Mechanistic Evidence for Ligand-Centered Electrocatalytic Oxygen Reduction with the Conductive MOF Ni₃(hexaiminotriphenylene)₂

Abstract

Establishing catalytic structure−function relationships introduces the ability to optimize the catalyst structure for enhanced activity, selectivity, and durability against reaction conditions and prolonged catalysis. Here we present experimental and computational data elucidating the mechanism for the O 2 reduction reaction with a conductive nickel-based metal−organic framework (MOF). Elucidation of the O 2 reduction electrokinetics, understanding the role of the extended MOF structure in providing catalytic activity, observation of how the redox activity and pK a of the organic ligand influences catalysis, and identification of the catalyst active site yield a detailed O 2 reduction mechanism where the ligand, rather than the metal, plays a central role. More generally, familiarization with how the structural and electronic properties of the MOF influence reactivity may provide deeper insight into the mechanisms by which less structurally defined nonplatinum group metal electrocatalysts reduce O 2 . KEYWORDS: O 2 reduction, electrocatalysis, metal−organic framework, porous catalysts, 2D materials ■ INTRODUCTIONUnderstanding catalytic kinetics and thermodynamics to construct a reasonable reaction mechanism is central for both elucidating the behavior of a given catalyst and gaining predictive power over structure−function relationships. This predictive power aids in efficiently optimizing catalyst performance by systematically tuning the structural and electronic properties of the catalyst. One class of materials that could benefit from mechanism-guided optimization is nonplatinum group metal (non-PGM) electrocatalysts for the O 2 reduction reaction (ORR) to water (4e − reduction) and/or hydrogen peroxide (2e − reduction). Such catalysts typically include abundant transition metals and/or heteroatoms such as N, O, and S doped into a carbonaceous matrix.1−6 Although quite active and stable during ORR, previously reported non-PGM catalysts often consist of amorphous carbon mechanically blended with transition metal macrocycles or other metal and main group heteroatomic sources. These relatively poorly defined materials do not lend themselves to facile mechanistic studies; the inhomogeneous dispersion and irregular orientation of the dopants throughout the carbon matrix engenders structural ambiguity that makes identification, experimental probing, and computational modeling of active sites difficult.Conversely, highly ordered metal−organic frameworks (MOFs) containing well-defined, spatially isolated active sites present an attractive platform for experimental and computational correlation between the chemical and electronic structure of a given catalyst and the electrocatalytic activity and mechanism, a feat that is traditionally restricted to homogeneous molecular systems. We previously showed that the electrically conductive MOF Ni 3 (HITP) 2 (HITP = 2,3,6,7,10, (Figure 1) functions as

[Cu3(C6Se6)]n: The First Highly Conductive 2D π–d Conjugated Coordination Polymer Based on Benzenehexaselenolate

“…[1,2] Recently, there are growing interests in design and synthesis of 2D covalent or noncovalent organic materials with in-plane π-conjugation structure. [17][18][19][20][21][22][23][24][25][26][27][28][29] The planar structure of these graphene analogues facilitates the synthesis of benzene-and triphenylene-polyselenols and CPs containing these ligands (relevant results are in preparation). [17][18][19][20][21][22][23][24][25][26][27][28][29] The planar structure of these graphene analogues facilitates the synthesis of benzene-and triphenylene-polyselenols and CPs containing these ligands (relevant results are in preparation).…”

“…The synthesis of protected BHS hexakis(tertbutylseleno)benzene (t-Bu 6 BHS) was reported by Turner and Vaid in 2012. [17][18][19][20][21][22][23][24][25][26][27][28][29] The planar structure of these graphene analogues facilitates the [33] An alternative approach is Lewis acid promoted deprotection protocol.…”

Metal–Organic Frameworks Based Electrocatalysts for the Oxygen Reduction Reaction

“…Copyright 2016, Nature Publishing Group. F) CV curves under O 2 atmosphere at pH 13, pH 8, and pH 4, and G) reaction mechanism of the 2‐electron transfer pathway for Ni 3 (hitp) 2 towards ORR . Copyright 2017, American Chemical Society.…”

“…As shown in Figure E, Ni 3 (hitp) 2 has a honeycomb‐like layered structure and exhibits good electrocatalytic activity in an extensive range of pH value (4–13) (Figure F). Intensive research indicated that Ni 3 (hitp) 2 catalyzed oxygen reduction through a two‐electron pathway by the ligands as active sites rather than metal centers (Figure G) . This mechanism shows the importance of electron delocalization for accessing key intermediates and should be applicable for designing other conductive MOFs.…”

Unpaired 3d Electrons on Atomically Dispersed Cobalt Centres in Coordination Polymers Regulate both Oxygen Reduction Reaction (ORR) Activity and Selectivity for Use in Zinc–Air Batteries

“…Dincă’s group have synthesized a series of two‐dimensional conductive MOFs including M 3 (HIB) 2 and M 3 (HITP) 2 (M=Cu, Ni; HIB=hexaiminobenzene; HITP=2,3,6,7,10,11‐hexaiminotriphenylene) and unveiled a reasonable activity of the Ni 3 (HITP) 2 in catalysing ORR . They have further indicated the β‐C on the HITP ligand, rather than the metal centre, and the delocalized valence band in the 2D framework synergistically contribute to the ORR activity . Yoon et al.…”