辛洋，郭鵬，李昊，陳仁德，孫麗麗，馬冠水，汪愛(ài)英,3

質(zhì)子交換膜燃料電池金屬雙極板改性碳基涂層技術(shù)研究進(jìn)展

辛洋1,2，郭鵬1，李昊1，陳仁德1，孫麗麗1，馬冠水1，汪愛(ài)英1,3

（1.中國(guó)科學(xué)院寧波材料技術(shù)與工程研究所 a.中國(guó)科學(xué)院海洋新材料與應(yīng)用技術(shù)重點(diǎn)實(shí)驗(yàn)室 b.浙江省海洋材料與防護(hù)技術(shù)重點(diǎn)實(shí)驗(yàn)室，浙江 寧波 315201；2.中國(guó)科學(xué)技術(shù)大學(xué) 納米科學(xué)技術(shù)學(xué)院，江蘇 蘇州 215123；3.中國(guó)科學(xué)院大學(xué) 材料與光電研究中心，北京 100049）

對(duì)比了碳基涂層改性金屬極板、未涂覆的金屬極板和傳統(tǒng)石墨極板性能的優(yōu)劣，闡述了碳基涂層在優(yōu)化金屬極板導(dǎo)電耐蝕性能方面取得的最新成果，以及在質(zhì)子交換膜燃料電池（Proton exchange membrane fuel cells, PEMFCs）環(huán)境長(zhǎng)期運(yùn)行后，碳基涂層出現(xiàn)性能失效及壽命受限等問(wèn)題。通過(guò)分析影響碳基涂層性能的因素，指出由于非晶碳材料設(shè)計(jì)、微觀結(jié)構(gòu)等對(duì)其性能影響規(guī)律的系統(tǒng)化研究不足，導(dǎo)致非晶碳涂層/金屬極板損傷及退化機(jī)理不明確。重點(diǎn)闡述了國(guó)內(nèi)外關(guān)于PEMFCs金屬極板改性碳基涂層材料技術(shù)的研究進(jìn)展，包括調(diào)控本征碳基涂層（a-C）微觀形貌優(yōu)化涂層性能；采用理論計(jì)算與實(shí)驗(yàn)相結(jié)合的方法制備金屬摻雜碳基涂層(a-C: Me)，解決涂層與特定金屬基體間粘附性差、壓應(yīng)力高等問(wèn)題；設(shè)計(jì)多層結(jié)構(gòu)碳基涂層以減少貫穿性缺陷。探討了幾類涂層失效機(jī)制，并對(duì)金屬極板改性用碳基涂層技術(shù)進(jìn)行了展望。

質(zhì)子交換膜燃料電池；雙極板；碳基涂層；導(dǎo)電性能；耐腐蝕性能

近年來(lái)，隨著工業(yè)的不斷發(fā)展，能源枯竭和環(huán)境污染等問(wèn)題日益突出，氫能與燃料電池技術(shù)是公認(rèn)的有效解決方案之一[1-2]。其中，質(zhì)子交換膜燃料電池（Proton exchange membrane fuel cells, PEMFCs）具有低運(yùn)行溫度、零排放、高比功率和高能量轉(zhuǎn)換率等優(yōu)點(diǎn)，在車用動(dòng)力電源、便攜設(shè)備和航空等領(lǐng)域前景廣闊[3-4]。PEMFCs主要由雙極板、膜電極組件、端板和密封件等組成[5]。雙極板作為PEMFCs的核心多功能組件，起到均勻分配氣體、排水、導(dǎo)熱、導(dǎo)電等作用，其質(zhì)量占整個(gè)燃料電池的60%，成本占15%～20%[6-9]，其性能和成本會(huì)直接影響電池的使用壽命和商業(yè)化進(jìn)程。傳統(tǒng)石墨極板由于體積大、制造成本高以及力學(xué)性能較差，逐漸被可加工性強(qiáng)、導(dǎo)電導(dǎo)熱性優(yōu)、力學(xué)性能好的金屬雙極板取代[10-12]。然而在電池酸性工作環(huán)境中，金屬極板易腐蝕，腐蝕過(guò)程中形成的金屬離子會(huì)導(dǎo)致質(zhì)子交換膜離子傳輸效率下降，同時(shí)在金屬雙極板表面形成的鈍化膜會(huì)增大界面接觸電阻（Interface contact resistance, ICR），從而導(dǎo)致燃料電池性能下降[13-15]。

表面涂層材料技術(shù)可以在保持金屬極板優(yōu)異的力學(xué)性能和加工性能的基礎(chǔ)上，提高金屬極板的電導(dǎo)率和耐蝕性[16]。Wang等[17]研究了金涂層改性鈦金屬雙極板的短期性能，可減少金屬氧化物的形成和金屬離子的溶出。Feng等[18]利用離子注入技術(shù)，將Ag離子注入到316L不銹鋼基體，成功制備出厚度45 nm富銀層。相比于基體，富銀表面層改善了基體耐腐蝕性和抗極化性。該團(tuán)隊(duì)[19]還通過(guò)相同的方法在SS316L表面制備了富鎳層，改善極板導(dǎo)電耐蝕性，但由于成本較高，貴金屬層技術(shù)不適合大批量商業(yè)生產(chǎn)。Mohammadi等人[20]利用二氧化鉛（PbO2）具有成本低和在H2SO4中電導(dǎo)率高的特點(diǎn)，通過(guò)電沉積技術(shù)在SS316L上制備PbO2涂層。在模擬PEMFCs環(huán)境下，PbO2涂層易發(fā)生局部腐蝕，難以滿足極板的耐蝕性能需求。Gonzalez-Rodriguez等人[21]利用電化學(xué)沉積法在SS304極板上制備的聚吡咯涂層具有較高的耐腐蝕性，但長(zhǎng)時(shí)間運(yùn)行后，出現(xiàn)涂層降解。A. Orsi等[22]通過(guò)物理氣相沉積工藝將氮化鈦（TiN）涂層沉積到SS316L基體上，并對(duì)其耐蝕性能和ICR進(jìn)行評(píng)估。在0.8~1.4 V（vs. SHE）的范圍內(nèi)測(cè)量電位對(duì)TiN涂層的影響，恒電位極化數(shù)據(jù)表明，電流密度隨電勢(shì)的降低而降低。另外，與未涂覆的SS316L相比，在1.4 V（vs. SHE）電位下，涂覆TiN涂層的SS316L的ICR從12.9 mW·cm2增大到287 mW·cm2。Zhang等[16]使用兩種表面改性技術(shù)制備TiN涂層，即通過(guò)磁控濺射技術(shù)制備SS304/Ti2N/TiN涂層和通過(guò)脈沖偏壓電弧離子鍍技術(shù)制備SS304/TiN涂層，兩種涂層均具有較好的耐蝕性能，但改性極板界面接觸電阻較大，不能滿足2020年美國(guó)能源部（United States Department of Energy, DOE）燃料電池技術(shù)指標(biāo)（腐蝕電流密度小于1.00 μA/cm2，ICR小于10 mW·cm2）。N. D. Nam等人[23]研究了射頻磁控濺射技術(shù)制備的TiN/CrN多層涂層的電化學(xué)行為與TiN/CrN涂層厚度比的關(guān)系。在模擬燃料電池（1 mol/L H2SO4+2 mg/L F–, 70 ℃）陰極環(huán)境下進(jìn)行10 h動(dòng)電位極化測(cè)試后，不同厚度比的TiN/CrN多層涂層的腐蝕電流密度均增加到15 μA/cm2左右。綜合考慮成本與性能改善，由于非晶碳涂層兼具優(yōu)異的導(dǎo)電性和耐腐蝕性能，且規(guī)模化成本優(yōu)勢(shì)顯著，在PEMFCs金屬極板的應(yīng)用中引起了廣泛關(guān)注。非晶碳（Amorphous carbon, a-C）[24]是一大類無(wú)定型碳的總稱，其結(jié)構(gòu)主要由sp2雜化（類石墨結(jié)構(gòu)）和sp3雜化（類金剛石結(jié)構(gòu)）的原子碳組成。sp3雜化能有效阻止腐蝕離子的侵蝕，而sp2雜化主要影響導(dǎo)電性。因此通過(guò)平衡sp2和sp3雜化的比例，可以獲得高導(dǎo)電、高耐蝕并且具有良好力學(xué)性能的非晶碳涂層。

目前，國(guó)內(nèi)外多個(gè)科研團(tuán)隊(duì)已經(jīng)開(kāi)發(fā)了多種方法制備金屬雙極板改性碳基涂層（見(jiàn)表1），主要包括物理氣相沉積[25-28]和化學(xué)氣相沉積[3,29]等方法。通過(guò)化學(xué)氣相沉積（CVD）法在SS304基體上制備a-C涂層，涂層耐蝕導(dǎo)電性能滿足美國(guó)DOE指標(biāo)。Chung等[29]使用CVD技術(shù)在SS304表面制備了碳涂層，發(fā)現(xiàn)涂層的表面形態(tài)取決于CVD沉積過(guò)程中氣體碳源的濃度。該方法可以通過(guò)調(diào)控氣相組成制備具有不同化學(xué)成分的涂層，從而獲得梯度或復(fù)合鍍層，但CVD技術(shù)的沉積功率較低，且成膜時(shí)工件溫度高，在應(yīng)用上受到一定限制。相比之下，物理氣相沉積（PVD）法制備的涂層ICR均小于CVD涂層（8.90 mW·cm2），具有較好的電導(dǎo)率[3]。Bi等[30]研究了偏置電壓對(duì)閉合場(chǎng)非平衡磁控濺射離子鍍（Closed field unbalanced magnetron sputter ion plating, CFUBMSIP）沉積的a-C涂層微觀結(jié)構(gòu)和性能的影響，成功制備出具有致密結(jié)構(gòu)的a-C涂層。Wu等[31]通過(guò)脈沖偏壓電弧離子鍍（PBAIP）在SS316L上沉積一系列含鉻的碳涂層。涂層中sp3和sp2碳原子的含量受摻雜鉻的影響很大，且ICR與sp3和sp2碳原子的含量密切相關(guān)。Cr0.23C0.77涂層獲得最低的ICR（壓實(shí)壓力為1.20 MPa，ICR為2.8 mW·cm2）以及最佳的耐腐蝕性。與CVD法相比，PVD沉積工藝過(guò)程簡(jiǎn)單，且成膜均勻致密。此外，Y. J. Ren等[9]通過(guò)高能微弧合金化（HEMAA）技術(shù)獲得的涂層比PVD方法制備的涂層結(jié)構(gòu)更致密，該涂層的腐蝕電流密度僅為0.034 μA/cm2，且在模擬PEMFCs的陰極工作環(huán)境下浸泡30天后，仍然具有較高的穩(wěn)定性。因此，與PVD涂層相比，由HEMAA制備的TiC涂層可以更有效地阻止腐蝕物質(zhì)到達(dá)基體，成為阻止腐蝕物質(zhì)向內(nèi)滲透的有效屏障?？傮w上，對(duì)于含氫、無(wú)氫非晶碳，采用PVD沉積工藝獲得的涂層材料具有更好的耐蝕性與更低的接觸電阻，這可能與H組分、電阻率有關(guān)。Asri等[32]充分探討了雙極板涂層與表面處理方法間的聯(lián)系，客觀評(píng)估了涂層制備方法和涂層性能。Yi等[33]、Bi等[30]和Wang等[34]從涂層結(jié)構(gòu)設(shè)計(jì)和組分調(diào)控等多方面研究了碳基涂層性能。在產(chǎn)業(yè)方面，上海YOOGLE（佑戈）公司和常州翊邁新材料科技有限公司等也開(kāi)發(fā)了系列碳基涂層改性的金屬雙極板產(chǎn)品。研究結(jié)果表明，采用非晶碳涂層材料技術(shù)可以顯著提高多種金屬極板的耐蝕性與導(dǎo)電性，同時(shí)部分產(chǎn)品已應(yīng)用于上汽集團(tuán)的榮威750、950及大通V80系列車型上，實(shí)測(cè)性能良好，市場(chǎng)應(yīng)用潛力巨大。

表1 不同沉積方法對(duì)雙極板碳基涂層性能的影響

Tab.1 Effects of different deposition methods on the performance of bipolar plate carbon-based coating

碳基涂層改性金屬極板具有良好的機(jī)械性、優(yōu)異的耐蝕導(dǎo)電性能和較低的價(jià)格，適合大批量商業(yè)生產(chǎn)。由于國(guó)內(nèi)外研究的具體應(yīng)用工況和需求不同，涉及的金屬極板種類多樣，包括SS316L、SS304、Ti、Ti6Al4V等金屬極板，研究團(tuán)隊(duì)針對(duì)不同的金屬極板所開(kāi)發(fā)的耐蝕導(dǎo)電非晶碳材料技術(shù)差異巨大。本文從非晶碳涂層材料組分、結(jié)構(gòu)設(shè)計(jì)角度出發(fā)，綜述了本征非晶碳涂層、金屬摻雜非晶碳涂層以及多層結(jié)構(gòu)非晶碳涂層在該領(lǐng)域的研究進(jìn)展，并對(duì)碳基涂層改性金屬極板的研究方向進(jìn)行了展望。

1 本征非晶碳涂層

Lee等[45]利用低溫加速C60離子束技術(shù)制備納米復(fù)合碳改性316L不銹鋼。如圖1所示，在模擬電池陰極環(huán)境（0.5 mol/L H2SO4+2 mg/L HF，80 ℃）下，涂層改性極板的腐蝕電流密度降低為0.23 μA/cm2；而在模擬電池陽(yáng)極環(huán)境下，碳涂層的腐蝕電流密度為0.05 μA/cm2，有效地抑制了基體的腐蝕。改性極板界面接觸電阻降低有限，仍大于10 mW·cm2。Afshar等[27]考察了基板溫度對(duì)碳涂層結(jié)構(gòu)和電化學(xué)性能的影響。如拉曼光譜（圖2a）所示，隨著基體溫度的增加，D峰強(qiáng)度增加，碳涂層中小尺寸納米晶石墨或短程有序石墨結(jié)構(gòu)增多。不同基板溫度下，經(jīng)碳涂層涂覆的316L不銹鋼的動(dòng)電位極化測(cè)試結(jié)果如圖2b所示?；鍦囟葧?huì)改變納米晶石墨的晶粒尺寸、缺陷和粗糙度，從不同程度影響了涂層導(dǎo)電耐蝕性能。類似地，Show等[46]研究對(duì)比了a-C涂覆鈦極板在550、600 ℃時(shí)的性能。與未處理的基體相比，a-C涂層可將接觸電阻降低1/2。

圖1 改性碳涂層與SS316L在陽(yáng)極和陰極環(huán)境下的動(dòng)電位極化曲線

圖2 不同溫度沉積的非晶碳涂層拉曼光譜和動(dòng)電位極化曲線

Feng等[47]利用閉合場(chǎng)非平衡磁控濺射技術(shù)在316L不銹鋼表面涂覆純a-C涂層，并在80 ℃下浸泡在0.5 mol/L H2SO4+2 mg/L HF的腐蝕液中進(jìn)行動(dòng)電位極化測(cè)試。與未涂覆SS316L相比，涂層改性的SS316L表現(xiàn)出更高的耐腐蝕潛能，腐蝕電流密度顯著降低至1.85 μA/cm2，ICR低于未處理的SS316L。同時(shí)，該團(tuán)隊(duì)[48]制備出連續(xù)且致密的a-C涂層，純a-C涂層改性不銹鋼極板具有比石墨極板更好的性能。

此外，在考慮到結(jié)合力等實(shí)際需求，本征非晶碳涂層在使用中涉及到過(guò)渡層的使用。Li等[49]采用模擬計(jì)算分析常用W、Ti、Cr等金屬層對(duì)于非晶碳的催化作用。如圖3所示，在a-C@Ti系統(tǒng)中，a-C結(jié)構(gòu)中的sp2-C含量隨著溫度升高到900 K而逐漸升高，隨著溫度進(jìn)一步升高到1500 K而降低。在a-C@Cr和a-C@W系統(tǒng)中，隨著溫度的變化，sp2-C含量的變化趨勢(shì)與a-C@Ti相反。因此，在較低的溫度（<900 K）下，Ti比Cr或W對(duì)a-C結(jié)構(gòu)的催化作用更好；而在較高的溫度（>900 K）下，Cr或W更易使a-C轉(zhuǎn)變?yōu)槭Y(jié)構(gòu)。人們從原子尺度上比較研究這三種過(guò)渡層與a-C涂層之間的界面結(jié)構(gòu)，明確了界面處金屬-碳作用情況以及沿涂層厚度方向sp3/sp2鍵態(tài)含量演變規(guī)律。Wu等人[50]通過(guò)直流磁控濺射技術(shù)在SS304上制備Cr/a-C涂層，發(fā)現(xiàn)在Cr過(guò)渡層與a-C涂層界面處形成了互鎖結(jié)構(gòu)，Cr/a-C涂層的內(nèi)部缺陷成功地從互鎖結(jié)構(gòu)中錯(cuò)開(kāi)，有效地阻止了腐蝕液到達(dá)基體，提高了電極相關(guān)性能。

圖3 溫度升高過(guò)程中三種過(guò)渡層與a-C涂層的雜化碳結(jié)構(gòu)演變

在本征非晶碳損傷機(jī)理方面，Bi等[30]通過(guò)對(duì)涂層沉積技術(shù)參數(shù)進(jìn)行調(diào)控，成功制備出具有不同微觀形貌的a-C涂層。結(jié)構(gòu)最致密涂層的腐蝕電流密度僅為0.6 μA/cm2，遠(yuǎn)小于2020年美國(guó)DOE技術(shù)指標(biāo)。涂層ICR值也僅為2 mW·cm2，是具有最疏松結(jié)構(gòu)涂層的ICR值的1/20。基于測(cè)試前后非晶碳表面組分分析，該團(tuán)隊(duì)提出了表層鈍化作用以及類石墨組分（sp2）氧化導(dǎo)致極板性能退化的機(jī)制，在金屬雙極板改性碳基涂層機(jī)理研究方面處于領(lǐng)先地位。Li等[51-52]利用直流磁控濺射技術(shù)制備了系列SS316L/a-C極板，發(fā)現(xiàn)在模擬PEMFCs工作條件下，a-C涂層可顯著提高SS316L的耐蝕導(dǎo)電性能。通過(guò)優(yōu)化濺射功率和磁場(chǎng)強(qiáng)度，發(fā)現(xiàn)在0.9 kW沉積的a-C涂層具有最低腐蝕電流密度（7.52×10–3μA/cm2）和初始接觸 電阻（2.91 mΩ·cm2）。12 h腐蝕測(cè)試后，ICR增至4.00 mΩ·cm2，且可在a-C/SS316L界面處觀測(cè)到Cr2O3的富集（如圖4所示）。這種氧化物具有良好的化學(xué)惰性和絕緣性，在抵抗基體腐蝕的同時(shí)，也一定程度上阻礙了電流的傳輸，是導(dǎo)致接觸電阻上升的主要原因之一，從而提出了界面損傷導(dǎo)致a-C/SS316L性能退化的機(jī)理。

2 金屬摻雜非晶碳涂層

上述非晶碳涂層雖使金屬極板具有良好的導(dǎo)電耐蝕性能，但仍然存在應(yīng)力高、易剝落等問(wèn)題。涂層中的殘余應(yīng)力會(huì)顯著影響到涂層的結(jié)合強(qiáng)度、抗疲勞、耐蝕等性能，也是引起涂層表面裂紋、剝落的重要因素。如果涂層中殘余應(yīng)力較高，則會(huì)在涂層中產(chǎn)生更多的裂紋缺陷，涂層也更容易脫落。尤其是在腐蝕過(guò)程中，腐蝕性介質(zhì)通過(guò)涂層中的裂紋、通孔等缺陷到達(dá)膜基界面位置，形成點(diǎn)蝕，進(jìn)而加速膜基界面失效。金屬摻雜后可有效地減少涂層中的殘余應(yīng)力，這是因?yàn)榻饘僭优c碳原子之間存在電負(fù)性的差別，使得鍵中存在離子部分的貢獻(xiàn)，降低了鍵的方向性和對(duì)鍵角畸變的敏感度[53-54]。金屬元素?fù)诫s非晶碳會(huì)產(chǎn)生離子鍵、共價(jià)鍵、非鍵和反鍵四類成鍵特征，鍵的離子相互作用導(dǎo)致殘余應(yīng)力大大降低[55-58]。因此，金屬（Ti、Mo、Cr、Al或W等）元素?fù)诫sa-C涂層的方法可解決涂層因應(yīng)力較高而剝落失效的問(wèn)題，同時(shí)也優(yōu)化了涂層與基體的電導(dǎo)率或化學(xué)親和力[59-61]。Hou等人[36]利用第一性原理計(jì)算揭示了Nb摻雜a-C涂層中sp2/sp3的變化規(guī)律，并對(duì)碳原子鍵合、結(jié)構(gòu)、摻雜碳原子狀態(tài)以及電荷密度分布進(jìn)行了模擬，并利用CFUBMSIP方法制備了Nb摻雜a-C涂層。結(jié)果表明，涂覆極板的腐蝕電流密度均低于未涂覆基體SS316L，經(jīng)過(guò)恒電位極化測(cè)試24 h后，腐蝕電流密度隨著摻雜Nb含量增加而降低。在1.40 MPa測(cè)試條件下，摻Nb樣品的ICR值均低于純a-C涂層。

Wang等人[62]在SS304表面制備了鈮摻雜a-C涂層，使改性極板具有8.47 mΩ·cm2的低界面接觸電阻和良好的耐腐蝕性（如圖5所示）。在模擬燃料電 池（0.5 mol/L H2SO4+2 mg/L HF，70 ℃）陰極環(huán) 境下進(jìn)行動(dòng)電位極化測(cè)試，涂層的腐蝕電流密度為0.051 μA/cm2。極化10 h后，接觸電阻增至9.04 mΩ·cm2。Andersson等[63]使用非反應(yīng)直流磁控濺射技術(shù)分別沉積了不同碳含量的金屬摻雜碳基涂層。不同碳含量涂層的微觀結(jié)構(gòu)有明顯不同（如圖6所示），并導(dǎo)致涂層的導(dǎo)電耐蝕性能具有較大差異。該課題組[15]將低成本電鍍工藝應(yīng)用到金屬摻雜碳基涂層制備中，隨著涂層中碳含量增加，ICR值降低。類似地，研究者[1]通過(guò)CFUBMSIP技術(shù)沉積了具有不同碳含量的金屬摻雜碳基涂層，其電化學(xué)性能如圖7所示。隨著Cr靶電流的增加，涂層中Cr含量增加，且金屬碳化物減少。在模擬電池陰極環(huán)境下，各涂層耐蝕性順序依次為Cr0.75C5>Cr0.5C5> Cr1C5>Cr1.5C5>Cr2C5。因此，通過(guò)工藝參數(shù)調(diào)控碳含量得到相似結(jié)論：涂層的耐蝕性與涂層組成密切相關(guān)，即隨著C含量增加，耐蝕性也增加。Wang等人[37]通過(guò)CFUBMSIP技術(shù)制備W摻雜非晶碳涂層，發(fā)現(xiàn)了涂層具備自鈍化能力。摻W的a-C涂層結(jié)構(gòu)致密，并且隨著W濃度的不同，涂層的相組成和表面形貌也發(fā)生了略微變化。在1.50 MPa壓實(shí)壓力下，摻雜濃度不同，ICR在6.25~ 7.21 mW·cm2內(nèi)波動(dòng)。在豐富的理論計(jì)算指導(dǎo)下和大量的實(shí)驗(yàn)研究中，研究人員成功制備出各種單元金屬摻雜非晶碳涂層。實(shí)驗(yàn)結(jié)果表明，通過(guò)改變工藝參數(shù)可制備出具有良好耐腐蝕性和導(dǎo)電性的金屬摻雜非晶碳涂層，且通過(guò)微量摻雜金屬粒子，可有效地改善極板涂層的表面電導(dǎo)率和耐腐蝕性能。

圖5 不同壓實(shí)壓力下Nb-C/SS304和SS304腐蝕前后的ICR

圖6 碳含量不同的CrxCy涂層的截面形貌

圖7 碳含量不同的金屬摻雜碳基涂層的電化學(xué)性能

基于單一元素?fù)诫s的理論研究與實(shí)驗(yàn)結(jié)果，研究人員對(duì)多元摻雜非晶碳涂層在金屬極板改性方面也做了探索。Li等[58]基于不同摻雜金屬之間的特性互補(bǔ)和材料計(jì)算學(xué)優(yōu)勢(shì)，進(jìn)行了多元摻雜復(fù)合涂層的組分優(yōu)化設(shè)計(jì)。Ti/Al、Cr/Al或W/Al共摻雜a-C涂層的殘余壓應(yīng)力和成鍵特征如圖8所示。與純a-C涂層相比，Ti/Al、Cr/Al或W/Al共摻雜的a-C涂層具有降低應(yīng)力的總體趨勢(shì)。Ti/Al、Cr/Al或W/Al共摻雜的a-C涂層會(huì)嚴(yán)重扭曲C—C鍵長(zhǎng)度，并形成弱的共價(jià)鍵，從而形成Ti—C、Cr—C、W—C以及Al—C鍵的離子相互作用，導(dǎo)致殘余應(yīng)力大大降低，如圖8b所示。Zhang等[64]將Ag和Cr原子摻雜到a-C涂層，結(jié)合分子動(dòng)力學(xué)模擬摻雜a-C涂層的沉積過(guò)程和涂層結(jié)構(gòu)變化。通過(guò)同時(shí)摻雜一定量Ag和Cr，可降低a-C涂層內(nèi)應(yīng)力，并提高致密性，如圖9所示。共摻雜Ag和Cr涂層樣品的ICR（0.87 mW·cm2）低于單獨(dú)摻雜Ag涂層（1.07 mW·cm2），經(jīng)過(guò)耐久性測(cè)試后，ICR約為2.14 mW·cm2。結(jié)果表明，將適量的金屬組元復(fù)合，可以進(jìn)一步提高a-C涂層的耐腐蝕性，相關(guān)研究還有待進(jìn)一步深入。

圖8 多元共摻雜涂層殘余應(yīng)力和成鍵特征

圖9 不同金屬含量下涂層內(nèi)應(yīng)力

3 多層結(jié)構(gòu)非晶碳涂層

與涂層腐蝕相關(guān)的大量研究表明，具有柱狀結(jié)構(gòu)和孔洞等缺陷是導(dǎo)致涂層耐蝕性能明顯降低的原因之一。人們普遍認(rèn)為多界面涂層可以減少貫穿性缺陷。因此，設(shè)計(jì)多層結(jié)構(gòu)涂層有利于提高耐蝕性。Yi等[41]采用CFUBMSIP技術(shù)在0.1 mm厚的SS316L表面制備了強(qiáng)結(jié)合梯度多層Cr/CrN/CrNC/a-C涂層，通過(guò)劃痕測(cè)試評(píng)估對(duì)比了該多層體系與SS316L以及a-C與SS316L之間的結(jié)合強(qiáng)度（如圖10所示）。在模擬電池（0.5 mol/L H2SO4+5 mg/L HF，70 ℃）陽(yáng)極（0.1 V（vs. SCE））和陰極（0.6 V（vs. SCE））環(huán)境下恒電位極化10 h后，改性極板的極化電流密度均低于1 μA/cm2，且界面接觸電阻為2.64 mΩ·cm2（壓實(shí)壓力為1.40 MPa）。Bi等人[42]采用相同技術(shù)在SS316L表面制備了多層Zr-C/a-C涂層。在模擬電池（pH=3, H2SO4+0.1 mg/L HF，80 ℃）條件下測(cè)試改性極板，與a-C涂層相比，陰極和陽(yáng)極環(huán)境中的腐蝕電流密度都降低了1個(gè)數(shù)量級(jí)。在1.40 MPa的測(cè)試條件下，Zr-C/a-C涂層的ICR僅為3.63 mΩ·cm2。

圖10 Cr-N-C和a-C涂層劃痕測(cè)試圖

Yi等[39]提出一種在磁控濺射過(guò)程中通過(guò)不同負(fù)襯底偏壓的協(xié)同作用沉積TiC/a-C多層涂層的新方法。不同負(fù)襯底偏壓使多層涂層表面形貌、sp2/sp3等發(fā)生改變，且抑制柱狀微結(jié)構(gòu)和微孔，可獲得ICR為1.85 mΩ·cm2、腐蝕電流密度為0.32 μA/cm2的涂層。在PEMFCs陰極環(huán)境下，單層涂層和多層涂層之間不同的腐蝕行為如圖11所示。在此基礎(chǔ)上，該團(tuán)隊(duì)[33]進(jìn)一步開(kāi)發(fā)了多層碳化鉻/鉻摻雜非晶碳（Cr-C/ a-C: Cr）新型涂層。在1.50 MPa時(shí)，Cr-C/a-C:Cr涂層改性的SS316L極板的ICR僅2.89 mΩ·cm2。在陰極工作電位（0.6 V（vs. SCE））下，多層涂層涂覆的SS316L的鈍化電流密度為0.276 μA/cm2。該多層涂層的耐蝕性比本征a-C涂層提高了1個(gè)數(shù)量級(jí)，滿足美國(guó)DOE的2020年技術(shù)指標(biāo)。Zhang等人[44]利用襯底偏置電壓逐層沉積的方法制備了多層TiC/a-C涂層。以15個(gè)交替周期沉積的涂層在0.6 V電勢(shì)下，腐蝕電流密度為0.297 μA/cm2，ICR值為3.58 mΩ·cm2（如圖12所示），在雙極板的商業(yè)應(yīng)用中顯示出巨大的潛力。以上研究表明，通過(guò)設(shè)計(jì)多層碳基涂層改性雙極板，可有效減少涂層缺陷而提高極板的耐腐蝕性能。

圖11 PEMFCs陰極環(huán)境中a-C單層和a-C多層涂層不同的腐蝕行為

圖12 TiCx/a-C多層涂層性能

4 結(jié)語(yǔ)

隨著工業(yè)的迅速發(fā)展，對(duì)質(zhì)子交換膜燃料電池性能的要求越來(lái)越高，通過(guò)表面改性技術(shù)，可賦予雙極板更加優(yōu)異的表面性能，從而滿足質(zhì)子交換膜燃料電池的使用壽命要求。研究人員通過(guò)不同制備技術(shù)，設(shè)計(jì)涂層結(jié)構(gòu)，調(diào)控制備工藝等方法研究改性雙極板，以保證改性后的極板具有良好的耐腐蝕能力和導(dǎo)電性能。以上研究表明，碳基涂層可表現(xiàn)出較好的耐腐蝕性能和低接觸電阻，具有一定的應(yīng)用前景，但仍然存在很多挑戰(zhàn)：

1）目前各類碳基涂層在PEMFCs腐蝕環(huán)境下，都面臨著ICR不斷增加的問(wèn)題，顯著影響涂層性能和壽命。因此開(kāi)發(fā)出性能優(yōu)異并實(shí)現(xiàn)穩(wěn)定低ICR的非晶碳涂層技術(shù)在該領(lǐng)域仍是巨大挑戰(zhàn)。同時(shí)長(zhǎng)時(shí)間運(yùn)行時(shí)，非晶碳/金屬極板性能退化與損傷機(jī)理不明確，仍需要進(jìn)一步系統(tǒng)研究涂層組分、結(jié)構(gòu)、致密性等與性能的相關(guān)性，闡明相關(guān)損傷機(jī)制。

2）圍繞具體工況需求，針對(duì)不同材質(zhì)與形狀的金屬極板材料，開(kāi)展針對(duì)性的a-C改性技術(shù)研發(fā)，突破強(qiáng)結(jié)合、耐蝕、良導(dǎo)電的低成本非晶碳涂層技術(shù)，對(duì)于商業(yè)化推廣十分必要。

3）在評(píng)價(jià)方法上，美國(guó)DOE提出金屬極板改性涂層性能的評(píng)估方法。然而，質(zhì)子交換膜燃料電池的實(shí)際運(yùn)行環(huán)境非常復(fù)雜，很難直接監(jiān)測(cè)沉積在雙極板上的涂層變化。國(guó)內(nèi)外研究中，以模擬電池腐蝕環(huán)境測(cè)試為主，與燃料電池實(shí)際運(yùn)行環(huán)境存在差異。因此，將非晶碳涂層改性的金屬極板組裝成電池，開(kāi)展實(shí)際工況下的性能評(píng)價(jià)是實(shí)現(xiàn)其商業(yè)化推廣的關(guān)鍵。

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Research Progress of Carbon-based Coating for Metal Bipolar Plates of Proton Exchange Membrane Fuel Cells

1,2,1,1,1,1,1,1,3

(1.a.Key Laboratory of Marine Materials and Related Technologies, b.Zhejiang Key Laboratory of Marine Materials and Protective Technologies, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Science, Ningbo 315201, China; 2.Nano Science and Technology Institute, University of Science and Technology of China, Suzhou 215123, China; 3.Research Center of Materials and Photoelectricity, University of Chinese Academy of Sciences, Beijing 100049, China)

In this work, the performances of bare metallic plate, traditional graphite plate and metallic bipolar plate modified with carbon-based coating used in the proton exchange membrane fuel cells (PEMFCs) were compared; the latest achievements of carbon-based coatings in improving the conductivity and corrosion resistance of metallic bipolar plates were reviewed; and issues such as performance degradation and limited life of carbon-based coatings were discussed, especially after long-term operation in PEMFCs environment. By analyzing the factors that affect the performance of carbon-based coatings, it was found that systematic researches on the relationship between microstructure of carbon-based coating and its performance were insufficient, leading to an unclear damage and degradation mechanism of a-C carbon coating/ metallic bipolar plate. The research progress of modified carbon-based coating materials for PEMFCs metal plates at home and abroad was mainly discussed, including improving the coating performance by adjusting the microstructure of carbon-based coatings (a-C); preparing metal-doped carbon coating (a-C:Me) by combing theoretical calculation with practice; solving the poor adhesion, high pressure stress and other issues between coating and special metallic matrix, design multi-layer structures to reduce penetrability defects. The failure mechanism of several kinds of coatings was discussed and the development trend of carbon-based coating technology for metallic plate modification was prospected.

proton exchange membrane fuel cell; bipolar plate; carbon-based film; conductivity;corrosion resistance

2020-04-29；

2020-05-22

XIN Yang (1995—), Female, Master, Research focus: electrochemical properties of carbon-based coatings.

汪愛(ài)英（1975—），女，博士，研究員，主要研究方向?yàn)楸砻鎻?qiáng)化涂層材料與功能改性。郵箱：aywang@nimte.ac.cn

Corresponding author：WANG Ai-ying (1975—), Female, Doctor, Professor, Research focus: strengthening and functional modification for surface coating materials. E-mail: aywang@nimte.ac.cn

辛洋, 郭鵬, 李昊, 等. 質(zhì)子交換膜燃料電池金屬雙極板改性碳基涂層技術(shù)研究進(jìn)展[J]. 表面技術(shù), 2020, 49(6): 22-33.

TM911.4

A

1001-3660(2020)06-0022-12

10.16490/j.cnki.issn.1001-3660.2020.06.003

2020-04-29；

2020-05-22

王寬誠(chéng)率先人才計(jì)劃盧嘉錫國(guó)際團(tuán)隊(duì)（GJTD-2019-13）；國(guó)家自然科學(xué)基金（51801226）；寧波市“科技創(chuàng)新2025”重大專項(xiàng)（2018B10014）

Fund：K. C. Wong Education Foundation (GJTD-2019-13); National Natural Science Foundation of China (51801226); Ningbo Science and Technology Innovation Project (2018B10014)

辛洋（1995—），女，碩士研究生，主要研究方向?yàn)樘蓟繉拥碾娀瘜W(xué)性能。

XIN Yang, GUO Peng, LI Hao, et al. Research progress of carbon-based coating for metal bipolar plates of proton exchange membrane fuel cells[J]. Surface technology, 2020, 49(6): 22-33.