Control over the architectural and electronic properties of heterogeneous catalysts poses a major obstacle in the targeted design of active and stable non-platinum group metal electrocatalysts for the oxygen reduction reaction. Here we introduce Ni3(HITP)2 (HITP=2, 3, 6, 7, 10, 11-hexaiminotriphenylene) as an intrinsically conductive metal-organic framework which functions as a well-defined, tunable oxygen reduction electrocatalyst in alkaline solution. Ni3(HITP)2 exhibits oxygen reduction activity competitive with the most active non-platinum group metal electrocatalysts and stability during extended polarization. The square planar Ni-N4 sites are structurally reminiscent of the highly active and widely studied non-platinum group metal electrocatalysts containing M-N4 units. Ni3(HITP)2 and analogues thereof combine the high crystallinity of metal-organic frameworks, the physical durability and electrical conductivity of graphitic materials, and the diverse yet well-controlled synthetic accessibility of molecular species. Such properties may enable the targeted synthesis and systematic optimization of oxygen reduction electrocatalysts as components of fuel cells and electrolysers for renewable energy applications.

Replacement of noble metals in catalysts for cathodic oxygen reduction reaction with transition metals mostly create active sites based on a composite of nitrogen-coordinated transition metal in close concert with non-nitrogen-coordinated carbon-embedded metal atom clusters. Here we report a non-platinum group metal electrocatalyst with an active site devoid of any direct nitrogen coordination to iron that outperforms the benchmark platinum-based catalyst in alkaline media and is comparable to its best contemporaries in acidic media. In situ X-ray absorption spectroscopy in conjunction with ex situ microscopy clearly shows nitrided carbon fibres with embedded iron particles that are not directly involved in the oxygen reduction pathway. Instead, the reaction occurs primarily on the carbon–nitrogen structure in the outer skin of the nitrided carbon fibres. Implications include the potential of creating greater active site density and the potential elimination of any Fenton-type process involving exposed iron ions culminating in peroxide initiated free-radical formation.

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

A Cu-azolate metal-organic framework uptakes stoichiometric loadings of Group 1 and 2 metal halides, demonstrating efficient reversible release and reincorporation of immobilized anions within the framework. Ion pairing interactions lead to anion-dependent Li + and Mg 2+ transport in Cu 4 (ttpm) 2 •0.6CuCl 2 , whose high surface area affords a high density of uniformly distributed mobile metal cations and halide binding sites. The ability to systematically tune the ionic conductivity yields a solid electrolyte with a Mg 2+ ion conductivity rivaling the best materials reported to date. This MOF is one of the first in a promising class of frameworks that introduces the opportunity to control the identity, geometry, and distribution of the cation hopping sites, offering a versatile template for application-directed design of solid electrolytes. ASSOCIATED CONTENT Supporting Information. Materials and Methods. Weight percentages of M n+ ions in the electrolytes. 1 H NMR spectra of MOF-MX n /PC. [110] reflection of Cu[(Cu 4 Cl)(ttpm) 2 ] 2 •CuCl 2. Electrolyte potential windows of MOF-LiX. SEM-EDS. I-V curve of Cu 4 (ttpm) 2 •0.6CuCl 2. Variable temperature EIS spectra of MOF-MX n. Li + transference number data of MOF-LiX. Li redox CVs with MOF-LiX. Arrhenius data of MOF-AlCl 3. The Supporting Information is available free of charge on the ACS Publications website.

Electrically conductive layered metal–organic frameworks, regardless of the metal or chelating atom identity, exhibit phase-dependent catalytic activity for O2 electroreduction.

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Phosphoric acid fuel cells are successfully used as energy conversion technologies in stationary power applications. However, decreased proton conductivity and lower oxygen permeability of phosphoric-acid-imbibed membranes require prohibitive loadings of the traditional noble-metal-based electrocatalyst, such as platinum supported on carbon. Additionally, specific adsorption of phosphate anions on the catalyst results in a surface poisoning that further reduces electrocatalytic activity. Here we report a nonplatinum group metal (non-PGM) electrocatalyst as an alternative cathode electrocatalyst for oxygen reduction in phosphoric acid fuel cells. The non-PGM was prepared in a one-pot synthesis using a metal organic framework and iron salt precursor. Phosphate anion poisoning was monitored electrochemically and spectroscopically in reference to the current state-of-the-art Pt-based catalyst at room temperature. Unlike Pt-based catalysts that are prone to phosphate poisoning, the non-PGM electrocatalyst exhibits immunity to surface poisoning by phosphate anions at room temperature. Imaging with microscopy reveals that the iron particles are isolated from the electrolyte by graphitic layers, which ultimately protect the iron from phosphate anion adsorption. The non-PGM electrocatalyst represents the highest performance to date in a high-temperature phosphoric acid membrane system, which is likely attributed to its immunity to phosphate adsorption at the harsher fuel cell environments.

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