Crystallographic and Geometrical Dependence of Water Oxidation Activity in Co-Based Layered Hydroxides

Abstract

Cobalt-based layered hydroxides (LHs) stand out as one of the best families of electroactive materials for the alkaline oxygen evolution reaction (OER). However, fundamental aspects such as the influence of the crystalline structure and its connection with the geometry of the catalytic sites remain poorly understood. Thus, to address this topic, we have conducted a thorough experimental and in silico study on the most important divalent Co-based LHs (i.e., α-LH, β-LH, and LDH), which allows us to understand the role of the layered structure and coordination environment of divalent Co atoms on the OER performance. The α-LH, containing both octahedral and tetrahedral sites, behaves as the best OER catalyst in comparison to the other phases, pointing out the role of the chemical nature of the crystalline structure. Indeed, density functional theory (DFT) calculations confirm the experimental results, which can be explained in terms of the more favorable reconstruction into an active Co(III)-based oxyhydroxide-like phase (dehydrogenation process) as well as the significantly lower calculated overpotential across the OER mechanism for the α-LH structure (exhibiting lower Egap). Furthermore, ex situ X-ray diffraction and absorption spectroscopy reveal the permanent transformation of the α-LH phase into a highly reactive oxyhydroxide-like stable structure under ambient conditions. Hence, our findings highlight the key role of tetrahedral sites on the electronic properties of the LH structure as well as their inherent reactivity toward OER catalysis, paving the way for the rational design of more efficient and low-maintenance electrocatalysts.

Transition metal incorporation: electrochemical, structure, and chemical composition effects on nickel oxyhydroxide oxygen-evolution electrocatalysts

“…Recent studies indicate that tetrahedral sites also play a role in shaping the electronic properties of layered hydroxides. 91 Further research is needed to clarify if tetrahedral sites occupied by transition metal cations affect the electrocatalytic and energy storage performance of MO x H y materials.…”

Influence of Crystallographic Structure and Metal Vacancies on the Oxygen Evolution Reaction Performance of Ni‐based Layered Hydroxides**

“…As we have recently reported for Co-based LHs, the role of crystallographic structure can play a crucial role in the OER performance, even in those phases containing Co II cations, exclusively. [23] Thus, considering our last finding and the importance of nickel in the development of OER catalysts, we decided to perform a comprehensive study on the most important Ni-based LHs: brucite-like (β-LH), hydrotalcite-like (LDH), and turbostratic-like (�-LH). In the case of the nonexpanded β-LH phase, cations adopt octahedral environments, M II (O h ), exclusively, and exhibit basal space distances (d BS ) lower than 5 Å.…”

“…In particular, fundamental aspects such as the specific role of the crystalline LH structure, as well as the coordination environment and cation distortions, have not been comprehensively studied for nickel centres. [23] This is quite striking given the variety of crystalline structures that LHs can exhibit, especially since these aspects have been thoroughly investigated for Ni-based spinels and perovskites, demonstrating that both Ni atom configuration, as well as the crystal structure, have a significant impact on the OER electrocatalysis. [24,25] For example, in the case of strainstabilised nickel hydroxide nanoribbons the modification of Ni coordination has a remarkable reduction in the overpotential for OER as well as methanol oxidation reaction (MOR).…”

Insight into the Anticorrosion Behaviors of Phase-Transformed Cobalt Hydroxides for Long-Term Electrocatalytic Water Oxidation in Saline Electrolytes

“…Cobalt hydroxide has been widely used as an OER catalyst in alkaline environments. − However, it is a precatalyst that experiences phase transformations to the active Co 3+ - or Co 4+ -containing species, such as amorphous Co oxy-hydroxides. , In this regard, extensive studies have been conducted to identify the OER active sites of Co­(OH) 2 both in α- and β-phases. − Nevertheless, the understanding in the structural evolution of Co­(OH) 2 during OER and CER processes is deficient, which is critical for the application and optimization of Co­(OH) 2 for superior catalytic performance in alkaline saline water. Herein, we focus on an electrodeposited α-Co­(OH) 2 on a nickel foam (NF) substrate and investigate the anticorrosion behaviors in alkaline saline electrolytes (1.0 M KOH and 0.5 M NaCl).…”