Oqmhula Kenji, Toma Takahiro, Maezono Ryo, Hongo Kenta
School of Information Science, Japan Advanced Institute of Science and Technology, Asahidai 1-1, Nomi, Ishikawa 923-1292, Japan.
Research Center for Advanced Computing Infrastructure, JAIST, 1-1 Asahidai, Nomi, Ishikawa 923-1292, Japan.
ACS Omega. 2023 Feb 9;8(7):6743-6752. doi: 10.1021/acsomega.2c07362. eCollection 2023 Feb 21.
Cobalt carbonate hydroxide (CCH) is a pseudocapacitive material with remarkably high capacitance and cycle stability. Previously, it was reported that CCH pseudocapacitive materials are orthorhombic in nature. Recent structural characterization has revealed that they are hexagonal in nature; however, their H positions still remain unclear. In this work, we carried out first-principles simulations to identify the H positions. We then considered various fundamental deprotonation reactions inside the crystal and computationally evaluated the electromotive forces (EMF) of deprotonation ( ). Compared with the experimental potential window of the reaction (<0.6 V (vs saturated calomel electrode (SCE)), the computed (vs SCE) value (3.05 V) was beyond the potential window, indicating that deprotonation never occurred inside the crystal. This may be attributed to the strong hydrogen bonds (H-bonds) that formed in the crystal, leading to structural stabilization. We further investigated the crystal anisotropy in an actual capacitive material by considering the growth mechanism of the CCH crystal. By associating our X-ray diffraction (XRD) peak simulations with experimental structural analysis, we found that the H-bonds formed between CCH planes (approximately parallel to the -plane) can result in 1-D growth (stacked along the -axis). This anisotropic growth controls the balance between the total "non-reactive" CCH phases (inside the material) and the "reactive" hydroxide (Co(OH)) phases (surface layers); the former stabilizes the structure, whereas the latter contributes to the electrochemical reaction. The balanced phases in the actual material can realize high capacity and cycle stability. The results obtained highlight the possibility of regulating the ratio of the CCH phase versus the Co(OH) phase by controlling the reaction surface area.
碱式碳酸钴（CCH）是一种具有极高电容和循环稳定性的赝电容材料。此前有报道称，CCH赝电容材料本质上是正交晶系的。最近的结构表征表明它们实际上是六方晶系的；然而，它们的H位置仍然不清楚。在这项工作中，我们进行了第一性原理模拟以确定H位置。然后我们考虑了晶体内部的各种基本去质子化反应，并通过计算评估了去质子化的电动势（EMF）。与反应的实验电位窗口（<0.6 V（相对于饱和甘汞电极（SCE）））相比，计算得到的EMF（相对于SCE）值（3.05 V）超出了电位窗口，这表明晶体内部从未发生去质子化。这可能归因于晶体中形成的强氢键（H键），从而导致结构稳定。我们通过考虑CCH晶体的生长机制，进一步研究了实际电容材料中的晶体各向异性。通过将我们的X射线衍射（XRD）峰模拟与实验结构分析相结合，我们发现CCH平面（大致平行于平面）之间形成的H键可导致一维生长（沿轴堆叠）。这种各向异性生长控制了材料内部总的 “非反应性” CCH相和 “反应性” 氢氧化物（Co(OH)）相（表面层）之间的平衡；前者使结构稳定，而后者有助于电化学反应。实际材料中的平衡相可以实现高容量和循环稳定性。所得结果突出了通过控制反应表面积来调节CCH相与Co(OH)相比例的可能性。
