0 引 言

化石燃料的大量使用导致土壤污染、空气污染、气候变化等严重威胁人类生存发展的环境问题[1]。开发新型清洁能源成为人类社会可持续发展的必经途径。氢气作为二次能源具有零污染零排放的特点,在工业生产和储能方面应用广泛。目前我国最常见的制氢方法是煤制氢,其产生的废水、废气、废渣会影响环境;通过天然气重整制氢相较煤制氢更环保,但仍会产生温室气体;目前绿色制氢的方式是通过风、光、水等可再生能源电力进行电解水制氢,在此过程中不产生环境污染和温室气体,是一种环境友好的产氢方式。

当前主要的电解制氢技术有碱性电解(AEM)、质子交换膜电解(PEM)和固体氧化物电解(SOEC)3种。与AEM和PEM相比,SOEC制氢的优势在于:① 高效:运行温度高(600～900 ℃)、耗能小、能量转化效率高;② 成本低廉:无需贵金属催化剂,具有良好的应用前景[2];③ 共电解:SOEC可电解混合气体(CO2和H2O)生成合成气,应用于二氧化碳回收、燃料气合成等领域;④ 可逆操作:固体氧化物电池(SOC)可在电解模式下制取氢气或合成气进行储能,在电池模式下将化学能转换成电能,可逆运行模式为其应用提供多种可能商业模式。

由于SOEC电解温度较高,因此电解反应动力学过程较快,过电位降低,所需电能逐渐减小,电解效率大幅提高,从而电能转化效率更高[3]。然而,SOEC所处的强腐蚀环境对其材料的性能带来巨大挑战:① 阴极的金属催化剂在高温高湿环境下极易迁移、团聚、挥发,反应气体扩散易导致其浓差极化升高并降低电解效率;② 电解质材料欧姆阻抗较高导致欧姆极化高造成电能损失;③ 高电流密度下阳极材料和电解质层会发生界面分离分层;④ 密封器件等材料机械性能降低,以上问题阻碍了其商业化进程。因此开发稳定性好,热膨胀系数匹配,成本低廉,使用寿命长的材料成为主要研究方向。针对SOEC常见材料类型及其改性方法进行总结,并对当前SOEC研究现状进行讨论和展望。

1 SOEC组成及工作原理

常见SOEC多为氧离子型电解池,由致密电解质层和两侧多孔电极组成,以电解水为例,将高温水蒸气通入阴极还原生成氢气和氧负离子,氧负离子通过致密电解质到达阳极,在阳极失电子生成氧气,其工作原理如图1所示。氧离子型电解池为主要电解池类型。

2 阴极材料

SOEC在高温下运行,反应气通入阴极发生还原反应生成氢气(或碳氢混合气体)和氧负离子,其中H2O/CO2得到电子分解为H2/CO和氧负离子(O2-)的过程是整个电解池阴极反应的速度控制步骤[4],因此提高反应气解离的速率是提高整个阴极性能的关键。Ni、Fe、Cu等金属及其合金可提供电解反应所需催化位点和电子传导[5],且在高温环境下不易自燃,价格低廉,可用于SOEC阴极,但纯金属与电解质材料热膨胀系数不匹配、发生反应的电解质-金属-气相三相界面(TPBs)区域有限且在高温条件下退化严重,因此常与电解质复合制备成具有多孔结构的金属陶瓷复合材料,如Ni-氧化钇稳定的氧化锆(YSZ)等。然而金属陶瓷复合材料并不能完全满足阴极所有要求,该复合材料中的金属单质在阴极高温高湿的条件下易发生元素迁移、氧化、损失等问题,在H2O和CO2共电解情况下Ni基金属陶瓷材料还会因积碳导致催化活性降低。而具有催化活性和导电性的钙钛矿型陶瓷材料也常用作阴极,但与传统金属陶瓷复合材料相比其催化性能和导电率较低。因此在高温高湿环境下保证材料长期运行的稳定性和催化活性,克服极化损失和性能衰减是阴极材料研究重点。目前常见的阴极材料包括金属陶瓷复合材料和钙钛矿型阴极材料。

2.1 金属陶瓷

Ni-YSZ是目前最常见的SOEC阴极材料,已实现商业化应用,YSZ在高温下具有优良的离子导电性和稳定性[6],是目前高温电解最常用的电解质材料,Ni的作用主要是提供催化反应位点和电子电导,在电解过程中表现出较高催化活性,将YSZ添加到Ni中可显著降低材料热膨胀系数,使其与电解质材料相匹配。Ni-YSZ在固体氧化物燃料电池(SOFC)模式下具有催化活性高、电化学性能优异、成本低、制作简单等优点。但SOEC电解模式下Ni元素扩散较快,Ni的迁移团聚是导致电解池初期性能下降的原因之一[7]。同时电解模式下阴极侧高浓度水蒸气还会导致单质Ni氧化生成Ni(OH)x进而蒸发造成Ni损失[8],这将导致电解反应的三相界面(TPB)减少从而降低电极的催化性能。提高Ni基金属陶瓷阴极的稳定性,防止Ni金属在长期运行中的氧化、损失、团聚是Ni基金属陶瓷电极研究的重点。更换不同的陶瓷基材料和降低反应温度对提高电极稳定性都有正面影响[9-10],但其内在机理仍需研究。如采用钐掺杂氧化铈作为Ni基金属陶瓷中的陶瓷相比Ni-YSZ电极具有更高的稳定性[11],钆掺杂氧化铈(GDC)作为Ni基陶瓷相的电极也同样表现出比传统Ni-YSZ较高的稳定性[12]。而较高的稳定性可归因于氧化铈基材料的混合离子和电子导电特性,在反应过程中,掺杂氧化铈中的Ce4 发生价态变化生成Ce3 ,并形成动态氧空位,有助于氧离子传递转移,扩大水解反应发生的范围,从而减少氧离子在Ni表面聚集,与此同时金属与掺杂氧化铈之间特定的相互作用也增加了氧化铈基金属陶瓷复合材料的稳定性。

由于反应物可直接在铈基氧化物上进行电化学反应,以Ni-YSZ作为骨架将掺杂氧化铈类材料浸渍烧结到Ni-YSZ骨架上,形成的纳米级电催化剂修饰的SOEC阴极[13-14]同样对电极性能起促进作用。由于浸渍形成的纳米级催化剂改变了电极表面的组成,可减少燃料气中有害物质对阴极影响[15],同时可提高电极电导率,增大三相界面(TPB)区域,因此较传统Ni-YSZ电极其耐久性和电化学性能显著增强。掺杂氧化铈类材料除具有稳定Ni金属作用外,覆盖的纳米级催化剂可能起到物理限域作用,可缓解电池在运行过程中的Ni金属团聚粗化现象[16]。除氧化铈类材料,负载铜等金属纳米颗粒[17]以及质子型导体陶瓷相[18]在Ni-YSZ表面均可有效提高电极催化性能。

Ni基金属陶瓷材料虽具有较高催化活性,但由于其在H2O和CO2共电解过程中易被CO2氧化、产生积碳等原因难以在共电解条件下长期稳定工作,因此诸多研究者尝试选择同样具有催化效果且在H2O和CO2共电解条件下更稳定的金属相如Cu、Fe等。Cu相较Ni具有不易积碳等优点,且与Fe相比有较高的电子导电性,常用作SOEC共电解H2O/CO2的阴极材料。但Cu作为电极材料催化活性较低,提高Cu基电极的催化活性是Cu基阴极材料的研究目标,目前铜基陶瓷金属常与掺杂氧化铈复合,且取得了较好稳定性和催化活性[19],这可能是由于掺杂氧化铈材料电导率更高,或由于掺杂增加了氧化铈氧空位,促进了在二氧化碳吸附/活化和氧扩散[20]。

除Ni、Cu等过渡金属基复合阴极,贵金属也可用作SOEC阴极材料。Ag具有导电性高和化学稳定等优点[21],被认为可用作SOEC阴极材料。XIE等[22]制备多孔Ag-GDC作为电解池阴极电解CO2,在800 ℃、1.59 V恒定电压下运行18 h,电流密度相对稳定,无明显衰退。贵金属Pt对于氢的吸附活化能低,在PEM电解水中表现出很好的催化活性,但Pt用作SOEC阴极容易在运行中出现Pt的活性退化、电极电阻增大等问题[23],稳定性较差。以贵金属为原料制备的金属陶瓷由于稳定性较差且价格高昂,不适合大规模工业应用;但基于贵金属(如Pt、Ru、Au、Ir)催化活性较好,可将其少量均匀地负载于Ni基金属陶瓷材料上,有效提高电极的性能。

Ni金属的合金化同样可提高SOEC整体电化学性能,研究表明Ni-Fe、Ni-Co、Ni-Pt、Ni-Cu等镍基合金均表现出较好的催化活性[24]和稳定性[25-26]。其中Ni-Fe合金陶瓷作为阴极材料在电解水方面表现出优异的性能,NIAKOLAS等[27]以负载少量Fe的Ni-GDC作为SOEC阴极电解50% H2O-50% H2,在1.6 V电解电压下的产氢速率可达4.3 mL/(cm2·min),产氢速率是纯Ni-GDC电极性能的3倍。针对镍合金基金属陶瓷相较单一金属基金属陶瓷具有更强催化活性和产氢速率这一现象,GU等[28]采用密度泛函理论计算结合微动力学模型研究Ni与另一种金属合金化对水电解过程的影响。研究发现Ni-Fe合金在所选Ni合金中表现出最强的电解特性,这可能是由于非均相催化剂对水的解离步骤更有利。合金的优越性不只体现在水的电解中,在CO2电解中亲氧金属对CO2电解的活性较高,但易与氧结合导致失活,合金化则可促进过渡金属电子结构调节,从而优化其电化学性能[29]。

除材料自身特性外,电极的微观结构形貌也会影响电解池性能。宏观来看阴极/电解质界面易发生元素扩散生成杂相并降低电解池的稳定性。虽然在电解质与阴极间增加阻挡层,可避免阴极/电解质界面之间元素扩散与电解质形成杂相[30],提高电解池稳定性,但阻挡层会增加阴极材料电极阻抗,因此为降低额外阻抗的影响,阻挡层应尽可能薄且致密,工艺要求较高。

电极孔隙率、孔径大小和分布将影响电解池性能[31],由于电极材料的多孔结构可增加发生反应的TPB区域,进而促进电极性能的增强[32],同时制备具有精细结构的Ni-YSZ可形成更稳定的TPB区域,从而降低其在运行过程中的极化损耗[33]。如MONACO等[34]制备了2种具有不同微观结构Ni-YSZ阴极的电池A、B并进行对比,电池A具有精细的微观结构,镍颗粒分布相当均匀,这意味着电池A具有更多TPB区域;电池B具有更粗糙的微观结构,镍颗粒和导电陶瓷颗粒较大,且团聚严重导致TPB区域较电池A少。长时间运行发现,在电解模式下具有粗糙微观结构Ni-YSZ阴极的电解槽性能下降更快,这可能与粗糙结构下Ni的损耗导致TPB反应区域减少有关。HAUCH等[35]在研究中也发现了类似现象,精细结构的Ni-YSZ电极在800 ℃、1 A/cm2的条件下表现出较低的衰减速率(0.3%～0.4%/1 000 h)。可见设计精细结构的阴极微观形貌对电解池稳定性的提高有显著促进作用。多孔结构不仅有助于增加反应位点还有利于气体的扩散,由于目前常见的固体氧化物电解池一般采用阴极支撑,支撑层增加了电极厚度,有利于提高电解池机械强度,防止电解池出现断裂等问题,但同时也会造成气体扩散变慢、浓差极化增大从而影响电池性能,这就需要制备多孔阴极层来保证气体的扩散率。目前常见的制备多孔结构的方法有:使用造孔剂如石墨、淀粉、PMMA等通过高温将造孔剂去除来制备多孔阴极材料,缺点是难以控制孔道尺寸和形状[36];使用冷冻干燥法可获得直孔结构,但冷冻干燥法成本较高难以大规模应用[37];也可通过相转化法制备三明治状微观多孔结构[38]。除增加孔隙率,选择大孔结构的阴极也可以实现气体的快速传输。LIN等[39]制备了一种含有指状直开孔结构的Ni-YSZ支撑层,由于孔隙较大的直孔结构能够实现快速气相传输,减小浓差极化,电池在750 ℃、1.3 V电解电压下电解50% H2O,产氢速率可达9.29 mL/(cm2·min),明显优于传统方法制备的弯曲多孔结构阴极支撑层。虽然高的孔隙率有利于气体扩散[34],且增大压力可提高产氢速率并使电解池动力学更快[29,40],但对于承担支撑电极作用的阴极,孔隙率的提高和孔径的扩大将影响其机械强度,提高压力将影响机械性能和稳定性。

综上,Ni-YSZ金属陶瓷复合材料作为SOEC阴极具有导电性好、催化活性高等优点,但由于导电材料和金属网络在高温环境下运行会出现金属粒子的迁移、团聚等现象,导致TPB区域减少进而影响催化活性和稳定性,同时由于Ni易积碳,应用于CO2电解时其性能较H2O电解时稍差,使用不同的金属基复合陶瓷提高催化活性和稳定性、增加阻挡层、制备精细多孔结构等可提高阴极材料的稳定性,但制备出长期运行下稳定性好的金属陶瓷阴极材料仍是当前研究的重点。

2.2 钙钛矿结构

钙钛矿型氧化物具有优异的氧化还原稳定性,简单的钙矿型氧化物可描述为ABO3,其中A为配位数为12的阳离子,B为配位数为6的阳离子,B离子位于氧离子所构成的八面体中心,其结构如图2所示。钙钛矿的A、B位可通过掺杂形成晶体缺陷结构,进而提高其电化学性能。具有混合离子电子导电性(MIEC)的钙钛矿型电极材料其反应活性位点为反应气体/钙钛矿型氧化物的气固界面,并不局限于三相界面,同时无需陶瓷材料与金属混合增加三相界面反应区域,从而减少烧结过程中杂相生成及金属单质在运行过程中损失的问题。然而相较金属陶瓷材料,其催化能力较差,目前常通过在A、B位掺杂过渡金属或通过浸渍法负载纳米级催化剂的方法提高其催化能力。此外,双钙钛矿材料由于具有比单钙钛矿材料更强的氧表面交换及在低温下优越的电化学性能受广泛关注。表1列出了目前常见的阴极钙钛矿材料及其组成在SOEC模式下的电化学性能,主要分为单钙钛矿型和双钙钛矿型。

铬酸镧基钙钛矿材料电导率随温度升高而增大[44],可在高温下保持稳定结构并在高电流密度下表现高性能,在SOEC模式下不需还原气氛的保护[52],被认为是一种有前景的阴极材料。然而铬酸镧基钙钛矿作为电极时,其导电率和催化活性均较差[53],且在强还原电势下易产生较大电极极化电阻,引起电极的化学和结构变化。将纳米活性金属与钙钛矿材料复合提高电导率和催化性能是目前常见的提高铬酸镧基钙钛矿电极性能的方法,如在电极上负载Ni、Fe等金属,可显著提高其电流效率和催化活性[54]。而将钙钛矿类催化剂以浸渍法负载到阴极陶瓷骨架中也可提高电极的稳定性和催化性能[55],通过负载其他高催化活性材料使铬酸镧基钙钛矿阴极表现出较高稳定性的同时具有较好催化性能。

以镧锶钒基钙钛矿材料为基体的电极在高温下具有稳定的相结构,且在氢气气氛中表现出较高的电导率,因此被诸多研究者用作SOEC阴极材料[56]。然而其催化活性较差,常通过负载金属粒子的方法改善催化性能。负载Ni、Fe纳米粒子后La0.7Sr0.3VO3电流密度大幅提高,法拉第效率可提高20%,这与Ni、Fe纳米粒子和La0.7Sr0.3VO3存在协同效应有关[41],通过与金属混合制备复合阴极材料是目前钙钛矿型陶瓷材料较常见的改性方法。

镧锶钛基钙钛矿材料在还原气氛下同样具有n型导体导电机制,适合在强还原条件下作为电极,同时镧锶钛基钙钛矿具有优异的耐积碳和抗硫中毒的能力,在高温条件下具有良好结构稳定性[57],可应用于共电解H2O和CO2领域。其同样具有与传统的Ni-YSZ阴极材料相比催化活性较低的问题,解决思路也可通过负载Ni、Fe、Cu等金属粒子提高其催化活性。除了可实现共电解生成H2和CO,镧锶钛基钙钛矿材料还可应用于实现H2O和CO2共电解制备甲烷,XIE等[58]采用La0.2Sr0.8TiO3 δ(LST)与Ce0.9Gd0.1O2-δ(CGO)制备的复合阴极,共电解CO2和H2O制备甲烷气体,其中阴极材料LST主要提供电子导电,CGO提供氧离子导电,在电解过程中H2O转化率达25%,CO2转化率达11.5%。

除负载过渡金属元素提高催化活性和导电性,在B位掺杂过渡金属利用原位析出法也可增加钙钛矿型材料的催化性能、导电性和长期运行的稳定性。如将Ni、Fe、Co、Cu等具有催化活性的金属离子掺杂到钙钛矿材料的B位中,采用原位析出工艺可原位构建具备活性纳米颗粒修饰的SOEC复合阴极材料。YANG等[59]报道了1种铜、锰共掺杂的镧锶钛基钙钛矿阴极材料(La0.2Sr0.8)0.95Ti0.65-xMn0.35CuxO3-δ,其对CO2电解表现出较高的催化活性和良好的吸附性。其单电池在800 ℃、1.8 V电解电压下的最大电流密度可达2.33 A/cm2。金属颗粒在运行过程中溶出可得到具有高催化活性和稳定性的电解池阴极。为研究B位掺杂金属颗粒运行中溶出规律,ARRIVÉ等[60-61]制备了一系列(La,Sr)(Ti,Ni)O3-δ材料作为SOEC的阴极,在运行过程中钛酸盐表面溶出的Ni纳米颗粒提高了电极的催化性能。同时,对La2xSr1-2xTi1-xNixO3-δ和La7x/4Sr1-7x/4Ti1-xNixO3-δ材料在高温还原条件下的稳定性研究发现,高温预还原有利于镍纳米颗粒的溶出,还发现经高温1 200 ℃还原后La7x/4Sr1-7x/4Ti1-xNixO3-δ的电导率提升,这可能是由于还原后Ti3 浓度的增加导致氧空位形成,提高了材料电导率。增加氧空位有利于提高材料的电导率,除前文提到的预还原生成氧空位,掺杂稀土元素同样有利于氧空位的形成,如YANG等[62]制备出的La0.2Sr0.8Ti1-xScxO材料,在掺杂钪后的钛酸盐离子导电性显著提高且表现出了良好的氧化还原稳定性。

Sr2Fe1.5Mo0.5O6-δ(SFM)是目前较常见的双钙钛矿阴极材料,与前文提到的单钙钛矿型材料相比,双钙钛矿材料中Fe/Mo和Fe 3d/O 2p态的电子结构易形成电子缺陷和氧空位,从而表现出良好的电子离子传输性能,在还原和氧化环境中均表现出良好的稳定性和高电导率[63]。同时具有混合离子-电子电导率的双钙钛矿氧化物,可为反应气的吸附和转化提供足够的活性位点,使其在高温SOEC方向表现出优异性能[64],进一步研究制备SFM基复合材料,增强阴极稳定性, LYU[43]等将Gd0.2Ce0.8O1.9(GDC)纳米粒子负载到SFM表面,构建了GDC-SFM为阴极的SOEC,由于TPB长度的增大、反应气的吸附和电还原活性位点的增加以及氧离子扩散能力的增强均使GDC负载后SFM电催化活性明显提升,极化电阻从0.46 Ω·cm2降至0.30 Ω·cm2。复合电极表现出更高的电催化活性和稳定性,更有利于SFM材料的实际应用。

综上,钙钛矿结构材料作为SOEC阴极,具有稳定性好、部分具有混合离子-电子导电性、抗积碳等优点。但钙钛矿结构材料的导电性较金属陶瓷材料仍有不足、催化活性较低,在A位掺杂稀土或金属元素可增加氧空位,提高材料导电性;直接负载金属或催化剂在钙钛矿骨架上也可提高其性能;将过渡金属掺杂在钙钛矿材料B位中,可实现金属粒子在反应过程中原位溶出,得到均匀分散的纳米金属粒子,有利于提高电极的催化活性。通过浸渍、掺杂等手段对钙钛矿材料进行设计,提高其导电性和催化活性仍然是目前研究重点。

总之,目前提高SOEC阴极材料的性能和耐久性仍是阴极材料研究重点,钙钛矿型材料由于其高温下稳定性好被广泛关注,但导电性和催化活性较差限制了其商业化应用,可通过负载催化剂、掺杂活性金属等方法提高其性能,但和目前商用的金属陶瓷材料仍然存在差距。与钙钛矿类材料相比,金属陶瓷材料具有催化活性高、与电解质材料相容性良好、制备简单工艺成熟等优势,通过对其微观结构进行改良修饰、负载纳米级催化剂、合金化等方法都可有效提高其电化学性能和耐久性,但仍无法避免金属颗粒迁移团聚等问题,可能会限制其未来大规模应用。

3 阳极材料

SOEC阳极上的化学反应主要为氧负离子发生氧化生成氧气,因此阳极除需具有良好的离子导电率和电子导电率、较好的催化活性,还需提供有利于氧负离子被氧化的环境、能够适应电解池阳极的氧化气氛、具有疏松多孔且适宜的微观结构。

钙钛矿类材料是目前较常见的阳极材料,具有与电解质相匹配的热膨胀系数、无需使用贵金属、高温下化学性质稳定等优点,但其导电率和催化能力相对较差,通过在材料A、B位中掺杂过渡金属或负载过渡金属氧化物等可提高其性能[65-66],表2列举了几种钙钛矿材料及其用作SOEC阳极的电化学性能。目前SOEC阳极面临的最大问题是长期运行过程中分层、裂纹等问题,较差的稳定性严重阻碍了SOEC商业化应用,因此提高阳极稳定性、减少运行过程中分层导致的性能衰减已成为SOEC阳极研究重点,镧锶锰基钙钛矿是研究较早的SOEC阳极材料之一,具有良好催化性能和电子导电性。镧锶锰基钙钛矿与常用电解质材料氧化钇稳定氧化锆(YSZ)具有很好的化学相容性,使其在电解池制备方面具有优势。但镧锶锰基钙钛矿材料在高电流密度下易分层,导致电极电阻增加、活性位点减少,引起镧锶锰基钙钛矿电极退化。阳极分层是阳极材料研究最需解决的问题之一,阳极/电解质界面的氧分压分布、电场梯度等存在较大差异,电解池在较高的电流密度下运行时,易在电极内部聚集形成局部高氧分压,从而导致第二相的形成或阳极/电解质界面的分离[84]。YSZ/LSM界面分层可能是因为运行过程中易形成钙钛矿纳米颗粒,此外局部拉伸应变[85]、阳极/电解质界面产生绝缘相[86]、电解质中La和Sr的取代缺陷[87]、其他有害相的生成都可能是高氧压位点形成的原因。

通过在致密的电解质和多孔阳极之间引入与电解质电极材料膨胀系数相近、相容性好的材料来制备多孔阻挡层,减小阳极与电解质的直接接触,能有效减少电解质/阳极界面的分层问题。SU等[88]在(La0.75Sr0.25)0.95MnO3-δ和电解质之间添加 YSZ多孔层,电池可在 0.5 A/cm2下运行100 h无明显衰减。研究发现多孔层不仅可抑制氧分压的升高也可加速氧离子的扩散,从而有效抑制 LSM 阳极降解。同时,研究发现在电解质/阳极界面添加(Sc2O3)0.10(ZrO2)0.90多孔层也可增强电池整体性能[89]。加入多孔层制备复合电极可减少分层现象,是由于多孔电解质材料扩大了反应的三相边界,更有利于氧气和氧离子的扩散,减轻了界面处的氧分压[90],并且由于多孔层的加入减少了高阻抗相的形成,从而减少了高氧分压位点。但也有研究认为这种增加多孔层对于阳极性能的提升有限,增加多孔层并不能抑制界面处生成镧锶锰基钙钛矿纳米颗粒[91],因此需寻找分层问题解决电解质/阳极界面的方法。

可逆运行同样可有效降低镧锶锰基钙钛矿电极的氧压力,研究表明通过固体氧化物燃料电池(SOFC)模式和SOEC模式循环操作的La0.8Sr0.2MnO3(LSM)/YSZ氧电极相较单一模式操作更稳定,说明循环操作有利于电池长期运行[92]。GRAVES等[93]研究发现在1.0 A/cm2较高电流密度下进行SOEC/SOFC循环操作,可有效抑制因镧锶锰基钙钛矿微观结构变化导致的氧电极性能衰减,在可逆循环4 000 h后,仍可保证阳极微观结构的稳定性。这是由于循环操作减少了O2在YSZ缺陷和孔隙内的堆积,从而减少了氧分压并减少了高阻抗相的生成和高氧分压位点的形成。

选择极化损耗更低的材料同样有助于增强电极的稳定性,在固体氧化物燃料电池(SOFC)的研究中发现基于Sr掺杂的LaCoO3(LSC)和Sr掺杂的LaFeO3(LSF)的阴极极化损耗低于常见镧锶锰基钙钛矿电极[94]。使用掺杂后钙钛矿具有更高的离子导电性,但是镧锶钴基钙钛矿面对Cr等杂质更易中毒,热膨胀系数增大,这为该材料应用于SOEC阳极带来了挑战[95]。WANG等[68]比较了常见3种阳极La0.8Sr0.2MnO3、La0.8Sr0.2FeO3和La0.8Sr0.2CoO3,结果表明在电解池模式下La0.8Sr0.2FeO3、La0.8Sr0.2CoO3材料作为电解池阳极更理想。

镧锶钴铁基钙钛矿材料由于具有很高的电子导电性,广泛应用于SOFC中,其在析氧反应(OER)中具有较高催化活性,在SOEC中也表现出良好电化学性能,是一种较理想的SOEC阳极材料,但在实际应用中镧锶钴铁基钙钛矿材料存在Sr偏析,导致电解池电化学活性下降[96],同时较高的热膨胀系数使其与常见的YSZ电解质层相容性较差[97],且含锶材料易被气体中存在的铬毒化导致性能下降。通过纯化通入气体和降低反应温度可减少毒化以及Sr偏析对镧锶钴铁基钙钛矿电极的影响。针对镧锶钴铁基钙钛矿热膨胀系数与电解质不匹配严重影响电极的长期运行稳定性并造成电解质与阳极的分层问题,一般通过与GDC、SDC复合制备复合电极可有效解决。与GDC、SDC等材料制备的复合电极还可将氧化反应从三相界面扩展到电极/气体界面使得电极的活化过电位降低[96,98]。

除镧锶钴铁基钙钛矿材料具有优良的导电性,铁基氧化物在高温环境下也表现出优异的导电性。常见的铁基氧化物BaFeO3-δ(BFO)因为具有较大阳离子半径的Ba2 和可变价的B位阳离子,表现出较高的氧空位浓度高和电导率。但由于只有立方相的BFO具有无序的氧空位,有利于氧离子导电率提高,成相困难阻碍了其在SOEC阳极材料中的应用[99]。KIM等[70]使用Nd3 部分替代Ba2 生成单一立方的钙钛矿BaFeO3-δ,当Nd3 掺杂量为3%时电导率最高,同时具有低的极化电阻,Nd3 掺杂材料中氧空位增加,使其用作SOEC阳极材料成为可能。

此外,具有优异的体相氧扩散和表面氧交换能力以及电子-离子导电率的双钙钛矿结构氧化物同样可应用于H2O/CO2共电解并受广泛关注[76]。TIAN等[78]将PrBa0.5Sr0.5Co1.5Fe0.5O5 δ(PBSCF)双钙钛矿材料用作SOEC阳极,在800 ℃、2 V的电解电压下电解90% H2O时的电解池产氢速率可达978 mL/(cm2·h),且在750 ℃、120 h循环测试中保持较好稳定性。证明了PBSCF双钙钛矿材料应用于SOEC阳极的优越性能。

除了ABO3型单钙钛矿和型双钙钛矿材料在SOEC阳极已得到广泛应用,目前研究发现R-P型钙钛矿具有独特的岩盐和钙钛矿交替结构,如图3所示,氧离子迁移率高,有利于减少氧分压[100],也可用于SOEC阳极。R-P型钙钛矿中最典型的是Ln2NiO4型(Ln为镧系元素)钙钛矿[101],该材料表现出较低极化电阻、较高导电性、氧空位浓度和氧传输能力,能够承受电极-电解质界面的过量氧气,且不含锶钴元素,不易被通入气体中的铬毒化,减少了不良相形成,具有较高稳定性[102-103]。

不同的镧系元素R-P型钙钛矿,由于八面体结构差异显示不同的化学特性,La2NiO4 δ氧化物与YSZ或GDC表现出合适的热膨胀系数,与电解质材料相容性好,且具有较高的稳定性;Pr2NiO4 δ(PNO)材料表现出较高的电化学性能,极化电阻较低,但其稳定性相比于La2NiO4 δ材料较差[104-105];Nd2NiO4 δ(NNO)在较低温度下仍具有离子导电性[106],与目前常见的阳极材料(LSM、LSF等)相比具有更优秀的电化学性能。CHAUVEAU等[72]使用镍酸钕(Nd2NiO4)制备SOEC阳极,将镍酸钕氧电极与商业镧锶锰基钙钛矿阳极进行比较,在1.3 V电压,750、800和850 ℃下测得其电流密度分别为0.40、0.64和0.87 A/cm2远高于同条件下以镧锶锰基钙钛矿材料为阳极的电解池。同时NNO材料具有较好的短期稳定性,但长期运行稳定性仍有待研究。为达到电化学性能和稳定性平衡,在A、B位上掺杂不同元素可调节其导电性、表面交换系数和稳定性,提高其电化学性能[107-108]。如Pr被La部分取代,稳定性可显著提升,且较PNO、LNO更不易发生分层,但电催化性能降低。或将R-P型材料与常见稳定性好、活性较差的电极材料复合,如使用Pr2Ni0.6Cu0.4O4浸渍到常见的LSCF阳极中,其极化电阻从0.98 Ω·cm2降至0.13 Ω·cm2 [109],电化学性能显著提高。目前研究制备长期运行稳定性、离子导电率和催化活性平衡的材料仍是R-P型钙钛矿材料研究重点。

SOEC阳极的电极极化较高和电解质/电极界面的分层、有害相生成、稳定性较差等问题影响其商业化运用。虽有许多材料可应用于SOEC阳极,但这些材料存在LSM材料的分层,LSCF材料长期运行形成高电阻相,Co、Sr基材料的元素扩散偏析问题等。寻找新材料或对现有材料改性,得到稳定性和催化活性平衡的阳极材料是研究重点。

4 电解质

电解质是SOEC中的重要组成部分,主要作用为传导离子同时起到分隔两极还原气体和氧化气体的作用。电解质按传导离子可分为氧离子导体电解质和质子导体电解质。氧离子导体电解质的迁移机制可用氧空位传导解释,是目前商业化应用最常见的电解质类型。

氧离子进入电解质并通过电解质中氧空位向阳极迁移,氧空位浓度对电解质传导氧离子的能力十分重要。氧离子导体电解质在高温下导电性良好,结构致密可隔绝反应气和氧气,不传导电子,同时在高温长期运行时性能稳定,在氧化气氛和还原气氛中保持化学、结构的稳定,电解质还需与其他电子元件有良好的匹配性和相容性,同时可提高电解池电解效率。目前主要电解质材料可分为萤石型和钙钛矿型2种,表3列举了几种电解质材料的电导率σ。

4.1 萤石型

目前常见的萤石型电解质为氧化锆基电解质和氧化铈基电解质,均具有高温下结构稳定、掺杂稀土和碱金属后电导率高的特点,是目前最常见的SOEC电解质类型。

氧化锆具有多种晶型,在常温下一般为单斜相,当温度达1 100～2 300 ℃时转变为四方相,2 370 ℃时转变为立方相。在ZrO2基体中通过掺杂2价或3价阳离子来保持不同温度下晶型的稳定,同时不同离子半径的氧离子取代Zr4 形成氧空位可提高氧化锆的离子导电性。目前最常见的SOEC电解质材料为8～10摩尔分数的氧化钇掺杂的氧化锆(YSZ)。其中氧化钇(Y2O3)掺杂ZrO2时氧化物的氧空位浓度和迁移率达到平衡,具有最佳的离子传输能力[116],因此8YSZ是目前最为常见的商业SOEC电解质材料。YSZ作为电解质具有离子导电性高、稳定性优异、与其他材料相容性好的优点,但在800 ℃以下离子导电率较差,800～1 000 ℃是保持其良好电化学性能的适宜温度。YSZ电解质的电阻率比常见电极材料大几个数量级,为解决YSZ电解质在电解池中阻抗过高问题,一般通过薄膜化技术来降低欧姆极化。常见YSZ薄膜制备方法包括流延成型法[117]、丝网印刷法[117]、化学气相沉积法[110]等,图4为几种常见的YSZ薄膜的制备方法。

钪稳定氧化锆(ScSZ)在中低温下具有高离子导电性,但在873 K下,易从立方相转变成四方相导致电导率降低[118-119],为在中低温下保持立方相稳定,可通过和其他氧化物共掺杂减少中低温下运行导致的不良相变,得到更稳定的立方晶格结构。氧化铈(CeO2)掺杂ScSZ虽然不会发生电荷补偿产生新的氧空位,但可通过引入半径更大的Ce4 离子稳定立方氧化锆结构,一般通过掺杂摩尔百分比1时氧化铈获得稳定的立方相ScSZ[120-121]。ScSZ掺杂氧化铋(Bi2O3)也可增强立方相的稳定性、增大晶粒尺寸、减少晶界电阻,目前掺杂Bi2O3的摩尔分数在0.5～3.0[122]。可掺杂氧化钇(Y2O3)来提高ScSZ的稳定性,ALFECHE等[111]采用滴涂沉积的方法制备了钪钇共掺杂氧化锆薄膜作为固体电解质,XRD检测显示制备出的4S4YSZ均为立方晶体结构,沉积的4S4YSZ薄膜在700 ℃时总电导率约为0.12 S/cm,活化能仅为0.73 eV。但氧化钇的掺杂会使电解质离子电导率下降[123]。通过掺杂氧化铝[124]、氧化钛[125]、氧化铌[126]等金属氧化物保持ScSZ立方晶相结构,但电解质并不能达到高离子传导率和耐老化性能的平衡,所制备的立方相掺杂钪稳定氧化锆无法应用于高温电化学方向。

铈(Ce)是一种我国储量丰富的稀土元素,CeO2的晶体结构为立方萤石型,可实现氧空位迁移。氧化铈经高温还原,其中的Ce4 被还原成Ce3 形成氧缺陷从而产生氧空位,具有一定的离子导电性,但在氧化气氛中纯立方萤石型结构不会产生氧缺陷,这意味着该状态下离子电导率较低。为提高离子电导率需在结构中制造缺陷,可以将稀土、碱金属元素掺杂到CeO2基中,部分取代Ce4 引入氧空位缺陷,这种CeO2基电解质,在中低温(500～800 ℃)下具有较高的电导率[112],在SOFC 中已经取得了较好结果,被认为可应用于中低温SOEC。但EGUCHI等[127]认为掺杂二氧化铈不适用于SOEC,因为产氢速率远低于预期,这可能是由于铈基电解质内部漏电或元素迁移造成。目前已开发出一些技术来解决这个问题,在基于氧化铈的电解质上制造稳定的氧化锆薄膜[128]、掺杂铋来保证电池的稳定性[129]、涂覆BaO薄膜防止掺杂氧化铈减少[130]。虽存在问题,但由于其在中低温环境下优异的电化学性能,仍作为电解质被普遍应用于中低温材料的开发研究。

4.2 钙钛矿型

钙钛矿一般具有离子-电子混合导电性,部分钙钛矿在中温条件下表现出高的氧离子导电性,可用作SOEC电解质材料,其中LaGaO3由于A、B位掺杂二价碱土氧离子会产生氧空位,是目前研究较多的钙钛矿型电解质。研究发现掺杂LaGaO3是一种纯氧化物离子导体,在较宽的氧分压范围内具有极高的电导率这使其可适应还原气氛和氧化气氛,可用作SOEC电解质材料,同时与Zr基电解质相比La位掺杂Sr、Ga位掺杂Mg的镧锶镓镁钙钛矿材料具有很高的氧离子导电率,尤其是应用于中低温环境下相较于传统YSZ电解质更具优势[113],YU等[131]发现在250～650 ℃下La0.8Sr0.2Ga0.8Mg0.2O3(LSGM)具有比锆基氧化物更加优异的电化学性能,在与Zr基SOEC相同入口气体成分下,在550 ℃、1.3 V条件下,CH4产率提高达50%以上,说明LSGM电解质在中低温电解下具有良好的性能。但由于LSGM难以致密化且极易与金属电极发生反应,导致电池性能降低,制备LSGM所需的原料价格昂贵,这对LSGM的大规模应用带来了挑战[114,131]。此外LaNbO4基、La2Mo2O9基都可用作SOEC电解质材料,也同样取得了较良好的效果[132-133],但从工艺和价格上来看与传统的萤石型电解质材料仍存在差距。综上所述钙钛矿类材料导电性优异,与电极材料的热膨胀系数匹配,但其价格高昂且工艺不够成熟,目前仍难适应商业化应用。

除上述氧离子导体电解质,以质子导体为电解质的质子传导固体氧化物电解池(PCEC)因为具有阴极只产生单一氢气,不需经后处理便可得到高纯度氢气,可与阳极共烧结有利于减少电解质和阳极界面的分层问题[134],且比离子导体具有更高的离子电导率,能在中低温下运行等优点受到广泛关注[135]。目前质子传导型电解质最常见的是BaCeO3-BaZrO3型材料[115,136]。质子型导体氧化物电解池通过质子导电,水蒸气含量高的富水环境更有利于提高其导电性,同时由于使用钙钛矿作为电解质材料中低温下仍具有较好的导电性,使其不仅可应用于高温电解,还可应用于中低温电解水,降低运行温度。

近年来使用混合离子-电子导体作为SOEC电解质同样受到关注,在使用该电解质的SOEC中水可在阴阳两极电解,提高产氢量。KIM等[137]制备了BaZr0.1Ce0.7Y0.1Yb0.1O3-δ为电解质的混合SOEC,在1.5 V、700 ℃条件下电流密度为4.61 A/cm2,较氧离子传导SOEC提高15%,较质子传导SOEC提高45%,具有十分优异的电化学性能。混合离子-电子电解质不仅在电解水方面取得较好的性能,运用于二氧化碳的还原性能同样优异,Pu等[138]制备了Zr掺杂的BaCe0.5Zr0.3Y0.2O3-δ(BCZY) 和二元碳酸盐(Li,Na)2CO3(LNCO)的复合电解质,在较低温度(500～600 ℃)下工作。复合电解质中的H 传输有助于促进非金属燃料电极上H2和甲烷的产生。综上,YSZ、ScSZ等作为电解质材料在高温电解方面由于其离子导电性强、价格低廉仍具有不可替代的优势,其主要研究方向为材料的薄膜化。氧化铈基、钙钛矿型电解质都具有更加优异的导电性能,可应用于中温SOEC,但这些材料在材料衰减、致密化等问题上仍面临挑战。质子传导型SOEC(H-SOEC)在阴极侧可得到干纯氢气,简化系统操作成本,混合离子传导型SOEC具有产氢速率高的优势,但是目前尚未达到商业化应用的标准,现在仍然处于实验室研发阶段,其中质子导体电解池具有很好的应用前景,是目前的研究热点。

5 结 语

SOEC已发展了半个世纪,随着化石燃料价格上涨,全球温室效应逐渐加剧,SOEC技术已经引起了各国的关注和重视,美国爱达荷国家实验室现已建成25 kW高温蒸汽电解制氢一体化装置,2021年9月康明斯从美国能源部获得500万美元拨款,用于建立年产能为94 MW的SOEC电解槽工厂,实现SOEC电堆自动化组装、生产。欧洲由欧盟委员会进行了一系列的科研架构计划,对SOEC项目进行资助,并预计在2030年系统效率达到90%, 2020 年欧盟启动的示范项目旨在5 a内将 SOEC 的技术成熟度由TRL7提升至TRL8,并在SOEC长期运行下的稳定性和机理、H2O、CO2共电解等方面都取得了较为优异的成果。虽然目前国内SOEC技术相较国外先进水平仍存在一定差距,但多家科研机构及企业已在加快推进SOEC研发和成果商业化应用。

SOEC商业化应用面临的挑战如下:① SOEC 关键部件材料的研发问题。如电解质/阳极的分层、长期运行电极电解质材料的损失和性能衰减问题等,开发能在高电流密度下可长期稳定运行的材料是目前研究的重点;② 电堆的组装仍面临挑战。如何开发可长期稳定运行的电堆是目前研究方向;③ SOEC商业化应用面临成本控制问题,未来SOEC应用最大的成本支出是电能的消耗, 若SOEC具有商业竞争力,需在电堆寿命内产氢价格和产量都具有优势,成本应低于其他电解技术,由于SOEC具有电能利用效率优势,因此找到低价电力资源和延长电解池使用寿命,将降低SOEC制氢成本,提高其竞争力。

综上,目前SOEC在长期运行中的衰减问题仍是面临的主要问题。SOEC电极、电解质材料仍存在金属粒子迁移团聚导致TPB反应区域减小、阳极与电解质界面的分层问题、平衡稳定性与活性等问题,制备具有良好催化活性和稳定性的SOEC是目前研究重点。需对反应机理、电极/电解质界面化学等进行研究,以指导新材料的研发和常用材料的改性,实现SOEC从实验室到商业化应用。

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Research status and development prospect of solid oxide electrolyzer cells materials

WANG Aoxue1,GUO Lin1,ZHENG Jinyu1,MI Wanliang1,ZHANG Panpan2,YANG Zhibin2

(1.SINOPEC Research Institute of Petroleum Processing,Beijing 100083,China;2.School of Chemical & Environmental Engineering,China University of Mining and Technology-Beijing,Beijing 100083,China)

Abstract：Hydrogen has the advantages of high energy density, clean and pollution-free, and is considered to be the most suitable strategic energy for low-carbon development. Electrolysis technology can be used for the conversion and storage of renewable energy such as solar energy and wind energy, which can convert electrical energy into chemical energy for the synthesis of hydrogen. Common electrolytic water technologies include alkaline water electrolysis (AWE), polymer electrolyte membrane (PEM) and solid oxide electrolysis cell (SOEC). SOEC has high efficiency and does not require the use of precious metal catalysts, making it the first choice for efficient large-scale hydrogen production. SOEC operates at high temperatures, requiring the materials used to have high conductivity and stability at high temperatures. At the same time, the materials used for different components should match the thermal expansion coefficient and have good compatibility. Therefore, the research status of cathode materials, anode materials and electrolytes in SOEC were summarized. The cathode materials are mainly metal ceramics and perovskite-type ceramics. The high temperature and humidity environment of the cathode can easily lead to the deactivation of cermets, and there are problems such as oxidation, loss, and agglomeration of metals during long-term operation. Therefore, the factors that led to the deactivation of metal ceramics were described, and the methods to improve the stability were discussed. The problems of low ionic conductivity and catalytic activity of perovskite-type materials were briefly described, and the methods to improve the properties of materials were introduced, such as doped transition metal in situ dissolution, loaded active metal nanoparticles, etc. Anode materials are mainly perovskite materials. The factors affecting the decline of neutral energy of anode operation were described, such as delamination, cracks, and other issues that lead to poor long-term stability of anode materials during long-term operation. The methods to improve anode durability were discussed, such as adding barrier layer, circulating operation, and preparing perovskite materials with high conductivity and catalytic activity. Electrolyte materials are mainly composed of fluorite-type or perovskite materials. Two methods of preparing high conductivity materials and thin-film process were introduced to reduce their impedance. Although SOEC is still considered to be in the stage of laboratory research and development, its electrolytic performance has been significantly improved with the continuous deepening of research on SOEC. Developed countries such as the United States, the European Union are laying out the commercial application of SOEC. The commercial application prospect of SOEC was discussed and prospected from the aspect of materials.

Key words：solid oxide electrolysis cells;hydrogen production;electrode material;electrolyte;high-temperature electrochemistry

中图分类号：TK91;TQ116.2

文献标志码：A

文章编号：1006-6772(2023)03-0062-16

收稿日期：2022-11-10;

责任编辑:张 鑫

DOI：10.13226/j.issn.1006-6772.H22111101

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基金项目：国家重点研发计划“政府间国际科技创新合作”重点专项资助项目(2021YFE0191200)

作者简介：王傲雪(1998—),女,河南许昌人,硕士研究生。E-mail:wangaoxue.ripp@sinopec.com

通讯作者:郑金玉(1974—),女,天津人,研究员,博士。E-mail:zhengjy.ripp@sinopec.com

引用格式：王傲雪,郭琳,郑金玉,等.固体氧化物电解池材料发展现状与展望[J].洁净煤技术,2023,29(3):62-77.

WANG Aoxue,GUO Lin,ZHENG Jinyu,et al.Research status and development prospect of solid oxide electrolyzer cells

materials[J].Clean Coal Technology,2023,29(3):62-77.