Complexes of cobalt(II) stabilized by lacunary polyoxometalates (CoPOMs) are highly discussed water oxidation catalysts (WOC). While their activity and stability in the oxygen evolution reaction (OER) have been widely explored, there is still no consensus between those claiming that CoPOMs are active and stable OER catalysts and those suggesting that they rather act as precatalysts, which degrade into OER-active heterogeneous CoO_x catalysts. In this work, we use operando X-ray absorption spectroscopy along with electrochemical methods (cyclic voltammetry, chronoamperometry) to assess the activity and stability of [Co₉(H₂O)₆(OH)₃(HPO₄)₂(PW₉O₃₄)₃]^16– (Co₉POM) under chemical and electrochemical operating conditions. First, we demonstrate that Co₉POM dissolved in a phosphate buffer quickly degrades during an electrochemical OER, leaving a Co(III)/Co(II)-containing layer on the electrode surface that acts as a heterogeneous OER catalyst. This degradation is detected using both the post mortem and operando X-ray absorption near-edge structure and extended X-ray absorption fine structure. Then, the electrochemical OER is studied in the presence of 2,2′-bipyridine, which is used to eliminate Co²⁺ aqua-complexes resulting from Co₉POM dissociation equilibrium. Yet, this does not avoid the formation of a Co-containing precipitate, albeit of a different composition. Finally, to differentiate between degradation associated with the catalytic cycle itself and the one provoked by local pH changes in the vicinity of the electrode during the OER, the Co₉POM is studied as a homogeneous WOC with NaClO as a chemical oxidant. An irreversible loss of Co from the Co₉POM is detected after the addition of two NaClO equivalents per Co ion. These combined insights provide clear operando evidence of Co₉POM instability under either electrochemical OER or chemical WOC operating conditions.

中文翻译：

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钴取代的多金属氧酸盐在析氧反应过程中的不稳定性：操作 X 射线吸收光谱研究

由缺位多金属氧酸盐 (CoPOM) 稳定的钴 (II) 配合物是备受关注的水氧化催化剂 (WOC)。虽然它们在析氧反应（OER）中的活性和稳定性已被广泛探索，但那些声称 CoPOM 是活性且稳定的 OER 催化剂的人和那些认为它们充当预催化剂（降解为 OER 活性）的人之间仍然没有达成共识非均相 CoO _x催化剂。在这项工作中，我们使用原位X射线吸收光谱以及电化学方法（循环伏安法、计时电流法）来评估[Co ₉ (H ₂ O) ₆ (OH) ₃ (HPO ₄ ) ₂ (PW ₉ O ₃₄ ) ₃ ] ^16– (Co ₉ POM) 在化学和电化学操作条件下。首先，我们证明溶解在磷酸盐缓冲液中的 Co ₉ POM 在电化学 OER 过程中快速降解，在电极表面留下含 Co(III)/Co(II) 的层，充当非均相 OER 催化剂。使用事后和操作X 射线吸收近边缘结构和扩展 X 射线吸收精细结构来检测这种退化。然后，研究了在2,2'-联吡啶存在下的电化学OER，其用于消除Co _{9 POM解离平衡产生的Co} ²⁺水络合物。然而，这并不能避免含Co沉淀物的形成，尽管其成分不同。最后，为了区分与催化循环本身相关的降解和 OER 过程中电极附近局部 pH 变化引起的降解，将 Co ₉ POM 作为均质 WOC 进行研究，并以 NaClO 作为化学氧化剂。在每个 Co 离子添加两个 NaClO 当量后，检测到Co ₉ POM中 Co 的不可逆损失。这些综合见解提供了Co ₉ POM在电化学 OER 或化学 WOC 操作条件下不稳定性的明确操作证据。