First-Principles-Based Insight into Electrochemical Reactivity in a Cobalt-Carbonate-Hydroxide Pseudocapacitor

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 (V _dp). Compared with the experimental potential window of the reaction (<0.6 V (vs saturated calomel electrode (SCE)), the computed V _dp (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 ab-plane) can result in 1-D growth (stacked along the c-axis). This anisotropic growth controls the balance between the total "non-reactive" CCH phases (inside the material) and the "reactive" hydroxide (Co(OH)₂) 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)₂ phase by controlling the reaction surface area.