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

Nickel-based layered hydroxides (LHs) are a family of efficient electrocatalysts for the alkaline oxygen evolution reaction (OER). Nevertheless, fundamental aspects such as the influence of the crystalline structure and the role of lattice distortion of the catalytic sites remain poorly understood and typically muddled. Herein, we carried out a comprehensive investigation on ɑ-LH, ß-LH and layered double hydroxide (LDH) phases by means of structural, spectroscopical, in-silico and electrochemical studies, which suggest the key aspect exerted by Ni-vacancies in the ɑ-LH structure. Density functional theory (DFT) calculations and X-ray absorption spectroscopy (XAS) confirm that the presence of Ni-vacancies produces acute distortions of the electroactive Ni sites (reflected as the shortening of the Ni-O distances and changes in the O-Ni-O angles), triggering the appearance of Ni localised electronic states on the Fermi level, reducing the Egap, and consequently, increasing the reactivity of the electroactive sites in the ɑ-LH structure. Furthermore, post-mortem Raman and XAS measurements unveil its transformation into a highly reactive oxyhydroxide-like phase that remains stable under ambient conditions. Hence, this work pinpoints the critical role of the crystalline structure as well as the electronic properties of LH structures on their inherent electrochemical reactivity towards OER catalysis. We envision Ni-based ɑ-LH as a perfect platform for hosting trivalent cations, closing the gap toward the next generation of benchmark efficient earth-abundant electrocatalysts.

Novel side chain functionalized polystyrene/O-PBI blends with high alkaline stability for anion exchange membrane water electrolysis (AEMWE).

We report the synthesis of a polystyrene-based anion exchange polymer bearing the cationic charge at a C6-spacer. The polymer is prepared by a functionalized monomer strategy. First, a copper halide catalyzed C-C coupling reaction between a styryl Grignard and 1,6-dibromohexane is applied, followed by quaternization with N-methylpiperidine and free radical polymerization. The novel polymer is blended with the polybenzimidazole O-PBI to yield mechanically stable blend membranes representing a new class of anion exchange membranes. In this regard, the ratio of the novel anion exchange polymer to O-PBI is varied to study the influence on water uptake and ionic conductivity. Blend membranes with IECs between 1.58 meq. OH- g-1 and 2.20 meq. OH- g-1 are prepared. The latter shows excellent performance in AEMWE, reaching 2.0 A cm-2 below 1.8 V in 1 M KOH at 70 °C, with a minor degradation rate from the start. The blend membranes show no conductivity loss after immersion in 1 M KOH at 85 °C for six weeks indicating high alkaline stability.

Upscaling the urea method synthesis of CoAl layered double hydroxides.

Research on two-dimensional materials is one of the most relevant fields in materials science. Layered double hydroxides (LDHs), a versatile class of anionic clays, exhibit great potential in photocatalysis, energy storage and conversion, and environmental applications. However, its implementation in real-life devices requires the development of efficient and reproducible large-scale synthesis processes. Unfortunately, reliable methods that allow for the production of large quantities of two-dimensional LDHs with well-defined morphologies and high crystallinity are very scarce. In this work, we carry out a scale-up of the urea-based CoAl-LDH synthesis method. We thoroughly study the effects of the mass scale-up (25-fold: up to 375 mM) and the volumetric scale-up (20-fold: up to 2 L). For this, we use a combination of several structural (XRD, TGA, and N2 and CO2 isotherms), microscopic (SEM, TEM, and AFM), magnetic (SQUID), and spectroscopic techniques (ATR-FTIR, UV-vis, XPS, ICP-MS, and XANES-EXAFS). In the case of the volumetric scale-up, a reduction of 45% in the lateral dimensions of the crystals (from 3.7 to 2.0 µm) is observed as the reaction volume increases. This fact is related to modified heating processes affecting the alkalinization rates and, concomitantly, the precipitation, even under recrystallization at high temperatures. In contrast, for the tenfold mass scale-up, similar morphological features were observed and assigned to changes in nucleation and growth. However, at higher concentrations, simonkolleite-like Co-based layered hydroxide impurities are formed, indicating a phase competition during the precipitation related to the thermodynamic stability of the growing phases. Overall, this work demonstrates that it is possible to upscale the synthesis of high-quality hexagonal CoAl-LDH in a reproducible manner. It highlights the most critical synthesis aspects that must be controlled and provides various fingerprints to trace the quality of these materials. These results will contribute to bringing the use of these 2D layered materials closer to reality in different applications of interest.

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

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, ß-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.