Materials Today Energy
Crystalline state transformation strategy for improving the catalytic performance of oxygen evolution reaction at high current density
Graphical abstract
The disordered iron-naphthalenedicarboxylic acid coordination complex has a low overpotential at high current density, which is favorable for water electrolysis reaction.
Introduction
The continuous consumption of fossil fuels has caused environmental pollution and energy shortage, promoting the development of renewable energy technologies, such as the metal-air battery, fuel cell, and hydropower, is extremely urgent [[1], [2], [3]]. Electrocatalytic water decomposition is considered a promising method to obtain clean hydrogen energy. However, the rate-determining steps controlled by the O–H cleavage and the O–O formation significantly affect the kinetics of the oxygen evolution reaction (OER), which is a multistep four-electron transfer reaction process [4]. The development of heterogeneous and homogeneous electrocatalysts promises to accelerate OER dynamics and reduce overpotential. Typically, the precious metal (Ru, Ir, and Pt)–based electrocatalysts show high OER catalytic activity. However, their wide application is limited by their high cost, richness in earth abundance, and low inactivation elasticity [5]. Therefore, there is an intense desire to develop electrocatalysts based on transition metals to achieve low overpotential and efficient OER performance [6].
To obtain stable and efficient catalysts, it is necessary to find a robust ligand system that can firmly fix the metal center and such that auto-oxidation does not occur in the oxidizing environment [7]. Coordination polymers with a designable and predictable structure have recently received much attention. However, their negative electrical conductivity and inferior structural stability severely restrict their practical applications [[8], [9], [10]]. Cheng et al. [11] demonstrated that binding of a naphthalenedicarboxylic acid (NDC) ligand to a metal could induce lattice strain, enabling a powerful bifunctional oxygen reduction reaction and OER. Xue et al. [12] suggested that the electronic structure of coordination polymers can be tuned by introducing missing linkers of carboxyferrocene, which improves OER performance of the catalyst. The complex materials have inherent molecular metal centers and can be used as potential active sites for electrocatalysis [13]. The coordinated unsaturated metal center formed in the process of complex catalysis can act as a typical Lewis acid center to accept electrons from the reactant, thereby promoting the related conversion [14]. Using complexes directly as electrocatalysts can provide a large number of molecular metal active sites [15,16].
Hence, we reported a simple and convenient solvent substitution method to design the iron-NDC (Fe-NDC) complex (see Scheme 1). The complexes were synthesized using ferric nitrate and 1,4-NDC as an organic ligand in a different solvent. Surprisingly, the complex with amorphous phase manifests high catalytic activity for OER, with a low overpotential of 225 mV at the low current density of 10 mA cm−2 and 333 mV at the high current density of 500 mA cm−2, with a low Tafel slope for amorphous-phase Fe-NDC (AP-Fe-NDC). Moreover, the characteristics of the low-cost and efficient non-precious catalysts display bright prospects to replace exorbitant noble metal catalysts in industry applications.
Section snippets
Results and discussion
To understand the structural characteristics of the synthesized AP-Fe-NDC, the X-ray diffraction (XRD) patterns are shown in Fig. 1a, which is in good agreement with the simulated standard XRD pattern of Fe-NDC, indicating the successful formation of the Fe-NDC complex. Interestingly, the crystallinity degree obtained from the solvent water is high (i.e., crystalline-phase Fe-NDC [C-Fe-NDC]), whereas the crystallinity degree is obviously decreased after immersing into ethanol for several hours.
Conclusions
In conclusion, the AP-Fe-NDC complex was designed using a facile and economic solvent substitution method. The prepared crystals deal with different solvents, and the crystal structure changed to the amorphous phase, making the particle size smaller and exposing more active sites, thereby promoting conduction. Whether it is under high or low current density, AP-Fe-NDC exhibits superb OER performance compared with C-Fe-NDC. The in situ generated FeOOH is considered active material for catalyzing
Credit author statement
Yuan Xu: Data curation, Writing - original draft; Muhammad Arif Khan: Methodology, Software; Zhe Chen: Visualization, Investigation, Software, Validation; Cong Chen: Visualization, Investigation, Software, Validation; Lei Zhang: Software, Validation; Daixin Ye: Supervision, Conceptualization, Writing-Reviewing and Editing; Kangning Zhao: Software, Validation; Hongbin Zhao: Supervision, Conceptualization, Writing-Reviewing and Editing; Xueliang Andy Sun: Methodology, Software, Writing-Reviewing
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
The authors gratefully acknowledge financial support from the National Key Research and Development Program of China (2017YFB0102900).
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