A Modular Approach to the Development of Molecular Electrocatalysts for H₂Oxidation and Production Based on Inexpensive Metals

Abstract

IntroductionThe development of inexpensive electrocatalysts for the production and oxidation of hydrogen will play a vital role in future energy storage and delivery systems. The generation of hydrogen from non -fossil energy sources such as solar, wind, geothermal, and nuclear energy is one approach being considered for storing the electrical energy generated by these sources for transportation and other uses that are not temporally matched to electrical energy production. In the reverse process, in which fuels are used to produce electricity, it is recognized that fuel cells have signifi cant thermodynamic advantages in terms of energy effi ciency compared to internal combustion engines and other Carnot processes. Both fuel generation and fuel utilization require electrocatalysts for effi cient interconversion of electrical energy and chemical energy.The best catalysts for the electrochemical oxidation and production of hydrogen are platinum metal and the hydrogenase enzymes. Both catalyze the reaction of two protons with two electrons to form H 2 , as shown in Equation 7.1 . Because of its superior catalytic rates and overpotentials compared to other metals and because of its high stability compared to hydrogenase enzymes, platinum is currently used as the catalyst for both half reactions (the oxidation of H 2 and the reduction of O 2 ) in polymer electrolyte membrane ( PEM ) fuel cells, which have been proposed for automotive transportation [1] . However, the high cost of platinum provides a strong impetus for developing less expensive alternatives. 2 2 2 H e H + − + ⇔ (7.1) The high activity of the hydrogenase enzymes containing iron or nickel, clearly demonstrates that catalytic activity for H + /H 2 interconversion is not unique to platinum [2 -5] . This observation has provided much of the impetus for attempting to develop small single site catalysts for hydrogen oxidation and production based on metals much less expensive than platinum. Artero and Fontecave have summarized these efforts in an excellent review [6] . Some notable developments since the time of that review have included progress on cobalt complexes containing 7 Catalysis Without Precious Metals. Edited by R. Morris Bullock

Potential Induced Protonation of the Second Coordination Sphere in a Hangman-type Iron Porphyrin Complex Promotes HER: Insights via in Situ Raman Spectroelectrochemistry

“…Complexes that promote the two-electron, two-proton reduction of H + to H 2 referred to as the hydrogen evolution reaction (HER), are of practical interest toward redefining the energy landscape of the future. Organometallic catalysts composed primarily of earth abundant elements also represent a sustainable avenue due to their relative ease in handling, efficiency in small molecule activation processes and nonscarcity in nature. , Development of single molecular catalysts to perform this electrochemically driven reaction allow the precise tuning of steric and electronic properties of the catalyst in order to improve overall efficiency. − Coordination complexes bearing a hanging group moiety in the second coordination sphere of the active site have been shown to facilitate proton-coupled electron transfer (PCET) within these catalytic transformations, promoting catalysis. − Bullock and co-workers have conducted exceptional work investigating the HER activity of Ni and Pt-based organometallic complexes containing amine-based substituents which act to facilitate intra- and intermolecular proton transfer for catalysis. , The neighboring amine residues in these complexes are subject to protonation and deprotonation, resulting in conformational changes within the complexes which further influence HER reactivity . Metal porphyrin-based molecular catalysts containing a hanging functional group have also garnered attention as they mimic naturally occurring iron-based metalloenzymes in biological systems. , The latter systems, although much more complex in their inner and outer coordination spheres, can facilitate HER or the carbon dioxide reduction reaction (CO 2 RR) through effective shuttling of protons and electrons via second coordination sphere effects. − Utilizing Raman spectroelectrochemistry the spin, oxidation state, and structural changes of iron–porphyrin-type molecular catalysts can be investigated, and a more in-depth understanding of the catalytic process governing HER can be understood, allowing for further refinement and development of molecular catalysts.…”

“…metallic complexes containing amine-based substituents which act to facilitate intra-and intermolecular proton transfer for catalysis. 11,12 The neighboring amine residues in these complexes are subject to protonation and deprotonation, resulting in conformational changes within the complexes which further influence HER reactivity. 13 Metal porphyrinbased molecular catalysts containing a hanging functional group have also garnered attention as they mimic naturally occurring iron-based metalloenzymes in biological systems.…”

Systematic Trends in the Electrochemical Properties of Transition Metal Hydride Complexes Discovered by Using the Ligand Acidity Constant Equation

“…We have uncovered important trends between the estimated p K a LAC ([MHL n ] + /ML n ) for the odd-electron complexes (p K a 2 in Scheme ) and the reversibility or the irreversibility of the corresponding reduction potential E ([MHL n ] + /MHL n ) ( E 1 in Scheme ) by examining the reported electrochemical behavior of hydride complexes from groups 5 to 10. Odd-electron hydride complexes are often implicated in the mechanisms of action of homogeneous electrocatalysts ,,,− and photoelectrocatalysts − for hydrogen production or oxidation and carbon dioxide reduction. Abundant 3 d transition metals are now being used to make active catalysts in the pursuit of more sustainable methods of hydrogen production, storage and utilization. ,,, Reports of the identification of paramagnetic hydrides of 3 d metals as active catalytic species in isomerization, hydrogenase-like action and asymmetric hydrogenation are appearing. − Therefore, the knowledge gained by our study is applicable for the rational design of superior catalysts for these processes.…”

Iron Complexes Bearing Diphosphine Ligands with Positioned Pendant Amines as Electrocatalysts for the Oxidation of H2

“…Two classes of enzymes have bimetallic active sites containing Fe atoms, the [FeFe]-hydrogenase, and the [NiFe]-hydrogenase. , These enzymes catalyze the reversible reduction of protons to form H 2 , and their high catalytic activity has inspired chemists to attempt to develop simpler synthetic catalysts using earth-abundant metals. − Of particular interest is the development of iron-based catalysts because of the abundance of this metal and its lack of toxicity to the environment. , Catalysts based on earth-abundant metals are of particular interest because most low-temperature fuel cells for oxidation of hydrogen are based on platinum. The study of model complexes has resulted in remarkable advances in our understanding of how the [FeFe]-hydrogenase functions, − and iron catalysts are now known for both hydrogen production − and oxidation. , Similarly, iron glyoximate complexes have recently been reported for H 2 production, and these complexes represent important extensions of previously reported cobalt catalysts − using this ligand platform.…”