Graphite-Conjugated Pyrazines as Molecularly Tunable Heterogeneous Electrocatalysts

Abstract

Condensation of ortho--phenylenediamine deriv--atives with ortho--quinone moieties at edge planes of graphitic carbon generates graphite--conjugated pyrazines (GCPs) that are active for oxygen reduction electrocatalysis in alkaline aqueous electrolyte. Catalytic rates of oxygen reduction are positively correlated with the electrophilicity of the active site pyrazine unit, and can be tuned by over 70 fold by appending electron--withdrawing substituents to the phenylenediamine precursors. Discrete molecular analogs containing pyrazine moieties display no activity above background under identical conditions. This simple bottom up method for constructing molecularly well--defined active sites on ubiquitous graphitic solids enables the rational design of tunable heterogeneous catalysts.The interconversion of electrical and chemical energy re--quires the coupling of electron transfer with substrate bond rearrangement. This can be achieved at surface exposed active sites of heterogeneous electrocatalysts 1 or via redox mediation facilitated by a homogeneous molecular electrocatalyst. 2 Mo--lecular electrocatalysts yield readily to synthetic alteration of their redox properties permitting systematic tuning of catalyst activity and selectivity. 3 Similar control is difficult to achieve with heterogeneous electrocatalysts because they typically exhibit a distribution of active site geometries and local elec--tronic structures, 4 which are recalcitrant to molecular--level synthetic modification. However, heterogeneous electrocata--lysts typically exhibit greater durability and are more readily integrated into functional energy conversion devices such as fuel cells and electrolyzers. In principle, the attractive features of heterogeneous and molecular catalysts could be combined if robust methods are developed for constructing well--defined, tunable active sites on the surfaces of conductive solids.Typically, molecular electrocatalysts are heterogenized by introducing an inert tether between the active site and the electrode surface. 5 However, there exist a paucity 6 of surface connection chemistries that are both robust and well--defined. For example, thiol--based self--assembled monolayers provide for a high degree of surface uniformity, 7 but exhibit a limited range of electrochemical stability. 8 In contrast, harsher liga--tion methods involving electrogenerated radicals 9 forge ro--bust covalent linkages with carbon surfaces but are prone to form ill--defined polymeric multilayers and are incompatible with sensitive molecular functionality. 9 Additionally, these methods inherently impose a tunneling barrier for electron transfer, limiting the rate of electron flux to the active site.Herein, we introduce an orthogonal strategy for constructing molecularly well--defined surface active sites that exploits the native surface chemistry of graphitic carbon, obviating the need for an exogenous linker. We show that condensation of ortho--phenylenediamines with ortho--quinone moieties pre--sent on the edge planes of ...

A General Two‐Step Strategy–Based High‐Throughput Screening of Single Atom Catalysts for Nitrogen Fixation

“…For NRR, several molecular catalysts, such as Fe‐, Co‐ and Mo‐nitride complexes, have been prepared, where the MN x moieties serve as the active centers ( Figure a). However, molecular catalysts generally exhibit poorer durability and are less readily incorporated into electrolyzers in comparison with heterogeneous catalysts . Among heterogeneous catalysts, N–G‐supported SACs possess similar active centers to that of the molecular catalysts for NRR (Figure b).…”

“…However, molecular catalysts generally exhibit poorer durability and are less readily incorporated into electrolyzers in comparison with heterogeneous catalysts. [48,49] Among heterogeneous catalysts, N-G-supported SACs possess similar active centers to that of the molecular catalysts for NRR ( Figure 1b). Therefore, investigation on the catalytic activity of N-G supported SACs for NRR is of great importance.…”

Molecular-Level Insights into Oxygen Reduction Catalysis by Graphite-Conjugated Active Sites

“…This methodology is similar to our previous report and full experimental details of electrode preparation are provided in the SI. 45 To examine the electrokinetic profile of these modified surfaces for ORR, we collected potentiostatic current-potential (Tafel) data across a range of potentials (0.83 -0.65 V; unless otherwise stated, all potentials are reported versus the reversible hydrogen electrode, RHE) spanning the catalytic wave, which correspond to overpotentials ranging from 0.40 to 0.58 V. All data were collected with electrode rotation at 2000 rpm to minimize transport limitations. Data at higher overpotentials were also explicitly corrected for residual transport limitations by extrapolating Koutecky-Levich (KL) plots of the reciprocal of the current density, j −1 , versus the reciprocal square root of the rotation rate, ω −1/2 , to the yintercept, corresponding to infinite rotation rate, to determine the activation-controlled current density ( Figure S1).…”

“…We have shown that this functionalization method generates graphite-conjugated catalysts (GCCs) that display high activity for the ORR in alkaline aqueous media, and that the activity of these GCCs is sensitive to the structure of the molecular fragment incorporated. 45,48 Our initial synthetic studies of structure-activity correlations on GCCs identified carbon surfaces treated with aryl-pyridinium-substituted diamines, dubbed N + -GCCs, to be particularly active for the ORR (Scheme 1). 45 With well-defined catalytic sites and appreciable activity, N + -GCCs are an ideal platform for detailed experimental investigations of the kinetics of ORR catalysis.…”

“…45,48 Our initial synthetic studies of structure-activity correlations on GCCs identified carbon surfaces treated with aryl-pyridinium-substituted diamines, dubbed N + -GCCs, to be particularly active for the ORR (Scheme 1). 45 With well-defined catalytic sites and appreciable activity, N + -GCCs are an ideal platform for detailed experimental investigations of the kinetics of ORR catalysis.…”