Hierarchically structured graphene-carbon nanotube-cobalt hybrid electrocatalyst for seawater battery
Introduction
Hierarchical architecturing of two-dimensional (2D) graphene to three-dimensional (3D) internetworked structure has been conducted to enhance the physical and structural properties [1]. The 3D internetworked graphene could be synthesized by various synthetic methods such as hydrothermal, chemical reduction, chemical vapor deposition, colloidal templating methods and so on [2], [3], [4], [5], [6], [7], [8], [9]. In particular, owing to their large surface-to-volume ratio, they provide excellent electrochemical properties in aspects of fast mass transport and high accessibility into active sites for an application in electrochemical energy storage [9], [10], [11], [12], [13], [14], [15], [16], [17], [18]. The electrochemical properties could be further improved by surface treatments, such as steam activation, KOH activation, thermal-annealing and plasma treatment [19], [20], [21], [22], [23], [24], [25]. Such surface-modified 3D graphene architectures can be also used as network-structured conductive matrix to hybridize with various functional materials, such as metals and metal oxides.
Taking advantages of such hierarchical architecture, surface modification protocol and hybrid composition, herein, we suggest a new type of 3D hybrid structure, which is comprised of one-dimensional (1D) carbon nanotubes (CNTs) with encapsulation and decoration of cobalt-based nanoparticles on the 3D internetworked reduced graphene oxide (rGO) architecture. The as-synthesized hybrid nanoparticles are examined as an electrocatalyst toward oxygen evolution reaction/oxygen reduction reaction (OER/ORR) for rechargeable seawater batteries. The seawater battery has recently been reported as a novel cost-effective rechargeable battery system utilizing natural seawater as the active material [26], [27]. To enhance the kinetics of the OER and ORR, seawater batteries require bifunctional oxygen electrocatalysts on the air-cathode. Typically, precious metal-based catalysts, such as Pt and RuO2/IrO2, have been used as oxygen electrocatalysts; however, their high cost and scarcity remain challenges to be resolved for widespread applications [28], [29], [30], [31], [32], [33], [34], [35], [36], [37], [38], [39], [40], [41]. Cobalt-based materials have recently attracted much interest as cost-effective oxygen electrocatalysts [42], [43], [44], [45], [46]. Nevertheless, steady degradation of the active sites as a result of particle aggregation during redox reactions and insufficient electrical conductivity should be improved for practical implementations. Moreover, existing synthetic routes, such as chemical vapor deposition and arc discharge, have limitations to construct such complicated hierarchical architectures [47], [48], [49], [50], [51], [52], [53], [54], [55], [56], [57].
In this work, we fabricated a hybrid hierarchical architecture, where CNTs are directly grown on the 3D surface-modified rGO (S-rGO)/Co nanoparticles by using a steam activation and microwave irradiation. The CNT branches grown on the 3D hierarchical architecture are expected to act as a conducting bridge between 3D carbon substrates and Co nanoparticles for high electrical conductivity, structural flexibility and strong tolerance [10], [47], [48], [49], [50]. The defect sites of S-rGO could act as the activated sites to firmly immobilize nanoparticles, when the Co catalysts are anchored onto the surface. Importantly, Co precursors are converted into Co nanoparticles to be encapsulated inside the densely grown CNT strands as well as decorated onto the surface of 3D S-rGO for high catalytic ability in seawater batteries. The battery using the hybrid nanoparticle electrocatalyst exhibits stable cycling performance without notable degradation at a current rate of 0.01 mA cm−2 during 50 cycles.
Section snippets
Synthesis of S-rGO
The graphene oxide (GO) was uniformly dispersed into the deionized water and blended with hypophosphorous acid (H3PO2, SAMCHUN Chemical, 50 wt%) and iodine (I2, DAEJUNG, 99.0%). Then, the mixture was located into the 80.0 °C oven for 12 h. After the mixture transformed to gel, the gel was washed by deionized water until it became pH 7. When the purified gel was placed into freeze-dryer for 72 h, 3D rGO was obtained.
As-prepared 3D rGO became stable in 900.0 °C with heating rate of 5.0 °C min−1
Results and discussion
In the synthetic process schematically illustrated in Fig. 1, the S-rGO and cobaltocene were used as the conductive substrate and catalyst precursor to grow CNTs directly onto the steam-activated surface. The cobaltocene precursors were decomposed into Co and cyclopentadienyl rings and immediately grown into Co nanoparticles through the microwave irradiation. It notes that the Co nanoparticles plays a catalytic role of growing CNTs, while the cyclopentadienyl rings serve as the carbon sources
Conclusions
In this work, we have successfully synthesized a novel 3D hybrid structure (S-rGO-CNT-Co), which is composed of encapsulating Co/CoOx nanoparticles along directly branched CNTs on 3D porous graphene sheets. When the hierarchical hybrid structured particles were employed as the electrocatalyst, the seawater battery exhibited good cell performance, such as cycling stability and rate properties, compared to bare carbon felt and Pt/C. The outstanding results were mainly attributed to as follows.
Acknowledgments
This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education(2017R1D1A1B03033428), Energy Technology Development Project(ETDP) funded by the Ministry of Trade, Industry & Energy (20172410100150) and Energy Efficiency & Resources program of the Korea Institute of Energy Technology Evaluation and Planning, and was granted financial resources from the Ministry of Trade, Industry & Energy, Republic of Korea
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These authors contributed equally to this work.