Understanding the potential-induced activation of a cobalt MOF electrocatalyst for the oxygen evolution reaction

Metal–organic frameworks (MOFs) are attractive porous materials for electrocatalytic applications associated with carbon-free energy storage and conversion. This type of material usually requires a post-treatment to be used as electrocatalyst. The present work comprehensively investigates the electrochemical activation of a cobalt-MOF@Nafion composite that produces outstanding electrocatalytic performance for the water oxidation reaction at neutral pH. A detailed electrochemical characterization reveals that the electroactivation of the composite requires the participation of the oxygen evolution reaction (OER) and leads to a significant increase in the electroactive population of cobalt centers. It is shown that an increase of the applied activation potential in the OER region results in a faster electroactivation of the Co-MOF without affecting the intrinsic electrocatalytic properties of the active cobalt centers, as evidenced by the unique linear correlation between the electrocatalytic OER current and the population of electroactive cobalt. In addition, at structural level, it is shown that the electrochemical activation causes the partial disruption of the Nafion adlayer, as well as morphological changes of the Co–MOF particles from a compact, rounded morphology, before electrochemical activation, to a more open and expanded structure, after electroactivation; with the concomitant increase of the number of surface–exposed cobalt centers. Interestingly, these cobalt centers retain their coordinative chemistry and their laminar distribution in the nanosheets at the nanoscale, which is consistent with the preservation of their intrinsic electrocatalytic activity after potential–induced activation. In this scenario, these results suggest that only the electroactivated cobalt centers with good accessibility to the electrolyte are electrochemically active. This work provides a better understanding of the processes and structural changes underlying the electrochemical activation at neutral pH of a Co–MOF for boosting the electrocatalytic water oxidation reaction9 página

Enzyme-like activity of cobalt-MOF nanosheets for hydrogen peroxide electrochemical sensing

[EN] Metal-organic frameworks (MOFs) are receiving increased attention as new functional nanomaterials for the development of electrochemical sensors. Herein, we develop an electrochemical platform for non-enzymatic hydrogen peroxide detection built with a composite of two-dimensional cobal t MOF nanosheets and Nafion (2D-Co-MOF@Nafion). The feasibility of the 2D-Co-MOF@Nafion composite as active material for high perfor-mance hydrogen peroxide sensor was investigated by usin g cyclic voltammetr y and chronoamperometry. Its voltammetric response reveals an efficient charge transport through the MOF composite, and rapid electron exchange between MOF and electrode. Notably, these MOF nanosheets exhibit enzyme-like activity for the non-common catalytic oxidation of hydrogen peroxide, leadin g to an electrochemical sensor with rapid quantitative detection, outstanding sensitivity, selectivity, stability, and durability at the desirable neutral pH. In particular, for a cobal t metal loading of 1.2 nanomol, the sensor yields amperometric H2O2 detection with characteristic electrocatalytic parameters ofima x = 5.7 mA cm(-2)andKM= 13 mM. Moreover, linear ranges of up to either 1 mM or 10 mM are achieved, with sensitivities as high as 570 +/-; 5 A cm(-2) mM(-1) or 395 +/- 10 A cm(-2) mM(-1) for the low and high concentration ranges, respectively. The particular coordination chemistr y of the MOF consisting of a regular arrangement of multiple Co(II) redox metal sites connected by appropriate organic ligands can provide inherent enzyme-mimicking properties, thereby explaining the higher oxidase-like activit y of the present MOF. This work raises the new idea of using two-dimensional cobalt-based MOFs as active nanozymes, offering exciting opportunities in the design of non-enzymatic electrochemical sensing devices.Acknowledgments Authors thank the financial support by the Spanish Government (RTI2018-096399-A-I00) and Junta de Andalucia (P20_01027 and PYC 20 RE 060 UAL).Portorreal-Bottier, A.; Gutiérrez-Tarriño, S.; Calvente, JJ.; Andreu, R.; Roldán, E.; Oña-Burgos, P.; Olloqui-Sariego, JL. (2022). Enzyme-like activity of cobalt-MOF nanosheets for hydrogen peroxide electrochemical sensing. Sensors and Actuators B Chemical. 368:1-9. https://doi.org/10.1016/j.snb.2022.1321291936