周宇，王宇新

（化學工程聯(lián)合國家重點實驗室，天津大學化工學院，天津化學化工協(xié)同創(chuàng)新中心，天津市膜科學與海水淡化重點實驗室，天津 300072）

雜原子摻雜碳基氧還原反應電催化劑研究進展

周宇，王宇新

（化學工程聯(lián)合國家重點實驗室，天津大學化工學院，天津化學化工協(xié)同創(chuàng)新中心，天津市膜科學與海水淡化重點實驗室，天津 300072）

氧還原反應(ORR)是質子交換膜燃料電池和金屬空氣電池等清潔能源轉化技術中的關鍵步驟，但其反應能壘高，動力學緩慢。目前，氧還原性能最佳的催化劑仍然是已經(jīng)商業(yè)化的碳載鉑（Pt/C）催化劑，但其價格高、資源儲量低，難以大規(guī)模應用。因此，近年來許多研究者致力于探尋低成本高性能非鉑ORR催化劑，以期降低催化劑成本，推進質子交換膜燃料電池和金屬空氣電池的商業(yè)化進程。雜原子摻雜碳材料屬于一種新型的非貴金屬ORR催化劑，具有優(yōu)異的電化學性能且成本低廉，顯示了廣闊的應用前景。以雜原子的不同作用原理為線索綜述最近幾年雜原子摻雜碳基ORR催化劑的研究進展，著重論述雜原子對于碳材料電子結構的影響，并討論了雜原子摻雜碳材料催化劑面臨的問題以及發(fā)展趨勢。

氧還原反應；雜原子摻雜；電催化劑；電子材料；催化；電子學

Key words: oxygen reduction reaction; heteroatom doping; electrocatalysts; electronic materials; catalysis; electrochemistry

引 言

質子交換膜燃料電池、金屬空氣電池等新型電源技術具有能量轉換效率高、清潔、可持續(xù)等突出優(yōu)點，有望在新能源汽車、分散式固定式電站、便攜式電子設備等重要領域廣泛應用[1]。然而，由于成本高、可選用材料范圍窄等因素，這些技術的大規(guī)模應用仍然面臨著巨大的挑戰(zhàn)。氧還原反應（oxygen reduction reaction, ORR）是燃料電池和金屬空氣電池等的關鍵步驟，但此反應過程復雜、能壘較高，通常需要較大的過電位驅動[2]。因此，氧還原反應通常需要高活性的電催化劑。目前，Pt催化劑的ORR催化活性最高，但是Pt易一氧化碳中毒失活，特別是其價格高昂、資源稀缺，故難以大規(guī)模應用。已經(jīng)有許多研究著眼于降低Pt負載量，通過Pt顆粒尺度與晶面調控、核殼結構、合金化等方法提高Pt利用率[3]。

但是更根本的方法應該是探求高性能非鉑系ORR催化劑。近年來，非鉑系ORR催化劑成為新能源和催化領域的研究熱點。文獻中大量報道了多類非鉑系ORR催化劑，包括雜原子摻雜碳材料、過渡金屬氧化物、金屬-氮-碳結構催化劑等。相比Pt催化劑，非鉑系催化劑價格優(yōu)勢明顯，日益受到研究者們的重視，論文發(fā)表數(shù)量顯著增加（圖1）。若干非鉑系ORR催化劑的性能已經(jīng)接近、達到甚至超過了商品化的Pt/C催化劑。關于非鉑系ORR催化劑近年來的進展，已經(jīng)有若干研究者做過很好的綜述。Higgins等[4]綜述了石墨烯相關的ORR電催化劑研究進展。Wood等[5]專門針對氮摻雜的多種納米碳材料在ORR催化和其他領域的近年研究成果做了綜述。Duan等[6]綜述了不同雜原子摻雜的石墨烯基催化劑在能源領域的研究進展。Zhu等[7]總結了3D多孔納米碳非貴金屬ORR催化劑的合理設計及合成方法。Zhou等[8]按照材料幾何形態(tài)進行劃分，分類總結了不同維度碳材料在雜原子摻雜以及金屬復合ORR催化劑研究方面的近期成就。He等[9]專門綜述了非貴金屬ORR催化劑的碳基體結構效應。

不同于前人的非貴金屬ORR催化劑綜述性文章，本文以摻雜元素的作用機理為線索和脈絡，分別從電荷極化、自旋極化以及復合作用等角度綜述雜原子摻雜碳基催化劑的最新研究進展。本文著重論述雜原子摻雜對碳材料電子結構的影響，并討論雜原子摻雜碳材料催化劑面臨的問題以及發(fā)展趨勢。

圖1 非鉑系ORR催化劑研究論文數(shù)量統(tǒng)計（數(shù)據(jù)來源：Web of Science）Fig.1 Statistic on number and year of publication of research papers on non-Pt ORR catalyst（source: Web of Science）

1 電荷極化作用

納米碳材料形態(tài)和結構多樣，包括零維的富勒烯、一維的碳納米管、二維的石墨烯以及多種不同的三維結構納米碳。許多納米碳材料由于具有高比表面積、高電導率、結構可調控等許多優(yōu)異的物理化學性質，加之碳資源豐富易得，在電化學能源轉化領域受到了廣泛的關注[1]。但是單純碳納米材料難于替代Pt系貴金屬用于ORR催化劑，因為普通碳材料電荷分布均勻，活性位點較少，催化活性較低[10-13]。為克服此缺點，研究者們通過引入雜原子摻雜的方法改變相鄰碳原子的電荷與自旋密度分布，增加活性位點，從而實現(xiàn)碳材料催化活性的提高[14-16]。目前，研究者普遍接受的主要有兩種摻雜作用機制[10,13,17]：其一，摻雜的雜原子與碳原子電負性差異較大，對相鄰碳原子產(chǎn)生電荷極化作用，改變電荷密度分布，形成對氧有較強吸附能力的活性位點；其二，雜原子的引入改變了sp2雜化碳原子的自旋密度，從而改變表面電子結構分布，形成活性位點。

雜原子摻雜碳材料一般以摻雜N、B、S、P、F等非金屬元素的碳材料為主。B、P電負性較小，與碳原子成鍵時存在電子排斥作用，對相鄰碳原子產(chǎn)生負電荷極化作用，自身帶正電，成為活性位點。N、F等元素電負性相對碳原子較大，與碳原子成鍵時存在電子吸引作用，對相鄰碳原子產(chǎn)生正電荷極化作用，提高了相鄰碳原子的正電荷密度，有利于氧分子的吸附[圖2(a)][9]。

圖2 原子電負性(χ)及原子半徑相對大小(a)和常見N鍵構型（b）[9]Fig.2 Electronegativity (χ) and relative radius of various atoms (a) and schematic diagram of common N bonding configurations (b)[9]

1.1 正電荷極化

在正電荷極化作用體系中，對N摻雜碳材料的研究最早也最為廣泛。2009年Gong等[18]在NH3氣氛下熱解酞菁鐵制備了垂直定向排列N摻雜碳納米管。所得材料不僅催化活性超過了商業(yè)Pt/C催化劑，穩(wěn)定性以及抗CO中毒性能相比Pt基催化劑也有顯著的提高。自此，面向ORR催化劑應用的雜原子摻雜碳納米材料開始受到廣泛關注。

N原子在碳材料中的摻雜有多種構型，不同的摻雜構型對催化性能有著不同的影響。人們發(fā)現(xiàn)在氮摻雜的碳材料中N原子與碳主要有4種鍵合類型，分別為石墨型氮（graphitic-N）、吡啶型氮（pyridinic-N）、吡咯型氮（pyrrolic-N）和氧化態(tài)氮（oxidized-N或pyridinic oxide-N）[9][圖2(b)]。不同前體和制備條件得到的催化劑中N摻雜構型不同，前體的選擇和制備方法對摻雜度和材料的石墨化程度也具有顯著的影響，而催化活性與這些因素密切相關[19-43]。因此，到底是哪種構型的摻雜氮能夠更好地提高氧還原催化活性尚沒有一致的結論。

Liu等[19]采用介孔二氧化硅SBA-15作為模板、含氮芳香染料PDI作為N源和C源，合成了N摻雜有序介孔石墨陣列。研究發(fā)現(xiàn)合成溫度對摻雜構型具有顯著影響。隨著石墨型氮的比例增加，催化活性以及四電子路徑選擇性得到了顯著的提高。Sheng等[20]采用一鍋法熱解氧化石墨烯和三聚氰胺制備得到了N摻雜碳材料，通過調控前體中C源和N源的比例控制摻雜N含量。ORR測試結果表明催化活性隨吡啶型氮含量增加顯著提高，故催化活性主要取決于吡啶型氮。Chung等[33]受到肺泡的啟發(fā)，采用多巴胺包覆的ZIF8沸石作為前體，隨后熱解，得到了內部具有互聯(lián)網(wǎng)絡的多孔碳三維結構，反應物的擴散與電子轉移速度顯著提高，ORR測試表明起始電位隨石墨型氮含量增加向正值方向移動。Unni等[43]制備了富含吡咯型氮的石墨烯，催化活性與吡咯型氮含量呈現(xiàn)良好的正相關性，因此認為催化活性的提高主要是由于吡咯型氮含量的提高。

碳材料的石墨化程度、比表面積、摻雜構型對催化活性具有顯著的影響，這些復雜因素增加了活性位點確定的困難。為了簡化問題，Guo等[40]設計合成了一種具有均勻π電子結構的高度取向熱解石墨HOPG作為模型催化劑，控制反應條件分別得到了以石墨型氮和吡啶型氮為主要摻雜類型的催化劑。研究發(fā)現(xiàn)起始電位隨吡啶型氮含量增加而增加。在不同的電位下，吡啶型氮含量與電流密度均存在線性關系，與制備方法無關，電催化活性只依賴于吡啶型氮含量（圖3）。二氧化碳吸附實驗證明了Lewis堿位點的存在，ORR測試結果表明活性位點就是與吡啶型氮相鄰的具有Lewis堿性的碳原子。

除了控制N摻雜構型外，研究者們還設法提高N摻雜程度、控制石墨化程度、增大比表面積、提高催化劑內部的傳質效率以及電導率，以提高所得摻雜碳材料的催化活性。Zhu等[41]通過無溶劑球磨法制備了N含量高達31.7%的有序介孔碳。Tang等[32]選擇雙嵌段共聚物膠束的自發(fā)共組裝作為軟模板，利用多巴胺的自聚得到球形結構，隨后熱解產(chǎn)生大量介孔結構。特殊的孔結構顯著提高了傳質效率，使得催化活性得到明顯提高，基本達到了商業(yè)Pt/C的水平。另外，微波輔助[37]、溶液等離子體處理[27]、有機縮合[22]等制備方法也顯示了合成不同結構類型的N摻雜碳催化劑的潛力。

圖3 N摻雜高度取向熱解石墨N-HOPG模型催化劑的ORR活性[40]Fig.3 Catalytic performance of N-HOPG model catalysts in ORR[40]

圖4 NC-A碳材料的形貌及電化學性能[31]Fig.4 Morphology and electrochemical performance of NC-A carbon materials[31]

在雜元素摻雜碳材料中，往往表現(xiàn)出3種“矛盾”現(xiàn)象：第一，雜元素摻雜度提高會增加活性位點數(shù)量，但是電導率會隨之下降；第二，隨著電導率的增加，比表面積和孔體積下降；第三，微孔結構能夠顯著提高反應比表面積，但會阻礙反應物的傳質，而大孔結構的作用則與之相反。因此，為獲得更高的催化性能，設計和實現(xiàn)合理的組成和結構，平衡各“矛盾”因素的影響，成為當前摻雜碳材料催化劑研究的焦點。He等[31]選擇聚多巴胺改性混合纖維素酯濾膜同時作為三維大孔模板以及內孔的生孔劑，設計了一種三維分級多孔N摻雜碳材料（NC-A）[圖4(a)、(b)]。大孔的存在提高了反應物的傳質效率，而大孔內部產(chǎn)生的介孔與微孔顯著提高了催化劑的比表面積。這種精確設計的分級多孔結構降低了反應物擴散的阻力，使得更多的活性位點暴露出來，催化活性得到了顯著提高[圖4(c)]。Sa等[26]利用碳納米管（CNTs）的高電導率與雜原子摻雜碳材料（HDC）的高催化活性制備得到了CNTs@HDC型催化劑（制備過程如圖5所示）。在保持原有結構的基礎上電導率得到顯著的提高，催化活性進一步增強。

圖5 CNT/HDC核殼納米結構的制備[26]Fig.5 Synthesis of CNT/HDC core-sheath nanostructures[26]

除了N原子摻雜外，F(xiàn)原子摻雜也具有正電荷極化作用[44-46]。F原子半徑略小于N原子、電負性大于N原子，在與C原子成鍵過程中電子嚴重地偏向F原子但并未完全脫離C原子，因此鍵型由共價鍵過渡到半離子鍵半共價鍵的形式。Sun等[44]采用價格低廉的炭黑和氟化氨作為前體，首次制備了F摻雜炭黑催化劑。測試表明，F(xiàn)原子的引入顯著地提高了起始電位與極限電流密度，電催化性能甚至超過了商業(yè)Pt/C催化劑(圖6)。另外，以F摻雜炭黑為陰極催化劑的堿性燃料電池功率密度可以達到商業(yè)Pt/C催化劑的1.6倍。如此高的催化活性歸于半離子鍵形式的C—F鍵的引入產(chǎn)生了大量的活性位點。除此之外，采用與N摻雜合成類似的模板法，在合成過程用含F(xiàn)前體替代N源，也能獲得具有較高催化活性的F摻雜碳材料[45-46]。

1.2 負電荷極化

B、P等原子的電負性較C原子的電負性更小，與碳原子成鍵過程中存在負電荷極化作用。當它們取代sp2碳晶格中的碳原子或者接枝到碳材料的基面和邊緣位時，均勻分布的電荷密度被打破，摻雜原子帶正電，成為吸附氧活性位點（圖7）[47]。N由于含有一對孤對電子，可以與sp2碳的大π鍵形成π共軛體系，顯著地影響碳原子的自旋密度。而B不含孤對電子，只能通過電負性的差異對碳原子產(chǎn)生負電荷極化作用，因此電催化活性一般低于同類型N摻雜碳材料。

圖6 F摻雜碳催化劑在0.1 mol·L-1KOH溶液中有O2或無O2條件下的循環(huán)伏安曲線(a)和不同摻雜量BP-F催化劑及商品質量分數(shù)20% Pt/C催化劑的線性掃描曲線(b)[44]Fig.6 CV curves of BP-F catalysts with or without O2in 0.1 mol·L-1KOH(a) and linear sweep curves of different BP-F catalysts and commercial 20% Pt/C catalyst(b)[44]

圖7 硼摻雜石墨烯BG表面的ORR電子轉移示意圖[47]Fig.7 Illustration of electron transfer for ORR on BG surface[47]

在B摻雜碳材料中，主要有BC3、B4C、BC2O以及BCO24種活性物種[48]。DFT理論計算表明，B通過電荷極化作用自身帶正電，增強了吸附氧分子的能力。同時，正離子吸引sp2共軛的π*電子填充到2pz軌道，隨后直接轉移到吸附態(tài)氧分子的反鍵軌道上。反鍵軌道電子的填充降低了氧分子的穩(wěn)定性，大大削弱O—O鍵的強度，反應所需活化能大大減少（降低了過電位），促進了氧還原反應的發(fā)生[49]（圖8）。因此，在保證電導率的同時盡可能提高B摻雜量，可以顯著地提高催化活性。Vineesh等[50]和Sheng等[47]分別合成得到了B摻雜石墨烯，所得材料催化活性均隨B含量提高而提高，但是催化性能均略低于Pt催化劑。

圖8 硼摻雜碳納米管BCNT上參與O2吸附的重要分子軌道[49]Fig.8 Important molecular orbitals involved in O2adsorption on BCNT[49]

與N摻雜碳材料類似，幾何結構、傳質速率和電子轉移速率也是B摻雜碳材料催化活性的關鍵影響因素。Panomsuwan等[51]采用溶液等離子體處理法制備了具有聯(lián)通孔結構的B摻雜碳納米顆粒。Zhou等[52]利用二氧化碳超臨界流體技術，采用一鍋法制備了具有分級多孔結構的三維還原氧化石墨烯（圖9）。特殊的幾何結構以及較高的B摻雜量[2.9%(atom)]使得催化活性顯著提高。

P摻雜方式與B有所不同。P原子由于半徑較大，插入sp2碳晶格較困難，通常接枝在碳基面或邊緣處。XPS測試結果表明P摻雜的活性主要由P—C和P—O產(chǎn)生[53]。Liu等[54]通過熱解甲苯和三苯基磷得到了表面富含帽狀凸起的石墨片層結構，這種特殊結構的形成與P摻雜形成的缺陷位有關。由于P原子難以直接插入sp2碳晶格，在原位摻雜過程中破壞了晶體結構，產(chǎn)生了缺陷，這種缺陷隨P摻雜含量增加愈發(fā)明顯[55]。EIS測試顯示，P原子的引入顯著降低了反應電阻，即提高了催化活性[56]。Zhang等[57]通過非模板熱解法在還原氧化石墨烯的過程中原位摻雜P，制備了P原子含量高達1.81%原子數(shù)的摻雜石墨烯。Raman光譜實驗結果表明，在G峰強度基本不變的情況下，D峰的強度得到了明顯增強，ID/IG比值增大，說明高含量的P摻雜在邊緣產(chǎn)生了大量缺陷位。但大量的P摻雜導致所得碳材料的導電性較差，需要加入一定量導電炭黑才可以使高催化活性得以顯現(xiàn)?；旌狭颂亢诘腜摻雜碳材料性能甚至超過了商業(yè)Pt/C催化劑。

圖9 硼摻雜三維還原氧化石墨烯的合成示意圖[52]Fig.9 Schematic illustration for synthesis of B-3DrGO[52]

2 自旋極化作用

與N、B等元素不同，S的電負性與C非常接近（分別為2.58和2.55），因此S原子的引入對于碳晶格的電荷密度分布影響較小。但與P原子類似，S原子半徑也明顯大于C，故也往往接枝在碳基面邊緣處。在S摻雜碳出現(xiàn)前，人們普遍認為雜原子摻雜主要是通過電荷極化作用提高催化活性[13,58-60]，研究主要集中在電負性與碳原子差異較大的元素與碳摻雜。Yang等[61]首先制備出具有高電催化活性的S摻雜石墨烯，從而證明S催化活性不僅僅由電負性差異產(chǎn)生。隨后，研究者們選取不同的前體，例如有機凝膠[62]、磺酸型陽離子交換樹脂[63]、碳納米管[64]、氧化石墨烯[61,65-66]以及雜環(huán)化合物[62]等，制備了各具不同結構特點的S摻雜碳材料。Jeon等[67]通過在三氧化硫氣氛下球磨石墨，在S摻雜的同時通過機械作用實現(xiàn)石墨片層的剝離，得到了邊緣選擇性功能化石墨烯納米片。Chen等[66]選擇Na2S作為硫源和還原劑，在低溫條件下同時實現(xiàn)了氧化石墨烯的還原和S元素摻雜。Yang等[65]利用氧化石墨烯對十六烷基三甲基溴化銨（CTAB）的吸附以及正硅酸乙酯（TEOS）的水解制備了氧化石墨烯-多孔二氧化硅片層，隨后在NH3或H2S氣氛下熱解，分別制備了N摻雜石墨烯和S摻雜石墨烯（圖10）。結果證明S元素主要以噻吩型S和氧化型S形式摻雜在石墨烯的邊緣處。

圖10 N和S摻雜石墨烯合成示意圖[65]Fig.10 Schematic illustration of fabrication of N-doped and S-doped graphene[65]

研究表明，摻雜S主要以表面吸附態(tài)S、鋸齒形邊緣S（SO2）、椅形邊緣S（SO2）、S環(huán)簇等構型存在。Jeon等[68]用DFT方法模擬計算了幾種摻雜S構型的電荷密度、自旋密度以及磁矩（表1）。相比初始狀態(tài)的石墨烯，表面吸附態(tài)S和環(huán)簇狀態(tài)S的自旋密度和電荷密度均未發(fā)生明顯改變，因此可以認為這兩種形式的S摻雜對于氧還原反應催化性能影響有限。而鋸齒形邊緣和椅形邊緣共價鍵合的S或SO2對自旋密度和電荷密度均產(chǎn)生了顯著影響，可以作為活性位點。軌道計算結果表明，石墨烯邊緣共價鍵合的S原子對最高已占軌道（HOMO）與最低未占軌道（LUMO）產(chǎn)生了強烈的極化作用（圖11），這些極化區(qū)域可以作為ORR的活性位點，對催化性能的提高起到關鍵作用。

表1 不同摻雜S構型的Mulliken自旋密度、電荷密度以及磁矩計算結果[68]Table 1 Mulliken spin densities, charge densities, and magnetic moments in structures of pristine graphene, SGnP, and SOGnP[68]

除了N、B、P、S、F等元素的摻雜之外，Cl、Br、I等鹵族元素[69-70]以及Se[71]、Sb[72]等元素單摻雜碳材料催化劑也已經(jīng)有研究者報道。研究結果表明這些雜元素摻雜催化劑電催化活性均得到了明顯的提高，然而具體作用機理還有待深入研究。

盡管若干單雜原子摻雜碳材料催化劑的性能已經(jīng)接近甚至超過現(xiàn)行商業(yè)Pt/C催化劑，但是依然未達到理想的目標[73]。對雜原子摻雜石墨烯進行模擬計算發(fā)現(xiàn)單雜原子摻雜石墨烯材料均位于火山型曲線右側，這顯示單元素摻雜無法達到理想的X-G催化劑的性能[10]（圖12）。

圖11 HOMO軌道分布[SGnP(a)和SOGnP(c)]以及LUMO軌道分布[SGnP(b)和SOGnP(d)][68]Fig.11 HOMO and LUMO distributions[68]

圖12 摻雜石墨烯催化劑理論交換電流密度和OOH*吸附能火山型曲線關系(紅色虛線)[10](藍色方塊為每種摻雜石墨烯催化劑的實驗交換電流密度和DFT計算出的OOH*吸附能)Fig.12 Volcano plot betweenj0theoryand ΔGOOH*(red dashed line)[10](blue hollow squares arej0exptand DFT derived ΔGOOH*for each doped graphene catalyst)

3 復合作用

單一雜元素摻雜對碳原子的電荷密度和自旋密度改變有限，為了得到更好的催化性能，研究者們將目光轉向多元素摻雜碳材料，希望通過不同組分之間的協(xié)同作用對催化劑的電子性質進行調控，獲得更理想的雜原子摻雜碳材料催化劑。

3.1 雜元素共摻雜

目前，研究者們已經(jīng)嘗試了多種雜元素共摻雜碳材料，例如N、B共摻雜[74-77]，N、S共摻雜[78-99]，甚至三元摻雜[100-104]等。相比單摻雜，多摻雜可以充分利用不同元素之間的極化作用。例如N、B摻雜不僅可以利用N元素的正電荷極化作用和B元素的負電荷極化作用，而且在形成N-C-B結構時產(chǎn)生協(xié)同效應，顯著提高活性碳位點的催化活性[74]。然而，共摻雜體系較單摻雜體系更為復雜，定向合成某種摻雜構型的碳材料也更為困難[58]。

研究者們通常采用在合成單摻雜碳材料過程中同時加入另一種原料作為前體實現(xiàn)共摻雜[81]，或者將單摻雜材料進行后處理，通過調整反應溫度以及原料配比控制摻雜元素含量。由于合成過程采用了不同的原料，反應過程相對復雜，通常需要采用合成模板[78,100]，增加了額外處理步驟。因此，為了簡化這一過程，同時能夠合成可控形貌的多摻雜碳材料，對于前體的優(yōu)化成了亟需解決的問題。一系列金屬有機化合物[105-107]、生物質材料[97-98,108-113]等，由于其特殊的結構特性以及豐富的元素組成，可以作為自犧牲模板合成得到具有豐富孔結構的多元素共摻雜碳材料催化劑，受到了廣泛關注。

Shao等[108]選擇富含C、N、P、S的雞蛋黃作為前體，通過簡單的一步熱解法實現(xiàn)了生物質的碳化和N、P、S 3種元素的原位摻雜。在熱解過程中，利用熔融KCl的作用形成了富含松散堆積的石墨納米片的層狀結構，這種結構顯著地提高了材料的比表面積（圖13）。電化學測試表明，此三元摻雜碳材料的ORR催化性能已經(jīng)達到了商業(yè)Pt/C催化劑的水平，而且有更高的穩(wěn)定性。

3.2 雜元素、金屬元素共摻雜

除了雜原子元素共摻雜外，雜原子與金屬原子共摻雜也能顯著提高催化活性。Razmjooei等[114]在P摻雜石墨烯的基礎上合成了Fe、P共摻雜石墨烯。由于Fe原子與P原子之間的相互作用，產(chǎn)生Fe-P活性物種的同時P元素摻雜量也得到顯著的提高（表2）。通過Fe原子與P原子之間的協(xié)同作用，F(xiàn)e、P共摻雜石墨烯催化活性相比單一P摻雜石墨烯得到了顯著的提高（圖14）。Guo等[115]通過一步熱解鐵鹽處理后的茶葉制備了N、P、Fe三元摻雜碳材料（圖15）。茶葉本身具有特殊的三維結構，熱解后形成分級三維多孔結構。多酚物質作為配體與Fe反應形成配合物，在隨后的熱解過程中產(chǎn)生了高催化活性的Fe-N活性位點。幾何結構與元素摻雜形成的協(xié)同效應顯著提高了催化活性，起始電位與極限電流密度均超過了Pt/C催化劑。

圖13 熱解雞蛋黃HDC催化劑的制備路線(a), Yolk-CN-800樣品的掃描電鏡照片(b)和透射電鏡照片(c)以及不同溫度下得到的HDC催化劑EDS譜圖(d)[108]Fig.13 Schematic illustration for synthesis process of yolk derived HDC catalysts (a), SEM (b) and TEM (c) images of sample Yolk-CN-800, and EDS spectra of yolk derived HDC catalysts at different temperatures(d)[108]

表2 不同樣品表面XPS元素分析結果[114]Table 2 Surface element contents obtained from XPS analysis for pristine RGO, GP, and GPFe[114]

圖14 P摻雜以及P、Fe共摻雜對還原氧化石墨烯ORR性能的影響[114]Fig.14 Effect of introduction of P into RGO and Fe into P-doped RGO on ORR[114]

除了金屬元素與雜元素共同摻入碳結構外，在雜原子摻雜碳材料上負載金屬氧化物或硫化物等的研究也有報道[116-121]。Zhu等[121]利用N摻雜碳納米管包覆碳化鐵納米顆粒形成核殼結構。內核Fe元素的加入并未產(chǎn)生新的活性位點，而是通過電子效應改變外層碳材料電子分布，從而提高催化活性。Liang等[116]采用N摻雜還原氧化石墨烯負載Co3O4納米顆粒，相比未摻雜復合材料催化活性有了顯著的提高（圖16）。X射線吸收近邊結構（XANES）表明石墨烯中的C原子與O原子或其他原子形成了新的化學鍵，這種化學鍵很有可能是Co—O—C和Co—N—C。這種協(xié)同耦合作用改變了原子的電荷密度，增加了反應的活性位點，因而顯著提高了復合材料的催化活性。

在雜元素摻雜碳材料的合成過程中難以避免微量金屬元素的引入。例如Hummer法制備的氧化石墨烯中含有少量金屬雜質[122]，熱解過程采用的催化劑[123]、前體中均可能含有金屬元素[18]。這些金屬通常為Fe、Ni、Co等存在孤對電子或外層空軌道的過渡金屬元素，對于sp2碳的電子結構具有顯著的影響，也有實驗研究證明[124]合成過程中微量金屬元素的引入對于雜元素摻雜碳材料的催化活性具有顯著的影響，因此不能忽略微量金屬元素對摻雜碳材料的作用。Guo等[125]在熱解聚（3,4-亞乙基二氧噻吩）-聚苯乙烯磺酸的過程中，由于氧化劑硝酸鐵的殘留，得到了S、N、Fe共摻雜的分級多孔泡沫碳結構。痕量Fe元素的引入不僅增加了活性位點數(shù)量，還通過與雜原子之間的協(xié)同作用提高了位點的反應活性。

圖15 原始茶葉熱解制備ORR電催化劑HDPC-X過程[115]Fig.15 Schematic illustration of fabrication of HDPC-Xelectrocatalysts towards ORR from pristine tea leaves[115]

圖16 催化劑的ORR性能以及穩(wěn)定性[116]Fig.16 ORR performance and stability of catalysts[116]

4 催化劑在單電池中的性能

對新催化劑的研究中，多數(shù)只用薄膜電極測試催化劑的性能。但薄膜電極與燃料電池電極的結構和工作條件有一定差別，在薄膜電極上性能優(yōu)異的催化劑不一定在電池中同樣性能優(yōu)異。為了綜合評價新催化劑，對其在電池中做測試十分必要。因此，為數(shù)不多的雜原子摻雜碳基催化劑在單電池中性能的數(shù)據(jù)尤有參考價值。

Venkateswara等[126]對N摻雜碳納米管進行了陰離子交換膜燃料電池測試。結果表明，摻雜碳材料的開路電壓略低于Pt催化劑，最大功率密度約為Pt催化劑的1/2（圖17）。Sa等[26]在碳納米管的基礎上引入N摻雜碳，制備得到了CNTs@HDC型催化劑。相比碳納米管陰極，電池功率密度提高了23.3倍，開路電壓接近Pt/C催化劑。

單摻雜碳材料由于活性位點相對較少，在實際電池測試中性能與Pt基催化劑仍有較大差距。研究者們通過引入多元素共摻雜的方法增加活性位點數(shù)量，通過協(xié)同效應提高材料在實際電池中的催化活性。Sun等[127]在N摻雜碳材料的基礎上引入F元素，活性位點數(shù)量得到了顯著提高。堿性直接甲醇燃料電池測試結果表明N、F共摻雜碳材料催化活性已經(jīng)達到Pt/C催化劑的水平。Xiao等[128]采用N摻雜多孔石墨層對Fe3C納米顆粒進行封裝。內層Fe元素的電子效應改變了外層碳原子的電荷密度，吸附氧分子的能力進一步提高，氧還原反應活性得到增強。盡管酸性條件下功率密度仍低于Pt基催化劑，但是穩(wěn)定性得到了顯著的提高。而在堿性條件下，無論是開路電壓還是功率密度，復合催化劑的性能均超過了Pt/C催化劑的水平（圖18）。

圖17 以N-CNT(藍)和Pt/C (紅)為陰極的膜電極的陰離子交換膜燃料電池性能[126]Fig.17 Anion-exchange membrane fuel cell performance of MEA with N-CNT (blue) and Pt/C (red) as cathode[126]

圖18 Fe3C/NG-800和Pt/C (60%)催化劑作為陰極的直接甲醇燃料電池在60℃下性能測試[128]Fig.18 DMFC performance at 60℃ using Pt/C (60%) or Fe3C/NG-800 as cathode[128]

5 小結與展望

相比于現(xiàn)行的鉑族貴金屬ORR催化劑，雜原子摻雜碳基催化劑來源廣泛、價格低廉，具有大規(guī)模廣泛應用的潛力。近年來若干雜原子摻雜碳基催化劑的活性和穩(wěn)定性已經(jīng)達到甚至超過了目前應用最多的商品Pt/C催化劑。對碳分子結構中雜原子摻雜的作用機制也已經(jīng)有了較明確的認識。被摻雜的雜原子主要是通過電荷極化以及自旋極化作用改變碳原子的電子結構，從而實現(xiàn)催化活性的提高。N、F等元素電負性遠大于碳原子，在與碳原子成鍵時對相鄰碳原子產(chǎn)生正電荷極化，提高了碳原子的電荷密度，成為更好吸附氧分子的位點。B、P元素電負性較小，與碳原子成鍵時存在電子排斥作用，對相鄰碳原子產(chǎn)生負電荷極化作用，自身成為活性位點。S元素電負性與C接近，對于電荷密度影響較小，主要通過自旋極化作用改變碳原子的自旋密度，從而形成活性位點。理論上，只要雜原子的摻雜能夠引起電荷密度或自旋密度足夠的非對稱分布，就能顯著提高催化活性。為了獲得更高性能的碳基ORR催化劑，還探索了包括金屬原子在內的多種原子的共摻雜碳材料以及這些材料與金屬氧化物、硫化物等的復合材料，期望通過多原子之間的協(xié)同效應形成高ORR活性位點。

雜原子摻雜碳基ORR催化劑已經(jīng)顯示了光明的前景，但距離開發(fā)出高性能、低價格的商業(yè)化產(chǎn)品還有較長路程。雜原子摻雜碳基ORR催化劑發(fā)展較晚，幾乎所有已經(jīng)探索的領域都值得進行更深入細致的探索，例如對N單摻雜高活性位點構型的理論和實驗確定，再如多摻雜復合型碳基催化劑的合理設計及高選擇性合成。另外，電催化劑的性能不但受限于活性點的多寡和活性高低，還取決于電荷和反應劑的傳遞。因此，應發(fā)展從量子化學到多孔結構中傳質的多尺度模擬計算方法，以更全面深入地認識雜原子摻雜碳基催化劑性能的影響因素并更合理地設計ORR催化劑。目前的研究中，許多雜原子摻雜碳基ORR催化劑與商品Pt/C催化劑的性能是用浸于液體電解質中的薄膜電極比較的。為更接近實際使用條件，今后新雜原子摻雜碳基催化劑性能應更多地用氣體電極和電池單池檢驗。而且，目前的膜電極技術是基于Pt/C催化劑，因此應當研究適用于新型的碳基催化劑的膜電極制備方法。最后，雜原子摻雜碳基ORR催化劑的商品化開發(fā)條件已經(jīng)基本具備，相應的開發(fā)工作有待開展，相關的技術難點有待挑戰(zhàn)。

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Recent progress on electrocatalysts towards oxygen reduction reaction based on heteroatoms-doped carbon

ZHOU Yu, WANG Yuxin
(State Key Laboratory of Chemical Engineering,Co-Innovation Center of Chemical Science & Engineering,Tianjin Key Laboratory of Membrane Science & Desalination Technology,School of Chemical Engineering,Tianjin University,Tianjin300072,China)

Electrochemical oxygen reduction reaction (ORR) is key to clean and sustainable energy technologies including proton exchange membrane fuel cells and metal-air batteries. However, the high-activation barriers in ORR often makes it the bottleneck of energy conversion processes, and thus high performance ORR electrocatalysts are desired. At present the best commercially available ORR catalyst is based on the precious metal Pt. But it suffers from resource scarcity and unsatisfactory operational stability, thus hindering a widespread and large-scale application of the clean and sustainable technologies. To tackle this problem, extensive efforts have been made in the last decade or so to search after efficient non-precious metal ORR catalysts. Among these, many heteroatoms-doped carbon (HDC) materials appear to be very promising, owing to their easy availability, low cost and excellent electrochemical performance. In this review article, recent advances in this active area are summarized, with the content being categorized according to the different underlying mechanisms of doped heteroatoms. Theoretical and experimental findings regarding HDC materials in ORR catalysis are reviewed, emphasizing the influence of heteroatom doping on the electronic structure of carbon materials. The problems facing HDC based ORR catalysts and further researches required are also discussed.

Prof. WANG Yuxin, yxwang@tju.edu.cn

O 643.36；TM 911.46

：A

：0438—1157（2017）02—0519—16

10.11949/j.issn.0438-1157.20161060

2016-07-26收到初稿，2016-09-07收到修改稿。

聯(lián)系人：王宇新。

：周宇（1993—），男，博士研究生。

國家自然科學基金項目（21120102039）。

Received date: 2016-07-26.

Foundation item: supported by the National Natural Science Foundation of China(21120102039).