備介孔摻氮的碳，具有高的雙功能ORR/OER活性和穩(wěn)定性，在鋰-空電池中展現(xiàn)出極低的充電過電勢(shì)(0.45 V)，是目前鋰-空電池非貴金屬催化劑中過電勢(shì)最低值.其實(shí)，電極結(jié)構(gòu)的設(shè)計(jì)對(duì)于提高能量轉(zhuǎn)換過程尤為重要[54-56].多孔碳材料通常用粘合劑緊密聚集在電極上，導(dǎo)致低的O2擴(kuò)散率和有限的Li2O2沉積空間，最終引發(fā)碳材料低的利用率以及鋰-氧電池低的容量和倍率性能.為此，ZHANG課題組[56]原位溶膠凝膠法構(gòu)造自支撐分級(jí)多孔的碳(FHPC)，最大化多孔碳材料的利用率和反應(yīng)物的傳輸.圖4a展示最初泡沫鎳的大孔骨架，原位合成后碳薄片垂直于骨架表面(圖4b)，形成相互連接的通道貫穿整個(gè)電極，高倍放大圖觀察到碳薄片由無數(shù)小的納米孔組成(圖4c和4d).FHPC作為鋰-氧電池陰極展現(xiàn)出高的比容量和優(yōu)異的倍率性能.電流密度為0.2 mA·cm-2時(shí)，容量高達(dá)11 060 mAh·g-1，是商業(yè)KB碳的兩倍；甚至當(dāng)電流密度為2 mA·cm-2時(shí)，容量達(dá)到2 020 mAh·g-1(圖4e).FHPC如此卓越的性能來源于自支撐結(jié)構(gòu)疏松填充的碳，為L(zhǎng)i2O2沉積提供充足的空隙容積且增加碳的有效利用.同時(shí)，分級(jí)多孔結(jié)構(gòu)加速O2擴(kuò)散、電解液的浸潤(rùn)和反應(yīng)物的物質(zhì)傳輸.類似的，ZHU等[57]用酚醛樹脂作為碳源，通過簡(jiǎn)單碳化法在泡沫鎳上直接生長(zhǎng)碳納米管(CNT/NF)，具有高的倍率性能和杰出的循環(huán)穩(wěn)定性.

圖4 SEM圖：(a) 初始泡沫鎳，(b-d) FHPC電極，(e) FHPC電極電流密度從0.2 mA·cm-2到2 mA·cm-2的放電曲線[56]Fig.4 SEM images of (a) the pristine nickel foam,(b-d) the FHPC electrode,(e) Discharge curves of FHPC electrode at different current densities ranging from 0.2 mA·cm-2 to 2 mA·cm-2 [56]

2.2.3 摻雜碳

碳材料摻雜定量的非金屬元素(例如N、B、S、P)可以提升電化學(xué)性能，因?yàn)楫愒訐诫s會(huì)改變碳材料的化學(xué)和電子性質(zhì)[58-60]，形成缺陷和官能團(tuán)[61].

碳納米管具有高比表面積，能充分地暴露活性位點(diǎn)，且嵌入氮會(huì)引入活性位點(diǎn)，參與氧分子O-O鍵的斷裂.YADAV等[62]通過改變前驅(qū)體和生長(zhǎng)條件制備不同直徑的竹狀碳氮納米管(CNNTs)，發(fā)現(xiàn)ORR/OER性能與納米管直徑和氮官能團(tuán)都密切相關(guān)，摻氮降低CNNTs氧吸附能且增強(qiáng)導(dǎo)電性.因此，需協(xié)調(diào)納米管直徑和氮官能團(tuán)兩種因素以獲得高效的雙功能催化劑.LI等[63]通過氨氣熱處理化妝棉合成3D彎曲多孔的摻氮碳微管海綿(NCMT)，由中空多孔的石墨化碳微管相互交錯(cuò)而成，如此獨(dú)特的結(jié)構(gòu)提供高密度活性位和快速電子轉(zhuǎn)移能力，因此NCMT是迄今為止最杰出的ORR/OER催化劑，在柔性能源存儲(chǔ)與轉(zhuǎn)換領(lǐng)域有巨大的應(yīng)用前景.

異原子摻雜能有效提升石墨烯作為雙功能催化劑的電化學(xué)穩(wěn)定性.LI等[61]首次介紹摻氮的石墨烯納米片(N-GNSs)應(yīng)用于鋰-空電池，電流密度為75、150 和300 mA·g-1時(shí)的放電容量分別為11 660、6 640 和 3 960 mAh·g-1，遠(yuǎn)超越純GNSs電極.摻雜其他元素(例如B、P和S)也可以增強(qiáng)電催化活性.VINEESH等[64]用B4C作為前驅(qū)體制備高產(chǎn)量的摻硼石墨烯(B-G)，展現(xiàn)出優(yōu)異的雙功能催化活性.B4C結(jié)構(gòu)轉(zhuǎn)化為石墨烯的過程還伴隨著原位摻雜B，形成來源于非電催化活性材料的電催化活性材料，擴(kuò)展在各種能源相關(guān)技術(shù)的應(yīng)用前景.近來，共摻雜已成為提高碳活性的研究方案.QU等[65]簡(jiǎn)單熱解氧化石墨烯、聚多巴胺和2-巰基乙醇混合物制備N、S共摻雜介孔碳納米片，展現(xiàn)出優(yōu)異的雙功能活性.DAI等[66]在植酸存在下熱解聚苯胺氣凝膠大規(guī)模生產(chǎn)三維氮磷共摻介孔碳泡沫(NPMC)，展現(xiàn)出優(yōu)異的ORR和OER電催化性能.ORR和OER過電勢(shì)分別為0.44 V和0.39 V，都低于最優(yōu)的催化劑(圖5a，5b)，而且在可充電鋅-空電池中展現(xiàn)出卓越的穩(wěn)定性(循環(huán)600圈，圖5d).通過密度泛函理論表明N、P共摻雜和石墨烯邊緣缺陷對(duì)雙功能電催化活性至關(guān)重要.XIA等[67]發(fā)現(xiàn)一個(gè)特性描述符可以準(zhǔn)確地描述共摻雜碳納米材料的ORR/OER性能，認(rèn)為共摻雜碳基催化劑性能的提升來源于雙摻雜物間的相互作用.當(dāng)兩個(gè)異原子摻雜在石墨結(jié)構(gòu)里且彼此接近時(shí)，p電子云發(fā)生重疊且相互作用，在相鄰碳原子上產(chǎn)生比單元素?fù)诫s更多的活性位，從而降低ORR/OER過電勢(shì)，表明共摻雜是發(fā)展高活性無金屬碳基雙功能催化劑的有效途徑.

圖5 摻N、P石墨烯 (a) ORR和 (b) OER過電勢(shì)對(duì)OH* 吸附能和對(duì) O*、OH* 吸附能差異的火山圖， 鋅-空電池空氣電極NPMC-1000 (c) 充放電極化曲線和 (d) 充放電循環(huán)曲線 (電流密度2 mA·cm-2) [66]Fig.5 (a) ORR and (b) OER volcano plots of overpotential η versus adsorption energy of OH* and the difference between the adsorption energy of O* and OH* for N,P-doped graphene,(c) Charge and discharge polarization curves and (d) Discharge/charge cycling curves of Zn-air battery using NPMC-1000 for the air electrodes at a current density of 2 mA·cm-2[66]

2.3 過渡金屬氧化物

過渡金屬氧化物具有高豐度、低成本和環(huán)境友好等優(yōu)點(diǎn)，已廣泛應(yīng)用為金屬-空氣電池陰極催化劑.過渡金屬元素具有多價(jià)態(tài)，可以形成各種不同晶體結(jié)構(gòu)的氧化物.本節(jié)中將根據(jù)過渡金屬氧化物的組分和結(jié)構(gòu)介紹4種氧化物電催化劑.

2.3.1 單金屬氧化物

錳氧化物由于價(jià)態(tài)可變、結(jié)構(gòu)豐富和環(huán)境友好已廣受關(guān)注.OGASAWARA等[68]在2006年首次將MnO2引入鋰-空電池陰極，自此以后許多研究致力于評(píng)估和優(yōu)化MnOx作為空氣電極催化劑[69-72].晶體結(jié)構(gòu)、形貌和結(jié)構(gòu)等影響錳氧化物催化性能的因素將在本節(jié)予以討論.

MnO2由于具有多種晶相和形貌被廣泛研究為陰極催化劑.MENG等[73]研究MnO2不同的晶體結(jié)構(gòu)(α-MnO2、β-MnO2、δ-MnO2和無定型MnO2(AMO))對(duì)雙功能氧催化活性的影響(圖6a).此外，各種形貌的MnO2(例如納米線、納米片和納米顆粒)在合成不同晶體結(jié)構(gòu)MnO2時(shí)被合成(圖6b-g).ORR/OER活性順序：α-MnO2> AMO >β-MnO2>δ-MnO2(圖6h和i)，表明晶體結(jié)構(gòu)和形貌是決定氧電化學(xué)整體活性的重要因素.為了進(jìn)一步提高M(jìn)nO2的催化性能，結(jié)構(gòu)修飾已經(jīng)被發(fā)展和探究.ZHANG等[74]以普魯士藍(lán)類配合物為前驅(qū)體合成分級(jí)多孔的δ-MnO2納米盒，形成的電極具有低過電勢(shì)、高倍率性能和優(yōu)異循環(huán)穩(wěn)定性，歸因于分級(jí)多孔的結(jié)構(gòu)和大比表面積.除此以外，ZHENG等[75]基于密度泛函理論研究α-MnO2表面，發(fā)現(xiàn)表面氧位點(diǎn)比暴露的金屬位點(diǎn)發(fā)揮更重要的作用.表面氧密度越大，Li2O2分散越均勻，形成顆粒小，利于ORR和OER.此報(bào)道為非質(zhì)子鋰-氧電池ORR催化機(jī)制提供新視角.

其他形式的錳氧化物也被應(yīng)用為金屬-空氣電池陰極催化劑.例如：GORLIN 等[76]制備納米結(jié)構(gòu)的錳氧化物(Ⅲ)薄膜，雙功能性能與貴金屬媲美，利用原位X射線吸收光譜研究MnxOy的活性位，發(fā)現(xiàn)無序相Mn3Ⅱ,Ⅲ,ⅢO4利于ORR，而混合的MnⅢ,Ⅳ氧化物與OER活性相關(guān)，表明催化劑表面的氧化態(tài)對(duì)于促進(jìn)高活性至關(guān)重要.KUO等[77]強(qiáng)調(diào)表面結(jié)晶度對(duì)電催化活性的影響，制備八面體納米顆粒、納米花和納米多豆莢結(jié)構(gòu)的MnO.其中，更易暴露(100)面的納米多豆莢的ORR/OER催化性能都優(yōu)于納米花結(jié)構(gòu)的MnO.

圖6 (a) MnO2各種晶相示意圖，MnO2納米結(jié)構(gòu)SEM圖：(b) α-MnO2-HT (水熱合成)，(c) α-MnO2-SF (無溶劑合成)， (d) 摻Ni的α-MnO2-HT (無溶劑合成)，(e) AMO (無定型)，(f) β-MnO2，(g) δ-MnO2，(h) ORR和 (i) OER極化曲線(在O2飽和的0.1 mol/L KOH溶液里，掃描速率5 mV·s-1和1 600 rpm轉(zhuǎn)速下獲得) [73]Fig.6 (a) Various crystal phases of manganese oxide,SEM images of manganese oxide nanostructures:(b) α-MnO2-HT (hydrothermal synthesis),(c) α-MnO2-SF (solvent-free synthesis),(d) Ni-doped α-MnO2-HT (solvent-free synthesis),(e) AMO (amorphous),(f) β-MnO2 and (g) δ-MnO2,(h) ORR and (i) OER polarization curves of MnO2 nanostructures obtained at 1 600 rpm with a scan rate of 5 mV·s-1 in O2-saturated 0.1 mol/L KOH solution[73]

2.3.2 尖晶石型金屬氧化物

尖晶石型氧化物的通式是AB2O4，其中A是二價(jià)金屬離子(例如Mg、Fe、Co、Ni、Mn、Zn)，B是三價(jià)金屬離子(例如Al、Fe、Co、Cr、Mn).尖晶石型氧化物由于制備簡(jiǎn)易、形貌多變和穩(wěn)定性高成為最受關(guān)注的金屬基雙功能催化劑之一.

Co3O4作為高效ORR/OER雙功能催化劑備受關(guān)注，由于多價(jià)態(tài)鈷離子充當(dāng)可逆吸-脫附氧過程中給-受體吸附位，具有雙功能活性[78].RIAZ等[79]制備納米片、納米針和納米花形貌的Co3O4電極，發(fā)現(xiàn)性能與Co3O4電極的結(jié)構(gòu)密切相關(guān).電流密度為20 mA·gcatalyst-1時(shí)的放電容量增加順序依次為：納米片(1 127 mAh·gcatalyst-1)< 納米花(1 930 mAh·gcatalyst-1)< 納米針(2 280 mAh·gcatalyst-1).ZHANG等[80]以普魯士藍(lán)類配合物納米立方體為前驅(qū)體合成分級(jí)多孔的Co3O4納米盒，在鋰-氧電池中展現(xiàn)出低過電勢(shì)、高倍率性能和優(yōu)異循環(huán)穩(wěn)定性.WU等[81]水熱合成自支撐分級(jí)多孔Co3O4超薄納米片，具有低過電勢(shì)和優(yōu)異的循環(huán)性能.納米片排列形成的大孔使得活性物質(zhì)與電解液充分接觸以及提供足夠的Li2O2存儲(chǔ)空間，介孔提供充足的ORR/OER催化活性位.基于這些研究表明：具有特殊結(jié)構(gòu)的單金屬尖晶石氧化物展現(xiàn)出高效的雙功能催化活性.

通常，用第二種金屬陽(yáng)離子對(duì)單金屬尖晶石氧化物進(jìn)行組分改性可以調(diào)整對(duì)氧催化起重要作用的性能(例如：晶體結(jié)構(gòu)和導(dǎo)電性).例如MnxCo3-xO4[82-83]、NixCo3-xO4[84-85]、CuxCo3-xO4[86]和ZnxCo3-xO4[87]已被報(bào)道為雙功能空氣電極材料.除成分以外，催化劑的納米結(jié)構(gòu)也極大地影響活性.PENG等[83]溶劑熱合成3D分級(jí)多孔NiCo2O4核殼微球，類似向日葵(圖7a)，由多孔納米片構(gòu)成(圖7b).如此獨(dú)特的結(jié)構(gòu)提高催化位點(diǎn)的利用率，同時(shí)加快電子和反應(yīng)物擴(kuò)散.SP(導(dǎo)電炭黑)的充放電過電勢(shì)為1.87 V，比NiCo2O4/SP高640 mV(圖7c)，而且NiCo2O4/SP電極在鋰氧電池中展現(xiàn)出優(yōu)異的循環(huán)穩(wěn)定性.類似的，WANG等[88]用硬模板法制備分級(jí)NiCo2O4中空納米球，由超薄納米片構(gòu)成，與普通刺猬狀NiCo2O4相比，具有更低過電勢(shì)和優(yōu)異循環(huán)穩(wěn)定性.此外，Mn基尖晶石氧化物也展現(xiàn)出卓越的ORR和OER雙功能催化活性，由于錳元素高豐度、低成本和環(huán)境友好等諸多優(yōu)點(diǎn).CHEN等[89]分別用NaH2PO2和NaBH4作為還原劑，室溫快速合成兩種不同晶體結(jié)構(gòu)的尖晶石(四方和立方CoxMn3-xO4).有趣的是，立方Co-Mn-O尖晶石的ORR催化能力比四方尖晶石強(qiáng)，而四方尖晶石的OER催化能力優(yōu)于立方相，DFT理論計(jì)算表明相依賴的電催化行為源于兩相表面不同的氧吸附結(jié)合能.這為合理設(shè)計(jì)尖晶石型ORR/OER雙功能催化劑提供重要的指導(dǎo).

2.3.3 鈣鈦礦型金屬氧化物

BRUCE等[26]首次使用La0.8Sr0.2MnO3作為非水系鋰-氧電池的催化劑，但效能不理想.后來， XU等[41]結(jié)合電紡和熱處理制備多孔的La0.75Sr0.25MnO3納米管(PNT-LSM)，展現(xiàn)出優(yōu)異的往返效率、倍率性能和循環(huán)穩(wěn)定性.在鋰-氧電池中，PNT-LSM/KB的充電電壓比KB低200 mV，庫(kù)侖效率約100%，而且在1 000 mAh·g-1容量限制下維持124圈.如此杰出的性能來源于PNT-LSM特殊多孔的結(jié)構(gòu)，多孔的管狀結(jié)構(gòu)提供更多傳輸氧和電解質(zhì)的通道，加速放電產(chǎn)物的分解，因此提高O2電極的可逆性.YANG等報(bào)道許多鈣鈦礦型氧化物作為雙功能催化劑[96-98].其中，電子和離子導(dǎo)體Ba0.5Sr0.5Co0.8Fe0.2O3-δ備受關(guān)注，摻雜或混合可以進(jìn)一步提高其ORR/OER活性.JUNG等[99]報(bào)道一種新型結(jié)構(gòu)摻La的Ba0.5Sr0.5Co0.8Fe0.2O3-δ催化劑(La0.3(Ba0.5Sr0.5)0.7Co0.8Fe0.2O3-δ)(圖9a)，菱形的LaCoO3納米顆粒(～10 nm)分布在立方Ba0.5Sr0.5Co0.8Fe0.2O3-δ表面，展現(xiàn)出與RuO2可比的ORR活性(圖9b)以及優(yōu)于IrO2的OER活性(圖9c).此后JUNG等[100]開創(chuàng)性研究納米鈣鈦礦Lax(Ba0.5Sr0.5)1-xCo0.8Fe0.2O3-δ氧化物,以納米粒子方式在可充電金屬-空氣電池領(lǐng)域?qū)で笸黄?眾所周知，鈣鈦礦氧化物的電化學(xué)氧催化程度與表面陽(yáng)離子密切相關(guān)，其實(shí)這取決于氧化物中氧原子的缺陷程度.在此方面，CHEN等[101]結(jié)合溶膠凝膠法和1 300 ℃真空熱處理制備具有氧缺陷的六邊形晶體結(jié)構(gòu)BaTiO3-x(h-BaTiO3-x)，與900 ℃ 空氣熱處理制備的四方相t-BaTiO3相比，h-BaTiO3-x具有部分占據(jù)的氧位點(diǎn)，展現(xiàn)出卓越的雙功能活性.中子分析表明：BaTiO3-x的真實(shí)化學(xué)式是BaTiO2.76，雙功能催化活性來源于氧缺陷，加速反應(yīng)物吸附和電荷轉(zhuǎn)移.這篇文章強(qiáng)調(diào)鈣鈦礦氧化物結(jié)構(gòu)和氧含量的重要性，其可以通過熱處理溫度和氛圍參數(shù)來控制.

圖7 多孔的NiCo2O4核殼微球 (a) 低倍和 (b) 高倍FESEM圖，(c) NiCo2O4/SP和純碳電極在鋰氧電池中首次充放電曲線 (電流密度為200 mA·gcarbon-1)[83]Fig.7 (a) Low-magnified and (b) high-magnified FESEM images of porous NiCo2O4 core-shell microspheres,(c) First charge-discharge curves of Li-O2 cells with NiCo2O4/SP and bare carbon electrodes at a current density of 200 mA·gcarbon-1[83]

盡管鈣鈦礦氧化物在金屬-空氣電池的應(yīng)用備受關(guān)注，陽(yáng)離子部分取代效應(yīng)的機(jī)理尚不明確.除ABO3以外，雙鈣鈦礦氧化物(A2B2O6，8b)[102]和層狀鈣鈦礦氧化物(圖8c)[103]很少被研究為金屬-空氣電池陰極催化劑.因此，未來需要更系統(tǒng)地研究鈣鈦礦型催化劑來提高ORR/OER的催化性能和促進(jìn)金屬-空氣電池的發(fā)展[104].

圖8 (a) 立方鈣鈦礦結(jié)構(gòu)[90]，(b) 雙鈣鈦礦結(jié)構(gòu)[102]，(c) 層狀鈣鈦礦結(jié)構(gòu)[103]Fig.8 (a) Cubic perovskite structure[90],(b) Double perovskite structure[102],(c) Layered perovskite structure[103]

圖9 (a) 雙功能催化劑La0.3(Ba0.5Sr0.5)0.7Co0.8Fe0.2O3-δ (La0.3-5582)示意圖，(b，c) 80% La0.3-5582與20% KB復(fù)合物和其他對(duì)比催化劑：(b) ORR和 (c) OER活性[99]Fig.9 (a) Illustration of the bifunctional catalyst:La0.3(Ba0.5Sr0.5)0.7Co0.8Fe0.2O3-δ (La0.3-5582),(b) ORR and (c) OER activ-ities of 80% La0.3-5582 on 20% Ketjen Black (KB) composite and other typical catalysts for comparison[99]

2.4 復(fù)合材料

如上所述，陰極材料需要催化ORR/OER的高效催化劑，還要具備大比表面積和大孔容量來儲(chǔ)存更多的放電產(chǎn)物.但是單材料很難完全滿足以上要求，因此，含復(fù)合材料的混合電極已成為提高金屬-空氣電池性能的趨勢(shì)[105].

2.4.1 貴金屬及其氧化物/納米碳復(fù)合物

貴金屬及其氧化物與納米碳復(fù)合滿足金屬-空氣電池陰極材料要求[106].功能化的碳在復(fù)合物中起重要作用，不僅提供大比表面積，而且促進(jìn)表面催化劑材料的分散，從而增強(qiáng)催化性能.而且，功能化的碳具有空位和缺陷促進(jìn)陰極反應(yīng).

碳納米管作為典型的功能化碳材料，已廣泛應(yīng)用于混合電極.LI等[107]用化學(xué)濕選法合成多壁碳納米管紙支撐的Ru催化劑(Ru/MWCNTP)，與MWCNTP電極相比，Ru/MWCNTP降低充電電壓約0.68 V.MA等[108]用測(cè)控濺射法制備貴金屬(Ru和Pd)催化的碳納米管，在鋰-氧電池中展現(xiàn)出超高的循環(huán)效率和持久的循環(huán)壽命.但原位電化學(xué)質(zhì)譜和非原位光譜分析表明：催化的鋰-氧電池并沒有按照預(yù)想的鋰-氧反應(yīng)進(jìn)行操作，充電過程中伴隨著CO2的產(chǎn)生，而且可逆性低于CNT.這篇報(bào)道為使用貴金屬催化劑的非質(zhì)子鋰-氧電池的氧電化學(xué)提供新的視角，同時(shí)例證定量測(cè)定氧電化學(xué)對(duì)于追求真正可充電鋰-氧電池的重要性.此外，鈦的碳化物和氮化物由于超強(qiáng)的耐腐蝕性和高導(dǎo)電性被研究為催化載體.ROCA-AYATS等[109]用乙二醇法合成碳氮化鈦擔(dān)載的 Pt3M(M = Ru、Ir、Ta)納米材料， Pt3Ru/TiCN 展現(xiàn)出最優(yōu)異的ORR/OER活性和穩(wěn)定性，歸因于支撐物的促進(jìn)作用，TiCN穩(wěn)定活性相和抑制釕溶解.Pt3Ru/TiCN 在一體式可再生燃料電池中具有很大的應(yīng)用潛力.

YILMAZ等[110]合成多壁碳納米管支撐的二氧化釕(RuO2/MWCNTs)，0.05 mA·cm-2電流密度下的充電電壓為3.48 V，低于MWCNTs的3.91 V.分析表明：RuO2納米顆粒促進(jìn)低結(jié)晶度的過氧化鋰層覆蓋在MWCNTs表面，而在純MWCNTs表面形成大的Li2O2顆粒(圖10).獨(dú)特的過氧化鋰結(jié)構(gòu)提供與導(dǎo)電CNT電極大的接觸面積和許多缺陷，在OER低電位下更易分解.這是首次發(fā)現(xiàn)金屬氧化物在轉(zhuǎn)變Li2O2晶體結(jié)構(gòu)和形貌方面發(fā)揮作用，顯著降低OER過程中的能量損失.

圖10 (a) CNT和 (b) RuO2/CNT陰極Li2O2形成示意圖，(c) CNT和 (d) RuO2/CNT首次放電TEM圖[110]Fig.10 Schematic illustration of the Li2O2 formation process in (a) CNT and (b) RuO2/CNT cathodes, TEM images of first-discharged (c) CNT and (d) RuO2/CNT cathodes[110]

2.4.2 過渡金屬氧化物/納米碳復(fù)合物

具有混合價(jià)的過渡金屬氧化物有替代貴金屬基礎(chǔ)電催化劑的潛力[111],但低的導(dǎo)電性和嚴(yán)重的團(tuán)聚限制其ORR/OER活性，因此，分散催化劑于導(dǎo)電基質(zhì)上形成復(fù)合物是有效的方法，而且催化劑和導(dǎo)電基質(zhì)間的協(xié)同作用能產(chǎn)生卓越的催化性能[112-114].

近來，CAO等[115]在石墨烯納米片上原位合成α-MnO2納米線(α-MnO2/GNSs)，展現(xiàn)出卓越的ORR和OER催化活性.電流密度200 mA·g-1時(shí)的放電容量為11 520 mAh·g-1，遠(yuǎn)高于α-MnO2和GNSs混合物的7 200 mAh·g-1.除了錳氧化物以外，F(xiàn)e氧化物[116-118]、Co氧化物[119-121]、Ni氧化物[122]和Zn氧化物[123]與G或CNT的復(fù)合材料也被研究為雙功能氧催化劑.

另一種復(fù)合型雙功能氧催化劑由尖晶石型化物和碳材料組成，例如 DAI課題組合成的Co3O4-N-rmGO(摻氮氧化程度低的還原氧化石墨烯)(圖11a)[124]，展現(xiàn)出卓越的ORR/OER活性，成為高性能非貴金屬雙功能催化劑(圖11c).X射線近邊結(jié)構(gòu)吸收測(cè)試(XANES)發(fā)現(xiàn)：與N-rmGO相比，Co3O4-N-rmGO復(fù)合物C的K邊緣峰強(qiáng)度在約288 eV處明顯增加(圖11d)，表明在復(fù)合物界面存在Co-O-C 和Co-N-C鍵，以及催化劑和基質(zhì)間形成協(xié)同作用.而且，GO的氧化程度也對(duì)復(fù)合物性能產(chǎn)生重要影響[125]，傳統(tǒng)的GO氧化不能兼顧無機(jī)物-碳間耦合作用和復(fù)合物的導(dǎo)電性，因此，調(diào)控石墨烯適中的氧化程度同時(shí)提供豐富的官能團(tuán)和導(dǎo)電性是達(dá)到高ORR/OER性能的關(guān)鍵.復(fù)合材料結(jié)構(gòu)的合理設(shè)計(jì)對(duì)氧電催化活性至關(guān)重要.LI等[126]制備石榴狀Co3O4納米晶-摻氮部分石墨化碳框架復(fù)合催化劑，獨(dú)特的結(jié)構(gòu)使其具有豐富活性位、強(qiáng)協(xié)同耦合作用和快速電子轉(zhuǎn)移能力，是高效的ORR/OER催化劑.類似的，YAN等[127]用SiO2球作為模板合成共價(jià)耦合的FeCo2O4-中空結(jié)構(gòu)還原氧化石墨烯球(FCO/HrGOS).與純FCO和HrGOS相比，F(xiàn)CO/HrGOS復(fù)合物展現(xiàn)杰出的電催化活性，ORR性能與20% Pt/C催化劑媲美，OER活性也勝過RuO2/C，歸因于FCO和HrGOS間共價(jià)耦合作用.而且，3D中空結(jié)構(gòu)的石墨烯球提供高比表面積，同時(shí)促進(jìn)電解液中氧和反應(yīng)物的有效傳輸.YAN等[128]簡(jiǎn)易制備CoFe2O4納米顆粒/氮、硫雙摻雜3D還原氧化石墨烯復(fù)合物(CFO/NS-rGO)，展現(xiàn)出優(yōu)異的雙功能活性，歸因于CFO和NS-rGO間耦合作用和分級(jí)多孔的結(jié)構(gòu).

鈣鈦礦氧化物和碳材料的復(fù)合物在氧電催化方面的應(yīng)用也備受關(guān)注.LEE等[129]合成相互交錯(cuò)的核冠結(jié)構(gòu)雙功能催化劑(IT-CCBC)，多孔交錯(cuò)的網(wǎng)狀NCNTs很好的覆蓋LaNiO3納米顆粒.與純LaNiO3納米顆粒和NCNT相比，IT-CCBC展現(xiàn)出高的ORR/OER活性和出色的電化學(xué)穩(wěn)定性，其獨(dú)特的形貌以及LaNiO3和NCNT間的協(xié)同作用提升可充電鋅-空電池的性能.此外，La0.58Sr0.4Fe0.2Co0.8O3/NCNT復(fù)合物[130]和Nd0.5Sr0.5CoO3-δ納米線/云狀石墨烯納米片[131]等都具有ORR/OER雙功能活性，應(yīng)用于可充電金屬-空氣電池.

圖11 (a) Co3O4-N-rmGO復(fù)合物的TEM圖，插圖是石墨烯擔(dān)載Co3O4納米晶的電子衍射圖，(b) Co3O4-N-rmGO復(fù)合物的XPS圖，(c)分散在碳紙上Co3O4-N-rmGO復(fù)合物、Co3O4納米晶和Pt/C催化劑的ORR和OER活性 (O2飽和的0.1 mol/L KOH)， (d) Co3O4-N-rmGO和N-rmGO的C的K邊緣XANES圖[124]Fig.11 (a) TEM images of the Co3O4-N-rmGO hybrid.The electron diffraction pattern of the Co3O4 nanocrystals on graphene is showed in the inset,(b) XPS spectrum of the Co3O4-N-rmGO hybrid,(c) ORR and OER activities of the Co3O4-N-rmGO hybrid,Co3O4 nanocrystal,and Pt/C catalysts dispersed on carbon fiber paper in O2-saturated 0.1 mol/L KOH,(d) C K-edge XANES of the Co3O4-N-rmGO hybrid and N-rmGO[124]

2.4.3 M-N/C復(fù)合物

過渡金屬和雜環(huán)氮配位化合物形成另一種復(fù)合物，展現(xiàn)優(yōu)異的ORR/OER催化活性[132-134].SUN等[135]報(bào)道有機(jī)電解液溶解的酞菁鐵(FePc)作為鋰-空電池溶液相雙功能催化劑，提出ORR和OER反應(yīng)機(jī)制：FePc是碳導(dǎo)體以及Li2O2位點(diǎn)間傳輸O2-和電子的載體，由于Li2O2的生長(zhǎng)和分解都未與碳接觸，電催化性能明顯提升.后來發(fā)現(xiàn)簡(jiǎn)單熱解含過渡金屬、碳和氮前驅(qū)體材料可以制備催化活性物種M-Nx/C，這為涉及廉價(jià)前驅(qū)體材料的研究提供了新方向.

LI等[136]通過高溫?zé)崽幚矸ㄖ苽涓缓?石墨烯管的N-Fe-MOF催化劑，含納米籠的金屬有機(jī)框架(MOF)和雙氰胺分別作為模板和碳氮前驅(qū)體，研究發(fā)現(xiàn)：N-Fe-MOF的放電電壓約2.80 V，遠(yuǎn)高于Pt/C催化劑(2.71 V).而且在電流密度為50 mA·g-1時(shí)，最高放電容量為5 300 mAh·g-1.這篇報(bào)道為使用MOF新模板合成碳基納米復(fù)合物高效ORR/OER催化劑提供新視野.MENG等[137]熱解長(zhǎng)在碳布上串有珍珠狀ZIF-67的聚吡咯納米纖維網(wǎng)，原位耦合Co4N和交錯(cuò)的N-C纖維，形成3D自支撐柔性氧電極Co4N/CNW/CC，具有優(yōu)異的ORR/OER活性和穩(wěn)定性，歸因于Co4N和Co-N-C間協(xié)同效應(yīng)以及3D連通導(dǎo)電網(wǎng)狀結(jié)構(gòu).自支撐柔性氧電極Co4N/CNW/CC在便攜式和可穿戴式電子設(shè)備領(lǐng)域具有廣泛的應(yīng)用前景.

3 結(jié)論

金屬-空氣電池具有超高能量密度已成為最有發(fā)展前景的能源存儲(chǔ)與轉(zhuǎn)換技術(shù)之一[138]，但在投入實(shí)際商業(yè)化應(yīng)用之前還有許多問題亟需解決[139-142]，如：放電容量低、實(shí)際能量密度低和循環(huán)性能差等.因此，尋找活性高和穩(wěn)定性好的ORR/OER雙功能催化劑至關(guān)重要.貴金屬及其合金通常具有高活性和穩(wěn)定性，但成本高和豐度低；對(duì)于碳基材料而言，適當(dāng)?shù)膿诫s能有效提高催化性能，但是碳在OER過程高電位下易腐蝕導(dǎo)致衰減；過渡金屬氧化物多價(jià)態(tài)、低成本、環(huán)境友好，但導(dǎo)電性差，因此，過渡金屬氧化物-納米碳復(fù)合材料成為新一代具有高催化活性的氧催化材料，兩者間的強(qiáng)耦合作用顯著提升電化學(xué)活性和穩(wěn)定性.

雙功能催化劑的開發(fā)及應(yīng)用面臨可充電金屬-空氣電池性能價(jià)格比和穩(wěn)定性問題.因此，尋求新的電極材料和設(shè)計(jì)特殊的結(jié)構(gòu)降低陰極過電勢(shì)，是未來發(fā)展可充電金屬-空氣電池的首要任務(wù).可充電金屬-空氣電池陰極的未來發(fā)展方向是：

1)通過新穎的制備方法，探索出新的陰極材料.

2)合理地設(shè)計(jì)雙功能催化劑的形貌和組分.設(shè)計(jì)形貌可以增加活性位暴露程度和孔隙率，從而提升催化性能.另一方面，微調(diào)催化劑的組分可以調(diào)整活性位的電子結(jié)構(gòu)，優(yōu)化反應(yīng)過程中與氧分子的相互作用.

3)電極結(jié)構(gòu)的合理設(shè)計(jì)也對(duì)催化劑的運(yùn)用和能量轉(zhuǎn)換效率的提高至關(guān)重要.金屬-空氣電池的電化學(xué)反應(yīng)包含氧擴(kuò)散和放電產(chǎn)物的沉積，因此，需要優(yōu)化空氣電極的多孔結(jié)構(gòu)和催化劑的分布以實(shí)現(xiàn)反應(yīng)物的快速傳輸.

4)理論計(jì)算和實(shí)驗(yàn)相結(jié)合，深入研究充放電過程催化劑表面的氧反應(yīng)機(jī)理，明確各種催化劑活性位，這是發(fā)展高效和長(zhǎng)壽命電池的先決條件.

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[責(zé)任編輯：吳文鵬]

Progress in cathodic bi-functional catalysts for metal-air battries

WANG Ya,LAI Qingxue,ZHU Junjie,LIANG Yanyu*

(JiangsuKeyLaboratoryofMaterialsandTechnologyforEnergyConversion,CollegeofMaterialsScienceandTechnology,NanjingUniversityofAeronauticsandAstronautics,Nanjing211106,Jiangsu,China)

Rechargeable metal-air batteries are considered as one of the most promising energy storage and conversion devices due to their ultrahigh energy density.Slow kinetics of cathodic electrochemical oxygen reduction/evolution reactions is a key factor affecting the performance of metal-air batteries,which requires a bi-functional catalyst to facilely realize the charge and discharge processes of a metal-air battery.In this review,we discuss the novel bi-functional catalysts developed in recent years,including precious metals,carbon materials,transition metal oxides,and hybrid materials.Among them,transition metal oxide/nanocarbon strong coupled hybrid materials have been developed as a new gene-ration of oxygen catalytic material with promising catalytic activity.Finally,several possible research directions in future are proposed based on the existing problem.

metal-air battery； air cathode； bi-functional catalyst； oxygen reduction/evolution reaction

2016-10-15.

國(guó)家自然科學(xué)基金項(xiàng)目(21273114)，中央高?；究蒲袠I(yè)務(wù)費(fèi)(NE2015003)，江蘇省“六大人才高峰”高層次人才項(xiàng)目(2013-XNY-010)，江蘇高校優(yōu)勢(shì)學(xué)科建設(shè)工程資助項(xiàng)目.

王 亞(1992-)，女，碩士生，研究方向?yàn)榛瘜W(xué)電源與電極材料.*

，E-mail:liangyy403@126.com.

O643.36

A

1008-1011(2017)01-0001-18