聶瑤,丁煒,魏子棟
(重慶大學(xué)化學(xué)化工學(xué)院,重慶 400044)
質(zhì)子交換膜燃料電池(PEMFCs)具有能量轉(zhuǎn)換效率高、無污染的特點(diǎn),非常適合作為綠色新能源汽車的動(dòng)力能源[1]。燃料電池電汽車可以解決汽車工業(yè)發(fā)展帶來的環(huán)境與能源問題,為汽車工業(yè)未來發(fā)展帶來新的契機(jī)。然而,燃料電池的高成本問題是動(dòng)力燃料電池大規(guī)模產(chǎn)業(yè)化與商業(yè)化道路上的巨大挑戰(zhàn)。目前,燃料電池所使用的催化劑是Pt 基催化劑。根據(jù)2010年DOE年度報(bào)告,若以現(xiàn)有技術(shù)進(jìn)行燃料電池汽車商業(yè)化,每年車用燃料電池對(duì)Pt資源的需求就高達(dá)1160 t,遠(yuǎn)遠(yuǎn)超過全球Pt 的年產(chǎn)量(約200 t)。從降低成本以及有限鉑資源角度考慮,開發(fā)高活性非貴金屬催化劑勢(shì)在必行[1-2]??諝怆姌O是燃料電池的正極,相對(duì)于負(fù)極氫電極,空氣電極上的氧還原反應(yīng)更為困難。在電流密度高達(dá)2 A·cm-2條件下,氫電極的過電位也不過50 mV。而空氣(O2)電極,在這樣大的電流密度下,即使以Pt 為催化劑,其過電位也達(dá)700~800 mV。因此,燃料電池的電壓損失主要來自于空氣電極。對(duì)氫電極,0.1 mg·cm-2或更少的Pt,即能滿足燃料電池工作需要,燃料電池膜電極(MEA)上鉑的需用量主要消耗在空氣電極[3-6]。近年來,許多研究著眼于提高Pt 基陰極氧還原(ORR)催化劑的穩(wěn)定性、利用率、改進(jìn)電極結(jié)構(gòu)以減少Pt 負(fù)載量,降低燃料電池成本[7]。但根本的出路應(yīng)當(dāng)是開發(fā)可以完全替代鉑的、低成本的、資源豐富的非鉑ORR 催化劑。本文結(jié)合本課題組的研究工作,綜述了燃料電池非鉑氧還原催化劑的最新研究進(jìn)展。
金屬Pd 具有儲(chǔ)量豐富、價(jià)格便宜等優(yōu)點(diǎn),被視為鉑的最理想替代金屬[8-10]。然而,Pd 基催化劑的催化活性遠(yuǎn)不及鉑類催化劑,無法滿足商業(yè)化使用的要求。調(diào)節(jié)Pd 基催化劑的表面電子結(jié)構(gòu)可使其獲得與Pt 基催化劑相當(dāng)?shù)拇呋钚?。通過與過渡金屬如Fe、Ni、Au 等形成Pd 合金是一種有效調(diào)節(jié)Pd 電子結(jié)構(gòu)的方法[11-12]。合金種類以及合金程度顯著影響Pd 的電子結(jié)構(gòu),產(chǎn)生兩種作用相異的效應(yīng),即晶格收縮效應(yīng)和表面配位效應(yīng)。其中,晶格收縮效應(yīng)降低Pd 的d 帶中心、減弱氧的吸附,被認(rèn)為是活性提高的主要原因[11]。近年來,研究人員制備了多種活性組分的高分散鈀基合金催化劑,在催化ORR 中顯示了可與鉑基催化劑相媲美的效果。Adzic 等[13]制備的Pd3Fe/C 催化劑,該催化劑的氧還原半波電位比商業(yè)化Pt/C 催化劑正約20 mV。Ding 等[14]以納米多孔銅作為模板和還原劑合成了納米管狀PdCu 合金,與商業(yè)化Pt/C 和Pd/C 催化劑相比,PdCu 合金催化劑在酸性溶液中表現(xiàn)出更優(yōu)異的ORR 性能和抗甲醇性能。Ferna?ndez 等[15]研究了Pd-Co-Au/C 以及Pd-Ti/C 作為陰極ORR 催化劑在 PEMFC 中的表現(xiàn)。在相同負(fù)載量下,Pd-Co-Au/C 以及Pd-Ti/C 的初始性能表現(xiàn)可與商業(yè)化Pt/C 催化劑相媲美;200 mA·cm-2電流密度下持續(xù)12 h 后,Pd-Co-Au/C 性能明顯衰減,而Pd-Ti/C性能基本沒有變化。Xu 等[16]通過脫除PdTiAl 合金中的Al 制備了具有相互交聯(lián)網(wǎng)狀結(jié)構(gòu)的納米多孔PdTi 合金。該催化劑不僅表現(xiàn)出比Pt/C 更優(yōu)異的氧還原和抗甲醇性能,而且在5000 次循環(huán)伏安(CV)老化實(shí)驗(yàn)中表現(xiàn)出較Pt/C 更優(yōu)異的穩(wěn)定性。DFT 理論研究表明,Ti 與Pd 合金化使Pd 的d 帶中心下降,從而削弱了Pd-O 鍵能。
另外,Pd 的電子結(jié)構(gòu)會(huì)隨暴露的晶面改變而改變。因此,調(diào)控Pd 的納米幾何形態(tài)以暴露不同的晶面也是一種有效調(diào)節(jié)Pd 金屬電子結(jié)構(gòu)的方法[8,17-19]。Kondo 等[20]研究表明催化氧還原反應(yīng)在以下Pd 單晶面上的活性遞減,即Pd(110) 圖1 Pd/C 四面體以及Pd/C 八面體在0.1 mol·L-1 HClO4溶液中的ORR 極化曲線Fig.1 ORR activities of Pd/C cubes and Pd/C octahedra in 0.1 mol·L-1 HClO4 利用載體和金屬納米顆粒之間的電子耦合效應(yīng)也是優(yōu)化金屬納米顆粒電子結(jié)構(gòu)的一種手段。金屬納米顆粒在載體上可以暴露出多種復(fù)合位點(diǎn),包括不同的晶面、邊緣、棱角以及缺陷。這些復(fù)合位點(diǎn)會(huì)與載體產(chǎn)生較強(qiáng)的相互作用,從而對(duì)金屬納米顆粒的電子結(jié)構(gòu)產(chǎn)生較大影響。Schalow 等[22]研究發(fā)現(xiàn),在金屬顆粒Pd 開始氧化時(shí),在Pd 與載體Fe3O4的接觸界面上形成了一層Pd 氧化物,并在載體的作用下穩(wěn)定存在。該界面氧化物可以導(dǎo)致Pd電子狀態(tài)或者是費(fèi)米能級(jí)上升或下降,改變Pd 的電子結(jié)構(gòu)。本課題組[23-24]通過采用具有單片層結(jié)構(gòu)的剝離蒙脫土片(ex-MMT)負(fù)載納米Pd 金屬顆粒,調(diào)節(jié)Pd 催化劑的電子結(jié)構(gòu),增強(qiáng)穩(wěn)定性和提高催化活性。蒙脫土的引入減少了因?yàn)樘驾d體的腐蝕而造成催化金屬從載體脫落和流失的可能性,從而提高了催化劑的穩(wěn)定性。此外,蒙脫土具有優(yōu)異的質(zhì)子傳導(dǎo)能力,可加速質(zhì)子在燃料電池催化層內(nèi)部的傳遞,提高催化活性。電化學(xué)測(cè)試表明,Pd/ex-MMT具有與Pt/C 相似的催化活性(圖2)。理論計(jì)算和實(shí)驗(yàn)數(shù)據(jù)表明催化劑活性、穩(wěn)定性的提高是由于在Pd 金屬顆粒與載體之間的界面上形成了一層界面氧化物PdOx或Pd-O-ex-MMT 價(jià)鍵。這種特殊的結(jié)構(gòu)改變了Pd/ex-MMT 催化劑電子結(jié)構(gòu),使Pd 的d帶寬化,d 帶中心負(fù)移,使其具有更趨近于Pt 的電子結(jié)構(gòu),表現(xiàn)出與Pt 相當(dāng)?shù)腛RR 催化活性以及酸性環(huán)境中良好的穩(wěn)定性,如圖2(a)所示。 圖2 Pd 和Pd/ex-MMT 催化劑中的d 帶結(jié)構(gòu)與d 帶中心以及Pd、Pd/ex-MMT 和Pt/C 在0.1 mol·L-1 HClO4 溶液中的ORR 活性Fig.2 D-band and relative center of Pt, Pd and Pd/ex-MMT; ORR activities of Pd/C, Pt/C and Pd/ex-MMT in 0.1 mol·L-1 HClO4 在眾多非貴金屬催化劑中,過渡金屬-氮-碳化合物(M/N/C)因其具有可觀的ORR 催化活性(在酸性溶液中)、低成本、壽命長(zhǎng)、抗甲醇和環(huán)境友好等特點(diǎn),被認(rèn)為是最具潛力替代鉑基催化劑的非貴金屬燃料電池催化劑之一。自從1964年Jasinski[25]首次報(bào)道過渡金屬卟啉和酞菁能有效催化ORR 后,M/N/C 便吸引了研究者的廣泛關(guān)注。金屬大環(huán)類催化劑具有較高的起始活性但穩(wěn)定性較差[26]。高溫處理后可提高催化劑的穩(wěn)定性,但催化劑易燒結(jié),導(dǎo)致比表面積減小,降低了催化劑的活性。該類催化劑主要以反應(yīng)速率較慢的2 電子過程催化氧還原。Yeager[27]首次報(bào)道了以非-N4 大環(huán)化合物為前軀體高溫?zé)峤庵苽銶/N/C 催化劑用于 ORR。之后各種不同形式的金屬、氮、碳前軀體被開發(fā)和應(yīng)用于制備M/N/C 催化劑。目前該類催化劑使用的氮源主要包括無機(jī)氮源(氨氣,sodium azide)、有機(jī)小分子(acetonitrile, pyrrole, 1-methylimidazole 等)和含氮有機(jī)聚合物(melamine resin,聚苯胺,聚吡咯,聚多巴胺等)[28-42]。與小分子前驅(qū)物相比,含氮有機(jī)聚合物有序化更高,可以在高溫?zé)峤膺^程中指導(dǎo)形成更有序穩(wěn)定的碳基活性層。聚吡咯是最早被應(yīng)用的聚合物,之后研究發(fā)現(xiàn)聚苯胺-衍生的M/N/C 催化劑活性更好且更穩(wěn)定[43]。最近,Wu 等[37]報(bào)道用聚苯胺結(jié)合鐵和鈷的熱處理制備一類M/N/C(圖3)。該類催化劑中催化活性最高的催化劑為PANI-Fe-C,其ORR 半波電位與Pt/C 相差60 mV;穩(wěn)定性最優(yōu)的催化劑為PANI-FeCo-C,其在0.4 V下穩(wěn)定運(yùn)行了700 h。Dodelet 等[33]于2011年報(bào)道了一種金屬框架類作為前驅(qū)體制備的M/N/C,該前驅(qū)物具有優(yōu)異的金屬-有機(jī)配位結(jié)構(gòu),在經(jīng)過兩次熱處理(一次在氮?dú)鈿夥障? h,再次為NH3氣氛中15 min)后,該催化劑表現(xiàn)出了優(yōu)異的催化性能,在0.8 V 下其體積活性高達(dá)230 A·cm-3(iR-free)[33],已經(jīng)非常接近DOE 2020年所設(shè)定的目標(biāo)(300 A·cm-3)。 圖3 PANI-FeCo-C 催化劑的制備Fig.3 Schematic diagram of synthesis of PANI-M-C catalysts 此類催化劑的催化機(jī)理和活性中心尚不明確,一直是研究的重點(diǎn)。目前,有兩條研究主線:① 催化劑表面的氮活性物種直接提供ORR 活性;② 含氮基團(tuán)與金屬配位成為活性中心。雖然此類材料的催化機(jī)理仍存在爭(zhēng)論,但不能否認(rèn)的是,過渡金屬的類型和含量,碳源、氮源的類型與含量,以及熱處理?xiàng)l件和持續(xù)時(shí)間對(duì)催化劑的性能有很大影響。許多研究工作致力于探究制備工藝條件與最終ORR 性能的關(guān)系[44-47]。就不同金屬種類來說,F(xiàn)e和Co 基M/N/C 催化劑活性一般比其他金屬基(如Zn、Ni、Mn、Cu、Cr)M/N/C 催化劑活性高[48]。而且,不同金屬的加入對(duì)活性位點(diǎn)形成所起的作用也不同。如對(duì)于有乙二胺或聚苯胺衍生的Co/N/C 催化劑,其表現(xiàn)的電化學(xué)性能(如起始電位、Tafel斜率)與無金屬摻雜的氮摻雜碳基催化劑類似,這意味著Co 物種的存在可能只是單純輔助氮原子更好地?fù)饺胩季Ц裰校⒉恢苯訁⑴c形成活性中心[41]。與Co 不同的是,F(xiàn)e 物種可以與周圍的氮配位(Fe-Nx),直接參與形成活性中心[30,44]。Kramm等[49]和Kattle 等[50]提出了幾種不同的Fe-Nx物種,其中,F(xiàn)eN4/C 和 N-FeN2+2/C 位點(diǎn)ORR 活性最高。實(shí)驗(yàn)研究表明,同時(shí)加入Fe、Co 物種可以顯著增強(qiáng)催化劑ORR 活性[51]。Xia 等[52]利用DFT 證明對(duì)于聚苯胺衍生的M/N/C 體系,其催化活性衰減次序依次為:CoFe-PANI > Fe-PANI > Co-PANI。這是由于摻入的不同金屬之間產(chǎn)生了協(xié)調(diào)作用,加快了電子向吸附氧物種的轉(zhuǎn)移。Co 的加入可能還降低了催化劑中最高占據(jù)分子軌道(HOMO)-最低占據(jù)分子軌道(LUMO) 帶,使得催化劑更加穩(wěn)定。 除了催化劑機(jī)理不明確,傳統(tǒng)熱解方法制備的M/N/C 還存在孔結(jié)構(gòu)少,比表面積低,暴露的活性位點(diǎn)有限等缺點(diǎn)。在M/N/C 中引入足夠的活性位點(diǎn),最常規(guī)的方法便是通過硬模板或柔模板增加催化劑的比表面積,如Liang 等[53]以硅膠球、介孔硅和蒙脫土為模板,VB12或PANI 為前驅(qū)體,制備了介孔的Fe/Co-N-C 材料,顯著提高了催化劑的比表面積。 在M/N/C 催化劑高溫制備過程中,金屬顆粒通常會(huì)包覆在石墨化碳?xì)ぶ?,而被包覆的金屬?duì)催化活性的貢獻(xiàn)已被探究[54]。包信和等[31,55-56]的一系列研究表明,當(dāng)金屬納米顆粒限域在碳納米管中時(shí)(如圖4 所示,是他們制備的金屬鐵納米粒子包裹在豆莢狀氮摻雜納米管催化劑),金屬顆粒不與酸性介質(zhì)、氧和硫等污染物直接接觸,也不妨礙活化氧分子電催化氧還原反應(yīng),它們之間特殊的電荷轉(zhuǎn)移降低了碳納米管表面的局部功函從而形成ORR 電催化活性中心。本課題組[47]開發(fā)了一種Co-N-C 殼層包覆鈷納米顆粒催化劑(Co@Co-N-C),其中高分散的Co@N-C 和表面Co-N 物種產(chǎn)生的電子效應(yīng)協(xié)同增強(qiáng)了氧還原活性,如圖5 所示。最近,Li 等[57]制備了一種空心球形的石墨碳層包覆Fe3C 納米催化劑。包覆在內(nèi)部的Fe3C 納米顆粒雖然沒有與外界電解液直接接觸,但它們卻使得周圍的石墨化碳層活化而更有利于ORR 的發(fā)生和進(jìn)行,這與包信和等的研究結(jié)果類似。此外,該催化劑表面的氮和金屬含量極少可忽略,卻在酸性和堿性溶液中表現(xiàn)出很好的ORR 活性,為此類包覆型催化劑活性位點(diǎn)的探究提供了新的模型。 圖4 Pod-Fe 催化劑的透射電鏡圖Fig.4 TEM images of Pod-Fe 圖5 Co@Co-N-C 催化機(jī)理Fig.5 Schematic diagram of ORR on Co@Co-N-C 過渡金屬氧化物,尤其是錳基和鈷基氧化物在堿性溶液中表現(xiàn)出很好的催化氧還原活性[58-60]。Dai等[61-62]通過水熱法制備了Co3O4、CoO 納米顆粒并擔(dān)載于氮摻雜碳類載體上(CNT,石墨烯),協(xié)同增強(qiáng)氧還原活性。通過X 射線近邊吸收精細(xì)結(jié)構(gòu)分析可知,該催化劑形成了金屬-碳-氧和金屬-碳-氮共價(jià)鍵,電子由氮傳至金屬氧化物,從而賦予了金屬氧化物好的導(dǎo)電性和電化學(xué)活性。將不同價(jià)態(tài)的過渡金屬氧化物復(fù)合形成尖晶石結(jié)構(gòu)的催化劑是過渡金屬氧化物催化劑研究的重點(diǎn)。Dai 等[63]發(fā)現(xiàn),用Mn3+取代部分 Co3+得到的具有尖晶石結(jié)構(gòu)的MnCo2O4可以顯著增強(qiáng)氧還原活性。Sun 等[64]通過熱解乙酰丙酮鹽前驅(qū)體,油胺油酸作穩(wěn)定劑,制備了單分散、粒徑小于10 nm 的= Fe, Cu, Co, Mn)納米顆粒。這些納米顆粒即便擔(dān)載在傳統(tǒng)碳載體上,也表現(xiàn)出與Pt/C 相當(dāng)?shù)难踹€原催化活性。近期,本課題組[65]開發(fā)了一種新型鈷基催化劑,堿式碳酸鈷(CCH),并發(fā)現(xiàn)催化劑相比于貴金屬催化劑Pt/C 具有更優(yōu)的氧還原催化性能。研究還發(fā)現(xiàn),隨著水熱時(shí)間的延長(zhǎng),所制備的催化劑發(fā)生了明顯的相變和形變,并具有不同的催化活性,如圖 6 所示。其中正交相的堿式碳酸鈷[Co(CO3)0.5(OH)·0.11H2O]同由單斜[Co2(OH)2CO3]和正 v c 交相組成的混合相的堿式碳酸鈷相比具有更高的氧還原活性。 圖6 堿式碳酸鈷催化機(jī)理以及反應(yīng)時(shí)間對(duì)堿式碳酸鈷 在0.1 mol·L-1 KOH 中ORR 活性的影響Fig.6 Schematic diagram of ORR and OER on CCH in presence of carbon powders and influence of reaction time on ORR activity for CCH in 0.1 mol·L-1 KOH 其他金屬氧化物,如TiO2、NbO2和 Ta2O5也具有ORR 催化活性[66-68]。近年來,鈣鈦礦型氧化物因其同時(shí)具有電子和離子導(dǎo)電性,越來越多地用作高溫燃料電池中的氧還原催化劑。鈣鈦礦型氧化物ABO3中稀土元素占據(jù)A 位,過渡金屬占據(jù) B 位。其中,通過陽離子取代很容易調(diào)控Ba0.5Sr0.5Co0.8Fe0.2O3-δ(BSCF5582)-基鈣鈦礦型氧化物組成,BSCF5582 被認(rèn)為是此類材料中最具潛力的氧還原催化劑[69]。Suntivich 等[69]提出鈣鈦礦型氧化物在燃料電池中的氧還原活性與eg(σ*-軌道占據(jù))和A-B-O 型中的B 位密切相關(guān),且eg-填充接近1的鈣鈦礦型氧化物可以表現(xiàn)出最好的氧還原活性。最近,Risch 等[70]采用脈沖激光沉積法制備了BSCF|LSMO|NSTO 催化劑,表現(xiàn)出很好的氧還原和析氧(OER)活性。 過渡金屬硫?qū)倩衔颩-X(其中,M=Co, Ru, Re, 或 Rh,X=S, Se, Te)高溫處理后能形成納米微晶[71],在酸性介質(zhì)中具有高的ORR 催化活性[72-73]。金屬硫化物(如Co9S8)被認(rèn)為是硫?qū)倩衔镏谢钚設(shè)RR最高的一類[74]。DFT 研究表明,在Co9S8中,氧氣的吸附是在硫元素上,且氧氣在(202)晶面上還原的過電勢(shì)與Pt 相當(dāng)[74]。此外,Co1-xS、Co4S3、CoSe2[73,75-77]等在堿性溶液中均可以表現(xiàn)出近4 電子過程,然而在酸性溶液中,這類催化劑通常表現(xiàn)為2 電子過程。Wu 等[78]開發(fā)的 Co9S8-N-C 催化劑,在0.1 mol·L-1NaOH 溶液中,其ORR 活性明顯優(yōu)于Pt/C 催化劑。Wang 等[79]以還原氧化石墨烯負(fù)載Co1-xS 納米顆粒,協(xié)同增強(qiáng)ORR 活性。 過渡金屬氮化物和氧氮化合物由于其較好的導(dǎo)電性和耐腐蝕性也被廣泛應(yīng)用于ORR。表面氮化物的形成可以調(diào)控催化劑的電子結(jié)構(gòu),使得d-帶收縮,電子密度增大更接近費(fèi)米能級(jí)。這樣加快了電子向氧吸附物種的轉(zhuǎn)移,從而使得活性金屬更容易還原氧[80]。之前,4~6 主族的單金屬氮化物/氧氮化合物被廣泛研究[81-84],如ZrOxNy和TaOxNy,它們?cè)诹蛩崛芤褐杏泻芎玫碾娀瘜W(xué)穩(wěn)定性; MoN 和Mo2N 表現(xiàn)出可觀的ORR 活性且反應(yīng)接近4 電子過程。之后研究者們開發(fā)了雙金屬氧氮化合物并發(fā)現(xiàn)它們發(fā)揮了協(xié)同增強(qiáng)的優(yōu)勢(shì)。如碳擔(dān)載雙金屬 Co-W-O-N 催化劑在0.5 mol·L-1H2SO4中ORR 起始電位為 0.749 V, 顯著優(yōu)于單金屬 W 或 Co 氧氮化合物催化劑[85]。最近,Cao 等[86]采用溶液浸漬法合成了CoxMo1-xOyNz催化劑,其在酸中表現(xiàn)出可觀的氧還原活性,其在堿性溶液中活性與Pt/C 相差0.1 V。 非金屬催化劑的研究主要是各種雜原子摻雜的納米碳材料,主要包括硼摻雜、氮摻雜、磷摻雜、硫摻雜以及多原子的雙摻雜或三摻雜[87-101]。研究表明,碳材料摻雜后,無論是否與過渡金屬復(fù)合,都顯示出明顯的氧還原催化活性。目前關(guān)于不同原子摻雜碳材料的催化劑機(jī)理仍不明確。Dai 等[88]認(rèn)為,對(duì)于氮原子摻雜碳材料,由于氮原子電負(fù)性較碳原子大(氮電負(fù)性為3.04;碳電負(fù)性為2.55),它的引入使得鄰近碳原子帶正電荷,這有利于氧氣的吸附從而保障氧還原反應(yīng)的進(jìn)行。然而這種解釋并不適用于電負(fù)性較碳原子小的磷原子和硼原子(磷電負(fù)性為2.19;硼電負(fù)性為2.04)。Hu 等[87]認(rèn)為,無論摻雜原子的電負(fù)性與碳原子相比是大還是小,只要破壞了sp2雜化的碳原子的電中性,生成了利于氧吸附的帶電位點(diǎn)就可以提升催化劑活性。對(duì)于電負(fù)性與碳接近的硫原子(硫電負(fù)性為2.58),Zhang等[102]認(rèn)為其催化活性增強(qiáng)的原因是自旋密度變化改變了表面電子結(jié)構(gòu)。 圖7 氮在石墨結(jié)構(gòu)中摻入位置以及相應(yīng)的結(jié)合能數(shù)據(jù)Fig.7 Schematic representation of common N bonding configurations. 各類雜原子摻雜碳類材料中,氮摻雜碳(NC)研究最多。氮原子的分子結(jié)構(gòu)對(duì)最終催化劑的性能具有至關(guān)重要的影響。摻氮碳材料中,氮有5 種鍵合結(jié)構(gòu),如圖7 所示,分別為石墨氮、吡啶氮、吡喏氮、氨基氮以及氧化氮。哪一種摻氮碳材料氧還原電催化活性最好,目前尚有爭(zhēng)議。吡啶氮摻雜的石墨烯,其ORR 的過程系2 電子還原過程,據(jù)此認(rèn)為吡啶氮不是有效的ORR 催化中心[103]。與此相反,還有發(fā)現(xiàn),酸性條件下催化劑氧還原活性隨吡啶氮含量增加而升高[104];在堿性介質(zhì),其電催化活性隨吡喏氮含量增加而升高[105]。故氮摻雜碳材料的活性中心,須考慮如下要點(diǎn):首先氮鍵合結(jié)構(gòu)不同時(shí),其催化劑的導(dǎo)電性是否處于同一水平;再者催化劑中sp2雜化C 含量、石墨化程度是否一致。通常,石墨氮形成的溫度較高,更有利于碳材料石墨化,也影響著材料的導(dǎo)電性和sp2雜化C 結(jié)構(gòu)。因此,“高石墨氮含量-高ORR 活性”可能與碳基材料的導(dǎo)電性有關(guān)。除了氮的分子結(jié)構(gòu)類型,摻入氮的總含量、碳邊緣位的含量、比表面積等也是影響最終NC 催化劑性能的重要因素。 納米碳材料氮摻雜的方法大致可分為3類[106-108]:①原位摻雜,即在納米碳材料期間摻入氮,如化學(xué)氣相沉積法(CVD),這種方法得到的產(chǎn)品摻雜率很高,但不適用于實(shí)際大規(guī)模批量生產(chǎn);②后摻雜,即合成納米碳材料后,再用含氮原子的前驅(qū)體對(duì)其進(jìn)行后處理,這種方法得到的產(chǎn)品氮摻雜率不高;③直接熱解含氮原子豐富的有機(jī)物,這種方法簡(jiǎn)單易操作,得到的產(chǎn)品摻雜率高,然而由于過高的含氮量,破壞了碳材料原共軛大π 鍵結(jié)構(gòu),使得產(chǎn)品電導(dǎo)率低。Bao 等[109]報(bào)道了大批量高質(zhì)量氮摻石墨烯的方法,如圖8 所示。其采用溶劑熱反應(yīng)將四氯化碳和氮化鋰直接反應(yīng)生成氮摻雜的石墨烯(NG),實(shí)現(xiàn)了克量級(jí)制備氮摻雜石墨烯。 圖8 溶劑熱法氮摻雜石墨烯制備及產(chǎn)品的電鏡照片F(xiàn)ig.8 Schematic representation of solvothermal synthesis of NG and TEM of NG 設(shè)計(jì)、制備含氮量高、導(dǎo)電性好且比表面積大的氮摻雜碳材料是提高氮摻雜類碳材料性能亟需解決的問題。通常采用軟模板或硬模板法可以顯著增加催化劑的比表面積。如通過多孔二氧化硅模板輔助法[110]、熱解具有優(yōu)異金屬配位效應(yīng)的金屬有機(jī)框架化合物(MOFs)或多孔有機(jī)聚合物(POP)制備得到的NC 材料[111-113],氮含量高,且比表面積大,然而在酸性溶液中,它們的氧還原活性與Pt 相比仍相差很遠(yuǎn)。這是因?yàn)?,在酸性介質(zhì)體系中,平面結(jié)構(gòu)的吡啶氮和吡喏氮氧還原電催化更為重要[114-116]。吡啶型和吡咯型的二維平面結(jié)構(gòu)使NG 保持了石墨烯原有的平面共軛大π 鍵結(jié)構(gòu),具有良好的導(dǎo)電性,因而具有優(yōu)異的ORR 催化活性;而石墨型氮為三維空間不平整結(jié)構(gòu),破壞了石墨烯原有的共軛大π鍵,導(dǎo)電性差,ORR 催化活性低,如圖9 所示。如何在高度石墨化的條件下選擇性的合成具有平面構(gòu)型的吡啶氮和吡喏氮(平面氮)并盡可能減少甚至 抑制石墨氮的形成則是獲得高活性O(shè)RR 催化劑的關(guān)鍵。 圖9 石墨氮和平面N 示意圖Fig.9 Schematic representation of quaternary N and planar N 針對(duì)上述問題,本課題組[117]在分子結(jié)構(gòu)的基礎(chǔ)上,認(rèn)識(shí)到“NG 分子結(jié)構(gòu)-NG 電導(dǎo)率-ORR 催化活性”的關(guān)聯(lián),利用層狀材料(LM)的層間限域效應(yīng),通過調(diào)制LM 層間距,在LM 層間插入苯胺單體,層間聚合,然后熱解的方法,獲得平面氮摻雜達(dá)90%以上的NG 材料,如圖10 所示。其催化ORR 的半波電位僅比Pt/C 催化劑落后60 mV,是傳統(tǒng)方法下獲得的NG 材料ORR 催化活性的54 倍,以該材料為正極催化劑的質(zhì)子交換膜燃料電池的輸出功率達(dá)320 mW·cm-2,如圖11 所示。LM 層間近乎封閉的扁平反應(yīng)空間不僅克服了傳統(tǒng)開放體系下合成的NG 以石墨型為主,導(dǎo)電性差,活性低的弊病,而且也克服了開放體系下因摻N 效率低而導(dǎo)致合成NG 成本高的問題。 除了增加活性位點(diǎn)數(shù)量,活性位點(diǎn)充分暴露在三相界面也是非常重要的。氧還原反應(yīng)是一個(gè)多相反應(yīng),涉及氧氣,質(zhì)子、電子和水的傳導(dǎo),因而一個(gè)高效的ORR 催化劑須含有足夠多的小孔以承載活性位點(diǎn),同時(shí)這些小孔還需聯(lián)通至能有效傳輸反應(yīng)氣體、生成水、電子導(dǎo)體以及質(zhì)子導(dǎo)體的中孔或大孔網(wǎng)絡(luò)結(jié)構(gòu)中。然而對(duì)于傳統(tǒng)直接熱解前驅(qū)體的方法,難以控制所制備的催化劑的孔結(jié)構(gòu),導(dǎo)致活性位點(diǎn)難以暴露到可以被ORR 催化反應(yīng)利用的區(qū) 域中[118-120]。 圖10 NG@MMT 制備Fig.10 Schematic representation of NG@MMT synthesis 圖11 NG@MMT 在0.1 mol·L-1 HClO4 中ORR 極化曲線以及以NG@MMT 為陰極催化劑制備的MEA 單電池測(cè)試 極化曲線Fig.11 ORR activity in 0.1 mol·L-1 HClO4 and MEA test of NG@MMT catalyst 圖12 基于形態(tài)控制通過鹽重結(jié)晶方法的示意圖和 MEA 單電池測(cè)試極化曲線Fig.12 Schematic diagram of “Shape Fixing via Salt Recrystallization” method and result of MEA test 圖13 PANI 三維網(wǎng)狀、PANI 納米管、PANI 納米殼 以及其相應(yīng)碳化后產(chǎn)品的掃描電鏡圖Fig.13 SEM images of 3D PANI network, PANI nanotubes, PANI nanoshell, and their corresponding carbonized products 上述本課題組設(shè)計(jì)的扁平納米反應(yīng)器制備平面氮摻雜的石墨烯,可有效地提高催化活性位的密度,增加反應(yīng)界面。但由于缺少傳質(zhì)通道,在制備成膜電極(MEA)后其活性位暴露的概率大大降低,影響了電池性能。在此工作的基礎(chǔ)上,本課題組進(jìn)一步開發(fā)了一種基于形態(tài)控制轉(zhuǎn)換納米聚合物制備高效氧還原碳納米材料催化劑的方法——“NaCl重結(jié)晶固型熱解法”[121],可以有效地使大量的活性位暴露在ORR 催化反應(yīng)的三相界面上,制備過程如圖12 所示。通過對(duì)含氮聚合物無機(jī)鹽水溶液混合物的蒸發(fā)重結(jié)晶,將含氮聚合物固化在無機(jī)鹽NaCl晶體中,利用無機(jī)鹽結(jié)晶的鹽封效應(yīng),避免了傳統(tǒng)直接碳化過程中活性位嚴(yán)重?zé)А⒏呤獡诫s和結(jié)構(gòu)坍塌等問題;避免了傳統(tǒng)模板法模板去除與納米催化劑分離的困難問題;巧妙地將低溫下聚合物的形態(tài)最大限度地保留到高溫碳化后的終極產(chǎn)品,如圖13 所示。此外,由于鹽封局域空間的限域效應(yīng),摻氮石墨烯中以具有二維平面結(jié)構(gòu)吡啶型和吡咯型為主,最大限度地抑制了撐開型石墨氮摻雜型NG;同時(shí),由于鹽封效應(yīng),在碳化過程中NG 內(nèi)部形成大量的氣蝕孔,NG 片邊沿和及內(nèi)孔邊沿的大量存在,有利于吡啶型和吡咯型氮參雜NG 的形成,使活性中心數(shù)量倍增。如圖14、圖15 所示,與沒有微孔生成的對(duì)比樣品相比,以NaCl 固型熱解法制備得到的催化劑其平面氮含量增加了68%,F(xiàn)eNx位點(diǎn)增加了130%。大量的活性位點(diǎn)結(jié)合高效的傳質(zhì)量通道使活性位暴露在三相界面的概率增高從而極大地提高活性位點(diǎn)的利用率。以該材料為正極催化劑的質(zhì)子交換膜燃料電池輸出功率達(dá) 600 mW·cm-2,較之前以扁平納米反應(yīng)器制備平面氮摻雜的石墨烯有大幅提高,為世界領(lǐng)先水平。加速老化實(shí)驗(yàn)顯示該催化劑非常穩(wěn)定。該方法具有廣泛的應(yīng)用性和通用性,并可以有效地控制碳材料的孔結(jié)構(gòu)、活性位點(diǎn)以及納米形貌。 圖14 邊緣位及孔內(nèi)平面氮活性位點(diǎn)示意圖Fig.14 Schematic diagram of planar N active sites on edges and in pores 圖15 邊緣位及孔內(nèi)FeNx 活性位點(diǎn)示意圖Fig.15 Schematic diagram of FeNx active sites on edges and in pores 碳材料之間的復(fù)合也是一種有效制備非金屬催化劑的方法。Chen 等[122]在氧化石墨烯表面,以Fe 催化三聚氰胺熱解,實(shí)現(xiàn)Fe-N 同時(shí)摻雜石墨烯和碳納米管的同步合成路線(N-CNT/N-G),如圖16 所示,該復(fù)合催化劑中納米管分散均勻,管徑均一,其特殊的3D 結(jié)構(gòu)有利于提高傳質(zhì)和電催化活性。最近,Wei 等[123]以FeMo-MgAl 層狀雙氫氧化物為模板,采用CVD 法制備了氮摻雜石墨烯/單壁碳納米管復(fù)合物(NGSHs)。FeMo-MgAl 層狀雙氫氧化物中的Fe 納米顆粒不僅可以作為氮摻雜單壁碳納米管生長(zhǎng)的催化劑,還可以作為氮摻雜石墨烯沉積的基底。以此制備得到的NGSHs 催化劑具有 高比表面積和高石墨化程度。研究發(fā)現(xiàn),NGSHs復(fù)合物表現(xiàn)出比其單組分更好的氧還原活性,因而氮摻雜石墨烯與單壁碳納米管的復(fù)合很有可能協(xié)同增強(qiáng)最終氧還原活性。 圖16 N-CNT/N-G 制備路線及其在0.1 mol·L-1 KOH 中氧還原活性Fig.16 Schematic illustration of formation of N-CNT/N-G and ORR activity of N-CNT/N-G in 0.1 mol·L-1 KOH 值得指出的是,大多數(shù)使用的碳材料在制備過程中都有一些金屬的參與,如通過 Hummers 法制備氧化石墨烯,CVD 法制備碳納米管和以生物或自然材料為前軀體或模板制備碳材料。以Hummers 法制備氧化石墨烯為例,最終石墨烯產(chǎn)品中的金屬雜質(zhì)可達(dá)到整個(gè)材料的2%(質(zhì)量分?jǐn)?shù))[124-129]。這些殘留在sp2碳材料中的金屬雜質(zhì)包括Fe、Ni、Co、 Mo、Mn、V 和 Cr,它們可以很大程度上影響最終碳材料的電化學(xué)性能[130-134]。因而,在制備過程卷入的痕量金屬(trace metal)對(duì)最終催化劑的氧還原活性的影響是不能忽略的。Masa 等[135]證明了無定形碳中的痕量金屬殘余對(duì)ORR 活性是有貢獻(xiàn)的。研究表明,在整個(gè)制備周期不涉及任何金屬參與的非金屬催化劑,其ORR 活性低于制備時(shí)有少量金屬參與的催化劑。而且,加入低至0.05% 含量的Fe 就會(huì)對(duì)最終ORR 活性和選擇性有很大影響。最近,Pumera 等[136]研究了痕量金屬雜質(zhì)對(duì)雜原子摻雜石墨烯ORR 性能的影響。為了探究錳基金屬雜質(zhì)的影響,他們采用Hummers 氧化法(得到的產(chǎn)品標(biāo)記為G-HU) 和Staudenmaier 氧化法(利用氯酸鹽氧化劑制備肼還原的石墨烯,得到的產(chǎn)品標(biāo)記為G-ST)制備兩組不同的石墨烯材料,并且利用耦合等離子質(zhì)譜法 (ICP-MS)分析制備原料及產(chǎn)品中的金屬雜質(zhì)含量。研究結(jié)果表明,富含錳基雜質(zhì)(>8000 mg·kg-1)的G-HU 催化劑的氧還原起始電位比含有少量錳基雜質(zhì)(約18 mg·kg-1)的G-ST 催化劑正50 mV,有力證明了錳基雜質(zhì)的ORR 催化作用。此外,即便是錳基雜質(zhì)含量低至18 mg·kg-1(0.0018%,質(zhì)量分?jǐn)?shù)),G-ST 催化劑表現(xiàn)的ORR 電位比裸露的玻碳電極(GC)正80 mV,進(jìn)一步證明痕量金屬雜質(zhì)足以改變石墨烯材料的氧還原電催化性能。 (1)由于鈀具有與鉑相媲美的催化性質(zhì),且Pd儲(chǔ)量遠(yuǎn)高于Pt,因而開發(fā)高效Pd 基催化劑是替代Pt,降低商業(yè)化成本的有效途徑。然而,迄今為止,在酸性條件下,Pd 基催化劑的活性和穩(wěn)定性很難與鉑基催化劑相當(dāng)。此外,由于需求/價(jià)格波動(dòng)的關(guān)系,用Pd 基催化劑完全替代Pt 不能從根本上擺脫貴金屬的資源限制。 (2)非貴金屬催化劑和非金屬催化劑完全擺脫了對(duì)貴金屬的依賴。在眾多的非貴金屬催化劑中,包含或不包含過渡金屬的氮摻雜碳基催化劑 (M/N/C 或 NC) 表現(xiàn)出可觀的ORR 催化活性。盡管對(duì)于金屬物種是否直接參與形成活性中心仍存在爭(zhēng)議,但碳結(jié)構(gòu)中氮原子的摻入對(duì)提高ORR 活性的作用是不可否認(rèn)的?;诋?dāng)前對(duì)非鉑催化劑的理論認(rèn)識(shí)和實(shí)驗(yàn)探究,非鉑催化劑的催化活性已有大幅提高,但其穩(wěn)定性仍與Pt 基催化劑有很大差距。探究非金屬催化劑的活性與原子組成、電子構(gòu)型、表面形貌的構(gòu)效關(guān)系,結(jié)合理論計(jì)算在分子、電子水平確定非金屬催化劑的活性位點(diǎn),開發(fā)提高活性位密度的技術(shù),構(gòu)筑高效新型非鉑催化劑結(jié)構(gòu),提高催化劑的穩(wěn)定性,是未來非鉑氧還原催化劑研究發(fā)展的主要方向。 [1]Yi Baolian(衣寶廉).Fuel Cells—Principle, Technologies and Applications (燃料電池——原理·技術(shù)·應(yīng)用)[M].Beijing: Chemical Industry Press, 2003. [2]Bashyam R, Zelenay P.A class of non-precious metal composite catalysts for fuel cell [J].Nature, 2006, 443: 63-66. [3]Xiong W, Du F, Liu Y.3-D carbon nanotube structures used as high performance catalyst for oxygen reduction reaction [J].J.Am.Chem.Soc., 2010, 132: 15839-15841. [4]Snyder J, Fujita T, Chen M W.Oxygen reduction in nanoporous metal-ionic liquid composite electrocatalysts [J].Nat.Mater., 2010, 9: 904-907. [5]Lim B, Jiang M, Cho E C.Pd-Pt bimetallic nanodendrites with high activity for oxygen reduction [J].Science, 2009, 324: 1302-1305. [6]Chen Z W, Waje M, Li W Z.Supportless Pt and PtPd nanotubes as electrocatalysts for oxygen-reduction reactions [J].Angew.Chem.Int.Ed., 2007, 46: 4060-4063. [7]Nie Y, Li L, Wei Z D.Recent advancements in Pt and Pt-free catalysts for oxygen reduction reaction [J].Chem.Soc.Rev., 2015,44: 2168-2201. [8]Jukk K, Alexeyeva N, Ritslaid P, Kozlova J, Sammelselg V, Tammeveski K.Electrochemical reduction of oxygen on heat-treated Pd nanoparticle/multi-walled carbon nanotube composites in alkaline solution [J].Electrocatalysis, 2013, 4(1): 42-48. [9]Sha Y, Yu T H, Merinov B V.Oxygen hydration mechanism for the oxygen reduction reaction at Pt and Pd fuel cell catalysts [J].J.Phys.Chem.Lett., 2011, 2(6): 572-576. [10]Antolini E.Palladium in fuel cell catalysis [J].Energy Environ.Sci., 2009, 2(9): 915-931. [11]Suo Y, Zhuang L, Lu J T.First-principles considerations in the design of Pd-alloy catalysts for oxygen reduction [J].Angew.Chem.Int.Ed., 2007, 46(16): 2862-2864. [12]Wei Y C, Liu C W, Wang K W.Improvement of oxygen reduction reaction and methanol tolerance characteristics for PdCo electrocatalysts by Au alloying and CO treatment [J].Chem.Commun., 2011, 47(43): 11927-11929. [13]Shao M H, Sasaki K, Adzic R R.Pd-Fe nanoparticles as electrocatalysts for oxygen reduction [J].J.Am.Chem.Soc., 2006, 128(11): 3526-3527. [14]Xu C, Zhang Y, Wang L, Xu L, Bian X, Ma H, Ding Y.Nanotubular mesoporous PdCu bimetallic electrocatalysts toward oxygen reduction reaction [J].Chem.Mater., 2009, 21(14): 3110-3116. [15]Ferna?ndez Jose? L, Raghuveer Vadari, Manthiram Arumugam, Bard Allen J.Pd-Ti and Pd-Co-Au electrocatalysts as a replacement for platinum for oxygen reduction in proton exchange membrane fuel cells [J].J.Am.Chem.Soc.,2005, 127(38) : 13100-13101. [16]Liu Y, Xu C.Nanoporous PdTi alloys as non-platinum oxygen-reduction reaction electrocatalysts with enhanced activity and durability [J].ChemSusChem, 2013, 1(6): 78-84. [17]Shao M, Yu T, Odell J H.Structural dependence of oxygen reduction reaction on palladium nanocrystals [J].Chem.Commun., 2011, 47(23): 6566-6568. [18]Shao M, Odell J, Humbert M.Electrocatalysis on shape-controlled palladium nanocrystals: oxygen reduction reaction and formic acid oxidation [J].J.Phys.Chem.C, 2013, 117(8): 4172-4180. [19]Zhang L, Hou F, Tan Y W.Shape-tailoring of CuPd nanocrystals for enhancement of electro-catalytic activity in oxygen reduction reaction [J].Chem.Commun., 2012, 48(57): 7152-7154. [20]Kondo S, Nakamura M, Maki N, Hoshi N.Active sites for the oxygen reduction reaction on the low and high index planes of palladium [J].J.Phys.Chem.C, 2009, 113(29) : 12625-12628. [21]Xiao L, Zhuang L, Liu Y, Lu J, Abruna H D.Activating Pd by morphology tailoring for oxygen reduction [J].J.Am.Chem.Soc., 2009, 131(2) : 602-608. [22]Schalow T, Brandt B, Starr D E, Laurin M.Size-dependent oxidation mechanism of supported Pd nanoparticles [J].Angew.Chem.Int.Ed., 2006, 45(22): 3693-3697. [23]Ding W, Xia M, Wei Z, Wan L.Enhanced stability and activity with Pd-O junction formation and electronic structure modification of palladium nanoparticles supported on exfoliated montmorillonite for the oxygen reduction reaction [J].Chem.Commun., 2014, 50: 6660-6663. [24]Xia M R, Ding W, Wei Z D.Anchoring effect of exfoliated-montmorillonite-supported Pd catalyst for the oxygen reduction reaction [J].J.Phys.Chem.C, 2013, 117 (20) : 10581-10588. [25]Jasinski R.A new fuel cell cathode catalyst [J].Nature, 1964, 201: 1212-1213. [26]Beck F.Redox mechanism of chelate-catalyzed oxygen cathode [J].J.Appl.Electrochem., 1977, 7: 239-245. [27]Yeager E.Electrocatalysts for O2reduction [J].Electrochim.Acta, 1984, 29(11): 1527-1537. [28]Liu H, Song C, Tang Y, Zhang J.High-surface-area CoTMPP/C synthesized by ultrasonic spray pyrolysis for PEM fuel cell electrocatalysts [J].Electrochim.Acta, 2007, 52(13) : 4532-4538. [29]Ren Qizhi(任奇志), Ma Xiaoxia(麻曉霞), Xie Xianyu(謝先宇),Yan Tao(閻陶), Ma Zifeng (馬紫峰).Heat-treated metalloporphyrin compounds supported on different carbons as electrocatalyst for oxygen reduction [J].Journal of Chemical Industry and Engineering(China)(化工學(xué)報(bào)), 2006, 57(11): 2597-2603. [30]Lefèvre M, Proietti E, Jaouen F, Dodelet J P.Iron-based catalysts with improved oxygen reduction activity in polymer electrolyte fuel cells [J].Science, 2009, 324: 71-74. [31]Deng D, Yu L, Chen X, Wang G, Jin L, Pan X, Deng J, Sun G, Bao X.Iron encapsulated within pod-like carbon nanotubes for oxygen reduction reaction [J].Angew.Chem.Int.Ed., 2013, 52(1): 371-375. [32]Wan Shuwei(萬術(shù)偉), Zhang Jing(張靖), Deng Peng (鄧棚).Research progress of non-platinum Fe/N/C and Co/N/C cathode electrocatalyst for fuel cell [J].Chinese Journal of Power Sources (電源技術(shù)), 2010, 34(10): 1087-1092. [33]Proietti E, Jaouen F, Lefèvre M, Larouche N, Tian J, Dodelet J, Herranz J P.Iron-based cathode catalyst with enhanced power density in polymer electrolyte membrane fuel cells [J].Nat.Commun., 2011, 2: 416. [34]Xiao H, Shao Z G, Zhang G, Gao Y, Lu W, Yi B.Fe-N-carbon black for the oxygen reduction reaction in sulfuric acid [J].Carbon, 2013, 57: 443-451. [35]Wohlgemuth S A, Fellinger T P, J?ker P.Tunable nitrogen-doped carbon aerogels as sustainable electrocatalysts in the oxygen reduction reaction [J].Journal of Materials Chemistry A, 2013, 1(12): 4002-4009. [36]Su P, Xiao H, Zhao J, Yao Y, Shao Z, Li C, Yang Q.Nitrogen-doped carbon nanotubes derived from Zn-Fe-ZIF nanospheres and their application as efficient oxygen reduction electrocatalysts with in situ generated iron species [J].Chem.Sci., 2013, 4: 2941-2946. [37]Wu G, More K L, Johnston C M, Zelenay P.High-performance electrocatalysts for oxygen reduction derived from polyaniline, iron, and cobalt [J].Science, 2011, 332: 443-447. [38]Zhang P, Sun F, Xiang Z, Shen Z, Yun J, Cao D.ZIF-derived in situ nitrogen-doped porous carbons as efficient metal-free electrocatalysts for oxygen reduction reaction [J].Energy Environ.Sci., 2014, 7: 442-450. [39]Wu Z S, Chen L, Liu J, Parvez K, Liang H, Shu J, Sachdev H, Graf R, Feng X, Müllen K.High-performance electrocatalysts for oxygen reduction derived from cobalt porphyrin-based conjugated mesoporous polymers [J].Adv.Mater., 2013, 26(9) : 1450-1455. [40]Lee J S, Park G S, Kim S T.A highly efficient electrocatalyst for the oxygen reduction reaction: N-doped ketjenblack incorporated into Fe/Fe3C-functionalized melamine foam [J].Angewandte Chemie, 2013, 125(3): 1060-1064. [41]Wu G, Johnston C M, Mack N H, Artyushkova K, Ferrandon M, Nelson M, Lezama-Pacheco J S, Conradson S D, More K L, Myers D J, Zelenay P.Synthesis-structure-performance correlation for polyaniline-Me-C non-precious metal cathode catalysts for oxygen reduction in fuel cells [J].J.Mater.Chem., 2011, 21: 11392- 11405. [42]Ai K, Liu Y, Ruan C, Lu L, Lu G.Sp2C-dominant N-doped carbon sub-micrometer spheres with a tunable size: a versatile platform for highly efficient oxygen-reduction catalysts [J].Adv.Mater., 2013, 25(7): 998-1003. [43]Shao M H, Adzic R R.Pd-Fe nanoparticles as electrocatalysts for palladium alloy electrocatalysts for oxygen reduction [J].Langmuir, 2006, 22: 10409-10415. [44]Li Shang(李賞), Zhou Yanfang(周彥方), Qiu Peng(邱鵬), et al.Preparation of Co-based non-noble metal catalyst and its electrocatalytic activity for oxygen reduction.[J].Chinese Sci.Bull.(科學(xué)通報(bào)), 2009, 54(7): 881-887. [45]Wu G, Zelenay P.Nitrogen-doped graphene-rich catalysts derived from heteroatom polymers for oxygen reduction in nonaqueous lithium—O2battery cathodes [J].ACS Nano, 2012, 6(11): 9764-9776. [46]Zhang Yuhui(張玉暉), Yi Qingfeng(易清風(fēng)).Effect of Fe/Co mass ratio on activity of non-noble metal catalyst for oxygen reduction reaction.[J].CIESC Journal (化工學(xué)報(bào)), 2014, 65(6): 2113-2119. [47]Wang Y, Nie Y, Wei Z D.Unification of catalytic oxygen reduction and hydrogen evolution reactions: highly dispersive Co nanoparticles encapsulated inside Co and nitrogen co-doped carbon [J].Chemical Communications, 2015, DOI: 10.1039/c5cc02400e. [48]Ohms D, Herzog S, Franke R, Neumann V, Wiesener K, Gamburcev S , Kaisheva A, Iliev I.Influence of metal ions on the electrocatalytic oxygen reduction of carbon materials prepared from pyrolyzed polyacrylonitrile [J].J.Power Sources, 1992, 38(3): 327-334. [49]Kramm U I, Dodelet J P.Structure of the catalytic sites in Fe/N/C-catalysts for O2-reduction in PEM fuel cell [J].Phys.Chem.Chem.Phys., 2012, 14: 11673-11688. [50]Kattel Shyam, Wang Guofeng.Reaction pathway for oxygen reduction on FeN4embedded graphene [J].J.Phys.Chem.Lett., 2014, 5(3): 452-456. [51]Nallathambi V, Lee J W, Kumaraguru S P, Wu G.Development of high performance carbon composite catalyst for oxygen reduction reaction in proton exchange membrane fuel cells [J].J.Power Sources, 2008, 183(1): 34-42 . [52]Chen X, Sun S, Xia D.DFT study of polyaniline and metal composites as nonprecious metal catalysts for oxygen reduction in fuel cells [J].J.Phys.Chem.C, 2012, 116(43): 22737-22742. [53]Liang H W, Feng X, Müllen K.Mesoporous metal—nitrogen-doped carbon electrocatalysts for highly efficient oxygen reduction reaction [J].J.Am.Chem.Soc., 2013, 135(43): 16002-16005. [54]Faubert G, Cote R, Dodelet J P, Lefèvre M, Bertrand P.Oxygen reduction catalysts for polymer electrolyte fuel cells from the pyrolysis of FeIIacetate adsorbed on 3,4,9,10-perylenetetracarboxylic dianhydride [J].Electrochim.Acta, 1999, 44(15): 2589-2603. [55]Zhang F, Pan X, Hu Y, Yu L, Chen X, Jiang P, Zhang H, Deng S, Zhang J, Bolin T B, Zhang S, Huang Y, Bao X.Tuning the redox activity of encapsulated metal clusters via the metallic and semiconducting character of carbon nanotube [J].Acad.Sci.USA , 2013, 110(37): 14861-14866. [56]Chen W, Fan Z, Pan X, Bao X.Effect of confinement in carbon nanotubes on the activity of Fischer-Tropsch iron catalyst [J].J.Am.Chem.Soc., 2008, 130(29): 9414-9419. [57]Hu Y, Xing W, Li Q.Hollow spheres of iron carbide nanoparticles encased in graphitic layers as oxygen reduction catalysts [J].Angew.Chem.Int.Ed., 2014, 53(14): 3675-3679. [58]Wu G, Li N, Zhou D R.Anodically electrodeposited Co+Ni mixed oxide electrode: preparation and electrocatalytic activity for oxygen evolution in alkaline media [J].J.Solid State Chem., 2004, 177(10): 3682-3692. [59]Liang Y, Dai H.Co3O4nanocrystals on graphene as a synergistic catalyst for oxygen reduction reaction [J].Nature Materials, 2011, 10: 780-786. [60]Liang Y, Dai H.Covalent hybrid of spinel manganese-cobalt oxide and graphene as advanced oxygen reduction electrocatalysts [J].J.Am.Chem.Soc., 2012, 134(7): 3517-3523. [61]Liang Y Y, Wang H L, D P, Chang Wesley, Hong G S, Li Y G, G M, Xie L, Zhou J, Wang J, Regier Tom Z, Wei F, Dai H.Oxygen reduction electrocatalyst based on strongly coupled cobalt oxide nanocrystals and carbon nanotubes [J].J.Am.Chem.Soc., 2012, 134 (38): 15849-15857. [62]Liang Y, Li Y, Wang H, Zhou J, Wang J, Regier T, Dai H.Co3O4nanocrystals on graphene as a synergistic catalyst for oxygen reduction reaction [J].Nat.Mater., 2011, 10: 780-786. [63]Liang Y, Li Y, Wang H, Zhou J, Wang J, Regier T, Dai H.Covalent hybrid of spinel manganese-cobalt oxide and graphene as advanced oxygen reduction electrocatalysts [J].J.Am.Chem.Soc., 2012, 134(7): 3517-3523. [64]Zhu H, Zhang S, Huang Y, Wu L, Sun S.Monodisperse MxFe3-xO4(M = Fe, Cu, Co, Mn) nanoparticles and their electrocatalysis for oxygen reduction reaction [J].Nano Lett., 2013, 13(6): 2947-2951. [65]Wang Yao, Ding Wei, Chen Siguo, Nie Yao, Xiong Kun, Wei Zidong.Cobalt carbonate hydroxide/C: an efficient dual electrocatalyst for oxygen reduction/evolution reactions [J].Chem.Commun., 2014, 50: 15529-15532. [66]Wu G, Zelenay P.Titanium dioxide-supported non-precious metal oxygen reduction electrocatalyst [J].Chem.Commun., 2010, 46: 7489-7491. [67]Sasaki K, Adzic R R.Niobium oxide-supported platinum ultra-low amount electrocatalysts for oxygen reduction [J].Phys.Chem.Chem.Phys., 2008, 10: 159-167. [68]Imai H.Structural defects working as active oxygen-reduction sites in partially oxidized Ta-carbonitride core-shell particles probed by using surface-sensitive conversion-electron-yield X-ray absorption spectroscopy[J]Appl.Phys.Lett., 2010, 96(19): 191905. [69]Suntivich J, Gasteige H A, Yabuuchi N, Nakanishi H, Goodenough J B, Shao-Horn Y.A perovskite oxide optimized for oxygen evolution catalysis from molecular orbital principles [J].Nat.Chem., 2011, 334 (6061): 1383-1385. [70]Risch M, Horn Y S.La0.8Sr0.2MnO3ˉδdecorated with Ba0.5Sr0.5Co0.8Fe0.2O3ˉδ: a bifunctional surface for oxygen electrocatalysis with enhanced stability and activity [J].J.Am.Chem.Soc., 2014, 136 (14): 5229-5232. [71]Feng Y J, Alonso-Vante N.Nonprecious metal catalysts for the molecular oxygen-reduction reaction [J].Phys.Status.Solidi.B, 2008, 245(9): 1792-1806. [72]Behret H, Binder H, Sandstede G.Electrocatalytic oxygen reduction with thiospinels and other sulphides of transition metals [J].Electrochim.Acta, 1975, 20(2): 111-117. [73]Feng Y J, He T, Alonso-Vante N.In situ free-surfactant synthesis and ORR-electrochemistry of carbon-supported Co3S4and CoSe2nanoparticles [J].Chem.Mater., 2007, 20(1): 26-28. [74]Sidik R A, Anderson A B.Co9S8as a catalyst for electroreduction of O2: quantum chemistry predictions [J].The Journal of Physical Chemistry B, 2006, 110(2): 936-941. [75]Ganesan P, Prabu M, Sanetuntikul J, Shanmugam S.Cobalt sulfide nanoparticles grown on nitrogen and sulfur codoped graphene oxide: an efficient electrocatalyst for oxygen reduction and evolution reactions [J].ACS Catal., 2015, 5 (6): 3625-3637. [76]Feng Y J, He T, Alonso-Vante N.Carbon-supported CoSe2nanoparticles for oxygen reduction reaction in acid Medium [J].Fuel Cells, 2010, 10(1): 77-83. [77]Zhou Y X, Yao H B, Wang Y.Hierarchical hollow Co9S8microspheres: solvothermal synthesis, magnetic, electrochemical, and electrocatalytic properties [J].Chemistry-A European Journal, 2010, 16(39): 12000-12007. [78]Wu G, Chung H T, Nelson M.Graphene-riched Co9S8-NC non-precious metal catalyst for oxygen reduction in alkaline media [J].ECS Transactions, 2011, 41(1): 1709-1717. [79]Wang H, Liang Y, Li Y.Co1ˉxS-graphene hybrid: a high-performance metal chalcogenide electrocatalyst for oxygen reduction [J].Angewandte Chemie International Edition, 2011, 50(46): 10969- 10972. [80]Ham D J, Lee J S.Transition metal carbides and nitrides as electrode materials for low temperature fuel cells [J].Energies, 2009, 2(4): 873-899. [81]Zhong H, Zhang H, Liu G.A novel non-noble electrocatalyst for PEM fuel cell based on molybdenum nitride [J].Electrochemistry Communications, 2006, 8(5): 707-712. [82]Xia D, Liu S, Wang Z.Methanol-tolerant MoN electrocatalyst synthesized through heat treatment of molybdenum tetraphenylporphyrin for four-electron oxygen reduction reaction [J].Journal of Power Sources, 2008, 177(2): 296-302. [83]Kim J H, Ishihara A, Mitsushima S.Catalytic activity of titanium oxide for oxygen reduction reaction as a non-platinum catalyst for PEFC [J].Electrochimica Acta, 2007, 52(7): 2492-2497. [84]Ishihara A, Lee K, Doi S.Tantalum oxynitride for a novel cathode of PEFC [J].Electrochemical and Solid-State Letters, 2005, 8(4): A201-A203. [85]Ando T, Izhar S, Tominaga H.Ammonia-treated carbon-supported cobalt tungsten as fuel cell cathode catalyst [J].Electrochimica Acta, 2010, 55(8): 2614-2621. [86]Cao B, Veith G M, Diaz R E.Cobalt molybdenum oxynitrides: synthesis, structural characterization, and catalytic activity for the oxygen reduction reaction [J].Angewandte Chemie, 2013, 125(41): 10953-10957. [87]Yang L, Jiang S, Zhao Y, Hu Z.Boron-doped carbon nanotubes as metal-free electrocatalysts for the oxygen reduction reaction [J].Angew.Chem.Int, Ed., 2011, 50(31): 7132-7135. [88]Gong K, Du F, Xia Z, Dai L.Nitrogen-doped carbon nanotube arrays with high electrocatalytic activity for oxygen reduction [J].Science, 2009, 323(5915): 760-764. [89]Qu L, Liu Y, Baek J B.Nitrogen-doped graphene as efficient metal-free electrocatalyst for oxygen reduction in fuel cells [J].ACS Nano, 2010, 4(3): 1321-1326. [90] Yu D, Zhang Q, Dai L.Highly efficient metal-free growth of nitrogen-doped single-walled carbon nanotubes on plasma-etched substrates for oxygen reduction [J].Journal of the American Chemical Society, 2010, 132(43): 15127-15129. [91] Sheng Z H, Shao L, Chen J J.Catalyst-free synthesis of nitrogen-doped graphene via thermal annealing graphite oxide with melamine and its excellent electrocatalysis [J].ACS Nano, 2011, 5(6): 4350-4358. [92] Liu R, Wu D, Feng X.Nitrogen-doped ordered mesoporous graphitic arrays with high electrocatalytic activity for oxygen reduction [J].Angewandte Chemie, 2010, 122(14): 2619-2623. [93] Xiong C, Wei Z, Hu B.Nitrogen-doped carbon nanotubes as catalysts for oxygen reduction reaction [J].Journal of Power Sources, 2012, 215: 216-220. [94] Yang Z, Yao Z, Li G.Sulfur-doped graphene as an efficient metal-free cathode catalyst for oxygen reduction [J].ACS Nano, 2011, 6(1): 205-211. [95] Yang D S, Bhattacharjya D, Inamdar S.Phosphorus-doped ordered mesoporous carbons with different lengths as efficient metal-free electrocatalysts for oxygen reduction reaction in alkaline media [J].Journal of the American Chemical Society, 2012, 134(39): 16127-16130. [96] Liu Z W, Peng F, Wang H J.Phosphorus-doped graphite layers with high electrocatalytic activity for the O2reduction in an alkaline medium [J].Angewandte Chemie, 2011, 123(14): 3315-3319. [97] Sun X, Zhang Y, Song P.Fluorine-doped carbon blacks: highly efficient metal-free electrocatalysts for oxygen reduction reaction [J].ACS Catalysis, 2013, 3(8): 1726-1729. [98] Choi C H, Park S H, Woo S I.Phosphorus-nitrogen dual doped carbon as an effective catalyst for oxygen reduction reaction in acidic media: effects of the amount of P-doping on the physical and electrochemical properties of carbon [J].Journal of Materials Chemistry, 2012, 22(24): 12107-12115. [99] Liang J, Jiao Y, Jaroniec M.Sulfur and nitrogen dual-doped mesoporous graphene electrocatalyst for oxygen reduction with synergistically enhanced performance [J].Angewandte Chemie International Edition, 2012, 51(46): 11496-11500. [100]Zheng Y, Jiao Y, Ge L.Two-step boron and nitrogen doping in graphene for enhanced synergistic catalysis [J].Angewandte Chemie, 2013, 125(11): 3192-3198. [101]Wang S, Zhang L, Xia Z.BCN graphene as efficient metal-free electrocatalyst for the oxygen reduction reaction [J].Angewandte Chemie International Edition, 2012, 51(17): 4209-4212. [102]Zhang L, Xia Z.Mechanisms of oxygen reduction reaction on nitrogen-doped graphene for fuel cells [J].The Journal of Physical Chemistry C, 2011, 115(22): 11170-11176. [103]Luo Z, Lim S, Tian Z.Pyridinic N doped graphene: synthesis, electronic structure, and electrocatalytic property [J].Journal of Materials Chemistry, 2011, 21(22): 8038-8044. [104]Rao C V, Cabrera C R, Ishikawa Y.In search of the active site in nitrogen-doped carbon nanotube electrodes for the oxygen reduction reaction [J].The Journal of Physical Chemistry Letters, 2010, 1(18): 2622-2627. [105]Unni S M, Devulapally S, Karjule N.Graphene enriched with pyrrolic coordination of the doped nitrogen as an efficient metal-free electrocatalyst for oxygen reduction [J].Journal of Materials Chemistry, 2012, 22(44): 23506-23513. [106]Jin Z, Yao J, Kittrell C.Large-scale growth and characterizations of nitrogen-doped monolayer graphene sheets [J].ACS Nano, 2011, 5(5): 4112-4117. [107]Gao F, Zhao G L, Yang S.Nitrogen-doped fullerene as a potential catalyst for hydrogen fuel cells [J].Journal of the American Chemical Society, 2013, 135(9): 3315-3318. [108]Zhao Y, Watanabe K, Hashimoto K.Self-supporting oxygen reduction electrocatalysts made from a nitrogen-rich network polymer [J].J.Am.Chem.Soc., 2012, 134 (48): 19528-19531. [109]Deng D H, Pan X L,Yu L, et al.Toward N-doped graphene via solvothermal synthesis [J].Chem.Mater., 2011, 23(5): 1188-1193. [110]Liu R L, Wu D Q, Feng X L, Müllen K.Nitrogen-doped ordered mesoporous graphitic arrays with high electrocatalytic activity for oxygen reduction [J].Angewandte Chemie, 2011, 122(14): 2619- 2623. [111]Proietti E, Jaouen F, Lefèvre M.Iron-based cathode catalyst with enhanced power density in polymer electrolyte membrane fuel cells [J].Nat.Commun., 2011, 2: 416. [112]Yuan S, Shui J L, Grabstanowicz L.A highly active and support-free oxygen reduction catalyst prepared from ultrahigh-surface-area porous polyporphyrin [J].Angew.Chem., 2013, 125(32): 8507-8511. [113]Tian J, Morozan A, Sougrati M T.Optimized synthesis of Fe/N/C cathode catalysts for PEM fuel cells: a matter of iron-ligand coordination strength [J].Angew.Chem.Int.Ed., 2013, 52(27): 6867. [114]Kundu S, Nagaiah T C, Xia W.Electrocatalytic activity and stability of nitrogen-containing carbon nanotubes in the oxygen reduction reaction [J].The Journal of Physical Chemistry C, 2009, 113(32): 14302-14310. [115]Dorjgotov A, Ok J, Jeon K Y.Activity and active sites of nitrogen-doped carbon nanotubes for oxygen reduction reaction [J].J.Appl.Electrochem., 2013, 43: 387-397. [116]Sidik R A, Anderson A B, Subramanian N P.O2reduction on graphite and nitrogen-doped graphite: experiment and theory [J].The Journal of Physical Chemistry B, 2006, 110(4): 1787-1793. [117]Ding W, Wei Z, Chen S.Space-confinement-induced synthesis of pyridinic and pyrrolic-nitrogen-doped graphene for the catalysis of oxygen reduction [J].Angewandte Chemie, 2013, 125(45): 11971-11975. [118]Ignaszak A, Ye S, Gyenge E.A study of the catalytic interface for O2electroreduction on Pt: the interaction between carbon support meso/microstructure and ionomer (Nafion) distribution [J].J.Phys.Chem.C, 2008, 113(1): 298-307. [119]Antolini E.Carbon supports for low-temperature fuel cell catalysts [J].Appl.Catal.B: Environ., 2009, 88(1): 1-24. [120]Jaouen F, Proietti E, Lefèvre M.Recent advances in non-precious metal catalysis for oxygen-reduction reaction in polymer electrolyte fuel cells [J].Energy Environ.Sci., 2011, 4(1): 114-130. [121]Ding W, Wei Z D.Shape fixing via salt recrystallization: a morphology-controlled approach to convert nanostructured polymer to carbon nanomaterial as a highly active catalyst for oxygen reduction reaction [J].J.Am.Chem.Soc., 2015, 137 (16): 5414-5420. [122]Zhang S M , Zhang H Y, Chen S L, et al.Fe-N doped carbonnanotube/graphene composite: facile synthesis and superior electrocatalytic activity [J].Journal of Materials Chemistry A, 2013, 1: 3302-3308. [123]Tian G L, Zhao M Q, Yu D, Wei F.Graphene hybrids: nitrogen-doped graphene/carbon nanotube hybrids: in situ formation on bifunctional catalysts and their superior electrocatalytic activity for oxygen evolution/reduction reaction [J].Small, 2014, 10(11): 2113-2113. [124]Liu S, Loper C R Kish.A source of crystalline graphite [J].Carbon, 1991, 29(8): 1119-1124. [125]Mayer H K.Elemental analysis of graphite//The American Carbon Society’s 24th Biennial Conference on Carbon–CARBON[C].1999: 99. [126]Koshino Y, Narukawa A.Determination of trace metal impurities in graphite powders by acid pressure decomposition and inductively coupled plasma atomic emission spectrometry [J].Analyst, 1993, 118(7): 827-830. [127]Zaghib K, Song X, Guerfi A.Purification process of natural graphite as anode for Li-ion batteries: chemical versus thermal [J].Journal of Power Sources, 2003, 119: 8-15. [128]McKee D W.Effect of metallic impurities on the gasification of graphite in water vapor and hydrogen [J].Carbon, 1974, 12(4): 453-464. [129]Heintz E A, Parker W E.Catalytic effect of major impurities on graphite oxidation [J].Carbon, 1966, 4(4): 473-482. [130]Dai X, Wildgoose G G, Compton R G.Apparent ‘electrocatalytic’ activity of multiwalled carbon nanotubes in the detection of the anaesthetic halothane: occluded copper nanoparticles [J].Analyst, 2006, 131(8): 901-906. [131]Batchelor-McAuley C, Wildgoose G G, Compton R G.Copper oxide nanoparticle impurities are responsible for the electroanalytical detection of glucose seen using multiwalled carbon nanotubes [J].Sensors and Actuators B: Chemical, 2008, 132(1): 356-360. [132]Jurkschat K, Ji X, Crossley A.Super-washing does not leave single walled carbon nanotubes iron-free [J].Analyst, 2006, 132(1): 21-23. [133]Dai X, Wildgoose G G, Salter C.Electroanalysis using macro-, micro-, and nanochemical architectures on electrode surfaces.Bulk surface modification of glassy carbon microspheres with gold nanoparticles and their electrical wiring using carbon nanotubes [J].Analytical Chemistry, 2006, 78(17): 6102-6108. [134]Wong C H A, Chua C K, Khezri B.Graphene oxide nanoribbons from the oxidative opening of carbon nanotubes retain electrochemically active metallic impurities [J].Angewandte Chemie, 2013, 125(33): 8847-8850. [135]Masa J, Zhao A, Xia W.Trace metal residues promote the activity of supposedly metal-free nitrogen-modified carbon catalysts for the oxygen reduction reaction [J].Electrochemistry Communications, 2013, 34: 113-116. [136]Wang L, Ambrosi A, Pumera M.“Metal-free” catalytic oxygen reduction reaction on heteroatom-doped graphene is caused by trace metal impurities [J].Angewandte Chemie International Edition, 2013, 52(51): 13818-13821.2 非貴金屬催化劑
2.1 金屬-氮-碳催化劑
2.2 過渡金屬氧化物、硫?qū)倩衔?、金屬氧氮化合物和金屬碳氮化合?/h3>
3 非金屬催化劑
4 結(jié) 論