惰性小分子的电催化还原如二氧化碳还原反应(CO2RR)和氮气还原反应(NRR),在改善能源利用方式、平衡化学元素循环和生产高附加值化学品等方面具有应用前景,近年来受到广泛关注[1,2,3,4]. 二氧化碳和氮气的高键能（C=O和N≡N键能分别为750和942 kJ·mol-1）和分子对称性导致反应动力学迟滞;CO2RR和NRR反应的电位区间（CO和NH3析出的理论平衡电位分别为-0.24和0.15 V vs. RHE）与氢析出（HER）相近,造成法拉第效率低;二氧化碳还原反应产物的种类繁多,反应的专一性和选择性面临巨大挑战[5,6,7,8]. 针对上述问题,人们发展了多种新型催化剂,如对二氧化碳有独特吸附特性的铜基催化剂、外层p轨道可与氮气强配合的铋基材料等,它们展现了可观的本征催化活性和选择性[9,10,11,12,13]. 然而,气/液/固三相催化体系存在固有的传质和电荷输运限制,若想达到实用化的电流密度同时维持高反应选择性,需兼顾整个催化反应过程的研究[14,15,16,17].
催化剂和电解液是惰性小分子电催化的重要研究对象,多数研究工作的重心是合理设计和调控电催化剂. 近来的研究表明,催化剂对局部反应环境高度敏感,电解液在催化反应中表现出独特的离子吸附和界面性质,并参与控制电极微环境,使得同种催化剂在不同电解液中活性和选择性不同[18,19,20]. 对电解液的深入认识与功能调控,是揭示反应机理、提升催化性能的重要途径. 电解液是电解质离子迁移及可溶反应物传质的载体,二氧化碳和氮气还原对电解液的要求苛刻. 溶剂作为质子供体既需保持一定的质子活性,又不能太活泼导致剧烈析氢副反应发生. 作为离子传导的枢纽,电解液在具备导电性的同时,需要宽电化学窗口维持选择性,还需提供更适合反应的微环境及更稳定的电化学活性界面. 此外,电解液作为气体储存和传质的媒介,需对氮气和二氧化碳等非极性分子具有合适的溶解和界面富集能力.
如图1所示,惰性小分子电催化还原的反应活性和选择性受到多个电解液的重要参数影响,如电解质强度、电极局域pH值(local pH)、电化学双电层(EDL)结构、电解液-催化剂相互作用和离子吸附效应等. 这些参数又相互影响和制约,某个参数的独特作用不甚清晰. 为满足特殊的反应微环境需求,有机电解液、高分子电解质、固态电解质及离子液体等多类电解液被用于惰性小分子还原,它们展现了独特的离子传输、界面调控和反应底物溶解特性[18]. 人们发展了多种原位谱学手段和电化学测试方法研究电解液-电极界面行为,结合密度泛函理论(DFT)及分子动力学(MD)等计算模拟方法,在微观尺度研究惰性分子电催化反应[21,22,23]. 本文对惰性小分子电催化电解液领域的研究进展进行综述,讨论电解液的离子效应、浓度效应以及与催化剂的相互作用,介绍非水系电解液的应用,并总结与展望提升二氧化碳和氮还原性能的电解液调控策略.
图1
图1
惰性小分子电催化还原反应的电解液环境条件
Fig. 1
Environmental conditions in electrolyte of stable molecule reduction reactions
1 离子效应
阳离子种类首先影响了电化学双电层(EDL)的组成和结构. 研究发现,在Li+系电解液中,氢析出优于二氧化碳还原,而在Na+、K+和Cs+溶液中,二氧化碳还原更为有利,且随着阳离子尺寸的增加,产物中C2H4的选择性超过CH4[27]. 一种合理的解释是,阳离子种类通过改变外亥姆霍兹平面(OHP)电位而改变反应选择性[28,29]. 半径较大的水合性低的阳离子容易吸附在催化剂表面上,使外亥姆霍兹平面的电势更正,从而降低双电层H+浓度,致使局域pH升高,而局域pH值的不同将导致产物选择性的改变[30,31]. Singh等人则认为半径较大的阳离子可提供更强的缓冲能力,使工作条件下电极保持较低的pH值,从而维持较高的二氧化碳局部溶解浓度（如图2A）[32]. 较高的二氧化碳浓度不仅可以降低电极极化,还能抑制析氢反应,改善催化剂对多碳产物的选择性. 此外还有一些观点认为,较高的阳离子浓度或更高价态的阳离子具有独特的场效应（如图2B）,它们会与具有大偶极矩的吸附物质之间发生较强的静电相互作用,从而稳定活性中间体并降低电荷转移阻力,提高二氧化碳还原的电流密度[33,34,35].
图2
图2
(A) 碱金属离子调节表面pH及二氧化碳局部浓度[32]. (B) 碱金属离子诱导表面电场法向分布[34]. (C), (D) 碱金属离子对二氧化碳还原中间体吸附能的调节[26]. (E) 卤离子的特性吸附[36].
Fig. 2
(A) Alkali metal ions adjusting surface pH and local concentration of CO2 (Reprinted with permission from Ref.[32]. Copyright (2016) American Chemical Society). (B) Surface electric field induced by alkali metal ions in normal distribution (Reprinted with permission from Ref. [34]. Copyright (2017) American Chemical Society). (C), (D) Adsorption energy of CO2 reduction intermediates tailored by alkali metal ions (Reprinted with permission from Ref.[26]. Copyright (2018) American Chemical Society). (E) Specific adsorption of halides (Reprinted with permission from Ref.[36]. Copyright (2018) American Chemical Society).
在惰性小分子电催化还原领域,阳离子的吸附作用影响催化剂的电化学行为. Hao等人使用第一性原理与实验结合的方法,深入研究了电解液离子对NRR性能的改善作用. 当钾离子吸附于铋纳米催化剂表面时,三种晶面对氮还原重要的中间体*NNH的吸附能均有不同程度的改变[11]. 不仅如此,在钾离子存在下,铋的氢吸附能也向析氢不利的方向移动. 实际固氮效果证实了钾离子对铋纳米催化剂的表面调节作用,且钾离子浓度越高,析氢受到更大程度抑制,产氨效率越高. 铋纳米晶与钾离子电解质的协同优化获得200 mmol·g-1·h-1的产氨速率以及接近66%的法拉第效率[26]. 在二氧化碳还原中,这种机制被研究得更深入. Gao等人使用密度泛函理论(DFT)计算了碱金属离子吸附条件下的分子还原路径,与无离子路径相比,阳离子促进了三种中间体*CO、*OCCO和*OCCOH的形成（如图2C）[26]. 他们的结果还表明,表面吸附的碱金属阳离子产生的近表面电场会促进极性或高极化性中间体的形成. 此外,阳离子在催化剂上的吸附强度遵循Li+ < Na+ < K+ < Cs+的趋势. 阳离子吸附强度越大,在界面产生的法向偶极矩越大,对于极性中间体的形成和稳定越有利（如图2D）[26].
碳酸氢根是电催化二氧化碳还原中独特的电解质阴离子,在水中以HCO3-/CO32-、H2CO3/HCO3-、CO2/HCO3-电离平衡的形式存在. 与氮还原不同,二氧化碳还原的活性底物问题一直存在争议. 虽然大多数研究者认为二氧化碳分子是发生还原的真正底物,但在碳酸氢根电解液中二氧化碳还原性能增强,这成为碳酸氢根还原的有利证据之一[37,38,39]. Dunwell等人开发了一种光谱方法,通过施加方波电势观察电化学表面原位的衰减全反射表面增强拉曼光谱(ATR-SEIRAS),并在金催化剂表面捕捉到碳酸氢盐-二氧化碳复合物,结合同位素标记和质谱技术证明,电解液中多数溶液相二氧化碳源自这种复合物的解离平衡而非气相二氧化碳扩散. 因此,碳酸氢盐的这种机制能够有效增加电极附近的二氧化碳浓度[40]. 可见,在二氧化碳还原体系中,碳酸氢盐不是普通的导通离子,它与二氧化碳独特的快速缔合/解离平衡为催化剂提供了高浓度的表面底物,且这种促进作用对于不同催化剂具有普遍性. 虽然碳酸氢盐体系受到广泛青睐,但Wuttig等人发现碳酸氢盐几乎不影响CO2RR的限速步骤,而是在限速步骤后起到提供质子和缓冲pH的作用[41]. 碳酸氢盐虽对传质有益,但不能促进二氧化碳的裂解和活化,且高电流下碳酸氢盐-二氧化碳复合物分解反应存在动力学迟滞等问题. 因此,寻找与碳酸氢盐具有相似作用的电解质或对其体系进行改进值得进一步的深入探究.
2 浓度效应
电催化反应是一种动态过程,随法拉第电流产生,电解液受传质限制将形成浓度梯度. 尤其在CO2RR和NRR体系中,阴极生成OH-,在电极附近pH值比本体溶液更高. 局域pH梯度不仅与电流密度有关,也取决于电解质的化学性质和缓冲容量. 调控电解液的浓度以及原位观察和建模计算电极局部浓度梯度对改善反应至关重要.
低浓度碳酸氢盐对CO2RR的促进作用被广泛认可. 早期Mul和Koper小组进行了深入探讨[43,44],在低浓度的KHCO3电解液中,由于电极附近具有较高的局部pH值,有利于C-C偶联而形成乙烯（如图3A）. Varela等人的工作证实了这一点,并且发现可以通过更改碳酸氢盐电解液的浓度来连续调节二氧化碳电化学还原过程中铜的活性和选择性,低浓度的KHCO3电解液具有较高C2选择性,而当HCO3-浓度较高时,电解液具有较高缓冲容量,有利于电子/质子耦合转移,反而会提高H2和CH4的法拉第效率[45]. Weber小组则从另一角度研究这个问题, 他们建立了一维等温稳态模型来模拟电极附近的流体动力边界层,研究电解液缓冲容量对边界层内的二氧化碳传输局部浓度的影响[46]. 结果表明,相同浓度下KHCO3比KCl电解液缓冲能力更高,使得局部pH值增加较慢,为电极提供了更高的二氧化碳传质通量. 不仅如此,缓冲容量还受浓度控制,通过增加碳酸氢盐浓度可更有效改善电解液中二氧化碳的传质. 电解液浓度不仅对CO2RR具有重要的影响,在近来的研究中,浓度调控在电化学氮还原中的作用也得到认可（如前所述）,通过调控K2SO4溶液的浓度,钾离子在表面的富集可以调节氢离子与氮气的竞争吸附,氮还原的选择性得到改善[11].
图3
图3
(A) 碳酸氢盐浓度连续调节二氧化碳还原选择性[44]. (B) Ag(111)表面EMIM+的Pourbaix图[55]. (C) 离子液体应用于氮气还原[54].
Fig. 3
(A) Tailoring CO2 reduction selectivity by bicarbonate concentration (Reprinted with permission from Ref.[44]. Copyright (2015) Wiley-VCH Verlag GmbH & Co. KGaA). (B) Pourbaix diagrams of EMIM+ on Ag(111) surface (Reprinted with permission from Ref.[55]. Copyright (2015) American Chemical Society). (C) Ionic liquids used for N2 reduction (Reprinted with permission from Ref.[54]. Copyright (2017) Royal Society of Chemistry).
高浓电解液较常规浓度电解液（0.1 ~ 1.0 mol·L-1）在溶剂和界面行为方面存在巨大差异,近来受到广泛的关注. 由于高局域pH有利于降低C-C耦合的活化能垒,高浓碱性电解液被用于二氧化碳的电化学还原[47,48,49]. 二氧化碳能与碱直接反应,因此电化学测试均在气体扩散电解池中完成. 当铜催化剂在10 mol·L-1 KOH中进行电化学二氧化碳还原时,反应过电位明显降低,乙烯几乎与CO同时析出. 在电位为-0.55 V vs. RHE时,以70%的法拉第效率快速生成乙烯[50]. 另外,高浓电解液较低的溶液欧姆降和传质限制有利于实际应用. 当配合使用具有异质结构的离聚物催化剂时[51],7 mol·L-1 KOH电解液中形成多碳产物的电流密度高达1.3 A·cm-2. 虽然强碱性的腐蚀作用对器件稳定性提出了挑战[14],提高浓度仍然代表了一种重要的催化电解液调控方式.
二氧化碳和氮气分子稳定性较高,在使用常规浓度电解液的催化体系中,反应物分子的裂解和活化几乎全部归功于催化剂和相间电势差,电解液的作用则不明显. 高浓度离子液体电解质可以通过络合的作用活化惰性小分子,降低初始还原势垒. Rosen等人最早使用18 mol·L-1的1-乙基-3-甲基咪唑四氟硼酸盐([EMIM] [BF4])作为电解质,原因是其与二氧化碳的络合作用较其它离子更适中[52]. 这种新型的高浓离子液体电解液表现出更低的起始过电位和接近96%的一氧化碳生成法拉第效率. 采用离子液体1-乙基-3-甲基咪唑双（三氟甲基磺酰基）酰亚胺([EMIM] [Tf2N])是调节二氧化碳电化学还原的新策略. 对Pb电极而言,该电解液不仅活化了二氧化碳分子,使反应起始过电位降低近0.18 V ,还可通过提升浓度改变反应机理. 具体而言,增加0.1 mol·L-1 [EMIM] [Tf2N]的浓度会抑制CO2-中间体的二聚化,从而将产物从草酸盐（Pb电极上CO2RR的主要产物）向CO和咪唑鎓羧酸盐配合物移动[53].
3 电解液-催化剂相互作用
大量证据表明,电极结构对电解液环境非常敏感,催化剂暴露于电解液和施加电势时可能经历动态变化过程[58,59]. 因此,关注催化反应前后催化剂的状态,或在反应条件下进行原位的光谱和显微结构表征,对于理解催化活性位点的真实性质至关重要. 例如,Kim等人使用原位电化学扫描隧道显微镜(EC-STM)观察到多晶铜电极表面的重构现象[60]. 在0.1 mol·L-1 KOH电解液环境中施加-0.9 V vs. SHE电位进行二氧化碳还原,多晶铜电极上发生逐步的表面重建,首先在30分钟内过渡到Cu(111),然后在30分钟内转化成为具有乙烯选择性的Cu(100)特性晶面. 该小组在相同条件下使用0.1 mol·L-1 KHCO3电解液进行二氧化碳还原,近3 h后铜催化剂从多晶表面直接转变为Cu(100)晶面[61],这种转变在0.1 mol·L-1 KOH中更加迅速,并存在一个以Cu(111)为中间取向的过渡阶段. 该发现不仅有助于解释多晶铜优异的C2产物选择性,还提供了一种廉价多晶铜电极定向转化为Cu(100)单晶电极的新方法. 虽然多晶铜向低晶面指数转化是施加电位的影响结果,但从不同电解液中反应路径和时间差异的角度分析,电解液作用是显著的. 因此,本领域值得关注非均相电催化剂表面的动态性质,探究催化剂结构、组成和反应活性与电解液和反应条件之间的关联.
图4
图4
(A) 电解液对铜纳米立方体催化剂表面粗化的促进作用[58]. (B) 阴、阳离子共吸附对催化剂形貌及性能影响[26]. (C) 浓KOH电解液阳极电化学氧化制备Cu(OH)2纳米线[62]. (D), (E) 操作条件下铜纳米立方体催化剂的表面熟化机理[63].
Fig. 4
(A) Promoting effect of electrolyte on the roughening of the surface of Cu nanocube catalysts (Reprinted with permission from Ref.[58]. Copyright (2018) Wiley-VCH Verlag GmbH & Co. KGaA). (B) The effects of anion and cation co-adsorption on the morphology and performance of catalysts (Reprinted with permission from Ref.[26]. Copyright (2018) American Chemical Society). (C) Electrochemical oxidation to prepare Cu(OH)2 nanowires in concentrated KOH electrolyte (Reprinted with permission from Ref.[62]. Copyright (2018) American Chemical Society). (D), (E) Surface maturation mechanism of cubic Cu catalyst under operating conditions[63].
卤离子效应是阴离子效应的重要部分. Gao等人探究了卤离子添加剂对等离子活化铜催化剂表面的作用,结果表明 :含有KI的电解液促进催化剂表面发生结构和形态的明显变化,即使在开路电势下,铜表面也可在含I-电解质中自发形成异质CuI颗粒,在电解液中引入不同阳离子还可改变异型颗粒的形态[26, 42]. 电解液中Cl-、Br-和I-的存在均可降低铜催化剂对二氧化碳的还原过电势并提高反应速率,促进作用顺序为Cl- < Br- < I-,与卤离子半径大小规律一致,并且加入卤离子不会牺牲催化剂本征的C2-C3产物选择性. 结合第一性原理计算,这种增强作用主要归因于反应过程中卤化物的特异性吸附,催化活性的增强规律可解释为高半径卤离子在铜表面具有更高的吸附能. 此外,阴、阳离子的共吸附也能改变铜的表面形貌[26]（图4B）. 在含I-的普通电解液中,铜表面形成的CuI被还原时易失去碘离子而形成多孔表面,而在含有大阳离子Cs+、K+的电解液中,铜表面CuI物种的稳定性更高,这些纳米CuI颗粒在反应过程中有助于维持铜的正化合价态,在维持高活性的同时增强催化稳定性.
受电解液诱导催化剂结构演变的启发,使用电解液驱动的纳米结构定向演化策略可以制备高效的CO2RR催化剂. Nilsson小组在含KCl的KHCO3电解液中原位制备出立方体铜催化剂, 连续的氧化还原循环将多晶铜的无序结构转变为立方体构型,KCl可能会在氧化条件下初步形成CuCl相来促进形成立方体结构[64]. Gao等人改进了这种制备方法,他们使用KCl溶液对铜箔进行循环电镀抛光,电极表面上生长出晶面和氧、氯离子含量可控的铜纳米立方体,且铜颗粒尺寸可以通过KCl浓度进行调整[65]. 该制备方法有利于保持所制备样品中Cu(100)晶面的稳定性,高稳定性Cu(100)晶面显示出较低的过电势和较高的乙烯、乙醇和正丙醇选择性,且C2和C3产物法拉第效率接近73%.
电解液的pH和反应中间体也会影响催化剂的表面结构. 早期文献报道了在酸性电解液HClO4中电化学氢吸附会诱导Cu(100)表面重建,使用原位扫描隧道显微镜(STM)观察到氢诱导始于铜表面原子,使表面晶格位置发生侧向位移,形成新的条纹状结构,最后沿条纹方向扩展表面晶格形成异质结构[66]. 而在浓碱性KOH中进行阳极电化学氧化工艺时,多晶铜箔会沿着表面生长Cu(OH)2纳米线,并在反应下进一步还原为具有混合价态的铜催化剂[62]（图4C）. 对这种催化剂施加温和的还原条件,可以实现约40%的乙烯生成效率,并在40小时内保持较高的稳定性. 除pH的影响外,Huang等人从理论计算的角度入手,使用DFT计算对铜纳米立方体的界面能和熟化过程进行了研究,结果发现,在二氧化碳还原过程中生成的*CO中间体及质子对催化剂存在共吸附效应（图4D, E）,不同晶面被吸附时相对稳定性规律为Cu(100) > Cu(110) > Cu(111),结果揭示了铜纳米立方体催化剂的表面熟化机理[63]. 由此可见,在小分子电催化还原反应的过程中,关注催化剂与电解液的相互作用和动态转变非常重要. 在电解液的驱动下,催化剂的动态转化可能是降解失活的诱因,也可能是真正活性位点暴露的前提. 目前相关的研究在氮还原领域里相对缺乏,电解液诱导催化剂定向转化策略可用于新型NRR纳米催化剂的设计.
4 非水系电解液
在惰性小分子催化还原领域,基于更低成本、更高设备兼容性以及更小环境影响原则,多数工作使用水系电解液. 有机电解液、离子液体（如前所述）、高分子电解液、固态电解液等非水系电解液也被用于氮气及二氧化碳还原. 例如,使用甲醇作为电解液进行均相氮还原电催化,可得到0.11%的产氨效率,合理使用碱(MeONa)和二价阳离子(Mg2+)等添加剂会提高氨产率[67]. Kim等人使用硫酸/2-丙醇混合物以及氯化锂/乙二胺作为电解液进行氮还原,分别获得0.89%和17.2%的产氨效率[68,69]. 作者证实,使用有机电解液可以拓宽电解液的电化学窗口,抑制析氢,提高反应选择性. 如图5A所示,有机溶剂可以选择性地搭配质子供体,例如,质子活性更低的四氢呋喃和乙醇溶剂改善了电化学氮还原性能[70]. 该电解液与超疏水MOF涂层相互促进,有效地抑制了析氢副反应,实现了90%的产氨选择性. 一些有机溶剂具有较高的二氧化碳溶解能力,有利于二氧化碳电化学还原. 在二甲基亚砜(DMF)、N,N-二甲基甲酰胺(DMSO)、乙腈(MeCN)和碳酸亚丙酯(PC)等溶剂中,CO2RR可生成两电子产物[71]. 然而,当使用甲醇作为电解液时,铜电极会析出乙烯、甲烷等多电子产物,这表明有机电解液对二氧化碳还原反应路径具有调控作用[71,72]. 值得注意的是,由于二氧化碳电化学还原会产生液态有机物,使用有机电解液会造成产物检测与分离不便等困难.
固态电解质的应用有助于解决析氢竞争反应以及产品与液态溶剂难以分离的问题. Sheets等人将聚丙烯酸均聚物与高浓度KOH溶液混合,制备出凝胶聚合物电解质,这种新型的聚合物凝胶被用于电催化氮还原（图5B）,通过限制电解池中水的传输和气体扩散管理,有效抑制了析氢反应,显著提高了NRR的选择性[73]. Cook等人则构建了一种钌/固体聚合物电解质界面,实现了将氮气直接气相电化学还原为氨[74]. CO2RR产生的液态产物与电解液难以分离,而使用固态电解质CsxH3-xPW12O40将电化学产生的阳离子（例如H+）和阴离子（例如HCOO-）耦合形成纯净的燃料产品,不引入其他杂质离子,当阴极使用二维Bi基催化剂时,该电解装置可以实现生产浓度高达12 mol·L-1的HCOOH溶液,阴极换用铜催化剂时,还可产出无电解质污染的乙酸、乙醇和正丙醇等高碳化合物液体[75].
图5
图5
(A) THF和EtOH混合液用于电化学氮还原电解液[70]. (B) 固态凝胶电解质用于电化学氮还原[73]. (C) 气相反应装置示意图[14].
Fig. 5
(A) THF and EtOH mixture as electrolyte solvent for electrochemical nitrogen reduction[70]. (B) Solid gel electrolyte for electrochemical nitrogen reduction (Reprinted with permission from Ref.[73]. Copyright (1996) Royal Society of Chemistry). (C) Demonstration of gas-phase reaction device[14].
尽管电解液的化学调控效果显著,气体溶解度低和传质慢等限制仍是巨大挑战. 研究表明,二氧化碳在水中的溶解能力仅支持最大~ 35 mA·cm-2的还原电流密度,对于氮还原则更低[14]. 对电解液的物理调控为解决此问题提供了新思路. 如图5C所示,使用气相反应装置提升了二氧化碳传质速率,达到了更高的二氧化碳还原电流密度[76,77,78,79]. 这种装置未采用催化剂层和离子交换膜之间流动的电解液,而将气体扩散层、催化剂和离子交换膜直接合并为一个单元,将水蒸气与二氧化碳/氮气混合作为反应气鼓入装置. 在工作状态下气相反应装置在多孔催化剂层中存在一层液态水膜[80],具有更高的气-液接触面积和表面动态更新特性. 与常用的H型电解槽相比,这种气相电解槽将二氧化碳扩散至催化剂表面的路径减少了大约3个数量级,有效地促进了二氧化碳的传质. 可见,电解液的调控能力可通过设计优化反应装置进一步提升.
5 结 论
本文概述了近年来运用电解液调控技术改善二氧化碳和氮气还原反应的研究工作,总结了从电解液角度出发研究影响机制、揭示反应机理的最新进展. 得益于理论计算和原位谱学的发展,人们逐渐阐明了一些独立的环境影响因素,如电解液强度、电极局域pH值、电化学双电层(EDL)结构、电解液-催化剂相互作用和离子吸附效应等.
目前改善二氧化碳还原的电解液调控方法包括 :1）降低缓冲容量,提升工作条件下的电极局域pH值;2)使用可造成高表面电场的离子,稳定具有高偶极距的反应中间体;3)改善电化学双电层结构,调控质子电子耦合的速率和竞争性;4)使用高浓电解液加速离子和二氧化碳传质,或使用高浓离子液体活化二氧化碳、屏蔽析氢反应;5）研究催化剂与电解液的相互作用和动态过程,利用某些具有特性吸附的离子原位改变催化剂的表面组成、形貌或性质;6)使用非水系电解液调控质子活性和电化学窗口,或提高二氧化碳溶解度;7)使用气相反应装置加速电解液表面更新与二氧化碳传质.
虽然上述策略可缓解反应活性和选择性差的问题,但CO2RR离实际工业应用尚远. 传统测试设备及电解液存在固有的传输限制,对工作状态下催化剂的动态变化以及电极/电解液相互影响机制理解仍有待深入. 催化剂的选择性和活性对局部反应环境高度敏感,局部反应环境又随反应速率改变而发生变化,操作条件可能导致催化剂活性中心的钝化甚至是活性晶面的降解. 虽然部分影响催化反应的关键参数被分离并被重新认识,但是某些参数忽略了催化界面的动态变化,研究结论可能会偏离实际情况. 因此,需要开发工况下电极/电解液界面的原位表征技术和多尺度时空模拟方法,研究惰性分子催化还原反应的微界面结构特性和演变过程,为高性能催化体系的开发提供有价值的信息.
电催化NRR和CO2RR存在相似性,可以预期,电解液的调控对于氮还原体系也有显著影响. 如果将氮还原产物氨(NH3)类比为二氧化碳还原的单碳产物甲烷(CH4),采用促进CH4生成的策略可能有助于氮还原产氨,如使用半径较小的碱金属阳离子(Li+, Na+)电解质、降低局部pH值或使用碳酸根和磷酸根等良质子供体电解质等. 此外,虽然目前氮还原领域鲜见高选择性产肼(N2H4)的报道,我们可将产物肼类比产物乙烯(C2H6),促进C-C耦合的反应条件也可能有利于选择性生成肼. 结合第一性原理、电化学双电层模型、离子特性吸附、扩散传质等理论与原位谱学和电化学测试相结合的方法,系统深入地研究电解液在电催化反应中的性质和性能强化机制,对于发展惰性小分子高附加值转化技术具有重要意义.
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Much effort has been devoted in the development of efficient catalysts for electrochemical reduction of CO2. Molecular level understanding of electrode-mediated process, particularly the role of bicarbonate in increasing CO2 reduction rates, is still lacking due to the difficulty of directly probing the electrochemical interface. We developed a protocol to observe normally invisible reaction intermediates with a surface enhanced spectroscopy by applying square-wave potential profiles. Further, we demonstrate that bicarbonate, through equilibrium exchange with dissolved CO2, rather than the supplied CO2, is the primary source of carbon in the CO formed at the Au electrode by a combination of in situ spectroscopic, isotopic labeling, and mass spectroscopic investigations. We propose that bicarbonate enhances the rate of CO production on Au by increasing the effective concentration of dissolved CO2 near the electrode surface through rapid equilibrium between bicarbonate and dissolved CO2.
Bicarbonate is not a general acid in Au-catalyzed CO2 electroreduction
[J].
DOI:10.1021/jacs.7b08345
URL
PMID:28978199
[本文引用: 1]
We show that bicarbonate is neither a general acid nor a reaction partner in the rate-limiting step of electrochemical CO2 reduction catalysis mediated by planar polycrystalline Au surfaces. We formulate microkinetic models and propose diagnostic criteria to distinguish the role of bicarbonate. Comparing these models with the observed zero-order dependence in bicarbonate and simulated interfacial concentration gradients, we conclude that bicarbonate is not a general acid cocatalyst. Instead, it acts as a viable proton donor past the rate-limiting step and a sluggish buffer that maintains the bulk but not local pH in CO2-saturated aqueous electrolytes.
Improved CO2 electroreduction performance on plasma-activated Cu catalysts via electrolyte design: halide effect
[J].
Formation of hydrocarbons in the electrochemical reduction of carbon dioxide at a copper electrode in aqueous solution
[J].
Manipulating the hydrocarbon selectivity of copper nanoparticles in CO2 electroreduction by process conditions
[J].
Controlling the selectivity of CO2 electroreduction on copper: the effect of the electrolyte concentration and the importance of the local pH
[J].DOI:10.1016/j.cattod.2015.06.009 URL [本文引用: 1]
Effects of electrolyte buffer capacity on surface reactant species and the reaction rate of CO2 in electrochemical CO2 reduction
[J].DOI:10.1021/acs.jpcc.7b11316 URL [本文引用: 1]
Combined high alkalinity and pressurization enable efficient CO2 electroreduction to CO
[J].
One-step electrosynjournal of ethylene and ethanol from CO2 in an alkaline electrolyzer
[J].DOI:10.1016/j.jpowsour.2015.09.124 URL [本文引用: 1]
Insights into the low overpotential electroreduction of CO2 to CO on a supported gold catalyst in an alkaline flow electrolyzer
[J].DOI:10.1021/acsenergylett.7b01096 URL [本文引用: 1]
CO2 electroreduction to ethylene via hydroxide-mediated copper catalysis at an abrupt interface
[J].
DOI:10.1126/science.aas9100
URL
PMID:29773749
[本文引用: 1]
Carbon dioxide (CO2) electroreduction could provide a useful source of ethylene, but low conversion efficiency, low production rates, and low catalyst stability limit current systems. Here we report that a copper electrocatalyst at an abrupt reaction interface in an alkaline electrolyte reduces CO2 to ethylene with 70% faradaic efficiency at a potential of -0.55 volts versus a reversible hydrogen electrode (RHE). Hydroxide ions on or near the copper surface lower the CO2 reduction and carbon monoxide (CO)-CO coupling activation energy barriers; as a result, onset of ethylene evolution at -0.165 volts versus an RHE in 10 molar potassium hydroxide occurs almost simultaneously with CO production. Operational stability was enhanced via the introduction of a polymer-based gas diffusion layer that sandwiches the reaction interface between separate hydrophobic and conductive supports, providing constant ethylene selectivity for an initial 150 operating hours.
CO2 electrolysis to multicarbon products at activities greater than 1 A·cm-2
[J].
DOI:10.1126/science.aay4217
URL
PMID:32029623
[本文引用: 1]
Electrolysis offers an attractive route to upgrade greenhouse gases such as carbon dioxide (CO2) to valuable fuels and feedstocks; however, productivity is often limited by gas diffusion through a liquid electrolyte to the surface of the catalyst. Here, we present a catalyst:ionomer bulk heterojunction (CIBH) architecture that decouples gas, ion, and electron transport. The CIBH comprises a metal and a superfine ionomer layer with hydrophobic and hydrophilic functionalities that extend gas and ion transport from tens of nanometers to the micrometer scale. By applying this design strategy, we achieved CO2 electroreduction on copper in 7 M potassium hydroxide electrolyte (pH approximately 15) with an ethylene partial current density of 1.3 amperes per square centimeter at 45% cathodic energy efficiency.
Ionic liquid - mediated selective conversion of CO2 to CO at low overpotentials
[J].
DOI:10.1126/science.1209786
URL
PMID:21960532
[本文引用: 1]
Electroreduction of carbon dioxide (CO(2))--a key component of artificial photosynthesis--has largely been stymied by the impractically high overpotentials necessary to drive the process. We report an electrocatalytic system that reduces CO(2) to carbon monoxide (CO) at overpotentials below 0.2 volt. The system relies on an ionic liquid electrolyte to lower the energy of the (CO(2))(-) intermediate, most likely by complexation, and thereby lower the initial reduction barrier. The silver cathode then catalyzes formation of the final products. Formation of gaseous CO is first observed at an applied voltage of 1.5 volts, just slightly above the minimum (i.e., equilibrium) voltage of 1.33 volts. The system continued producing CO for at least 7 hours at Faradaic efficiencies greater than 96%.
Switching the reaction course of electrochemical CO2 reduction with ionic liquids
[J].
DOI:10.1021/la5009076
URL
PMID:24851903
[本文引用: 1]
The ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([emim][Tf2N]) offers new ways to modulate the electrochemical reduction of carbon dioxide. [emim][Tf2N], when present as the supporting electrolyte in acetonitrile, decreases the reduction overpotential at a Pb electrode by 0.18 V as compared to tetraethylammonium perchlorate as the supporting electrolyte. More interestingly, the ionic liquid shifts the reaction course during the electrochemical reduction of carbon dioxide by promoting the formation of carbon monoxide instead of oxalate anion. With increasing concentration of [emim][Tf2N], a carboxylate species with reduced CO2 covalently bonded to the imidazolium ring is formed along with carbon monoxide. The results highlight the catalytic effects of the medium in modulating the CO2 reduction products.
Electro-synjournal of ammonia from nitrogen at ambient temperature and pressure in ionic liquids
[J].
Theoretical study of EMIM+ adsorption on silver electrode surfaces
[J].
Monodisperse Au nanoparticles for selective electrocatalytic reduction of CO2 to CO
[J].
DOI:10.1021/ja409445p
URL
PMID:24156631
[本文引用: 1]
We report selective electrocatalytic reduction of carbon dioxide to carbon monoxide on gold nanoparticles (NPs) in 0.5 M KHCO3 at 25 degrees C. Among monodisperse 4, 6, 8, and 10 nm NPs tested, the 8 nm Au NPs show the maximum Faradaic efficiency (FE) (up to 90% at -0.67 V vs reversible hydrogen electrode, RHE). Density functional theory calculations suggest that more edge sites (active for CO evolution) than corner sites (active for the competitive H2 evolution reaction) on the Au NP surface facilitates the stabilization of the reduction intermediates, such as COOH*, and the formation of CO. This mechanism is further supported by the fact that Au NPs embedded in a matrix of butyl-3-methylimidazolium hexafluorophosphate for more efficient COOH* stabilization exhibit even higher reaction activity (3 A/g mass activity) and selectivity (97% FE) at -0.52 V (vs RHE). The work demonstrates the great potentials of using monodisperse Au NPs to optimize the available reaction intermediate binding sites for efficient and selective electrocatalytic reduction of CO2 to CO.
The mechanism of room-temperature ionic-liquid-based electrochemical CO2 reduction: a review
[J].DOI:10.3390/molecules22040536 URL [本文引用: 1]
Dynamic changes in the structure, chemical state and catalytic selectivity of Cu nanocubes during CO2 electroreduction: size and support effects
[J].
Beyond copper in CO2 electrolysis: effective hydrocarbon production on silver-nanofoam catalysts
[J].DOI:10.1021/acscatal.8b01738 URL [本文引用: 1]
The evolution of the polycrystalline copper surface, first to Cu(111) and then to Cu(100), at a fixed CO2RR potential: a study by operando EC-STM
[J].
DOI:10.1021/la504445g
URL
PMID:25489793
[本文引用: 1]
A study based on operando electrochemical scanning tunneling microscopy (EC-STM) has shown that a polycrystalline Cu electrode held at a fixed negative potential, -0.9 V (vs SHE), in the vicinity of CO2 reduction reactions (CO2RR) in 0.1 M KOH, undergoes stepwise surface reconstruction, first to Cu(111) within 30 min, and then to Cu(100) after another 30 min; no further surface transformations occurred after establishment of the Cu(100) surface. The results may help explain the Cu(100)-like behavior of Cu(pc) in terms of CO2RR product selectivity. They likewise suggest that products exclusive to Cu(100) single-crystal electrodes may be generated through the use of readily available inexpensive polycrystalline Cu electrodes. The study highlights the dynamic nature of heterogeneous electrocatalyst surfaces and also underscores the importance of operando interrogations when structure-composition-reactivity correlations are intended.
Surface reconstruction of polycrystalline Cu electrodes in aqueous KHCO3 electrolyte at potentials in the early stages of CO2 reduction
[J].DOI:10.1007/s12678-018-0469-z URL [本文引用: 1]
Mixed copper states in anodized Cu electrocatalyst for stable and selective ethylene production from CO2 reduction
[J].
DOI:10.1021/jacs.8b02173
URL
PMID:29913063
[本文引用: 3]
Oxygen-Cu (O-Cu) combination catalysts have recently achieved highly improved selectivity for ethylene production from the electrochemical CO2 reduction reaction (CO2RR). In this study, we developed anodized copper (AN-Cu) Cu(OH)2 catalysts by a simple electrochemical synthesis method and achieved approximately 40% Faradaic efficiency for ethylene production, and high stability over 40 h. Notably, the initial reduction conditions applied to AN-Cu were critical to achieving selective and stable ethylene production activity from the CO2RR, as the initial reduction condition affects the structures and chemical states, crucial for highly selective and stable ethylene production over methane. A highly negative reduction potential produced a catalyst maintaining long-term stability for the selective production of ethylene over methane, and a small amount of Cu(OH)2 was still observed on the catalyst surface. Meanwhile, when a mild reduction condition was applied to the AN-Cu, the Cu(OH)2 crystal structure and mixed states disappeared on the catalyst, becoming more favorable to methane production after few hours. These results show the selectivity of ethylene to methane in O-Cu combination catalysts is influenced by the electrochemical reduction environment related to the mixed valences. This will provide new strategies to improve durability of O-Cu combination catalysts for C-C coupling products from electrochemical CO2 conversion.
Potential-induced nanoclustering of metallic catalysts during electrochemical CO2 reduction
[J].
DOI:10.1038/s41467-017-02088-w
URL
PMID:29317637
[本文引用: 3]
Many studies have shown how pigments and internal nanostructures generate color in nature. External surface structures can also influence appearance, such as by causing multiple scattering of light (structural absorption) to produce a velvety, super black appearance. Here we show that feathers from five species of birds of paradise (Aves: Paradisaeidae) structurally absorb incident light to produce extremely low-reflectance, super black plumages. Directional reflectance of these feathers (0.05-0.31%) approaches that of man-made ultra-absorbent materials. SEM, nano-CT, and ray-tracing simulations show that super black feathers have titled arrays of highly modified barbules, which cause more multiple scattering, resulting in more structural absorption, than normal black feathers. Super black feathers have an extreme directional reflectance bias and appear darkest when viewed from the distal direction. We hypothesize that structurally absorbing, super black plumage evolved through sensory bias to enhance the perceived brilliance of adjacent color patches during courtship display.
High selectivity for ethylene from carbon dioxide reduction over copper nanocube electrocatalysts
[J].
DOI:10.1002/anie.201412214
URL
PMID:25728325
[本文引用: 1]
Nanostructured surfaces have been shown to greatly enhance the activity and selectivity of many different catalysts. Here we report a nanostructured copper surface that gives high selectivity for ethylene formation from electrocatalytic CO2 reduction. The nanostructured copper is easily formed in situ during the CO2 reduction reaction, and scanning electron microscopy (SEM) shows the surface to be dominated by cubic structures. Using online electrochemical mass spectrometry (OLEMS), the onset potentials and relative selectivity toward the volatile products (ethylene and methane) were measured for several different copper surfaces and single crystals, relating the cubic shape of the copper surface to the greatly enhanced ethylene selectivity. The ability of the cubic nanostructure to so strongly favor multicarbon product formation from CO2 reduction, and in particular ethylene over methane, is unique to this surface and is an important step toward developing a catalyst that has exclusive selectivity for multicarbon products.
Plasma-activated copper nanocube catalysts for efficient carbon dioxide electroreduction to hydrocarbons and alcohols
[J].
DOI:10.1021/acsnano.7b01257
URL
PMID:28441005
[本文引用: 1]
Carbon dioxide electroreduction to chemicals and fuels powered by renewable energy sources is considered a promising path to address climate change and energy storage needs. We have developed highly active and selective copper (Cu) nanocube catalysts with tunable Cu(100) facet and oxygen/chlorine ion content by low-pressure plasma pretreatments. These catalysts display lower overpotentials and higher ethylene, ethanol, and n-propanol selectivity, resulting in a maximum Faradaic efficiency (FE) of approximately 73% for C2 and C3 products. Scanning electron microscopy and energy-dispersive X-ray spectroscopy in combination with quasi-in situ X-ray photoelectron spectroscopy revealed that the catalyst shape, ion content, and ion stability under electrochemical reaction conditions can be systematically tuned through plasma treatments. Our results demonstrate that the presence of oxygen species in surface and subsurface regions of the nanocube catalysts is key for achieving high activity and hydrocarbon/alcohol selectivity, even more important than the presence of Cu(100) facets.
Reconstruction of Cu(100) electrode surfaces during hydrogen evolution
[J].
DOI:10.1021/ja904033t
URL
PMID:19588964
[本文引用: 1]
Electrochemical hydrogen evolution on (100)-oriented copper electrodes is shown to induce a novel surface reconstruction, which substantially influences the rates of this electrochemical reaction. As revealed by in situ video-STM the formation of this phase starts with lateral displacements of Cu surface atoms from lattice positions, resulting in stripe-like structures, followed by expansion of the surface lattice along the stripe direction.
Nitrogen fixation: Part I. Electrochemical reduction of titanium compounds in the presence of catechol and N2 in MeOH or THF
[J].DOI:10.1016/0022-0728(87)80138-9 URL [本文引用: 1]
Communication-electrochemical reduction of nitrogen to ammonia in 2-propanol under ambient temperature and pressure
[J].DOI:10.1149/2.0231607jes URL [本文引用: 1]
Electrochemical synjournal of ammonia from water and nitrogen in ethylenediamine under ambient temperature and pressure
[J].DOI:10.1149/2.0741614jes URL [本文引用: 1]
Favoring the unfavored: selective electrochemical nitrogen fixation using a reticular chemistry approach
[J].
Electrochemical reduction of CO2 in methanol with aid of CuO and Cu2O
[J].DOI:10.1016/j.cattod.2009.07.077 URL [本文引用: 2]
Electrochemical reduction of high pressure CO2 at a Cu electrode in cold methanol
[J].
DOI:10.1016/j.electacta.2006.01.032
URL
[本文引用: 1]
AbstractThe electrochemical reduction of high pressure CO2 with a Cu electrode in cold methanol was investigated. A high pressure stainless steel vessel, with a divided H-type glass cell, was employed. The main products from CO2 by the electrochemical reduction were methane, ethylene, carbon monoxide and formic acid. In the electrolysis of high pressure CO2 at low temperature, the reduction products were formed in the order of carbon monoxide, methane, formic acid and ethylene. The best current efficiency of methane was of 20% at −3.0 V. The maximum partial current density for CO2 reduction was approximately 15 mA cm−2. The partial current density ratio of CO2 reduction and hydrogen evolution, i(CO2)/i(H2), was more than 2.6 at potentials more positive than −3.0 V. This work can contribute to the large-scale manufacturing of fuel gases from readily available and inexpensive raw materials, CO2-saturated methanol from industrial absorbers (the Rectisol process).]]>
Electrochemical nitrogen reduction to ammonia under mild conditions enabled by a polymer gel electrolyte
[J].
DOI:10.1039/c8cc00657a
URL
PMID:29521392
[本文引用: 3]
A novel gel electrolyte approach has been implemented to enable the electrochemical reduction of nitrogen to ammonia at low temperature and pressure.
Ambient temperature gas phase electrochemical nitrogen reduction to ammonia at ruthenium/solid polymer electrolyte interface
[J].DOI:10.1007/BF00766163 URL [本文引用: 1]
Continuous production of pure liquid fuel solutions via electrocatalytic CO2 reduction using solid-electrolyte devices
[J].DOI:10.1038/s41560-019-0451-x URL [本文引用: 1]
Electrolysis of CO2 to syngas in bipolar membrane-based electrochemical cells
[J].DOI:10.1021/acsenergylett.6b00475 URL [本文引用: 1]
Electrolysis of Gaseous CO2 to CO in a flow cell with a bipolar membrane
[J].DOI:10.1021/acsenergylett.7b01017 URL [本文引用: 1]
Electrochemical generation of syngas from water and carbon dioxide at industrially important rates
[J].
Catholyte-free electrocatalytic CO2 reduction to formate
[J].
DOI:10.1002/anie.201803501
URL
PMID:29660257
[本文引用: 1]
Electrochemical reduction of carbon dioxide (CO2 ) into value-added chemicals is a promising strategy to reduce CO2 emission and mitigate climate change. One of the most serious problems in electrocatalytic CO2 reduction (CO2 R) is the low solubility of CO2 in an aqueous electrolyte, which significantly limits the cathodic reaction rate. This paper proposes a facile method of catholyte-free electrocatalytic CO2 reduction to avoid the solubility limitation using commercial tin nanoparticles as a cathode catalyst. Interestingly, as the reaction temperature rises from 303 K to 363 K, the partial current density (PCD) of formate improves more than two times with 52.9 mA cm(-2) , despite the decrease in CO2 solubility. Furthermore, a significantly high formate concentration of 41.5 g L(-1) is obtained as a one-path product at 343 K with high PCD (51.7 mA cm(-2) ) and high Faradaic efficiency (93.3 %) via continuous operation in a full flow cell at a low cell voltage of 2.2 V.
Design of an electrochemical cell making syngas (CO + H2) from CO2 and H2O reduction at room temperature
[J].DOI:10.1149/1.2801871 URL [本文引用: 1]
