Facile Functionalization of Carbon Electrodes for Efficient Electroenzymatic Hydrogen Production

Abstract

Enzymatic electrocatalysis holds promise for new biotechnological approaches to produce chemical commodities such as molecular hydrogen (H₂). However, typical inhibitory limitations include low stability and/or low electrocatalytic currents (low product yields). Here we report a facile single-step electrode preparation procedure using indium-tin oxide nanoparticles on carbon electrodes. The subsequent immobilization of a model [FeFe]-hydrogenase from Clostridium pasteurianum ("CpI") on the functionalized carbon electrode permits comparatively large quantities of H₂ to be produced in a stable manner. Specifically, we observe current densities of >8 mA/cm² at -0.8 V vs the standard hydrogen electrode (SHE) by direct electron transfer (DET) from cyclic voltammetry, with an onset potential for H₂ production close to its standard potential at pH 7 (approximately -0.4 V vs. SHE). Importantly, hydrogenase-modified electrodes show high stability retaining ∼92% of their electrocatalytic current after 120 h of continuous potentiostatic H₂ production at -0.6 V vs. SHE; gas chromatography confirmed ∼100% Faradaic efficiency. As the bioelectrode preparation method balances simplicity, performance, and stability, it paves the way for DET on other electroenzymatic reactions as well as semiartificial photosynthesis.

Scheme 1. Schematic Illustration of Electrode Functionalization, Hydrogenase Loading, and Electron Transfer/Catalytic Turnover at NanoITO|Hydrogenase|Electrolyte Interfaces

Figure 1

(a) Side-by-side photograph of a nanoITO-modified (left) and unmodified glassy carbon electrodes (scale bar: 10 mm). (b) Atomic force microscopy images of nanoITO-GCE.

Figure 2

(a) Cyclic voltammetry (third scan, scan rate: 10 mV/s, 5 consecutive scans in Figure S4) of GCE-nanoITO-hydrogenase (red solid line) and PGE-nanoITO-hydrogenase (blue solid line) with corresponding hydrogenase-free electrodes (dashed lines). (b) Amperometric j–t curve of GCE-nanoITO-hydrogenase at −0.6 V vs SHE over 120 h (5 days) of continuous operation. (c) Online gas chromatography measurement with 1.5 h electrolysis at −0.6 V vs. SHE for Faradaic efficiency (FE) determination (black spheres). Accumulated H₂ (μmol, red circles) and electrons (blue circles) from online gas chromatography enzymatic electrocatalysis experiments (mean ± standard deviation, n = 3). Conditions: Ar-saturated 100 mM MOPS buffer, pH 7, under stirring. (d) Radar plot for comparison of current density (j), FE, stability, and remaining j after stability test with hydrogenase DET on representative metal oxide electrodes (table of comparison in Table S2).

Figure 3

(a) Illustration of RDE-nanoITO-hydrogenase under working conditions. (b) Amperometric i–t experiment of RDE-nanoITO-hydrogenase with different rotation rate (rpm) at −0.56 V vs. SHE. (c) Cyclic voltammetry (second scan) of RDE-nanoITO-hydrogenase at 2500 rpm, before and after adding 50% D₂O. (d) Amperometric i–t at −0.56 V vs. SHE, 2500 rpm, before and after adding 50% D₂O.

Figure 4

(a) Galvanostatic electrolysis of iRDE-GC nanoITO-hydrogenase system at −1.53, −2.55, and −3.56 mA/cm² with corresponding FE. (b) Potentiostatic electrolysis of iRDE-GC nanoITO-hydrogenase at −0.53, −0.59, and −0.64 V vs. SHE with corresponding FE (experiments performed at 500 rpm).