張晗明 張少飛 孫金峰











摘要:氫能具有高能量密度、綠色可持續的優點,是人類社會的理想能源。電解海水制氫是未來氫能產業的戰略方向,其中陰極析氫催化劑的活性和穩定性對電解海水制氫的發展至關重要。貴金屬鉑雖具備優異的析氫催化性能,但價格昂貴、資源有限,限制了其大規模應用。因此,對非貴金屬催化劑的研究備受關注。從析氫反應原理入手,介紹了過電位、塔菲爾斜率、法拉第效率、比活性和質量活性等評價催化性能的幾個重要參數,綜述分析了多種非貴金屬催化劑作為電解海水制氫陰極催化劑的研究現狀,指出了目前電解海水制氫面臨的問題,認為未來非貴金屬催化劑規?;娊夂K帢O析氫研究應從以下幾方面開展:1)設計高活性和穩定性的非貴金屬催化劑;2)優化非貴金屬催化劑的制備工藝;3)利用先進測試表征手段輔助構建反應模型;4)深化理論計算機理方面的研究。
關鍵詞:氫能;電解海水;氫析出反應;非貴金屬催化劑;氫吸附自由能;電子結構;異質原子
中圖分類號:O643.36文獻標識碼:A
DOI:10.7535/hbkd.2022yx03013
Review of nonprecious metal electrocatalysts in cathode hydrogen generation by seawater electrocatalysis
ZHANG Hanming ZHANG Shaofei SUN Jinfeng
(1.School of Materials Science and Engineering,Hebei University of Science and Technology,Shijiazhuang,Hebei 050018,China;2.Hebei Key Laboratory of Flexible Functional Materials,Shijiazhuang,Hebei 050018,China)
Abstract:Hydrogen energy has the advantages of high energy density,green and sustainability,which make it an ideal energy source for human society.Hydrogen generation by seawater electrocatalysis becomes the strategic direction of hydrogen energy industry in the future.The activity and stability of cathode hydrogen evolution catalyst are vital to this hydrogen energy strategy.Although the precious metal Pt-based electrocatalysts have shown excellent catalytic performance on hydrogen evolution,the high price and scarce resources limit their large-scale application.Therefore,nonprecious metal electrocatalysts have attracted much research attention.Starting from the principle of hydrogen evolution reaction,important parameters of overpotential,Tafel slope,Faraday efficiency,specific activity and mass activity,etc.were introduced to evaluate catalytic performances.The review summarized recent progress on multiple nonprecious metal electrocatalysts for cathode hydrogen generation by seawater electrocatalysis,and the problems of current research was clarified.Additionally,the future research of nonprecious metal electrocatalysts in cathode for large-scale hydrogen generation by seawater electrocatalysis was prospected from the following aspects:1) Designing nonprecious metal catalysts with high activity and stability;2) Optimizing production and preparation process of nonprecious metal catalyst;3) Using advanced test and characterization to construct reaction models;4) Deepening theoretical calculation for the mechanism research.
Keywords: hydrogen energy;seawater electrocatalysis;hydrogen evolution reaction;nonprecious metal catalyst;hydrogen adsorption free energy;electronic structure;heteroatom
隨著當今社會對化石能源需求的日益增長,溫室效應、環境污染以及能源危機等問題日漸凸顯,探索新型清潔能源代替傳統化石能源迫在眉睫。氫能,因具有高能量密度、高熱值、綠色清潔、可持續等優點,被認為是人類社會的理想能源[1]。氫,可以作為綠色燃料直接燃燒放熱,也可以用于氫燃料電池發電。此外,氫氣在石化、電子、冶金、食品等領域也有著重要的用途。氫能的開發利用已經受到世界各國的重視[1-2]。國際能源署在《能源技術展望2020》中預測,未來50年全球對氫能源的需求量將增至現在的5倍。2020年中國發布的《新時代的中國能源發展》白皮書中也提出了加速發展綠氫制取、儲運和應用等氫能產業鏈技術裝備,促進氫能燃料電池技術鏈、氫燃料電池汽車產業鏈的發展。國家十四五規劃綱要中進一步強調了加速氫能產業發展,要在2030年前碳排放達峰,2060年前實現碳中和。目前,商用氫氣96%以上來自化石燃料,制氫的同時會伴有大量CO排放,被稱為灰氫。電解水制氫過程為零碳排放,產生的氫為綠氫,是規模化制氫的潛在方案。但是,電解水技術大多依賴純水,而淡水資源僅占全球總水量的3.5%,淡水資源緊缺是繼全球氣候變暖之后的世界第二大環境問題。廣闊無垠的海水占全球總水量的96%以上,利用電解海水制氫有助于減緩對淡水資源的消耗。因此,電解海水制氫是未來氫能產業規?;?、低成本發展的戰略方向[3]。
電解海水電解槽中陽極發生氧析出反應(OER),陰極發生氫析出反應(HER)。較早的研究工作主要集中在制備高效、高選擇性的陽極OER催化劑,抑制析氯競爭反應(ClER)[4-8]。但一些因素導致電解海水制氫的成本很大:一是目前陰極析氫反應依然依賴貴金屬Pt基催化劑;二是海水中的鈣離子和鎂離子易形成Ca(OH)和Mg(OH)沉淀,沉積在電極表面,覆蓋活性位點,導致催化劑失活[9];三是海水中高濃度的氯離子(~0.5 mol/L)對陰極催化劑具有很強的腐蝕作用,會使催化劑發生降解[10]。因此,開發高效、穩定、廉價的陰極HER催化劑對電解海水制氫的發展至關重要。近年來,許多科研人員致力于穩定、廉價非貴金屬催化劑的研究,通過增大催化劑比表面積[11]、提高金屬價態形成磷化物、硫化物[12]、釋放陰離子以排斥氯離子[13]等方法,或采用較穩定的單原子、合金,或在催化劑表面覆蓋碳層、MnO和CrO保護層等手段,提高非貴金屬催化劑的抗腐蝕性能[9]。在此基礎上,人們對非貴金屬電解海水析氫催化劑在提升活性和穩定性方面的研究也取得了進展,對其析氫機理也進行了深入探討,有望替代貴金屬Pt基催化劑[11,14-15]。
1析氫反應機理及相關參數
1.1析氫反應機理
1.2催化劑的性能參數
1.2.1過電位
1.2.2塔菲爾斜率
1.2.3法拉第效率
法拉第效率指外電路提供的電荷變成氫氣分子的效率,即實際氫析出量與理論氫析出量的比值。實際氫析出量可以通過氣驅水法或氣相色譜檢測獲得,理論氫析出量可由法拉第定律計算反應中的電量而得。理想催化劑的法拉第效率為100%,但是副反應的存在會造成一定的損失。
1.2.4比活性和質量活性
1.2.5穩定性
除了催化活性,催化劑的長循環穩定性也是實際應用中非常重要的一個參數。通常有2種穩定性測試:加速循環伏安法和計時電位法或計時安培法。加速循環伏安法通過比較循環伏安測試前后線性掃描伏安曲線中過電位的變化情況,衡量催化劑的穩定性;計時電位法或計時安培法是固定電流或電壓,通過一段時間后根據電流或電壓變化情況判斷穩定性的好壞。
2非貴金屬催化劑的研究現狀
近年來,電解海水制氫領域已經受到國內外研究工作者的廣泛關注,許多文獻報道了非貴金屬作為陰極析氫催化劑的研究工作,并對其催化機理進行了較為深入的研究[3, 10, 11, 14, 23-24]??蒲腥藛T通過在非貴金屬鉬、鈷、鎳、鐵及銅中引入異質原子,如磷、氮、氧、碳、硫、硒原子等非金屬原子或其他非貴金屬原子,形成非貴金屬磷化物、氮化物、氧化物/氫氧化物、碳化物、硫族化物以及非貴金屬單原子、復合催化劑[25-50]。異質原子的引入不但能提高非貴金屬催化劑的穩定性和抗腐蝕性,還能調節活性中心原子的電子結構,使其d帶中心的能級靠近反應費米能級,從而調節氫吸附自由能使其接近0,增強非貴金屬催化劑的催化活性。
2.1非貴金屬磷化物
在多種非貴金屬催化劑中,非貴金屬磷化物具備優異的導電性和結構穩定性,同時大半徑的磷原子溶入金屬晶體結構會導致晶體表面存在較多不飽和原子,提高其本征催化活性。因此,非貴金屬磷化物在電解海水陰極析氫方面展現出巨大的應用潛力[51]。催化過程中,非貴金屬磷化物中金屬和P位點可分別作為質子和氫氧根的吸附位點,加速析氫反應動力學?;谝陨蟽烖c,非貴金屬磷化物作為理想的海水陰極析氫催化劑近年來備受關注,人們研發出雙金屬磷化物、磷化物異質結和元素摻雜的磷化物等高效電解海水陰極析氫催化劑。
2.1.1雙金屬磷化物
2.1.2磷化物異質結
2.1.3元素摻雜
2.2非貴金屬氮化物
非貴金屬氮化物中較小原子半徑的氮原子溶入金屬晶體的間隙中,使得晶體結構中的原子排布更緊密,導電性更高、抗腐蝕性更強;加之氮原子與金屬原子的協同作用,可調節過渡金屬d帶中心,提高催化活性,因而使得非貴金屬氮化物也成為具有發展前景的電解海水陰極析氫催化劑[52]。近年來,大量研究者致力于研發性能優異的非貴金屬氮化物,如碳包覆氮化物、貧氮中氮富氮氮化物、雙金屬氮化物。
2.2.1碳包覆氮化物
2.2.2貧氮中氮富氮氮化物
2.2.3雙金屬氮化物
2.3非貴金屬氧化物/氫氧化物
非貴金屬氧化物/氫氧化物因廉價易得、本征活性高,已成為最有潛力的電解水析氫非貴金屬催化劑之一[54-55]。但是,非貴金屬氧化物/氫氧化物導電性差,在電解海水陰極析氫反應中的催化性能仍然不是很理想。因此,科研工作者采用合金化、元素摻雜等策略來提高催化劑的電子傳輸能力和陰極析氫的催化活性。
2.3.1雙金屬氧化物/氫氧化物
2.3.2元素摻雜
2.4非貴金屬硫族化物
非貴金屬硫族化物資源豐富、結構特殊、催化活性優異,是一類應用廣泛的電解水析氫催化劑[57],硫族化物在催化過程中會放出含硫的多原子陰離子,排斥氯負離子,提高抗腐蝕性[13]。元素摻雜硫族化物和硫族化物異質結在催化電解海水陰極析氫領域具有很好的發展前景。
2.4.1元素摻雜硫族化物
2.4.2硫族化物異質結
2.5非貴金屬單原子催化劑
為了充分利用非貴金屬,降低成本,增強催化活性,科研人員研發了非貴金屬單原子催化劑,并廣泛應用于各種電催化領域[58-59]。非貴金屬單原子通常與非金屬原子(N,S等)配位,穩定金屬單原子的同時,還實現了金屬中心電子結構的調控,提高催化活性。近年來,非貴金屬單原子催化劑已經應用于催化電解海水陰極析氫反應,如N-配位單原子和N,S-配位單原子催化劑都取得了令人驚喜的結果。
2.5.1N-配位單原子
2.5.2N,S-配位單原子
2.6其他非貴金屬催化劑
除上述幾種催化劑外,其他非貴金屬催化劑,如混合型金屬化合物異質結、雜化催化劑,也被廣泛應用于電解海水陰極析氫。通過不同化合物之間的電子相互作用,可調節其電子結構,優化析氫催化性能。
2.6.1混合型金屬化合物異質結
2.6.2雜化催化劑
3問題與展望
氫能產業是國家的重大戰略發展方向,也是實現碳達峰、碳中和宏偉目標的重要舉措。電解海水制氫是規?;茪涞臐撛诜桨浮D壳?,制約電解海水制氫產業發展的瓶頸是電解能耗過高、催化活性不高、催化劑穩定性差等問題。因此,降低制氫成本、提高催化劑活性和穩定性是發展電解海水制氫的關鍵。近年來,一些研究人員致力于開發廉價、高效、穩定的電解海水陰極析氫非貴金屬催化劑,但非貴金屬催化劑自身的催化性能較低、穩定性較差。通過在非貴金屬催化劑中引入異質原子,可顯著提高其性能及穩定性。一方面,異質原子的引入可提高非貴金屬催化劑的穩定性和抗腐蝕性;另一方面,異質原子能調節活性中心原子的電子結構,增強催化活性。其中,非貴金屬磷化物、氮化物、氧化物/氫氧化物、碳化物、硫族化物、非貴金屬單原子、合金以及多種復合催化劑已經廣泛應用于電解海水陰極析氫的研究,并取得了令人驚喜的成果。但是,當前電解海水制氫仍處于研究階段,還存在一些亟待解決的問題。
3.1電解海水制氫面臨的問題
1)大部分研究工作均沒有考慮大電流析氫反應時的能耗問題,法拉第效率也多是小電流下的測試結果[28,49-50]。而電解海水制氫規模化的要求是工作電流>1 A/cm,能耗小于4.4 kW·h /m(H)。
2)催化劑活性和穩定性距離工業電解水制氫標準還存在很大差距。研究工作中析氫電流密度大部分小于1 A/cm,測試時間大多幾十小時。電流密度超過1 A/cm [28,34]時測試達到上千小時[13,38]的報道屈指可數。而工業電解海水電流密度高達1 A/cm,要求電極工作2 000 h性能衰減要小于5%。
3)目前非貴金屬電解海水陰極析氫催化劑的制備工序較復雜,多數需要經過多步工藝制得。此外,催化劑的制備僅限于實驗毫克或克級的研究,不能滿足催化劑大規模批量生產的要求。
4)催化劑的抗腐蝕機理以及抗腐蝕性與催化活性之間的關系仍有待研究,催化劑的晶體結構、金屬價態以及負離子種類與抗腐蝕性和催化活性之間的作用機制仍需探明。
5)海水體系中析氫機理的研究需考慮海水體系的影響。目前,對于電解海水析氫催化機理的研究大多沒有考慮海水體系中氯離子、鎂離子等對催化機理的影響。電解水制氫通常采用堿性或酸性的純水體系,其催化活性高于電解海水的性能。由此可見,氯離子、鎂離子等對催化反應中涉及的反應底物和中間體(水分子、氫離子、氫氧根離子等)之間的靜電作用力,其對催化反應機理的影響也需深入研究。
3.2電解海水制氫研究展望
加速氫能產業革命,推動電解海水制氫的發展仍然任重道遠,迫切需要研發新型、高效、穩定的非貴金屬催化劑。未來,對電解海水析氫研究的重點可概括為以下幾方面。
1)設計高活性和穩定性的非貴金屬催化劑
綜合利用電子工程、缺陷工程、界面工程等策略,調節原子軌道能級、電子結構,增強催化劑本征活性與自身穩定性,利用納米工程構筑納米多孔結構,增加活性位點,加速物質傳輸,最終通過降低產氫能耗,實現大電流密度下超長時間的持續工作。
2)優化非貴金屬催化劑的制備工藝
盡量采用簡便易行的操作工藝規?;a非貴金屬催化劑,使用簡單的儀器設備、廉價易得的原料、溫和的反應條件,避免使用高溫、高壓工藝和強腐蝕、易致毒及易致爆等危險化學品。同時,控制制備工藝過程中廢氣、廢液、廢固的產生和排放,保護操作人員的生命安全,保護環境,實現催化劑的大規模量產。
3)利用先進測試表征手段輔助構建反應模型
利用先進原位測試表征技術和常規表征手段,檢測反應過程中活性位點的反應歷程、價態和結構變化,檢測不同類型催化劑的腐蝕情況和電解液中雜質離子的變化情況,確定催化反應過程和腐蝕過程,構建催化劑模型和電解液模型。
4)深化理論計算機理研究
利用理論計算深入探究海水環境中的析氫反應機理、抗腐蝕機理及電解液對催化反應的影響機制。計算催化劑模型中活性中心反應中間體和產物的吸/脫附能力,確定析氫反應機理與構效關系;計算催化劑模型的腐蝕能壘,確定腐蝕機理;優化催化劑組分及結構;根據電解液模型中雜質離子的變化及遷移情況,確定其對催化反應的影響機制,指導電解海水析氫非貴金屬催化劑的研發工作,推動電解海水制氫工業的發展。
此外,沿海地區可充分利用風能、太陽能、潮汐能等優勢,內陸鹽湖地區可利用風能、太陽能和地熱能,通過研發實現可再生能源電解海水制氫,提高電解海水制氫的經濟性。
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