Non‐Covalent Integration of a [FeFe]‐Hydrogenase Mimic to Multiwalled Carbon Nanotubes for Electrocatalytic Hydrogen Evolution

Abstract

Surface integration of molecular catalysts inspired from the active sites of hydrogenase enzymes represents a promising route towards developing noble metal-free and sustainable technologies for H 2 production. Efficient and stable catalyst anchoring is a key aspect to enable this approach. Herein, we report the preparation and electrochemical characterization of an original diironhexacarbonyl complex including two pyrene groups per catalytic unit in order to allow for its smooth integration, through π-interactions, onto multiwalled carbon nanotube-based electrodes. In this configuration, the grafted catalyst could reach turnover numbers for H 2 production (TON H2 ) of up to 4 � 2 × 10 3 within 20 h of bulk electrolysis, operating at neutral pH. Post operando analysis of catalyst functionalized electrodes revealed the degradation of the catalytic unit occurred via loss of the iron carbonyl units, while the anchoring groups and most part of the ligand remained attached onto multiwalled carbon nanotubes.

Electrode Integration of Synthetic Hydrogenase as Bioinspired and Noble Metal-Free Cathodes for Hydrogen Evolution

“…XPS spectra recorded after one CV cycle of 2c/MWNT confirmed the presence of 2c on electrode by revealing peaks at 710 eV and 723.5 eV (Fe 2p), 164.2 eV (S 2p), 532.5 eV (O 1s), 400.2 eV (N 1s), and 285 eV (C 1s in C�CH), 286.2 eV (C 1s in C−O) consistent with previous reports for complex 3 (Figures 4B, C and S28). 58 The small peak around 730 eV in the Fe 2p core region was also observed, attributable to impurities present in the nanotubes. 63 Moreover, two additional peaks centered at 168.2 eV (blue trace) and 162.6 eV (green trace) were observed in the S 2p region which likely correspond to various sulfonate, sulfinate (RSO n − , n = 2 or 3) and thiolate (RS − ) moieties, respectively (Figure 4C), revealing some possible degradation of the catalytic core during the first CV cycle as previously reported.…”

“…A redox event peak was observed at E 1/2 = −1.28 V versus Fc +1/0 and assigned to the Fe I Fe I /Fe 0 Fe 0 (2e − reduction) redox event by analogy to parent diiron complex 3. 58 A linear correlation between the peak current and the scan rate could be observed, characteristic of a surface bound redox species (Figures S16−S20). Integration of the aforementioned anodic waves lead to surface concentrations between 1.1 ± 0.1 and 1.6 ± 0.1 nmol cm −2 of active sites for polymers 2a−e (Table 2).…”

“…A similar behavior was previously observed for reference diiron complex 3 immobilized on MWNT. 58 Thus, possible pathways leading to the observed loss of catalysis include (a) the leaching of the diiron center from the metallopolymer following hydrolysis of the ester linkage connecting the diiron center to the polymer matrix, (b) the leaching of a large part of the metallopolymer leaving the pyrene units at the MWNT film following hydrolysis of ester groups, or (c) the degradation of diiron core or metallopolymer backbone during catalysis.…”

“…immobilized on multiwall CNTs (MWNT) affording sustained electrochemical hydrogen production activity for more than 20 h at neutral pH. 58 Herein, we report on the design and characterization of a series of "synthetic hydrogenases", that is, pyrene-modified metallopolymers 2b−e (Figure 1), based on the recently proposed 2a. 42 Their ability to perform sustained HER catalysis as well as their decomposition pathways was studied using a combination of electrochemical and spectroscopic techniques.…”

“…The synthesis and characterization of the reference complex [μ-2,3-(naphthalene-1,4-diylbis(4-(pyren-1-yl)butanoate)-dithiolato]bistricarbonyliron (complex 3) was performed following previously reported work. 58 1 H NMR spectra of 2a showcased presence of active sites by showing peaks at the aromatic region between 7.64 and 7.49 ppm. In addition, multiple peaks at non-aromatic region (0−7 ppm) were observed, that is, peaks at 4.06 ppm and 2.54 and 2.19 ppm attributed to the presence of DMAEMA, which characterize the composition of the metallopolymer backbone (Figure S3).…”