(627h) Augmenting the Performance of Hydrogenase for Aerobic Photocatalytic Hydrogen Evolution Via Solvent Tuning

Natural enzymes are among the most efficient electrocatalysts for solar fuel production with hydrogenases and carbon monoxide dehydrogenases capable of reversibly catalysing the reduction of protons (HER) and CO₂-to-CO (CO₂RR) conversion, respectively, at minimal overpotentials and high current densities.¹ These enzymes offer a low cost, more sustainable method of co-catalysis in photocatalytic systems. [NiFeSe] hydrogenases are viewed as some of the most active group of hydrogenases due to their efficient shuttling of electrons through a Ni active site.² However, their inhibition by oxygen, which is generated during water splitting, limits the usefulness of these enzymes. Additionally, conventional methods of inducing O₂ tolerance such as shielding, or using anti-oxidant reagents may not be compatible with the sensitive nature of enzymes.

We present a novel approach to making these enzymes oxygen tolerant beyond the established concepts of encapsulation or genetic engineering. Instead, we use the solvent to act as a simple yet effective dynamic oxygen shield. We show that deep eutectic solvents (DESs), a low-cost and non-toxic type of ionic liquids can be tailored to impede O₂ diffusion.³ Hindering O₂ diffusion allows HER to outcompete O₂ inhibition. With this approach, the residual photocatalytic HER activity of [NiFeSe] hydrogenase/TiO₂ under a constant air purge increases from ~4% in water to ~90% in DES (relative to the activity under N₂).⁴ Electrochemical measurements using protein film voltammetry further demonstrate that this concept is equally applicable to achieving high current densities for proton reduction in the presence of O₂. The electrochemical measurements correlate well to photocatalytic activity, allowing for the design of catalytic systems for fully O₂ tolerant H₂ production.

1. 1. R. Cammack, Nature 1999, 397, 214–215
   2. Wombwell et al., Acc. Chem. Res. 2015, 48, 2858–2865.
   3. M. G. Allan, et al., Energy Environ. Sci.2021, 14, 5523–5529
   4. M. G. Allan, et al., Angew. Chem. Int. Ed., 62, e202219176