On the Stability of Water Oxidation Catalysts: Challenges and Prospects

Abstract

Future requirements for water splitting technologies need highly efficient water oxidation catalysts that are sufficiently stable for operation over many years. Recent research has achieved significant progress in improving the electro-catalytic activities of these catalysts. However, there has not been a strong research focus on their long-term mechanical and chemical stability, yet this is critical for commercial application. In this paper we discuss some of the chemical and thermodynamic challenges confronting this goal, as well as some of the strategies that are available to overcome them. The challenge becomes even greater in the area of photo-active electromaterials; fortunately some of the same strategies may allow progress in this area also.

Water Oxidation by a Cytochrome P450: Mechanism and Function of the Reaction

“…Interestingly, the latter MnO x catalyst stops the water oxidation process at H 2 O 2 , because the peroxide is solvated and stabilized by hydrogen bonding to ethylamine and/or water in the electrolyte. [14], [15] Analogously, here we propose that hydrogen bonding within the water cluster in the hydrophobic P450 cam active site is essential for stabilization of the various reactive intermediates and of the H 2 O 2 formed. The turnover numbers with regard to H 2 O 2 formation we have observed are ∼7, whereas the electrocatalytic systems give turnover numbers of 20–1500 for complete water oxidation to O 2 .…”

“…The involvement of OH-radicals in H 2 O 2 formation has been proposed previously for electrolytic catalysts that oxidize water to O 2 (via an intermediate peroxide) [13] and also for a recently discovered water oxidation catalyst that produces H 2 O 2 during electrolysis [14], [15]. Interestingly, the latter MnO x catalyst stops the water oxidation process at H 2 O 2 , because the peroxide is solvated and stabilized by hydrogen bonding to ethylamine and/or water in the electrolyte.…”

“…Catalytic water oxidation is difficult to achieve, because the reaction is endothermic and has a large barrier. [13], [14], [15] To our knowledge, this is the first description of a cytochrome P450 oxidizing water.…”

“…[13] This difference arises because P450 cam only has access to thermal energy to perform this “uphill” reaction, whereas the electrocatalytic systems are run at overpotentials. [13], [14], [15].…”

Towards Global Artificial Photosynthesis (Global Solar Fuels): Energy, Nanochemistry, and Governance

“…Examples of approaches for increasing mechanical stability include inclusion in the electrolyte of components that have the capacity to reform the catalytic layer via oxidation and precipitation, or surface modification to create a secondary lattice network that modifies the thermodynamics of the phases involved or absorbs the lattice strain during catalytic cycling. [26] Biological Approaches to Solar Fuels The next group of papers focus on the important biological approach to solar fuel development. Kastoori Hingorani and Warwick Hillier of the ANU ('Perspectives for Photobiology in Molecular Solar Fuels') discuss prospects for bio-inspired solar energy conversion.…”