非催化瞬态铱脱溶—筛选铱基钙钛矿析氧催化剂时不可忽视的实验现象

Non‐catalytic,instant iridium(Ir)leaching:A non‐negligible aspect in identifying Ir‐based perovskite oxygen‐evolving electrocatalysts

传统化石燃料的过度开采、消耗在推动各国工业化进程的同时,也导致了能源枯竭、环境污染和气候恶化等问题.为应对全球环境治理等难题,推进能源变革,构建脱碳化的能源体系势在必行.质子交换膜电解槽(PEMWE)能够在高电流密度下运行,其体积小,效率高,具有更高的灵活性,更有利于反应进行,能够克服可再生能源(太阳能、风能和水电等)的间歇性和波动性,因而非常适合与可再生能源供电系统耦合.目前,商业PEMWE中的析氧催化剂主要为二氧化铱(IrO_(2)),因为它能够在酸性环境中表现出较好的催化活性与稳定性.然而,铱元素的地壳丰度极低,其价格近期大幅提升至每克约200美元(约为贵金属铂的6倍),极大地限制了PEMWE的大规模应用.因此,开发低铱含量的高性能析氧催化剂成为当务之急.铱基双钙钛矿氧化物(A_(2)B’IrO_(6):A为碱金属、碱土金属或稀土金属,B’为过渡金属或稀土金属)的晶体结构骨架包容性强,元素组成多样,能够更好地稀释铱的用量,有望取代IrO_(2),成为低铱含量、高性能的催化剂.虽然已有研究证实铱基双钙钛矿会在催化过程中发生阳离子脱溶,导致催化剂表面重构,但对其结构演化机制的研究还有待深入,相应的催化剂设计原则也有待进一步完善.本文系统研究了六种代表性铱基双钙钛矿氧化物(包括Sr_(2)TiIrO_(6),Sr_(2)CoIrO_(6),Ba_(2)PrIrO_(6),La_(2)LiIrO_(6),Ba_(2)YIrO_(6)和Sr_(2)YIrO_(6))的结构稳定性,观察到了一些超出预期的现象:部分铱基双钙钛矿氧化物(例如Ba_(2)PrIrO_(6)和Sr_(2)YIrO_(6))一旦与酸性电解液接触,就会发生瞬态铱元素脱溶.进一步考察了六种铱基双钙钛矿氧化物在催化与非催化条件下的结构演化过程,结果发现:(1)非催化条件下的酸腐蚀会引起金属阳离子的大量脱溶,导致钙钛矿结构坍塌.其中,非铱组分的脱溶量决定了铱元素的脱溶量.(2)铱基双钙钛矿是否会发生铱元素脱溶,不是由热力学稳定性决定的,而是由晶体结构骨架坍塌影响的动力学稳定性决定的.(3)对于结构稳定性较差的铱基双钙钛矿催化剂,A和B'元素会在非催化条件下完全脱溶,演化成IrO_(x),表现出类水合氧化铱的活性.其中,非铱元素没有改善催化剂的成本与性能,因而不能称之为低铱催化剂.所以,本文认为酸腐蚀测试是评价酸性水氧化催化剂结构稳定性的必要环节,由此提出了更为全面的低铱酸性析氧催化剂筛选方案.综上,本文证实了电解液酸腐蚀性对铱基双钙钛矿催化剂的结构稳定性的影响,加深了对铱基双钙钛矿催化剂在催化与非催化条件下结构演化机制的理解,进一步完善低铱析氧催化剂的筛选方案,为今后催化材料的综合评价提供新依据.

The large‐scale application of proton exchange membrane water electrolysis technology requires the development of high‐performance oxygen evolution electrocatalysts with as little iridium(Ir)as possible.Ir‐based double perovskite oxides(A_(2)B’IrO_(6);A=alkaline,alkaline‐earth,or rare‐earth elements;B’=transition metal or rare‐earth elements)represent a class of oxides with great potential to replace the commercial catalyst IrO_(2).However,the structural evolution of Ir‐based double perovskite oxides in electrolytes is incompletely understood,and foundational knowledge of the design principle of the“ideal”material is lacking.In this work,we report the unexpected phenomenon of instant Ir leaching from Ir‐based double perovskite oxides in acid under non‐catalytic conditions and discuss the implications of this phenomenon for mechanism investigation and material identification.Some well‐known Ir‐based double perovskite oxides,such as Ba_(2)PrIrO_(6) and Sr_(2)YIrO_(6),undergo instantaneous Ir leaching when they come into contact with acidic electrolytes.The Ir‐leaching process is found to be non‐persistent and non‐thermodynamically determined,and its extent is correlated with the leaching of other B’‐site elements in the perovskite oxides.Based on this observation,we revisit the Ir dissolution‐precipitation process for surface IrO_(x) formation during the perovskite‐electrolyzed oxygen evolution reaction,emphasizing the non‐negligible role of Ir species owing to acid corrosion in the electrolyte.Finally,we modify a screening protocol for low‐Ir oxygen evolution electrocatalysts and propose an instant acid corrosion test as an indispensable process to evaluate the structural stability of potential catalysts.