Water electrolysis under alkaline conditions is of interest due to the applicability of non-precious metal-based materials for electrocatalysts. However, the successful design and synthesis of earth-abundant and efficient catalysts for the oxygen evolution reaction (OER) remain a significant challenge. This work presents cost-effective and straightforward ways to improve the OER activity under alkaline conditions by activating the catalyst–support and reactant–support interaction. Micro/nano-sized fibrous poly(vinylidene fluoride- co -hexafluoropropylene) (PVdF-HFP) was synthesized via simple and scalable electrospinning and subsequently coated with Cu by electroless deposition to obtain the electrocatalyst with a large specific surface area, enhanced mass transport, and high catalyst utilization. Scanning electron microscopy, infrared spectroscopy, and X-ray diffraction confirmed the successful synthesis of the series of Cu/PVdF-HFP fibrous catalysts with varied ferroelectric polarizability of the PVdF-HFP support in the order of stretch-anneal > anneal > stretch > without pre-treatment of the catalyst. The best OER activity was confirmed for the Cu/PVdF-HFP catalyst with stretch and annealed treatment among the catalysts tested, suggesting that both the reaction kinetics and energetics of stretch-annealed Cu/PVdF-HFP catalysts were optimal for the OER. The electron delocalization between Cu and PVdF-HFP substrates (electron transfer from Cu to the negatively charged (δ – eff ) PVdF-HFP region at the Cu|PVdF-HFP interface) and the enhanced transport of reactive hydroxide species and/or the increase in the local pH by positively charged (δ + eff ) PVdF-HFP region concertedly accelerate the OER activity. The overall activity for the prototype water electrolyzer increased 10-fold with stretch-anneal treatment compared to the one without pre-treatment, highlighting the effect of tuning the catalyst–support and reactant–support interaction on improving the efficiency of the water electrolysis.
Water electrolysis is an ideal method for producing hydrogen, which is expected as a next-generation energy carrier. However, the sluggish reaction kinetics of the oxygen evolution reaction (OER) limits the overall energy efficiency.1,2 Therefore, designing efficient electrocatalyst for the OER is crucial and has been studied extensively for more than a few decades.3 Precious metal catalysts such as RuO2 and IrO2 exhibit high activity,4,5 but are expensive, and transition metals have been attracted as alternative catalysts.6 In this study, a fibrous Cu catalyst was synthesized as a highly active and low-cost OER catalyst. An electrospinning method was selected to synthesize a fibrous polymer substrate, which was subsequently coated with Cu by electroless deposition. We applied Cu, which is a low cost and has excellent electrical conductivity as an electrocatalyst. As a geometric structure, fiber shape was adopted to increase the specific surface area and improving the diffusion efficiency of the catalyst. We selected PVdF-HFP as catalyst support, to promote the diffusion of the ions in the electrolyte and enhance the activity as a catalyst by its ferroelectricity. In order to clarify the effect of geometric structure on the OER activity, Cu catalysts were synthesized by various methods, including drop-casting, impregnation, and electrospinning. It was found that the fibrous Cu catalyst exhibited the best OER activity amongst all the geometric structures tested in this study, probably due to the increased active surface area as well as better mass transport and diffusion. Furthermore, the effect of the dielectric properties of PVdF-HFP on the OER activity was clarified. The molecular orientation of PVdF-HFP was evaluated by infrared (IR) spectroscopy, which indicates that the stretching and annealing treatment can effectively increase its electric polarizability. Linear sweep voltammetry (LSV) confirmed that the fibrous Cu catalyst with stretching and annealing treatment showed superior OER activity compared to the pristine fibrous Cu catalyst, indicating the large electric polarizability also enhanced the OER. Based on these results, it was clarified that the improvement of the OER activity of the fibrous Cu catalyst was due to the following reasons; (i) improvement of mass transport by fibrous structure, and (ii) optimization of the OER reaction energetics by catalyst(Cu)-support(PVdF-HFP) interaction. Optimizing both geometry and electronic structure of the electrocatalyst provides a new design strategy to achieve improved activity towards oxygen evolution reaction. 1 Seh, W. Z. et al., Science. 2017, 355, eaad4998. 2 Fabbri, E. et al., Catal. Sci. Technol. 2014, 4, 3800–3821. 3 Jiao, Y. et al., Chem. Soc. Rev. 2015, 44, 2060–2086. 4 Lee, Y. et al., J. Phys. Chem. Lett. 2012, 3, 399–404. 5 Paoli, A. E. et al., Chem. Sci., 2015, 6, 190–196. 6 Hong, T. W. et al., Energy Environ. Sci. 2015, 8, 1404–1427.
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