CeO2/Co(OH)2 hybrid electrocatalysts for efficient hydrogen and oxygen evolution reaction
Graphical abstract
Introduction
In recent years, many studies have bene performed to minimize the use of fossil fuels to reduce environmental pollution [1]. Among all solutions, electrochemical water splitting can produce hydrogen energy with high energy density. This can reduce environmental pollution problems caused by oil, coal and gases [2,3]. Electrochemical water splitting is classified into hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) [4].
Theoretical operation potentials of OER and HER are 1.23 V and 0.0 V (vs. RHE) [5], respectively. Considering kinetic factors in these theoretical calculations, practical application of OER and HER becomes complicated. As a result, additional voltage known as overpotential is required for OER and HER to have continuous water splitting [6]. Previous studies have attempted to reduce the overpotential by using precious metals such as Pt, Ag, IrO2, and RuO2. However, it is difficult to use them as catalysts because of their high cost and insufficient reserves [7,8]. Thus, various non-precious metals such as transition metals [9,10], transition metal oxides [11,12], and metal–free carbonaceous materials [13,14] have been noted for replacing precious metals due to their abundance and low cost. Transition metal oxides are expected to have high catalytic properties through nano-structuring and hybrid-structuring [15,16].
Through hybrid-structuring, CeO2 is very useful as co-catalyst due to its stability in alkaline solution. CeO2 has excellent ionic conductivity, high oxygen storage capacity, and reversible oxidation state of Ce3+ and Ce4+. It also exhibits enormous oxygen utilization capability, suggesting that it might be able to improve OER activity [17,18]. According to previous literature [19], FeOOH/CeO2 exhibits superior OER performance to pure FeOOH. Due to the uniquely high oxygen storage capacity of CeO2, FeOOH/CeO2 can easily absorb oxygen generated during OER and promote OER activity accordingly. In HER reaction, CeO2 serves as a water dissociation promoter to produce hydrogen intermediates [20]. It has been reported that hybridization of CeO2 with transition metal catalysts can improve HER activity. CeO2/Ni composites also exhibit good HER performance because the CeO2/Ni interface can decrease electrochemical reaction resistance (Rct + Rp) and hydrogen binding energy [21,22].
The objective of this study was to investigate OER/HER catalytic effects between Co(OH)2 and CeO2 for water splitting. Co(OH)2 has higher OER/HER activity than cobalt oxides because Co(OH)2 catalyst has high active site density [15,23]. Mixtures of Co(OH)2 nanostructure inserted between CeO2 nanoplates were synthesized by a simple two-step wet chemistry method. In the initial step, CeO2 nanoplates were synthesized into a hexagonal–shape with a hydrothermal method. And then, CeO2/Co(OH)2 hybrid catalysts (HCs) were fabricated by a precipitation of Co(OH)2 nanostructure between CeO2 nanoplates. High electrocatalytic properties and stability of CeO2/Co(OH)2 HCs were confirmed through HER and OER tests and chronoamperometry.
Section snippets
Preparation of CeO2 nanoplates
CeO2 nanoplates were synthesized by a facile hydrothermal process. Briefly, 2.5 g of Ce(NO3)3∙6H2O (ACROS, purity 99.5%) and 8 mL of NH4OH (SAMCHUN, purity 28.0–30%) were dissolved in 62.5 mL of distilled water with vigorous stirring at room temperature. The prepared solution was sealed in a 100 mL Teflon-lined stainless-steel autoclave and heated at 90 °C for 6 h. The obtained sample was filtered and washed with a nylon membrane (Durapore, 0.22 mm, Millipore, USA) using distilled water and
Characterization of CeO2/Co(OH)2 hybrid catalysts
CeO2 can be synthesized in various shapes such as nanoplates, nanotubes, and nanorods. In order to find the optimal condition for the synthesis of hexagonal CeO2, we have confirmed the morphology evolution through the TEM analysis of hydrothermal reaction products synthesized at 90 °C for various reaction times for 0.5, 1, 6, and 12 h in the presence of NO3− and NH4OH. Fig. S1 shows the microstructural change of CeO2 nanoparticles. As the reaction time increased up to 0.5 h,
Conclusion
We systematically investigated the enhanced OER/HER performance of CeO2/Co(OH)2 HCs through analysis of crystal structure, morphology, and chemical composition. In the morphology and chemical composition of CeO2/Co(OH)2 HCs, CeO2 nanoplates with an average diameter of 18.6 nm were hybridized with Co(OH)2 as efficient electrocatalysts. When CeO2 nanoplates contacted with Co(OH)2, the ratio of Ce3+ was increased due to formation of oxygen vacancies in the CeO2/Co(OH)2 HCs caused by charge
Acknowledgement
This work is supported by the National Research Foundation of Korea (NRF) Grant funded by the Ministry of Science and ICT, South Korea (2016M3A7B4909318), by the R&D Center for Valuable Recycling (Global-Top R&BD Program) of the Ministry of Environment, South Korea (Project No.: R2-17_2016002250005), and by a Korea University Grant, South Korea.
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These authors contributed equally to this article.