Impactful Role of Cocatalysts on Molecular Electrocatalytic Hydrogen Production

Abstract

Hydrogen evolution from a weak acid, acetic acid, occurs at extremely high rates for iron tetraphenylporphyrin in the presence tertiary amine as a cocatalyst. Kinetic analysis of H2 evolution for a range of amines with varying coordinative and acid–base properties reveals a dramatic rate enhancement derived from enhanced proton activity of a heteroconjugated adduct between acetic acid and the amine. Additionally, nonbulky tertiary amines (quinuclidine and diazabicyclooctane) result in a further increase in activity resulting from a trans effect for a TPPFe­(II)–H hydride intermediate. These findings reveal the design principle of a cocatalyst for creating more efficient molecular systems for hydrogen evolution electrocatalysis.

Proton Relay in Iron Porphyrins for Hydrogen Evolution Reaction

“…In the presence of acids with a wide range of p K a values [e.g., tosic acid (TsOH; p K a = 8.64), p -bromoanilinium tosylate (BA-H + ; p K a = 9.43), diisopropylanilinium tosylate (DIPA-H + ; p K a = 10.62), N , N -diethylanilinium tosylate (DEA-H + ; p K a = 11.42), and collidinium tosylate (Col-H + ; p K a = 14.98)] into the acetonitrile solution of 1 mM FeTPP, an electrocatalytic HER is observed (the tosylate salts of the conjugate acids are generated in situ by adding TsOH to the corresponding amine). − The Fe III/II potential is not affected in the presence of these bases because they are all noncoordinating due to the presence of bulky groups around the potentially coordinating nitrogen atom (Figure S1). This is in contrast to recent investigations using iron porphyrins, where coordinating axial ligands were demonstrated to substantially affect the kinetics of the HER …”

“…This is in contrast to recent investigations using iron porphyrins, where coordinating axial ligands were demonstrated to substantially affect the kinetics of the HER. 64 The data suggest that the electrochemical response along with the HER current is dependent on the pK a value of the external acid source used. For the addition of a strong acid like TsOH (pK a = 8.64) in 1 mM FeTPP [0.1 M tetrabutylammonium perchlorate (TBAP) in acetonitrile], the Fe II/I redox wave becomes irreversible and HER is observed with a peak potential of −1.50 V. This HER current corresponds to the process where a Fe I species is protonated to form Fe III -H − , which is then reduced to Fe II -H − and becomes further protonated to evolve H 2 similar to previously reported steps in related systems.…”

“…As was already established, another possible route to HER after saturation of the catalytic current or in the absence of catalysis from the Fe II/I redox state is reduction of the Fe III -H – species, generating a Fe II -H – species, which is further protonated to eliminate H 2 . The latter process was investigated in detail earlier ,,,, and is not the focus of this investigation, which is directed to the HER from the Fe I state.…”

Through‐Space Electrostatic Effects of Positively Charged Substituents on the Hydrogen Evolution Reaction

“…Molecular complexes have the benefits for studying structure‐function relationships [26–29] . Recent efforts have led to the identification of a variety of molecular complexes, made of earth‐abundant transition metals, as active HER electrocatalysts [30–51] . By studying these complexes, it is learned that introducing proton relays [31,52,53] and 2 nd sphere redox‐active groups [54] can boost electrocatalytic HER.…”

Molecular Z-Scheme for H2 Production via Dual Photocatalytic Cycles

“…The regeneration of ROH from ROmay also prevent homoconjugation between ROH and its conjugate base (RO -… H-OR), an interaction that is known to impede catalytic H2 generation. (33,34) The proposed photocatalytic mechanism for the production of H2 from a solution of PC, ROH, TEOA, and PhOH is shown in Figure 3. This reaction generates H2 from the relatively unreactive TEOA and PhOH, a thermodynamically unfavorable process by 2.65 eV, but is enabled by the additive effects of two independent excitation events in a dual catalytic Z-scheme.…”