DOI: 10.1002/slct.201701303 Full Papers 1 2 z Energy Technology & Environmental Science 3 4 5 Bioinspired Copper Coordination Polymer Catalysts for 6 Oxygen Reduction Reaction 7 8 Rupali Mishra,[a] Bhushan Patil,[b] Ferdi Karadas¸,*[a, b] and Eda Yılmaz*[b] 9 10 11 Non–noble metal catalysts have recently emerged as promising catalysts for ORR. The electrochemical analysis demonstrates 12 alternatives to the expensive platinum catalysts for the oxygen that [Cu–A] catalyses ORR with 3.24 numbers of electrons with 13 reduction reaction (ORR). In this study, a new domain of Tafel slopes of 122/83 mV dec À1 while it is 2.37 numbers of 14 materials, copper based coordination polymers, has been electrons with Tafel slopes of 131/84 mV decÀ1 for [Cu–B]. 15 investigated as promising catalysts for ORR. The study was Rotating disk electrode measurements and evaluation of Tafel 16 inspired by copper incorporating biomolecules, which effi- slopes reveal that acetate moieties attached to Cu site shift the 17 ciently catalyse the oxygen reduction reaction in nature. Two onset potential of ORR anodically (ca. 40 mV) compared to the 18 coordination polymers, [Cu2(mÀ AcO)4Po)]n (shortened as[Cu– one with benzoate bridging groups. The effect of bridging 19 A]) and [Cu2(mÀBzO)4Po)]n (shortened as[Cu–B]), incorporating ligands to the stability and activity of catalysts in alkaline media 20 one–dimensional chains of Cu(II) paddle wheel units bridged was also evaluated. This study opens a new perspective for the 21 with phosphineoxide ligands were combined with multi À development of non–platinum ORR catalysts. 22 walled carbon nanotubes (MCNTs) to prepare hybrid electro- 23 24 operating conditions such as neutral pH, narrow temperature 25 Introduction range, and etc. Interestingly, ORR mechanism and potential are 26 The oxygen reduction reaction (ORR) is a crucial reaction in different for each biomolecule participating in the process even 27 numerous applications including fuel cells[1À3] and metalÀair though they contain copper as the active site reducing the 28 batteries.[4,5] Platinum is one of the most efficient electro- oxygen. This suggests that the ligands attached to Cu have a 29 catalysts for ORR.[6À10] However, its high cost, scarcity, and vital role in the ORR mechanism. During the adsorption and 30 tendency of the surface to poison, constrain its applications. dissociation of O molecules, Cu is significantly affected by the 31 2 This triggers the interest of researchers to develop alternative ligands attached to it. 32 electrocatalysts for ORR including carbon–based materials such A 1D coordination polymer incorporating copper paddle– 33 as carbon nanotubes,[11,12] graphene,[13À15] and transition metal wheel units has recently been investigated as an active 34 compounds such as cobalt,[16,17] iron,[18À21] and copper.[22,23] Cu is electrocatalyst for water oxidation.[60] The previous study shows 35 a highly abundant element in earth’s crust and it is used in a that copper sites in the paddle–wheel unit can activate water 36 wide range of areas such as host Àguest systems,[24À29] to form O . Given the microscopic reversibility of ORR and OER 37 2 catalysis,[30À34] magnetism,[35À39] biological systems,[40À44] and mechanisms, the electrocatalytic activity of copper paddle– 38 electrochemistry.[45À47] wheel unit in ORR has been investigated in this work. The 39 Cu can also be found in enzymes participating in several previously reported 1D coordination polymer together with a 40 reactions. Bio–inspired copper catalysts[48,49] and CuÀbased new derivative have been used in the study to investigate the 41 enzymes have been reported to be active catalysts in ORR such effect of surrounding bridging ligands to the catalytic activity 42 as ascorbate oxidase,[50] bilirubin oxidase,[51À54] and laccase[55À59] and stability of copper systems. 43 etc. The use of such enzymes for the practical systems is 44 however, not possible due to the requirement of very limiting 45 Result and discussion 46 47 [a] Dr. R. Mishra, Prof. F. Karadas¸ Crystal Structure Department of Chemistry, Bilkent University, 06800, Ankara, Turkey 48 [b] Dr. B. Patil, Prof. F. Karadas¸, Prof. E. Yılmaz Copper acetate was reacted with phosphine ligands in 49 Institute of Materials Science and Nanotechnology, National Nano- methanol/dichloromethane mixture in peroxide media, which 50 technology Research Center (UNAM) Bilkent University, 06800, Ankara, results in the formation of dark blue crystals of [Cu–A] and [Cu– 51 Turkey Tel:+ 90-312-290 19 97 B]. The structure of [Cu–A] was already described in our 52 Tel:+ 90-312-290-8028 previously reported work.[60] The crystal structure of the new 53 Fax: +90-312-266 40 68 copper catalyst, [Cu–B] was successfully solved in the mono- 54 E-mail: [email protected] clinic space group, P121/n1. Crystallographic data and struc- 55 [email protected] tural refinement parameters for the compound are given in 56 Supporting information for this article is available on the WWW under https://doi.org/10.1002/slct.201701303 (Table S1). 1D chain of both compounds consists of independ- 57 ChemistrySelect 2017, 2, 8296–8300 8296 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Wiley VCH Donnerstag, 21.09.2017 1727 / 99023 [S. 8296/8300] 1 Full Papers ent copper paddleÀwheel units linked by phosphineoxide (P ) Electrochemical measurements 1 o bridging ligands in Figure 1(b). Each metal ion is surrounded by 2 Cyclic voltammetry (CV) measurements were obtained at bare 3 GC, MWCNT/GC, [Cu–A], and [Cu–B] under N (Figure S5) and 4 2 O (Figure 2) atmosphere in saturated 0.1 M KOH solution. 5 2 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 Figure 2. Cyclic voltammograms of GC (in green), MWCNT/GC (in blue), [Cu– 23 A] (in red) and [Cu–B] (in black) performed in 0.1 M KOH, under saturated O2 Figure 1. Asymmetric unit of [Cu–B] and hydrogen atoms are omitted for at 10 mV/s. 24 clarity in (a). In fig (b) showing both different 1D chains of [Cu–A] and [Cu– 25 B].Colour code: Cu =blue; P=green; O=red; C=black. 26 27 Figure S6 shows the CVs of individual catalyst under N and O 28 2 2 five oxygen atoms, one of which belongs to oxygen of bis saturated solutions. Anodic peaks for[Cu–B] under N saturated 29 2 (diphenylphosphino)phosphine dioxide ligand while the re- electrolyte obtained at 0.92 V (peak I) and À0.88 V (peak II) are 30 maining four belong to four different benzoate bridging attributed to Cu0–Cu1+ and Cu1+–Cu2+ oxidation processes, 31 moieties. In [Cu–B], copper centre is in square pyramidal respectively. An anodic shift of 0.02 V in both of these anodic 32 coordination environment Figure 1(a). In both compounds the peaks were observed for [Cu–A] compared to those obtained 33 paddle–wheel units have different metal–to–metal distance. for [Cu–B]. Furthermore cathodic peak attributed to Cu2+–Cu0 34 Cu···Cu distance is 2.6224 A˚ in [Cu–B] and while it is 2.658 A˚ for reduction at 0.7 and 0.72 V were observed for [Cu–B] and [Cu– 35 [Cu–A]. This difference in the metal–to–metal distance obtained A], respectively. The overall anodic shift of 0.02 V observed for 36 for [Cu–B] could be attributed to the presence of a bulkier [Cu–A] compared to [Cu–B] is likely due to the difference 37 bridging group. The crystal packing diagrams of compounds between ligands attached to the Cu active site. An increase in 38 displayed in Figures S3 (a) and (b) clearly indicate that the the cathodic current density compared to anodic current 39 shortest distance between copper centres in neighbouring density was observed, which is also in good agreement with 40 chains is much larger for [CuÀB] 8.086 A˚ for [CuÀA] and the assignment of redox processes involving two one–electron 41 11.104 A˚ for [CuÀB] as a result of bulky benzoate groups. The reduction and a two–electron oxidation processes, where the 42 supramolecular framework of [CuÀB] is stabilized by C–H···p oxidation bands are assigned as one electron processes. 43 interactions between H atoms of the phenyl ring of benzoate In comparison to MWCNT/GC, an anodic shift of 35 mV and 44 molecules (2.822–2.848 A˚) with other aromatic ring of benzoate 65 mV was observed for the ORR onset potentials obtained at 45 groups. The supramolecular structure of [CuÀB] is further form the [Cu–B] and [Cu–A], respectively. The difference between 46 an overall 3D structure in Figure S2 (b). Both compounds show onset potentials for ORR is likely due to the difference in the O 47 2 3D structure in Figure S3 (a) and (b). Selected bond distances dissociation mechanism of these two compounds and change 48 (A˚) and bond angles (8) for [CuÀ B] are summarized in Table S2. in the bonding energy of O (or reduced intermediates) at the 49 2 All the MÀO distances are within the normal ranges allowing Cu active sites. The anodic current intensity was decreased 50 for statistical errors.[61,62] remarkably under O than the N atmosphere obtained for [Cu– 51 2 2 In order to examine the thermal stability and bulk purity of B], which might be because of strong and almost irreversible 52 the both coordination polymers [CuÀA] and [CuÀB] we have bonding of O or intermediates with the Cu site.