Electrocatalysts For H2 Production Research Articles

Water-soluble and air-stable [2Fe-2S]-metallopolymers: A new class of electrocatalysts for H2 production via water splitting

[2Fe–2S] organometallic complexes are effective electrocatalysts for the reduction of weak acids in acetonitrile to generate molecular hydrogen (H2) (hydrogen evolution reaction, HER). Incorporation of such a [2Fe–2S] catalytic site into metallopolymers has been achieved using atom transfer radical polymerization (ATRP). These well-defined polymers have been structurally characterized by SEC, 1H NMR and IR spectroscopy. These metallopolymers catalyze the electrochemical reduction of acetic acid in acetonitrile. In addition, [2Fe–2S] metallopolymers appended with amino groups were shown to be water-soluble and potent electrocatalysts for H2 production at pH 7. In addition to the exceptional activity of these [2Fe–2S] metallopolymers in water, these catalysts maintain catalytic activity in air unlike comparable molecular catalysts.

Electrochemical Detection of Transient Cobalt Hydride Intermediates of Electrocatalytic Hydrogen Production.

A large variety of molecular cobalt complexes are used as electrocatalysts for H2 production, but the key cobalt hydride intermediates are frequently difficult to detect and characterize due to their high reactivity. We report that a combination of variable scan rate cyclic voltammetry and foot-of-the-wave analysis (FOWA) can be used to detect transient Co(III)H and Co(II)H intermediates of electrocatalytic H2 production by [Co(II)(P(tBu)2N(Ph)2)(CH3CN)3](2+) and Co(II)(dmgBF2)2(CH3CN)2. In both cases, reduction of a transient catalytic intermediate occurs at a potential that coincides with the Co(II/I) couple. Each reduction displays quasireversible electron-transfer kinetics, consistent with reduction of a Co(III)H intermediate to Co(II)H, which is then protonated by acid to generate H2. A bridge-protonated Co(I) species was ruled out as a catalytic intermediate for Co(II)(dmgBF2)2(CH3CN)2 from voltammograms recorded at 1000 psi of H2. Density functional theory was used to calculate Co(III)-H and Co(II)-H bond strengths for both catalysts. Despite having very different ligands, the cobalt hydrides of both catalysts possess nearly identical heterolytic and homolytic Co-H bond strengths for the Co(III)H and Co(II)H intermediates.

Diiron Complexes Bearing Bridging Hydrocarbyl Ligands as Electrocatalysts for Proton Reduction

Diiron complexes with bridging hydrocarbyl ligands and containing CO and Cp ligands (Cp = η5-C5H5) have been investigated as possible electrocatalysts for H2 production. In particular, studies included the vinyliminium complexes [Fe2{μ-η1:η3-C(R′)=CRC═NMe2}(μ-CO)(CO)(Cp)2][SO3CF3] (R′ = Tol (4-MeC6H4), R = H, 1a; R′ = CH2OH, R = H, 1b; R′ = CH2OH, R = SPh, 1c), the vinylalkylidene [Fe2{μ-η1:η3-C(Tol)CH═CHNMe2}(μ-CO)(CO)(Cp)2] (2), the aminoalkylidyne [Fe2{μ-CN(Me)(R)}(μ-CO)(CO)(L)(Cp)2][SO3CF3] (R = Me, L = CO, 3a; R = Me, L = NCMe, 3b; R = Xyl (2,6-Me2C6H3), L = CO, 3c), [Fe2{μ-CN(Me)2}(μ-L′)(CO)(L)(Cp)2] (L′ = CO, L = CN, 4; L′ = H, L = CO, 5), the thiocarbyne complexes [Fe2(μ-CSEt)(μ-CO)(CO)2(Cp)2][BF4], (6) and [Fe2(μ-CSMe)(μ-CO)(CO)(CN)(Cp)2] (7), and the alkylidene complexes [Fe2{μ-C(CN)(SMe)}(μ-CO)(CO)2(Cp)2][SO3CF3] (8) and [Fe2{μ-C(CN)(PMe2Ph)}(μ-CO)(CO)2(Cp)2][SO3CF3] (9). Cyclic voltammograms (CV) of the above complexes in CH3CN have been recorded in the presence of increasing amounts of acetic acid to evidence electrocatalytic proton reduction. In spite of the fact that the above diiron complexes do not resemble the typical diiron dithiolate model systems, the aminocarbyne 4 and the thiocarbyne complex 7 exhibit significant electrocatalytic properties for proton reduction (e.g., for 4, the turnover number (TON) is 15.5).

Electrocatalytic Hydrogen Production by [Ni(7PPh2NH)2]2+: Removing the Distinction Between Endo- and Exo-Protonation Sites

A new Ni(II) complex, [Ni(7PPh2NH)2H]3+ (7PPh2NH = 3,6-diphenyl-1-aza-3,6-diphosphacycloheptane), has been synthesized, and its electrochemical properties have been reported. The 7PPh2NH ligand features an NH, ensuring properly positioned protonated amine groups (N–H+) for electrocatalysis, regardless of whether protonation occurs exo or endo to the metal center. The compound is an electrocatalyst for H2 production in the presence of organic acids (pKa range 10–13 in CH3CN), with turnover frequencies ranging from 160 to 780 s–1 at overpotentials between 320 and 470 mV, as measured at the potential of the catalytic wave. In stark contrast to [Ni(PPh2NR′2)2]2+ (PPh2NR′2 = 3,7-diphenyl-1,5-diaza-3,7-diphosphacyclooctane) and other [Ni(7PPh2NR′)2]2+ complexes, catalytic turnover frequencies for H2 production by [Ni(7PPh2NH)2]2+ do not show catalytic rate enhancement upon the addition of H2O. This finding supports the assertion that [Ni(7PPh2NH)2]2+ eliminates the distinction between the endo- and exo-protonation isomers.

Water-assisted proton delivery and removal in bio-inspired hydrogen production catalysts.

Electrocatalysts for H2 production are envisioned to play an important role in renewable energy utilization systems. Nickel-based catalysts featuring pendant amines functioning as proton relays in the second coordination sphere of the metal center have led to catalysts achieving turnover frequencies as high as 10(7) s(-1) for H2 production. The fastest rates are observed when water is present in solution, with rates up to 10(3) times faster than those found in dry solvent. The focus of this paper is to provide mechanistic insight into the unexpected enhancement due to water. Addition of H2 to [Ni(P(Cy)2N(R')2)2](2+) was previously shown to give three isomers of a Ni(0) product with two protonated amines, where the N-H can be endo or exo to the Ni. By investigating the deprotonation of these two N-protonated Ni(0) intermediates resulting from the addition of H2 to [Ni(P(Cy)2N(R')2)2](2+), we observe by NMR spectroscopy studies an enhancement in the rate of deprotonation for protons positioned on the pendant amine next to the metal (endo) vs. protons that are positioned away from the metal (exo). Computational studies suggest that for smaller bases, the desolvation energy of the exogenous base is the primary contribution limiting the rate of endo deprotonation, while steric accessibility and facile proton movement also contribute. For more bulky bases, steric accessibility can play the dominant role. The significant reduction in these barriers observed in the presence of water has important implications for disfavoring less productive catalytic pathways and increasing catalytic rates.

Comparison of [Ni(PPh2NPh2)2(CH3CN)]2+ and [Pd(PPh2NPh2)2]2+ as Electrocatalysts for H2 Production

The complexes [Ni(PPh2NPh2)2(CH3CN)]2+ and [Pd(PPh2NPh2)2]2+, where PPh2NPh2 is 1,5-diphenyl-3,7-diphenyl-1,5-diaza-3,7-diphosphacyclooctane, are compared as electrocatalysts for H2 production under identical experimental conditions. With [(DMF)H]+ as the acid in acetonitrile solution, [Pd(PPh2NPh2)2]2+ afforded a turnover frequency (TOF) of 230 s–1 for formation of H2 under dry conditions and a TOF of 640 s–1 when H2O was added. These rates are similar to the TOFs of 590 s–1 (dry) and 720 s–1 (wet) that were previously measured for [Ni(PPh2NPh2)2(CH3CN)]2+ using [(DMF)H]+. The [Ni(PPh2NPh2)2(CH3CN)]2+ and [Pd(PPh2NPh2)2]2+ complexes both exhibited large current enhancements when treated with trifluoroacetic acid (TFA). At a TFA concentration of 1.8 M, TOF values of 5670 and 2060 s–1 were measured for [Ni(PPh2NPh2)2(CH3CN)]2+ and [Pd(PPh2NPh2)2]2+, respectively. The fast rates observed using TFA are, in part, attributed to homoconjugation of TFA in acetonitrile solutions, which decreases the effective pKaMeCN of the acid. In support of this hypothesis, dramatically lower rates of H2 production were observed using p-anisidinium, which has a pKaMeCN value comparable to that of TFA but does not homoconjugate significantly in acetonitrile solutions.

A new reactivity pattern of heterodinuclear complexes [MnRe(CO)6(μ-S2CPR3)] with nBuLi/protonation and its electrochemistry properties investigation as structure and function models for the Fe-only hydrogenase active site

A novel reaction pattern of heterodinuclear complexes was reported. Namely, nucleophilic attack of nBuLi/protonation of complexes [MnRe(CO)6(μ-S2CPR3)] occurs with the cleavage of the Mn–Re, (P)C–S bond and formation of new (Bu)C–S bond, to afford the novel complexes [MnRe(CO)6(μ-SnBu)(μ-S-C(H)PR3)] (L) (L = PR3, C5H5N) 3 and 4 which are different from previous complexes (1 and 2 or other complexes) obtained by the reduction with Na[Hg] or superhydride reagent. Additionally, in order to develop these complexes application, several similar structure Mn–Re binuclear complexes, MnRe(CO)6(μ-H)[μ-S(SnBu)CC(PiPr3)S] (1), MnRe(CO)6(μ-H)[μ-S(SCH3)CC(PiPr3)S] (2), MnRe(CO)6(μ-SnBu)[μ-SC(H)PCy3PPh3] (3), MnRe(CO)6(μ-SnBu)[μ-SC(H)PCy3Py] (4), which serve as structural models for the active site of Fe-hydrogenase, were chosen to investigate their possible electrochemical properties. Consequently, they are indeed shown to be electrocatalysts for H2 production in the presence of acetic acid in acetonitrile. The electrocatalysis is initiated by electrochemical reduction of these Mn–Re complexes, which subsequently, under weak acid conditions, undergo protonation of the reduced rhenium center to produce H2. And two probable electrochemical mechanisms (EECC and ECCE) are also proposed.

In situ determination of the ‘real are factor’ in H2 evolution electrocatalysis at porous NiFe composite electrodes

Abstract Composite cathode materials, such as electrodeposited NiFe, NiMo or NiMoCd, are of considerable practical interest as possible electrocatalysts for H2 production in water electrolysers and for cathodes in the chlor-alkali process. The present work extends the examination of the real electrode surface area factor in electrocatalysis at materials havinhg porous or irregular surfaces in relation to evaluation of the role of true electrocatalytic behaviour of a 65 at.% Fe + 35 at.% Ni electrodeposited material. The results also enable the behaviour of the adsorbed H intermediate in the cathodic H2 evolution reaction to be examined. Two distinct regions of steady state behaviour, exhibiting different Tafel slopes over the temperature range 298 to 348 K, are observed. The real to apparent electrode surface area ratio was evaluated from the double-layer capacitance values that were determined by means of a recently published method based on a curve-fitting extrapolation procedure applied to the initial, short-time, regions of potential-relaxation transients recorded after current interruptions. The double-layer capacitance values are derived under conditions of sufficient electrolyte conductivity that the effects of distributed ohmic resistance in the pores of the electrode matrix are minimized. The adsorption behaviour of the H species was also studied by means of a.c. impedance and potential-relaxation measurements.