Electrocatalytic Proton Reduction Research Articles

Mechanism-Guided Kinetic Analysis of Electrocatalytic Proton Reduction Mediated by a Cobalt Catalyst Bearing a Pendant Basic Site.

Cobalt polypyridyl complexes stand out as efficient catalysts for electrochemical proton reduction, but investigations into their operating mechanisms, with broad-reaching implications in catalyst design, have been limited. Herein, we investigate the catalytic activity of a cobalt(II) polypyridyl complex bearing a pendant pyridyl base with a series of organic acids spanning 20 pKa units in acetonitrile. Structural analysis, as well as electrochemical studies, reveals that the Co(III) hydride intermediate is formed through reduction of the Co(II) catalyst followed by direct metal protonation in the initial EC step despite the presence of the pendant base, which is commonly thought of as a more kinetically accessible protonation site. Protonation of the pendant base occurs after the Co(III) hydride intermediate is further reduced in the overall ECEC pathway. Additionally, when the acid used is sufficiently strong, the Co(II) catalyst can be protonated, and the Co(III) hydride can react directly with acid to release H2. With thorough mechanistic understanding, the appropriate electroanalytical methods were identified to extract rate constants for the elementary steps over a range of conditions. Thermodynamic square schemes relating catalytic intermediates proposed in the three electrocatalytic HER mechanisms were constructed. These findings reveal a full description of the HER electrocatalysis mediated by this molecular system and provide insights into strategies to improve synthetic fuel-forming catalysts operative through metal hydride intermediates.

The rigidity and chelation effect of ligands on the hydrogen evolution reaction catalyzed by Ni(II) complexes.

With increasing interest in nickel-based electrocatalysts, three heteroleptic Ni(II) dithiolate complexes with the general formula [Ni(II)L(L')2] (1-3), L = 2-(methylene-1,1'-dithiolato)-5,5'-dimethylcyclohexane-1,3-dione and L' = triphenylphosphine (1), 1,1'-bis(diphenylphosphino)ferrocene (DPPF) (2), and 1,2-bis(diphenylphosphino)ethane (DPPE) (3), have been synthesized and characterized by various spectroscopic techniques (UV-vis, IR, 1H, and 31P{1H} NMR) as well as the electrochemical method. The molecular structure of complex 2 has also been determined by single-crystal X-ray crystallography. The crystal structure of complex 2 reveals a distorted square planar geometry around the nickel metal ion with a NiP2S2 core. The cyclic voltammograms reveal a small difference in the redox properties of complexes (ΔE° = 130 mV) while the difference in the catalytic half-wave potential becomes substantial (ΔEcat/2 = 670 mV) in the presence of 15 mM CF3COOH. The common S^S-dithiolate ligand provides stability, while the rigidity effect of other ligands (DPPE (3) > DPPF (2) > PPh3 (1)) regulates the formation of the transition state, resulting in the NiIII-H intermediate in the order of 1 > 2 > 3. The foot-of-the-wave analysis supports the widely accepted ECEC mechanism for Ni-based complexes with the first protonation step as a rate-determining step. The electrocatalytic proton reduction activity follows in the order of complex 1 > 2 > 3. The comparatively lower overpotential and higher turnover frequency of complex 1 are attributed to the flexibility of the PPh3 ligand, which favours the easy formation of a transition state.

Exploring the effect of a pendent amine group poised over the secondary coordination sphere of a cobalt complex on the electrocatalytic hydrogen evolution reaction.

A CoIII complex (2) of a bispyridine-dioxime ligand (H2LNMe2) containing a tertiary amine group in the proximity of the Co center is synthesized and characterized. One of the oxime protons of the ligand is deprotonated, and the amine group remains protonated in the solid-state structure of the CoII complex (2a). The acid-base properties of 2 showed pKa values of 5.9, 8.4, and 9.6, which are assigned to the dissociation of two consecutive oxime protons and amine protons, respectively. The electrocatalytic proton reduction of 2 was investigated in an aqueous phosphate buffer solution (PBS), revealing a catalytic hydrogen evolution reaction (HER) at an Ecat/2 of -1.01 V vs. the SHE, with an overpotential of 673 mV and a kobs value of 2.6 × 103 s-1 at pH 7. For comparison, the HER of the Co complex (1) lacking the tert-amine group at the secondary sphere was investigated in PBS, which showed a kobs of 1.3 × 103 s-1 and an overpotential of 577 mV. At pH 4, however, 2 revealed a ∼3 times higher kobs value than 1, which suggests that the protonated amine group likely works as a proton relay site. Notably, no significant change in the reaction rate was observed at different pH values for 1, implying that oxime protons may not be involved in the intramolecular proton-coupled electron transfer reaction in the HER. The kobs values for Co complexes at pH 7.0 are significantly higher than those of the [Co(dmgH)2(pyridine)(Cl)] complex, implying that the primary coordination sphere around 1 or 2 enhances the HER and offers better catalyst stability in acidic buffer solutions.

Aminophosphine-substituted Fe/E (E = S, Se) carbonyls related to [FeFe]-hydrogenases: Synthesis, protonation, and electrocatalytic proton reduction

Aminophosphine-substituted Fe/E (E = S, Se) carbonyls related to [FeFe]-hydrogenases: Synthesis, protonation, and electrocatalytic proton reduction

FeFe] Hydrogenase: 2‐Propanethiolato‐Bridged {FeFe} Systems as Electrocatalysts for Hydrogen Production in Acetonitrile‐Water

AbstractDiiron bis(monothiolato)‐bridged hydrogenase mimics [Fe2(μ‐SiPr)2(CO)6] A (iPr=isopropyl) and their phosphine derivatives [Fe2(μ‐SiPr)2(CO)5(L1/2)] and [Fe2(μ‐SiPr)2(CO)4(L3)] {tricyclohexylphosphine (L1, PCy3) 1, triphenylphosphine (L2, PPh3) 2 and cis‐1,2‐bis(diphenylphosphino)ethylene (L3, dppv) 3} have been synthesized and investigated for electrocatalytic proton reduction activity in CH3CN−H2O. Based on the FTIR data (red‐shift), the electron‐donating ability of the phosphines followed the order: dppv>PCy3>PPh3 (average shift value with respect to complex A was 61 (1), 51 (2) and 80 (3) cm−1). From CV measurements, it was seen that the catalytic reduction potential (acetic acid as proton source) shifted considerably towards positive potentials (anodic shift) on changing the solvent from pure CH3CN to CH3CN−H2O (4 : 1 v/v). An increase in the percentage of water in CH3CN led to the decomposition of complexes 1 and 3. Though complexes A and 2 were stable in CH3CN−H2O (3 : 2 v/v), no improvement in electrocatalytic currents was observed.

Bis(benzimidazole)amino thio- and selenoether Iron(II) complexes as proton reduction electrocatalysts

Two novel Iron (II) complexes featuring tetrapodal bis(benzimidazole)amino thio- and selenoether ligands (LS and LSe) were synthesized, characterized, and tested as electrocatalysts for the hydrogen evolution reaction. The bromide complexes [Fe(LS,LSe)Br2] (1–2) are highly insoluble, but their DMSO solvates were characterized by single crystal X-ray diffraction, revealing an octahedral coordination environment that does not feature coordination of the chalcogen atoms. The corresponding triflate derivatives [Fe(LS,LSe)(MeCN)3]OTf2 (1c-2c) were employed for electrocatalytic proton reduction, with 1c exhibiting higher activity, thus suggesting that the thioether may participate as a more competent pendant ligand for proton transfer.

Synthesis of cobalt(II) phenolate selenoether complexes to mimic hydrogenase-like activity for hydrogen gas production.

Selenium-derived electrocatalysts have been well explored for electrocatalytic hydrogen evolution reactions to mimic hydrogenase-like activity; however, the stability of these synthetic mimics is yet to be enhanced. In this study, we report the synthesis and characterization of a series of 1,10-phenanthroline-cobalt(II) phenolate selenoether complexes using 1,10-phenanthroline and 6-nitro-1,10-phenanthroline-Co(II)-dichloride and substituted bis-selenophenolate ligands. The synthesized cobalt(II) phenolate selenoether complexes have been characterized by CHN analysis, mass spectrometry, single crystal XRD, and UV-visible absorption spectroscopy. These complexes show electrocatalytic proton reduction from acetic acid at an overpotential of 0.45-0.56 V vs. Fc+/Fc and surpass previously reported selenium and sulfur-containing electrocatalysts. Furthermore, gas analysis from control potential electrolysis confirms that the cobalt(II) selenoethers act as electrocatalysts to produce H2 with a faradaic efficiency of 43-83% and show a turnover number of 3.24-58.60 molcat-1. The hydrogen evolution reaction (HER) was probed using deuterated acetic acid, which demonstrates an inverse kinetic isotopic effect (KIE) and is consistent with the formation of metal hydride intermediates. Furthermore, control experiments (post-dip analysis and multiple CV studies) have been performed to support the catalysis being due to a homogeneous process. Acid titration using UV-visible spectroscopy reveals that protonation is the prior step for electrocatalysis and assists in the formation of a cobalt hydride intermediate, which upon reaction with a proton generates hydrogen gas.

FeFe]‐Hydrogenase models featuring dithiolato‐bridgehead functionality: Preparation, structures, and electrocatalytic proton reduction

AbstractTo further develop the active site models of [FeFe]‐hydrogenases, a new series of diiron model complexes [{(μ‐SCH2)2CHCH2OC(O)C6H4I)}Fe2(CO)6] (I is located at ‐para for 1, ‐meta for 2, and ‐ortho for 3) bearing dithiolato‐bridgehead functionality were successfully prepared by treatments of bridghead‐hydroxyl‐containing parent complex [{(μ‐SCH2)2CHCH2OH)}Fe2(CO)6] (A) and three different iodobenzoic acids (IC6H4CO2H) in the presence of 4‐dimethylaminopyridine (DMAP) as catalyst and dicyclohexylcarbodiimide (DCC) as dehydrating reagent. All these new complexes 1–3 have been fully characterized through elemental analysis, FT‐IR and NMR (1H, 13C) spectroscopies, and X‐ray crystallography. Further electrochemical and electrocatalytic properties of complexes 1–3 are studied and compared in the absence and presence of acetic acid (HOAc) as a proton source by cyclic voltammetry (CV), indicating that they may be considered as the active biomimetic electrocatalysts for proton reduction to hydrogen (H2).

Carboxylate pentapyridines: Pathway to surface modification and tuneable catalytic proton reduction

The ability to synthetically tune catalytic performance is a key advantage in the use of molecular catalysts. A series of cobalt pentapyridine carboxylate esters [Co(Py5Me2COOMe)(CH3CN)]2+(Co-Me), [Co(Py5Me2COOn-Pr)(CH3CN)]2+(Co-Pr) and [Co(Py5Me2COOPh)(CH3CN)]2+(Co-Ph) were successfully synthesised and characterised through HD-MS, FT-IR and cyclic voltammetry. Electrochemical studies reveal the complexes are active for electrocatalytic proton reduction in acetonitrile with acetic acid as the proton source. At an acid concentration of 2 mM, the required overpotential for proton reduction was calculated to be 465 mV for Co-Me, 485 mV for Co-Pr and 360 mV for Co-Ph. The rate constant is calculated to be 9.41 s−1, 4.09 s−1 and 7.25 s−1 for complexes Co-Me, Co-Pr and Co-Ph respectively. The cobalt carboxylate complex [Co(Py5Me2COO−)(OH−)]1+ (Co-COOH) was used to prepare chemically modified electrodes from fluorine doped tin oxide (FTO) coated glass and glassy carbon with a calculated surface coverage of 6.38 × 10−11 mol/cm2 and of 6.18 × 10−11 mol/cm2 respectively. The successful reduction of catalytic overpotential through esterification offers an alternative pathway for the design of molecular catalysts.

Bulky oxadithiolate-bridged [FeFe]‑hydrogenase mimics [Fe2(μ-R2odt)(CO)4(κ2-diphosphine)] (R = Ph and H) with chelating diphosphines

In order to develop an attractive generation of bulky oxadithiolate-bridged [FeFe]‑hydrogenase mimics with chelating diphosphines, two new series of asymmetrically diphosphine-substituted diiron model complexes [Fe2(μ-R2odt)(CO)4(κ2-diphosphine)] (3–5) with bulky Ph2odt bridge and their reference counterparts (6–8) with common odt bridge were obtained from the Me3NO-assisted substitutions of diiron hexacarbonyl precursors [Fe2(μ-R2odt)(CO)6] (R2odt = (SCHR)2O, R = Ph (1) and H (2)) with different diphosphines such as (Ph2P)2NBn (labelled PNBnP, Bn = benzyl), (Ph2PCH2)2NBn (PCNBnCP), and (Ph2PCH2)2CH2 (DPPP)), respectively. All the as-prepared complexes have been characterized by elemental analysis, IR plus NMR spectroscopies, and particularly by X-ray crystallography for 3–8. It is interesting to note that complexes 3 and 6 chelating by small bite-angle PNBnP diphosphine have the favorable dibasal isomer whereas analogues 4, 5 and 7, 8 chelating by flexible backbone PCNBnCP or DPPP ligands possess the main apical-basal isomer in solution or in the solid state. Further, the electrochemical properties of two pairs of representative complexes 3, 6 and 5, 8 are explored and compared by cyclic voltammetry (CV) in the absence and presence of trifluoroacetic acid (CF3CO2H) as proton source, indicating that the complete protonations of 3, 6 and 5, 8 with higher concentration of CF3CO2H lead to two new catalytic waves for the electrocatalytic proton reduction to hydrogen (H2).

A PEGylated Tin Porphyrin Complex for Electrocatalytic Proton Reduction: Mechanistic Insights into Main-Group-Element Catalysis.

Electrocatalytic proton reduction to form dihydrogen (H2 ) is an effective way to store energy in the form of chemical bonds. In this study, we validate the applicability of a main-group-element-based tin porphyrin complex as an effective molecular electrocatalyst for proton reduction. A PEGylated Sn porphyrin complex (SnPEGP) displayed high activity (-4.6 mA cm-2 at -1.7 V vs. Fc/Fc+ ) and high selectivity (H2 Faradaic efficiency of 94 % at -1.7 V vs. Fc/Fc+ ) in acetonitrile (MeCN) with trifluoroacetic acid (TFA) as the proton source. The maximum turnover frequency (TOFmax ) for H2 production was obtained as 1099 s-1 . Spectroelectrochemical analysis, in conjunction with quantum chemical calculations, suggest that proton reduction occurs via an electron-chemical-electron-chemical (ECEC) pathway. This study reveals that the tin porphyrin catalyst serves as a novel platform for investigating molecular electrocatalytic reactions and provides new mechanistic insights into proton reduction.

A PEGylated Tin Porphyrin Complex for Electrocatalytic Proton Reduction: Mechanistic Insights into Main‐Group‐Element Catalysis

AbstractElectrocatalytic proton reduction to form dihydrogen (H2) is an effective way to store energy in the form of chemical bonds. In this study, we validate the applicability of a main‐group‐element‐based tin porphyrin complex as an effective molecular electrocatalyst for proton reduction. A PEGylated Sn porphyrin complex (SnPEGP) displayed high activity (−4.6 mA cm−2 at −1.7 V vs. Fc/Fc+) and high selectivity (H2 Faradaic efficiency of 94 % at −1.7 V vs. Fc/Fc+) in acetonitrile (MeCN) with trifluoroacetic acid (TFA) as the proton source. The maximum turnover frequency (TOFmax) for H2 production was obtained as 1099 s−1. Spectroelectrochemical analysis, in conjunction with quantum chemical calculations, suggest that proton reduction occurs via an electron‐chemical‐electron‐chemical (ECEC) pathway. This study reveals that the tin porphyrin catalyst serves as a novel platform for investigating molecular electrocatalytic reactions and provides new mechanistic insights into proton reduction.

Substituent effects of tertiary phosphines on the structures and electrochemical performances of azadithiolato‐bridged diiron model complexes of [FeFe]‐hydrogenases

In this work, a new series of azadithiolato‐bridged diiron complexes [Fe2(μ‐adtNOH)(CO)5{P(C6H4R)3}] (adtNOH = (SCH2)2N(C6H4CH2CH2OH‐p) and R = Cl‐p, 1; Me‐m, 2; OMe‐p, 3 supported by tertiary phosphines, which may be considered as the active site models of [FeFe]‐hydrogenases, has been prepared in 73–81% yields by the Me3NO‐assisted decarbonylating reactions of all‐CO precursor [Fe2(μ‐adtNOH)(CO)6] (A) with several tertiary phosphines [(P(C6H4R)3] in MeCN at room temperature. All the new complexes 1–3 are fully characterized by elemental analysis, spectroscopic techniques (Fourier transform‐infrared spectroscopy [FT‐IR] and nuclear magnetic resonance [NMR]), and especially for 1 and 3 by X‐ray crystallography. The IR and 31P{1H} NMR spectroscopic analyses have shown that the electron density of diiron center in 1–3 may be adjusted by the introduction of the different substituents (R) of the P(C6H4R)3 ligands into the [2Fe2S] cluster. The X‐ray crystallographic investigation has displayed that the typical Fe‐Fe, Fe‐P, and Fe‐C distances of 1–3 are significantly influenced by the electronic effects of the P(C6H4R)3 phosphines coordinated apically to one Fe atom, whereas their Papical‐Fe‐Fe and Capical‐Fe‐Fe angles are mainly controlled by the steric interaction between the phosphine ligands and the dithiolate bridges. Further, the electrochemical performances of 1–3 are studied and compared in the absence or presence of acetic acid (HOAc) as proton source by using cyclic voltammetry, suggesting that they are active for the electrocatalytic proton reduction to hydrogen (H2).

Elucidating Two Distinct Pathways for Electrocatalytic Hydrogen Production Using CoII Pincer Complexes.

Hydrogen gas is a sustainable energy source with water as the sole combustion product. As a result, efforts to catalyze H2 production are pertinent and widespread. The electrocatalytic H2 generating capabilities of two CoII complexes, [Co(κ3 -2,6-{Ph2 PNR}2 (NC5 H3 ))Br2 ] with R=H (I) or R=Me (II), were presented for a variety of proton sources including trifluoroacetic acid (TFA), acetic acid (AA), and trifluoroethanol (TFE). Cyclic voltammetry and controlled potential coulometry demonstrated that electrocatalysis from I and II occurred at two different potentials and are associated with different reduction processes. Density functional theory analysis provided insight into the identities of the catalyst and supported two distinct reaction pathways for electrocatalytic proton reduction. Specifically, stronger acids (e. g., AA, TFA) proceeded at -1.31 to -1.45 V through a MI /MIII pathway while sources with higher pKa values (e. g., TFE, H2 O) generated hydrogen at -2.4 V via M0 /MII ligand-assisted metal-centered reduction.

Molecular and Electronic Structures of a Series of Dinuclear CoII Complexes Varied by Exogeneous Ligands: Influence of π‐Bonding on Redox Potentials

AbstractFour dinuclear CoII complexes have been synthesized and structurally characterized with the dinucleating ligand susan (susan=4,7‐dimethyl‐1,1,10,10‐tetra(2‐pyridylmethyl)‐1,4,7,10‐tetraazadecane) varying in the exogeneous ligands: [(susan){CoII(CH3CN)2}2](PF6)4, [(susan){CoIICl}2](ClO4)2, [(susan){CoIIBr}2](ClO4)2, and [(susan){CoII(μ‐OH)CoII}](ClO4)3. The CoII ions are six‐coordinate with CH3CN ligands and five‐coordinate with anionic ligands. The electronic absorption spectra reflect the differences in the electronic structures, not only between the six‐ and five‐coordinate complexes, but also between the five‐coordinate complexes. These variations are also reflected in the magnetic properties with the highest orbital angular momentum contribution in six‐coordinate [(susan){CoII(CH3CN)2}2](PF6)4. The lower symmetry in the five‐coordinate complexes reduces the orbital angular momentum contribution. The hydroxo‐bridged complex [(susan){CoII(μ‐OH)CoII}](ClO4)3 exhibits an additional antiferromagnetic interaction. The electrochemical characterization reveals that the π‐acceptor ligand CH3CN facilitates not only reduction from CoII to CoI but also oxidation to CoIII, while the π‐donor ligands Br−, Cl−, and μ‐OH− impede both reduction to CoI and oxidation to CoIII. [(susan){CoII(CH3CN)2}2](PF6)4 and [(susan){CoIICl}2](ClO4)2 show electrocatalytic proton reduction at a potential of ≈−1.9 V vs Fc+/Fc that is associated with a ligand‐centered reduction.

Characterization of electrocatalytic proton reduction and surface adsorption of platinum nanoparticles supported by a polymeric stabilizer on an ITO electrode

Pt nanoparticles stabilized by a polymeric stabilizer of polyacrylic acid were stably adsorbed on an ITO electrode to work effectively and stably for electrocatalytic proton reduction.

Electrocatalytic proton reduction by dinuclear cobalt complexes in a nonaqueous electrolyte

Two dinuclear CoII complexes 1 and 2 have been synthesized and characterized using various spectroscopic methods. Both the complexes were employed for H+ reduction in organic media. Faradaic efficiency of 82–90% was obtained for the H2 evolution.

Electrocatalytic reduction of protons to dihydrogen by the cobalt tetraazamacrocyclic complex [Co(N4H)Cl2]+: mechanism and benchmarking of performances.

The cobalt tetraazamacrocyclic [Co(N4H)Cl2]+ complex is becoming a popular and versatile catalyst for the electrocatalytic evolution of hydrogen, because of its stability and superior activity in aqueous conditions. We present here a benchmarking of its performances based on the thorough analysis of cyclic voltammograms recorded under various catalytic regimes in non-aqueous conditions allowing control of the proton concentration. This allowed a detailed mechanism to be proposed with quantitative determination of the rate-constants for the various protonation steps, as well as identification of the amine function of the tetraazamacrocyclic ligand to act as a proton relay during H2 evolution.

Synthesis, characterization, structural, redox and electrocatalytic proton reduction properties of cobalt polypyridyl complexes

A monoanionic amido pentadentate ligand bpaqH (2-(bis(pyridin-2-ylmethyl)amino)-N-(quinolin-8-yl)acetamide) and its corresponding cobalt(III) chloro complex [Co(bpaq)Cl]Cl: 1 and aqua derivative [Co(bpaq)(OH2)](ClO4)2: 2 were successfully synthesized and fully characterized by different analytical and spectroscopic techniques such as FT-IR, 1H NMR, UV–vis spectroscopy, ESI mass spectra. The structures of 1 and 2 have been determined by the single-crystal X-ray diffraction. Spectral and redox properties were investigated along with free ligand under electrochemical conditions. Both complexes performed proton reduction activity under soluble, diffusion-limited conditions in acetonitrile with acetic acid as an external proton source with overpotentials of 0.412 V for 1 and 0.394 V for 2. The stability of the catalysts was inspected by the time-dependent UV–vis spectroscopy; 1 and 2 were found to be highly stable in the absence and presence of acetic acid. There was no significant spectral change before and after the controlled potential electrolysis suggesting no change in molecular integrity during electrocatalysis.

Electroconvective Instability in Water Electrolysis: An Evaluation of Electroconvective Patterns and Their Onset Features

In electrochemical systems, an understanding of the underlying transport processes is required to aid in their better design. This includes knowledge of possible near-electrode convective mixing that can enhance measured currents. Here, for a binary acidic electrolyte in contact with a platinum electrode, we provide evidence of electroconvective instability during electrocatalytic proton reduction. The current-voltage characteristics indicate that electroconvection, visualized with a fluorescent dye, drives current densities larger than the diffusion transport limit. The onset and transition times of the instability do not follow the expected inverse-square dependence on the current density, but, above a bulk-reaction-limited current density are delayed by the water dissociation reaction. The dominant size of the electroconvective patterns is also measured and found to vary as the diffusion length scale, confirming previous predictions on the size of electroconvective vortices.