Crystal Growth of Bimetallic Oxides CuMnO₂ with Tailored Valence States for Optimum Electrochemical Energy Storage

Abstract

Bimetallic oxides ABO x (x = 2, 3, and 4) with multiple lattice sites have shown numerous properties, blossoming into diverse applications, while regulating the valence state at the A or B site usually causes a dramatic change in the crystal structure, giving rise to uncertainties in comprehending the structure–property relationships. Herein, we synthesized bimetallic layered crednerite CuMnO2 with double-coordinate Cu cations at the A site and hexa-coordinate Mn cations at the B site via a cetyltrimethylammonium bromide (CTAB) modified hydrothermal method. By controlling the crystal growth temperature, valence states tailoring was implemented along with the stabilization of the monoclinic layered structure. The regulation process was followed by morphology changes of the CuMnO2 crystal in a sequence: triangular sheets (140 °C), nanowires (160 °C), hexagonal prisms (180 °C), and octahedrons (over 200 °C). Interestingly, the oxidation states of Cu2+ and Mn2+ were found for triangular sheets, which transformed to the mixed valency Cu+/Cu2+ and Mn3+/Mn2+ for nanowires, and then to the dominant Cu+ and Mn3+ oxidation states for hexagonal prisms and octahedrons. Among all morphologies, nanowires showed a higher aspect ratio, preferentially growing along the (002) plane, which when first applied to a supercapacitor, exhibited the highest specific capacitance of 921 F/g at the current density of 1 A/g. The synergistic effect between the special redox equilibrium and controlled one-dimensional crystal architecture is uncovered to optimize the electrochemical energy storage. The methodology reported in this work creates a new path of functional bimetallic oxides ABO x with tailored valence states, controllable crystal growth, and stable crystal structure for optimum energy storage applications.

Recent Trends in Bimetallic Oxides and Their Composites as Electrode Materials for Supercapacitor Applications

“…Various morphologies of bimetallic oxides synthesized via different synthetic routes have been reported by many researchers. [59] Das et al, reported an octahedron morphology for CuCo 2 O 4 , synthesized via ethylenediaminetetraacetic acid (EDTA) assisted hydrothermal approach that is most suited for supercapacitor application. The electrochemical studies on the obtained single-crystalline CuCo 2 O 4 octahedron with a surface area of 61.97 m 2 g À 1 , revealed the specific capacity of 989 C g À 1 at 5 mV s À 1 .…”

Relationship between morphological and physical properties in nanostructured CuMnO2

“…In the last few decades, there has been a renewed interest in the research on the CuMnO 2 material due to its an exceedingly rich spectrum of potential applications. As a p-type semiconductor [5][6][7], electrode material [8], CuMnO 2 has also been examined regarding photocatalytic activities for hydrogen production or hydrogen storage [9][10][11], oxygen storage [12,13], energy storage [14,15] as well as in the context of multiferroic peculiarities [16]. Remarkably, by the use of a hydrothermal method, nanoparticles with diameters of 50 -100 nm [6], or nanowires [15], can be obtained.…”

“…The most natural method of preparing different morphologies of nanoparticles is that which can modify condition of reaction, e.g., by varying factors such as precursor solution and its concentration, reaction temperature, reaction time and pH value of the solvents. A successful synthesis upon changes of reaction condition has been documented for delafossite CuGaO 2 [27] or for crednerite CuMnO 2 [15,28]. Fu et al [15] showed that the valence states of samples consisting of Cu 1+ , Cu 2+ , Mn 2+ and Mn 3+ are possibly adjusted varying reaction temperatures.…”

“…A successful synthesis upon changes of reaction condition has been documented for delafossite CuGaO 2 [27] or for crednerite CuMnO 2 [15,28]. Fu et al [15] showed that the valence states of samples consisting of Cu 1+ , Cu 2+ , Mn 2+ and Mn 3+ are possibly adjusted varying reaction temperatures. As a result, morphologies of the CuMnO 2 nanoparticles are changed in a consequence triangular sheets -nanowires -hexagonal prisms -octahedrons with creasing temperature.…”

“…In this article, we report the synthesis of nanostructured CuMnO 2 samples using a hydrothermal method employed in previous studies, e.g. as described by Krause and Gawryck many years ago [29], and reported in the literature for synthesis of delafossites [15,28,30,31]. Because, the reaction between (OH) and Mn + plays an important role in the formation of MnO octahedron, we use several amounts of NaOH mineralizator, i.e., 1g, 2g and 3g per 35 ml of solution, to turn the reaction rates and the degree of nucleation.…”