Unveiling the occurrence of Co(III) in NiCo layered electroactive hydroxides: the role of distorted environments

MARTÍN MIZRAHI; VICTOR OESTREICHER; DIEGO HUNT; FÉLIX G REQUEJO; MATIAS JOBBAGY

Congreso; 30th Annual Users Meeting of LNLS; 2020

CNPEM

The current requirements of reliable and environmentally friendly energy resources, electrochemical systems have gathered great interest. Two dimensional (2D) materials, as the layered double hydroxides (LDHs), have received tremendous attention in the last years, especially due to their upstanding physical and chemical properties, including in the field of energy storage and conversion.[1-4] The structure of the LDHs consists of stacks of positively charged metal hydroxide layers with charge-balancing anions situated between the layers. They exhibit a formula M_(1-x)^II M_x^III 〖(OH)〗_2^(x+) A_(x/n)^(n-)?m〖 H〗_2 O, where M^II and M^III are divalent and trivalent metals, respectively. In particular, Co- and Ni-based layered hydroxides constitute a unique class of 2D inorganic materials with exceptional physicochemical properties and outstanding performance as supercapacitors and for water splitting catalysts.[5-6] Recently, the occurrence of Co(III) in these phases has been proposed as a key factor that enhance their electrochemical performance. However, the origin of this centers and the control over its contents, remains as an open question. In this work, we employed the Epoxide Route to synthesize a full set of α-NiCo layered hydroxides. The structural characterization (PXRD and XAS), alerts about the distortion over the Co environment mediated by the increment the Ni(II) in the solid phase. The lattice compression promote a shortening over the Co-O distance triggering the oxidation of Co(II) to Co(III) as a function of Ni content. These experimental results are contrasted by DFT+U calculations, confirming the oxidation by the double degeneration of the 3d Co orbital in a distorted environment. This study combines a multiscale crystallochemical analysis supported by a microscopic interpretation, regarding the so called synergistic redox behavior of Co and Ni, offering fundamental tools for the controllable design of highly efficient electroactive materials.[1] Nguyen, T. et al.; Advanced Science 2019, 6 (9), 1801797. [2] Shao, Y. et al.; Chemical Reviews 2018, 118 (18), 9233-9280. [3] Wang, Y. et al.; Advanced Science 2018, 5 (8).[4] Wang, J. et al.; Advanced Materials 2016, 28 (2), 215-230.[5] Wang, W. et al.; ACS Nano 2019, 13 (10), 12206-12218.[6] Yang, P. et al.; Cryst Eng Comm, 2020, 22, 1602-1609.