孫煉, 顧全超, 楊雅萍, 王洪磊, 余金山, 周新貴

二維過渡金屬硫?qū)倩衔镅踹€原反應(yīng)催化劑的研究進(jìn)展

孫煉, 顧全超, 楊雅萍, 王洪磊, 余金山, 周新貴

(國(guó)防科技大學(xué) 空天科學(xué)學(xué)院, 新型陶瓷纖維及其復(fù)合材料重點(diǎn)實(shí)驗(yàn)室, 長(zhǎng)沙 410073)

氧還原(ORR)反應(yīng)是燃料電池等清潔能源陰極的關(guān)鍵反應(yīng), 其反應(yīng)動(dòng)力學(xué)復(fù)雜, 陰極需使用Pt等貴金屬催化劑。然而Pt價(jià)格昂貴, 且載體炭黑在高電位環(huán)境下穩(wěn)定性欠佳, 導(dǎo)致電池部件成本高且壽命短。二維過渡金屬硫?qū)倩衔?2D TMDs)具有高比表面積與可調(diào)節(jié)的電學(xué)性能, 且穩(wěn)定性強(qiáng), 有望在維持活性的同時(shí)提高燃料電池陰極的耐久性。本文梳理了近年來2D TMDs在ORR催化劑領(lǐng)域的最新研究進(jìn)展: 首先概述了2D TMDs的結(jié)構(gòu)、性質(zhì)及ORR反應(yīng)機(jī)理; 其次分析了調(diào)控2D TMDs的ORR性能策略, 包括異質(zhì)元素?fù)诫s、相轉(zhuǎn)變、缺陷工程與應(yīng)力工程等, 介紹了2D TMDs基異質(zhì)結(jié)構(gòu)對(duì)ORR性能的提升作用; 最后, 針對(duì)該領(lǐng)域目前存在的挑戰(zhàn)進(jìn)行展望與總結(jié)。

氧還原反應(yīng)；二維材料；過渡金屬硫?qū)倩衔?；電催化；綜述

以燃料電池[1]與金屬–空氣電池[2]為代表的新型能源器件具有環(huán)境友好性與能量轉(zhuǎn)換高效性, 對(duì)于實(shí)現(xiàn)“碳達(dá)峰”、“碳中和”目標(biāo)具有重大意義。上述能源器件陰極均涉及氧還原反應(yīng)(Oxygen Reduction Reaction, ORR), 該反應(yīng)可分為直接四電子路徑與兩電子路徑[3]。在電池運(yùn)行中, 研究者期望陰極ORR沿直接四電子路徑反應(yīng)以提高能源轉(zhuǎn)化效率, 其具體反應(yīng)式如下：

直接四電子路徑

O2+ 4H++ 4e–→ 2H2O (酸性)

O2+ 2H2O + 4e–→ 4OH–(堿性)

兩電子路徑

O2+ 2H++ 2e–→ H2O2

H2O2+ 2H++ 2e–→ 2H2O (酸性)

O2+ H2O + 2e–→ HO2–+ OH–

HO2–+ H2O + 2e–→ 3OH–(堿性)

目前, 炭載鉑(Pt/C)以其優(yōu)異的活性成為應(yīng)用最廣泛的ORR催化劑[4-7]。然而實(shí)際應(yīng)用中, 電池陰極常處于高電位與強(qiáng)酸/堿環(huán)境, 使炭黑載體發(fā)生嚴(yán)重電化學(xué)腐蝕, 造成表面Pt顆粒溶解或團(tuán)聚, 進(jìn)而影響催化劑的長(zhǎng)期穩(wěn)定性[8-10]。因此, 研究者們期望找到一種活性高且耐腐蝕的ORR催化劑, 以延長(zhǎng)能源器件壽命[11-13]。近年來, 以石墨烯[14]為代表的層狀二維納米材料因其尺寸效應(yīng)帶來的獨(dú)特電子結(jié)構(gòu)與電磁性質(zhì), 在電催化劑領(lǐng)域獲得了大量關(guān)注[15-20]。其中, 二維過渡金屬硫?qū)倩衔?Two- dimensional Transition Metal Dichalcogenides, TMDs, 2D TMDs)能夠形成獨(dú)特的晶體與能帶結(jié)構(gòu), 具有可比肩石墨烯的電學(xué)性能[21]。同時(shí), TMDs優(yōu)異的強(qiáng)度與穩(wěn)定性使其在嚴(yán)苛環(huán)境中能夠維持自身結(jié)構(gòu), 保證了催化劑的耐久性[22]。雖然目前已有部分關(guān)于2D TMDs電催化劑的綜述, 但尚未見到聚焦于2D TMDs的ORR催化性能的總結(jié)文獻(xiàn)。本文綜述了2D TMDs在ORR催化領(lǐng)域的應(yīng)用進(jìn)展, 主要包括: 1) 2D TMDs的結(jié)構(gòu)與性質(zhì)；2) 2D TMDs 片層ORR催化劑的性能調(diào)控; 3) 2D TMDs異質(zhì)結(jié)構(gòu)ORR催化劑的性能調(diào)控, 并對(duì)2D TMDs在ORR催化劑領(lǐng)域的應(yīng)用前景及挑戰(zhàn)進(jìn)行總結(jié)和展望。

1 2D TMDs的結(jié)構(gòu)與性質(zhì)

1.1 2D TMDs的晶體結(jié)構(gòu)與電學(xué)性質(zhì)

TMDs的化學(xué)式為MX2, 其中M為過渡金屬元素, 包含IV族(如Ti)、V族(如V、Nb)、VI族(如Mo、W)、VII族(如Re、Tc)與X族元素(如Pd、Pt), X代表硫?qū)僭?如S、Se)。2D TMDs具有與石墨烯類似的片層狀結(jié)構(gòu), 其相態(tài)包含1T, 2H與3R[23], 其中數(shù)字代表晶胞中X-M-X層的數(shù)量, 字母代表晶系(四方晶系、六方晶系、菱方晶系), 如圖1所示。1T相與2H相是2D TMDs較為常見的兩種晶型, 1T相在發(fā)生扭曲的情況下還可以轉(zhuǎn)變?yōu)?T’, 1T’’等亞穩(wěn)相[24]。

TMDs的電學(xué)特性與其d軌道中的電子密切相關(guān)：對(duì)于2H型TMDs, 其過渡金屬原子的未成鍵d軌道無孤對(duì)電子, 呈現(xiàn)半導(dǎo)體性質(zhì)；而對(duì)于1T型TMDs, 未成鍵的d軌道存在孤對(duì)電子, 呈現(xiàn)類似金屬的導(dǎo)電性[25]。隨厚度減小, 2D TMDs的能帶結(jié)構(gòu)因量子限域效應(yīng)發(fā)生了巨大改變。以MoS2為例[26](圖2)：對(duì)于塊體MoS2, 其帶隙寬度為1.29 eV, 屬于間接帶隙半導(dǎo)體；隨著MoS2原子層數(shù)不斷減少, 其帶隙不斷增大, 直至單原子層MoS2的帶隙達(dá)到最大, 變?yōu)橹苯訋栋雽?dǎo)體。同時(shí), 2D TMDs的邊界效應(yīng)使其邊緣不飽和配位原子產(chǎn)生催化反應(yīng)能力[27-28]。因此, 2D TMDs較其塊體材料而言在催化領(lǐng)域具有更廣闊的應(yīng)用前景。人們不斷深入研究2D TMDs的電子結(jié)構(gòu), 特別是利用多電子光子效應(yīng)[29]、分子動(dòng)力學(xué)[30]等手段研究其激子動(dòng)力學(xué)模式, 不斷挖掘2D TMDs的催化性能與電子結(jié)構(gòu)間的內(nèi)在聯(lián)系。

圖1 2D TMDs的結(jié)構(gòu)與晶型

Fig. 1 Structures and crystal forms of 2D TMDs

圖2 (a) 塊體結(jié)構(gòu)MoS2, (b) 三層結(jié)構(gòu)MoS2, (c) 雙層結(jié)構(gòu)MoS2與(d) 單層結(jié)構(gòu)MoS2經(jīng)過計(jì)算后得到的帶隙[26]

目前合成2D TMDs的策略可歸類為“自上而下”法與“自下而上”法兩種。其中, “自上而下”法是利用TMDs層間的弱范德瓦爾斯力作用, 從塊體中剝離二維層狀結(jié)構(gòu)的方式, 包含超聲輔助剝離法[31]、機(jī)械剝離法[32]、溶劑插層剝離法[33]等。但該方法耗時(shí)長(zhǎng)、產(chǎn)率低, 且產(chǎn)物尺寸與形貌難以控制。“自下而上”法則是含過渡金屬與硫?qū)僭氐幕衔镌唇?jīng)溶解/升華、沉積與反應(yīng)得到2D TMDs的方式。其中, 水熱合成法因操作簡(jiǎn)便等優(yōu)勢(shì)成為最常用的合成手段之一[34-35]。然而, 受反應(yīng)溫度限制(<220 ℃), 水熱合成產(chǎn)物難于調(diào)控組分與結(jié)構(gòu)?；瘜W(xué)氣相沉積法(Chemical Vapor Deposition, CVD)是另一種得到廣泛應(yīng)用的自下而上合成方式, 其最大優(yōu)勢(shì)是可通過調(diào)節(jié)參數(shù), 如原料比、沉積溫度、載氣流量與成分等, 較好地控制產(chǎn)物的組成與形貌[36-38]。但目前僅有少數(shù)2D TMDs可通過CVD法得到, 如MoS2、MoSe2、MoTe2、WS2、WSe2等[39]?？傊? 目前合成2D TMDs的方法呈現(xiàn)多樣化。然而, 上述方法仍然存在效率低、產(chǎn)物結(jié)構(gòu)不符合期望等缺陷, 在一定程度上制約了2D TMDs的產(chǎn)業(yè)化。

1.2 2D TMDs在ORR催化劑領(lǐng)域的應(yīng)用潛力

根據(jù)不同的反應(yīng)環(huán)境, ORR反應(yīng)機(jī)理也有所區(qū)別(協(xié)同機(jī)制與解離機(jī)制), 但均涉及O2分子的活化、含氧中間體(一般為O*或OH*)的生成與還原等步驟(圖3(a))[3]。一般認(rèn)為, 催化劑對(duì)中間體的吸附與解離是ORR反應(yīng)的決速步驟[40]：根據(jù)氧吸附能與ORR活性間的關(guān)系[41], 催化劑對(duì)含氧分子的吸附能應(yīng)處于“火山曲線”頂端附近才能達(dá)到最佳效果(圖3(b))。目前, 已有多位學(xué)者[42-44]通過理論計(jì)算研究了一系列2D TMDs在多種電催化反應(yīng)中的作用機(jī)制, 結(jié)果表明片層邊緣配位不飽和原子可通過優(yōu)化中間體吸附能降低反應(yīng)勢(shì)壘, 被視為催化劑的活性位點(diǎn)。

對(duì)于2D TMDs而言, 單原子層/多原子層結(jié)構(gòu)使其具有較大的比表面積, 對(duì)催化反應(yīng)中間體更加敏感。同時(shí), 雖然單一2D TMDs結(jié)構(gòu)僅有邊緣原子處于配位不飽和態(tài), 但通過修飾手段, 大量面內(nèi)原子也可以對(duì)含氧中間體產(chǎn)生活化與吸附作用。此外, 二維材料的模型較為簡(jiǎn)單, 使得采用分子動(dòng)力學(xué)、第一性原理等計(jì)算手段預(yù)測(cè)2D TMDs的ORR性能成為可能[23]。綜上所述, 2D TMDs具有尚未被發(fā)掘的巨大ORR催化潛力。

圖3 ORR催化劑的設(shè)計(jì)理論[3, 41]

(a) ORR mechanism with blue arrow representing dissociative mechanism, red arrows representing associative mechanism, and purple arrows representing the parts involving both mechanisms[3]; (b) “Volcano plots” showing relationship between oxygen binding energy and maximal activity[41]

Colorful figures are available on website

2 2D TMDs層狀結(jié)構(gòu)的ORR性能調(diào)控

如上節(jié)所述, 盡管2D TMDs具有能夠比肩石墨烯的優(yōu)異電學(xué)性能, 但較弱的導(dǎo)電能力和較少的活性位點(diǎn)極大制約了其電催化性能。為此, 研究者們不斷通過各種方式激發(fā)2D TMDs的ORR催化活性。提高2D TMDs層狀結(jié)構(gòu)的催化活性可從增加邊緣活性原子數(shù)目和激活面內(nèi)原子催化活性兩個(gè)方面入手。典型的幾種手段包含異質(zhì)元素?fù)诫s、相轉(zhuǎn)變、缺陷工程、應(yīng)力工程等。表1總結(jié)了一些代表性2D TMDs層狀結(jié)構(gòu)ORR催化劑的性能指標(biāo)。

2.1 異質(zhì)元素?fù)诫s

向2D TMDs的面內(nèi)或邊緣摻雜異質(zhì)元素能夠改變?cè)性拥碾娮咏Y(jié)構(gòu), 進(jìn)而改變其對(duì)含氧中間體的作用行為, 達(dá)到調(diào)控ORR性能的目的[50]。同時(shí), 一些異質(zhì)元素(特別是貴金屬)本身對(duì)含氧中間體的吸附能處于“火山曲線”頂點(diǎn)附近, 起到活性位點(diǎn)作用[51-52]。根據(jù)異質(zhì)元素在MX2結(jié)構(gòu)不同的摻雜位置, 異質(zhì)元素?fù)诫s可分為M位摻雜與X位摻雜兩種 形式。

表1 典型2D TMDs層狀結(jié)構(gòu)ORR催化劑的性能

M位摻雜常選用金屬元素, 而具有高催化活性的貴金屬是首選。Upadhyay等[53]通過DFT計(jì)算發(fā)現(xiàn)向原本具有ORR催化惰性的單原子層2D MoSe2表面負(fù)載少量Pt原子后, 由于Pt的內(nèi)層5d電子與基體相互作用, MoSe2的禁帶寬度可由2.21 eV驟降至～0 eV, 呈現(xiàn)金屬態(tài)。隨后他們計(jì)算ORR反應(yīng)中間過程的勢(shì)壘, 發(fā)現(xiàn)Pt-MoSe2從熱力學(xué)和動(dòng)力學(xué)角度均適合ORR反應(yīng)(圖4(a))。Hwang等[54]預(yù)測(cè)了48種以2D TMDs為基底的單原子催化劑的ORR性能, 結(jié)果表明Pt或Pd單原子與MoS2或NbS2復(fù)合時(shí)ORR活性最高。在上述理論研究的基礎(chǔ)上, 研究者們探索出多種向2D TMDs摻雜貴金屬的方法。如Shi等[55]采用電置換反應(yīng), 在預(yù)先電沉積Cu的一系列2D TMDs基底上負(fù)載了單原子Pt1, 其負(fù)載量可達(dá)質(zhì)量分?jǐn)?shù)4.1%～5.1%(圖4(b))。其它M型摻雜的實(shí)現(xiàn)方式還包括物理混合[56]、水熱還原[57]、外延生長(zhǎng)法[58]等。一些非貴金屬在2D TMDs基底中也展現(xiàn)出可與貴金屬媲美的催化活性。Tian等[59]研究了Fe、Ni與Co三種非貴金屬在IVB-VIIB族TMDs上的ORR催化活性。上述非貴金屬在基底中均為帶正電的活性中心, 特別是Fe@TMDs的電荷轉(zhuǎn)移作用較強(qiáng), 使*OOH/*OH中間體的吸附能增大, ORR活性最高。基于此, 他們研究了一系列M@TMDs結(jié)構(gòu)的ORR熱力學(xué)性能。

X位摻雜則選用與硫?qū)僭叵嘟髯宓脑?如O、N、P、B等)對(duì)2D TMDs進(jìn)行改性。由于這些元素的外層電子排布與硫?qū)僭卮嬖诓町? 因此摻雜后2D TMDs的電子結(jié)構(gòu)發(fā)生改變, 進(jìn)而導(dǎo)致帶隙與電導(dǎo)率變化, 同時(shí)與摻雜元素相鄰的面內(nèi)原子催化活性也能夠得到激發(fā)。然而, 目前對(duì)X位摻雜2D TMDs的ORR具體機(jī)制尚存爭(zhēng)議。以P摻雜2D MoS2為例, Zhang等[60]的DFT研究表明P-MoS2更容易沿協(xié)同機(jī)制(O2*+H++e–→OOH*, 勢(shì)壘0.11 eV)而非解離機(jī)制(O2*→2O*)進(jìn)行, 同時(shí)*OOH→O*的轉(zhuǎn)化步驟自由能變化為正(+2.01 eV), 使P-MoS2不適宜作為ORR催化劑。然而Liu等[61]發(fā)現(xiàn)P-MoS2的ORR活性與P摻雜量密切相關(guān)：當(dāng)P摻雜量達(dá)到原子分?jǐn)?shù)3.7%時(shí), 與摻雜原子相近的Mo原子展現(xiàn)出強(qiáng)氧吸附能力, 與Huang等[45]的實(shí)驗(yàn)結(jié)果吻合(圖5(a)), 這是由于P摻雜使周圍的Mo原子將O2*有效解離成O*, 且P原子在面內(nèi)或邊緣位置均能起到相同效果(圖5(b))。上述工作為長(zhǎng)期以來解決困擾研究者們的重要問題——元素?fù)诫s量與ORR作用機(jī)制的關(guān)系提供了一定指導(dǎo)。除摻雜單個(gè)元素外, 摻雜多種異質(zhì)元素可發(fā)揮各種元素間的協(xié)同作用, 還能夠有效減少貴金屬用量[62-63]。然而, 由于2D TMDs的大部分面內(nèi)原子處于穩(wěn)定結(jié)構(gòu), 外來元素?fù)饺刖Ц裨趯?shí)驗(yàn)角度上存在一定挑戰(zhàn), 因此目前相關(guān)研究大多仍然處于理論預(yù)測(cè)階段。今后的工作首先要解決高質(zhì)量2D TMDs的合成問題, 以便更好地調(diào)控其性能。同時(shí)仍需要聚焦于開發(fā)更有效的策略引入外來元素, 如Gong等[64]報(bào)道的插層法將異質(zhì)元素引入2D TMDs的片層間。另外, 先進(jìn)表征技術(shù)(如原位觀測(cè)技術(shù)、同步輻射技術(shù)等)也有助于更加深入了解摻雜后2D TMDs組成與電子結(jié)構(gòu)之間的關(guān)系。

2.2 相轉(zhuǎn)變

2013年, Voiry等[65]發(fā)現(xiàn)使用1T相WS2取代傳統(tǒng)2H相結(jié)構(gòu)后, 電解池的電導(dǎo)率與電流密度提高了5倍以上。從此, 1T相TMDs的電催化性能得到了廣泛關(guān)注[66-67]。但由于其熱力學(xué)的亞穩(wěn)態(tài), 大部分純凈1T相2D TMDs的制備存在一定困難。Sadighi等[68]使用鈉離子插層法在削弱2H-MoS2層間范德瓦爾斯力的同時(shí)晶體發(fā)生2H→1T相轉(zhuǎn)變, 得到的1T-MoS2納米片與碳納米管(CNT)復(fù)合后具有與Pt/C比肩的ORR催化活性(圖6)。但這種2D 1T-MoS2在空氣中極易轉(zhuǎn)變?yōu)?H相, 導(dǎo)致催化劑的穩(wěn)定性欠佳。近來, 研究發(fā)現(xiàn)一些金屬元素(如Re, Mn等)可通過電子供體作用穩(wěn)定1T-TMDs[69-70]。同時(shí), 通過等離子體轟擊、磁場(chǎng)誘導(dǎo)轉(zhuǎn)化、熱電子轟擊[71-72]等方式能夠獲得較穩(wěn)定的1T/2H雜化相。Wang等[73]通過將2H-MoS2置于Ar/P混合氣氛煅燒, 利用P原子向MoS2插層得到1T-2H雜化態(tài)2D MoS2, 其電導(dǎo)率相較2H MoS2塊體提升了500倍以上。

圖4 2D TMDs的M位摻雜[53, 55]

(a) ORR energy barrier for Pt-MoSe2with insets showing crystal structures[53]; (b) TEM images of Pt-SAs/2D TMDs prepared by galvanic replacement[55]

圖5 2D TMDs的X位摻雜[45, 61]

(a) ORR polarization curves of P-MoS2in 0.1 mol·L–1KOH[45]; (b) Corresponding possible reaction mechanism[61]

圖6 由2H-MoS2相轉(zhuǎn)變制備1T-MoS2納米片的(a)示意圖及其(b)在0.1 mol·L–1 KOH的ORR極化曲線與(c)K-L曲線[68]

Colorful figures are available on website

目前2H相TMDs向1T相轉(zhuǎn)化的方法大多條件嚴(yán)苛, 且不同相的比例難于精確控制, 限制了1T- TMDs的廣泛應(yīng)用。今后相關(guān)研究仍需要致力于探索較為簡(jiǎn)便的相轉(zhuǎn)變手段, 尤其是針對(duì)VB族TMDs的相轉(zhuǎn)變手段至今幾乎未見報(bào)道[24]。同時(shí), 將相轉(zhuǎn)變與相穩(wěn)定手段(如摻雜異質(zhì)元素)結(jié)合能夠得到更加穩(wěn)定的雜化相2D TMDs產(chǎn)物。需要指出的是, 2D TMDs的相轉(zhuǎn)變過程不可避免會(huì)引入缺陷與應(yīng)力, 在評(píng)估相關(guān)性能時(shí)需要考慮上述因素。

2.3 缺陷工程

2D TMDs的面內(nèi)結(jié)構(gòu)完整, 難以產(chǎn)生催化活性與高電導(dǎo)率。為此, 研究者們嘗試引入缺陷破壞2D TMDs的面內(nèi)結(jié)構(gòu), 以暴露更多配位不飽和原子, 提高活性位點(diǎn)數(shù)目。X空位是一種最常見的缺陷[34, 74], 這是由于X空位通常是一系列電催化反應(yīng)的有效活性位點(diǎn)[75-76]。Koh等[46]采用鋰離子(Li+)插層法從PdSe2塊體剝離出2D納米片。由于Li+的體積大于PdSe2的層間距, 在PdSe2晶格中引入殘余應(yīng)力, 最終形成大量Se空位。X射線光電子能譜(XPS)表征證實(shí)了Se形成了空位及其對(duì)氧氣的增強(qiáng)吸附作用(圖7(a, b))。該2D PdSe2的ORR塔菲爾斜率為64 mV·dec–1, 小于Pt/C的67 mV·dec–1, 表明ORR的動(dòng)力學(xué)得到有效促進(jìn)。通過改變?nèi)毕菀敕绞娇梢栽?D TMDs表面引入不同比例的缺陷。以MoS2為例, 目前報(bào)道的S空位引入量在原子比12.5%至15.62%(表面)之間[23]。

引入缺陷也使異質(zhì)元素?fù)诫s更為簡(jiǎn)便。Huang等[49]利用石墨相氮化碳作為自犧牲模板得到O-MoS2結(jié)構(gòu)(圖7(c))。由于氮化碳模板在燒結(jié)過程中釋放大量氣體, 得到的MoS2納米片具有多孔形貌, 暴露了大量邊角活性原子, 有助于提升電催化活性。在堿性環(huán)境中, O-MoS2的ORR半波電位達(dá)到0.80 V (圖7(d))。然而, 引入缺陷在一定程度上會(huì)破壞2D TMDs的完整結(jié)構(gòu), 降低原有電導(dǎo)率。目前雖然可以通過理論計(jì)算的方式預(yù)測(cè)缺陷的最優(yōu)存在形式與數(shù)量, 尤其是機(jī)器學(xué)習(xí)技術(shù)使得識(shí)別與構(gòu)建缺陷更加簡(jiǎn)便, 但在實(shí)際操作中仍然缺少向2D TMDs精確定量引入缺陷的方法, 特別是向大面積2D TMDs引入規(guī)整缺陷對(duì)其實(shí)際功能器件應(yīng)用具有重要意義。通過“自下而上”法在合成2D TMDs過程中構(gòu)建缺陷可作為未來研究的重要方向。另外, 表征與識(shí)別缺陷需要借助更加精確的觀測(cè)手段, 如球差校正透射電子顯微鏡、超景深顯微鏡等。

圖7 2D TMDs的缺陷工程與應(yīng)力工程調(diào)控[46, 49, 65]

(a) TEM images and (b) O1s XPS spectra of defected I-PdSe2[46]; (c) Schematic illustration of synthesis of O-MoS2; (d) ORR polarization curves in 0.1 mol·L–1KOH of O-MoS2[49]; (e) AFM (left) and TEM (middle and right) images of 2H-1T WS2showing the formation of strain[65]

2.4 應(yīng)力工程

催化劑中存在的應(yīng)力能夠改變體系態(tài)密度, 進(jìn)而影響催化劑對(duì)氧吸附行為[77]。而2D TMDs的單原子層/多原子層結(jié)構(gòu)使得應(yīng)力的產(chǎn)生與引入更加容易[78]。Zhao等[79]的理論計(jì)算表明拉伸應(yīng)力使硫?qū)僭豴軌道中心更加接近費(fèi)米能級(jí)位置, 增強(qiáng)了2D TMDs對(duì)ORR含氧中間體的吸附, 反之壓縮應(yīng)力能夠減弱這種作用。實(shí)際上, 上述所提到的異質(zhì)元素?fù)诫s、缺陷形成及多相復(fù)合等方式均可在2D TMDs內(nèi)部引入一定程度的應(yīng)力。如Voiry等[65]在WS2發(fā)生2H→1T相轉(zhuǎn)變的過程中發(fā)現(xiàn)兩相間的錯(cuò)配界面處存在各向同性的應(yīng)力作用(圖7(e)), 應(yīng)變值約為3%。應(yīng)力降低了催化反應(yīng)過程的勢(shì)壘, 且降低程度與應(yīng)變值正相關(guān)。Meng等[76]在研究空位對(duì)2D TMDs的ORR性能影響過程中發(fā)現(xiàn)不同空位在TMDs晶格中產(chǎn)生的應(yīng)力存在差異, 由此他們建立了應(yīng)力與含氧中間體吸附能之間的關(guān)系曲線, 發(fā)現(xiàn)空位產(chǎn)生壓縮應(yīng)力時(shí),*OOH中間體的吸附與解離是反應(yīng)的決速步驟, 而*OH中間體吸附能的變化則對(duì)拉伸應(yīng)力較為敏感。

其他引入應(yīng)力的方式還有基底外延生長(zhǎng)法[80]、構(gòu)建異質(zhì)結(jié)[81]等。與上述策略同樣, 雖然理論方式可以預(yù)測(cè)合適的應(yīng)力以達(dá)到調(diào)控效果, 但是目前實(shí)驗(yàn)手段產(chǎn)生的應(yīng)力難于精確達(dá)到理論值, 這主要是由于2D TMDs的單層/多層結(jié)構(gòu)使應(yīng)力難于控制, 需要更加細(xì)化應(yīng)力的引入方式。此外, 在相同應(yīng)力下, 不同的應(yīng)力取向?qū)?D TMDs結(jié)構(gòu)與性能的影響差異巨大, 在研究過程中同樣不可忽略。

總體而言, 2D TMDs優(yōu)異的電學(xué)性質(zhì), 及其單層/多層結(jié)構(gòu)帶來的高比表面積與暴露的大量面內(nèi)原子等優(yōu)勢(shì), 使得調(diào)控其性能滿足ORR應(yīng)用需求成為可能。采用理論計(jì)算方式預(yù)測(cè)2D TMDs的調(diào)控手段已經(jīng)較為完善, 但受限于實(shí)驗(yàn)條件, 上述調(diào)控策略的可操作性尚未達(dá)到成熟階段。

3 2D TMDs異質(zhì)結(jié)構(gòu)的ORR性能調(diào)控

異質(zhì)結(jié)構(gòu)能夠充分發(fā)揮每種組分本身的特點(diǎn), 同時(shí)通過協(xié)同作用提高總體的活性與穩(wěn)定性[82]。2D TMDs異質(zhì)結(jié)構(gòu)能夠利用其高比表面積的優(yōu)勢(shì), 同時(shí)提高導(dǎo)電性與催化性能。異質(zhì)結(jié)構(gòu)能夠帶來原半導(dǎo)體材料不具有的電學(xué)特性。近來, 研究者們已能夠通過光電子能譜儀與掃描隧道顯微鏡直接觀察到2D TMDs異質(zhì)結(jié)界面極化子的形成[83]。Liu等[84]闡明了半導(dǎo)體異質(zhì)結(jié)構(gòu)的電子聚集能力相較普通范德瓦爾斯力接觸提高4倍以上, 且提高了電子/空穴對(duì)分離效率。目前, 研究者們已探索出許多2D TMDs基異質(zhì)結(jié)構(gòu)設(shè)計(jì)策略, 包括2D TMDs@碳材料, 半導(dǎo)體@2D TMDs材料與大環(huán)化合物@2D TMDs等。表2總結(jié)了一些代表性2D TMDs異質(zhì)結(jié)構(gòu)的催化性能。

表2 典型2D TMDs異質(zhì)結(jié)構(gòu)ORR催化劑的性能

3.1 2D TMDs@碳材料異質(zhì)結(jié)

碳材料具有優(yōu)異的導(dǎo)電性, 但耐久性較弱, 制約了其大規(guī)模應(yīng)用。2D TMDs的穩(wěn)定性高, 有望延長(zhǎng)碳基ORR催化劑的使用壽命。Anwar等[85]在還原氧化石墨烯(rGO)上垂直生長(zhǎng)了2H-MoS2納米片, 呈現(xiàn)二維多層狀結(jié)構(gòu), 暴露了大量邊緣活性位點(diǎn)(圖8(a))。負(fù)載Pt納米顆粒后, Pt/MoS2-rGO經(jīng)10000次ORR循環(huán)后的活性損失為25%, 相較Pt/C(75%)提升明顯。此外, 相同負(fù)載量下Pt/MoS2-rGO的初始ORR催化活性較Pt/C提高了10%, 這得益于2D MoS2的高比表面積使負(fù)載的Pt充分暴露于ORR催化環(huán)境中(圖8(b))。

調(diào)控碳材料的微觀形貌能夠提高2D TMDs的負(fù)載量。Xin等[91]利用rGO表面含氧官能團(tuán)在水熱條件下的自組裝趨勢(shì)制備了三維多級(jí)MoSe2@rGO結(jié)構(gòu)。該復(fù)合材料呈玫瑰花狀結(jié)構(gòu), 且超薄MoSe2納米片均勻分布于高導(dǎo)電性rGO片層上。掃描電化學(xué)顯微鏡(Scanning electrochemical microscope, SECM)結(jié)果表明在氧氣飽和電解液中, 當(dāng)電壓由0.57 V增至0.77 V時(shí), MoSe2@rGO始終保持較高的電流密度, 而Pt/C、rGO與MoSe2在高電壓下產(chǎn)生明顯的電流衰減, 表明MoSe2@rGO具有較低的ORR反應(yīng)過電位。

一些含碳化合物模板(如石墨相氮化碳、功能高分子等)在高溫下能夠轉(zhuǎn)變成碳材料, 在此過程中加入其它原料可制備多種碳基異質(zhì)結(jié)構(gòu)。Hao等[92]利用F127高分子在水溶液中的定向自組裝制備了二維硫脲/三聚氰胺/F127模板, 并與MoS2混合煅燒后得到多孔氮摻雜MoS2/C納米片(圖8(c))。該結(jié)構(gòu)較高的孔隙率使得面內(nèi)存在大量邊界不飽和配位S原子, 同時(shí)提高了傳質(zhì)效率, 增強(qiáng)ORR動(dòng)力學(xué)。使用N-MoS2/C組裝的微生物燃料電池, 其最高功率密度為0.805 W·m–2, 遠(yuǎn)高于Pt/C的0.465 W·m–2(圖8(d))。此外, 碳納米管(CNT)[86]、碳納米纖維[93]等碳材料也常用于2D TMDs的ORR催化劑基底。但目前2D TMDs@碳材料異質(zhì)結(jié)多采用水熱法制備, 需要探索其它合成手段來提高2D TMDs的結(jié)構(gòu)完整度。

圖8 2D TMDs@碳材料異質(zhì)結(jié)ORR催化劑[85, 92]

(a) TEM images and (b) activity variations of Pt/MoS2-rGO[85]; (c) SEM image and (d) power density of microbial fuel cells based on N-MoS2/C catalysts[92]

Colorful figures are available on website

3.2 半導(dǎo)體@2D TMDs異質(zhì)結(jié)

兩種半導(dǎo)體形成異質(zhì)結(jié)時(shí), 在費(fèi)米能級(jí)達(dá)到平衡的過程中發(fā)生電子轉(zhuǎn)移, 導(dǎo)致在兩種材料界面處形成電子濃度較小的耗盡層[94-95]。耗盡層對(duì)電子濃度的變化較為敏感, 使其產(chǎn)生更明顯的中間體吸附效應(yīng)[96]。Mao等[88]利用這種界面作用在MoS2納米片上負(fù)載Ni3S2, 并在透射電鏡下觀察到大量Ni3S2/ MoS2界面(圖9(a))。邊界Mo原子與Mo-Ni-S共同起到ORR活性位點(diǎn)作用, 使Ni3S2/MoS2納米片的ORR半波電位較Pt/C提高了22 mV(圖9(b)), 且在30000 s持續(xù)通電測(cè)試后電流密度僅衰減了10%。Roy等[89]采用液相插層剝離法構(gòu)建了h-BN/MoS2異質(zhì)結(jié)(圖9(c))。h-BN與MoS2之間的晶格錯(cuò)配使h-BN面內(nèi)富集了B空位, 帶隙減小的同時(shí)極大提升了ORR動(dòng)力學(xué)。與此同時(shí), h-BN/MoS2保留了兩種二維材料優(yōu)異的穩(wěn)定性, 經(jīng)2000次循環(huán)后活性仍有92.80%(圖9(d)), 同時(shí)抗甲醇毒化能力強(qiáng), 成本低廉, 適用于金屬–空氣電池。

其它關(guān)于半導(dǎo)體TMDs異質(zhì)結(jié)構(gòu)ORR催化劑的報(bào)道還包括CoO/mC@MoS2[97]、Fe3O4@MoS2[98]、Co9S8@MoS2[99]等。然而, 上述異質(zhì)結(jié)構(gòu)微觀形貌大多為納米顆粒形式, 在2D TMDs搭建半導(dǎo)體異質(zhì)結(jié)用于ORR催化劑的研究目前相對(duì)較少。

3.3 2D TMDs@大環(huán)化合物異質(zhì)結(jié)構(gòu)

大環(huán)化合物可設(shè)計(jì)性強(qiáng), 通過配體調(diào)控能夠改變電子結(jié)構(gòu)[100]。但大環(huán)化合物本征電導(dǎo)率較低, 且高分子容易在氧還原環(huán)境中降解, 導(dǎo)致穩(wěn)定性較差。將大環(huán)化合物與高穩(wěn)定性基底復(fù)合可在一定程度上提高耐久性。Kwon等[90]通過水熱法將鐵酞菁(FePc)固定在1T’-MoS2層間。FePc與MoS2間的電荷相互作用使FePc帶正電荷, 且DFT計(jì)算表明FePc-MoS2面內(nèi)電子濃度較MoS2更高(圖9(e,f))。該復(fù)合結(jié)構(gòu)的ORR半波電位達(dá)到0.89 V, 遠(yuǎn)高于Pt/C的0.84 V, 并且連續(xù)測(cè)試20 h后活性僅衰減9.4%。Samanta等[101]同樣研究了銅酞菁(CuPc)- MoS2異質(zhì)結(jié)的ORR活性, 其在8000 s循環(huán)后ORR活性仍有97.6%, 而Pt/C只有87.4%。

由于異質(zhì)結(jié)構(gòu)合成手段多樣, 操作簡(jiǎn)便, 有關(guān)TMDs異質(zhì)結(jié)構(gòu)的ORR催化劑的研究呈不斷增長(zhǎng)趨勢(shì)。然而一些合成方式得到的產(chǎn)物結(jié)構(gòu)并非二維納米材料, 2D TMDs異質(zhì)結(jié)構(gòu)的ORR性能仍亟待探索。此外, 異質(zhì)結(jié)構(gòu)的形成對(duì)產(chǎn)物的ORR性能影響機(jī)制的研究仍然缺乏。

圖9 其它2D TMDs基異質(zhì)結(jié)構(gòu)ORR催化劑[88-90]

(a) TEM image of Ni3S2/MoS2and (b) corresponding ORR polarization curves in 0.1 mol·L–1KOH[88]; (c) TEM image of hBN/MoS2and its (d) ORR durability test[89]; (e) Structure of FePc-MoS2and (f) its integrated partial density of states IPDOS[90]

4 總結(jié)與展望

現(xiàn)有Pt/C等貴金屬ORR催化劑成本高昂, 且穩(wěn)定性有待提高, 亟需開發(fā)高耐久性、低成本的替代品。相較傳統(tǒng)碳材料與其它半導(dǎo)體(如TiO2, SnO2等), 2D TMDs材料具備二維材料特有的高比表面積、高比容量和可調(diào)帶隙, 且維持了TMDs優(yōu)異的結(jié)構(gòu)與化學(xué)穩(wěn)定性。同時(shí)相比其它二維材料(如石墨烯), 單層2D TMDs由于量子限域效應(yīng)能夠產(chǎn)生特殊的電學(xué)性質(zhì), 這是2D TMDs與其他電催化劑材料的最大區(qū)別。然而, 面內(nèi)活性位點(diǎn)缺乏及電導(dǎo)率低下, 制約了其在ORR領(lǐng)域的應(yīng)用。大量理論研究表明異質(zhì)元素?fù)诫s、相轉(zhuǎn)變與缺陷/應(yīng)力引入能夠改變2D TMDs的電子結(jié)構(gòu), 使氧吸附行為發(fā)生變化, 進(jìn)而提高催化活性。為了進(jìn)一步提高2D TMDs的催化性能, 研究者們還探索了2D TMDs同碳材料、半導(dǎo)體材料與功能高分子材料等的異質(zhì)結(jié)構(gòu), 期望通過協(xié)同效應(yīng)提高其導(dǎo)電性與ORR性能。目前, 已經(jīng)能從理論計(jì)算較好地預(yù)測(cè)與模擬2D TMDs的ORR活性, 實(shí)驗(yàn)性能也得到了一定提升。未來工作可從以下角度進(jìn)行改進(jìn)。

1)材料制備方面。目前合成高質(zhì)量2D TMD仍然是困擾相關(guān)領(lǐng)域研究的難題。“自下而上”合成與“自上而下”合成方式均存在產(chǎn)率低、條件苛刻的不足; CVD等方式制備的2D TMDs結(jié)構(gòu)完整, 成分形貌可調(diào), 但目前CVD法制備的結(jié)構(gòu)種類仍然受限。開發(fā)高通量CVD合成策略能夠從一定程度上解決此問題。近來, Zhou等[102]通過表面等離子轟擊主動(dòng)干預(yù)2D TMDs的微結(jié)構(gòu), 調(diào)控其生長(zhǎng), 觀察到表面產(chǎn)生了大量含氧活性物種(·O2?與·OH等), 為精確合成與調(diào)控2D TMDs結(jié)構(gòu)提供了新思路。

2)反應(yīng)機(jī)理方面。目前大量理論研究以氧中間體吸附能作為描述符預(yù)測(cè)2D TMDs的ORR性能, 然而實(shí)際ORR過程會(huì)產(chǎn)生多種中間體, 每種體系的決速步驟涉及的中間體不盡相同, 需要采用更加精確的描述符為ORR性能的調(diào)控提供指導(dǎo)。此外, 采用原位表征技術(shù)實(shí)時(shí)觀察ORR反應(yīng)過程中催化劑表面電子態(tài)的變化能夠更加準(zhǔn)確判斷2D TMDs的活性 位點(diǎn)。

3)性能調(diào)控方面。目前從實(shí)驗(yàn)角度仍然沒有較為有效的摻雜、相調(diào)控與缺陷/應(yīng)力引入方式。鑒于此, 使用原位透射電鏡技術(shù)觀察2D TMDs的生長(zhǎng)模式可以幫助研究者們更好地了解性能調(diào)控手段。此外, 構(gòu)建異質(zhì)結(jié)構(gòu)也是發(fā)揮TMDs特性的有效策略, 但針對(duì)2D TMDs異質(zhì)結(jié)構(gòu)的ORR性能研究還很缺乏, 需要在今后的工作中予以關(guān)注。

4)反應(yīng)應(yīng)用方面。雖然四電子反應(yīng)是燃料電池常用的氧還原反應(yīng)路徑, 但其它氧還原反應(yīng), 如兩電子反應(yīng)生成過氧化氫(H2O2)也是目前研究較多的類型。一般通過調(diào)控催化劑中間體的產(chǎn)生途徑與吸附能可有效切換2D TMDs ORR催化劑的反應(yīng)類型。受篇幅與主題限制, 該部分未進(jìn)行過多描述, 但同樣值得研究。

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Two-dimensional Transition Metal Dichalcogenides for Electrocatalytic Oxygen Reduction Reaction

SUN Lian, GU Quanchao, YANG Yaping, WANG Honglei, YU Jinshan, ZHOU Xingui

(Science and Technology on Advanced Ceramic Fibers and Composites Laboratory, College of Aerospace Science and Engineering, National University of Defense Technology, Changsha 410073, China)

Oxygen reduction reaction (ORR) is the key reaction in cathode for fuel cells. Because of the sluggish kinetics, platinum (Pt) is widely used as the electrocatalysts for ORR. However, the high cost of Pt and poor stability of carbon black support under high voltage limit the commercialization and durability of fuel cells. Two-dimensional transition metal dichalcogenides (2D TMDs) possess large specific area, tunable electronic structure, and high chemical stability, making them a good candidate for ORR catalysts with high activity and durability. This paper reviews the recent progress of 2D TMDs-based ORR electrocatalysts. First, crystal structure, electronic properties, and ORR reaction mechanism are briefly introduced. Then some strategies for adjusting ORR performance of 2D TMDs are summarized, including heteroatom doping, phase conversion, defect engineering, and strain engineering. Meanwhile, the ORR activity enhancement arising from 2D TMDs-based heterostructures is also analyzed. Finally, perspectives are given for current issues and their possible solutions.

oxygen reduction reaction; two-dimensional material; transition metal dichalcogenide; electrocatalysis; review

1000-324X(2022)07-0697-13

10.15541/jim20220128

O614

A

2022-03-08;

2022-04-12;

2022-04-26

國(guó)家重點(diǎn)研發(fā)計(jì)劃(2018YFB1900603) National Key R&D Plan (2018YFB1900603)

孫煉(1993–), 男, 博士研究生. E-mail: sunlian12@nudt.edu.cn

SUN Lian (1993–), male, PhD candidate. E-mail: sunlian12@nudt.edu.cn

周新貴, 教授. E-mail: zhouxinguilmy@163.com

ZHOU Xingui, professor. E-mail: zhouxinguilmy@163.com