Engineering a solid-state metalloprotein hydrogen evolution catalyst

Abstract

Hydrogen has the potential to play an important role in decarbonising our energy systems. Crucial to achieving this is the ability to produce clean sources of hydrogen using renewable energy sources. Currently platinum is commonly used as a hydrogen evolution catalyst, however, the scarcity and expense of platinum is driving the need to develop non-platinum-based catalysts. Here we report a protein-based hydrogen evolution catalyst based on a recombinant silk protein from honeybees and a metal macrocycle, cobalt protoporphyrin (CoPPIX). We enhanced the hydrogen evolution activity three fold compared to the unmodified silk protein by varying the coordinating ligands to the metal centre. Finally, to demonstrate the use of our biological catalyst, we built a proton exchange membrane (PEM) water electrolysis cell using CoPPIX-silk as the hydrogen evolution catalyst that is able to produce hydrogen with a 98% Faradaic efficiency. This represents an exciting advance towards allowing protein-based catalysts to be used in electrolysis cells.

Figure 1

Scheme outlining the preparation of biologically-inspired hydrogen evolution catalysts. CoPPIX – cobalt protoporphyrin IX.

Figure 2

Preparing and testing CoPPIX-silk films for hydrogen evolution. (a) The molecular structures of cobalt protoporphryin (CoPPIX), hemin (FePPIX) and nickel protoporphyrin (NiPPIX). (b) Cyclic voltammetry of CoPPIX-silk, FePPIX-silk and NiPPIX–silk films on a glassy carbon electrode. Scan rate = 10 mV s⁻¹, pH 7.

Figure 3

Using protein engineering to enhance hydrogen evolution (a,b) Cyclic voltammetry of CoPPIX-silk films on a glassy carbon electrode when the axial coordination was altered. Scan rate = 10 mV sec⁻¹, pH 7. (c–f) 3-dimensional model of CoPPIX binding to honeybee silk variants to rationalise results observed in cyclic voltammetry. (c,d) The Tyr76His variant showing the two rotamers of His76 that allow CoPPIX coordination without steric clashes. (e) Leu72Asp Tyr76His variant which could potentially form a hydrogen bond to His76. (f) The Tyr76His Ser80Asp variant which cannot adopt conformations that allow hydrogen bonding to His76.

Figure 4

Characterization of optimal carbon paper electrode preparation, CoPPIX-silk film mixed with carbon black (0.5 mg cm⁻²) 5 wt% PTFE coating. (a) High resolution FE-SEM image with (b) SEM image and corresponding EDX elemental mapping. (c) XRD pattern of the CoPPIX-silk catalyst layer.

Figure 5

Testing a PEM water electrolysis cell using a CoPPIX-silk as the HER catalyst. (a) Polarization curves in a standard water electrolysis cell using CoPPIX-silk based catalyst as a cathode under different temperatures in the range of 40–80 °C. Theoretical and measured hydrogen production rate is plotted (right y-axis) (b) Chronopotentiometry loadings of constant current of 3 A at 80 °C of CoPPIX-silk based HER catalyst in the water electrolysis cell.

Figure 6

Stability of CoPPIX-silk films in acidic conditions. (a,b) UV/Vis spectra of CoPPIX-silk before and after the films were immersed in (a) 100 mM sodium acetate buffer (pH 3) and (b) 0.5 M H₂SO₄ for 48 h. (c) Photographs of CoPPIX-silk films after immersion in solutions of varying pH.