Copper-Modified Covalent Triazine Frameworks As Efficient Electrocatalysts for Oxygen Reduction Reactions
Authors: Iwase Kazuyuki , Kamiya Kazuhide , Nakanishi Shuji , Hashimoto Kazuhito 
Published: February 2020

1. Introduction
Electrochemical oxygen reduction reaction (ORR) is important as a cathode reaction of various types of fuel cells. At present, platinum (Pt) is used as a practical ORR electrocatalyst. However, as Pt is scarce and expensive, development of electrocatalysts composed only of abundant elements is strongly required. One of the effective approaches to synthesize non-noble metal catalysts is to learn from enzymes. Cu ions in Cu-containing enzymes such as laccase serve as the active centers for ORR. Especially, multi-copper oxidases (MCOs) are representative enzymes that catalyze ORR with almost no activation energy [1]. For MCOs, the modulation of the electronic state of the copper (Cu) active sites through the coordination with amino acid residues is known to be essential for the superior ORR catalytic activity. Therefore, many studies have been conducted on developing Cu-complex based electrocatalysts that mimic the molecular structure of MCOs [2]. However, their catalytic activity and stability developed to date are not satisfactory.
Covalent triazine framework (CTF) is a promising material as a novel platform for non-noble metal based electrocatalysts since the CTF is robust and possesses abundant nitrogen (N) atoms with electron lone pair for metal coordination. Recently, our laboratory has successfully synthesized Pt-modified CTF hybridized with conductive carbon nanoparticles (Pt-CTF) as methanol-tolerant ORR electrocatalyst [3]. However, modification of CTFs by non-noble metals has not been achieved yet. In this work, it was attempted to synthesize Cu modified CTF hybridized with carbon particles as an ORR electrocatalyst.
2.Experimental
The CTF hybridized with conductive carbon particles was synthesized by in situ polymerization of 2,6-dicyanopyridine in the presence of carbon particles. Briefly, the mixture of ZnCl2 molten salt, 2,6-dicyanopyridine and carbon particles were placed in a vacuum-sealed pyrex glass tube and heated at 400 °C for 40 h. The modification of Cu atoms was performed by an impregnation method in CuCl2 solutions (Cu-CTF). Electrocatalytic activity was evaluated by conducting cyclic voltammetry (CV) in 0.1 M phosphate buffer solutions (PBS) at room temperature using a rotating ring disk electrode. The molecular structure of the catalyst was characterized by an extended X-ray absorption fine structure (EXAFS) and a transmission electron microscope (TEM).
3.Result and discussion
The EXAFS and TEM results (not shown here) demonstrated that Cu atoms were coordinated to N atoms located in pores of the CTFs (Figure 1(a)). Figure 1(b) shows CVs for Cu-CTF and CTF in deaerated PBS (pH 7). The single pair of redox peaks corresponding to the Cu(I)/Cu(II) couple was observed for Cu-CTF. In the presence of dissolved oxygen, ORR current started to flow at the potential where Cu(II) is reduced to Cu(I) (Figure 1(c)), indicating that Cu(I) is the key species for the catalytic activity. Notably, both the redox potential of Cu(I)/Cu(II) (0.57 V) and the ORR onset potential (0.81 V) for the Cu-CTF were higher than those for the other Cu-complexes reported so far [4].
To clarify the molecular mechanism of ORR for Cu-CTF, we calculated the adsorption energy of O atom (ΔEo) as the indicator of interaction with ORR intermediates by the density functional theory (DFT) method. The result showed that Cu-CTF has similar ΔEo to platinum cluster which is the best artificial ORR catalyst. It strongly suggests that the regulation of the coordination structure by CTF modulates the interaction between reactant and metal centers [4][5].
Next, the stability of Cu-CTF during ORR was evaluated by conducting continuous CV cycles. For comparison, the Cu-(3,5-diamino-1,2,4-triazole) complex that shows similar ORR onset potential to the Cu-CTF [2] was also tested as a reference sample. For the conventional Cu-complex, the current density at 0.6 V decreased by 84 % after 500 cycles. However, the current decreased only by 19 % for the Cu-CTF, clearly indicating the superior stability of the Cu-CTF.
4.Conclusion
Thus, it was demonstrated that newly-synthesized Cu-CTF could serve as an efficient ORR electrocatalyst in neutral solutions. Notably, the Cu-CTF exhibited the best activity and stability among the Cu-complex based electrocatalysts reported so far.
5. References
[1] N. Mano et al., J. Phys. Chem. B, 2006, 110, 11180. [2] M. S. Thorum et al., Angew. Chem. Int. Ed., 2009, 48, 165. [3] K.Kamiya et al., Nat. Commun, 2014, 5, 5040. [4] K. Iwase et al., Angew. Chem. Int. Ed.
2015, 38, 11068. [5] T. Yoshioka, K. Iwase et al., J. Phys. Chem. C DOI: 10.1021/acs.jpcc.5b10962.

Figure 1