Semiartificial approaches to renewable fuel synthesis
exploit the
integration of enzymes with synthetic materials for kinetically efficient
fuel production. Here, a CO
2
reductase, formate dehydrogenase
(FDH) from
Desulfovibrio vulgaris
Hildenborough,
is interfaced with carbon nanotubes (CNTs) and amorphous carbon dots
(
a
-CDs). Each carbon substrate, tailored for electro-
and photocatalysis, is functionalized with positive (−NHMe
2
+
) and negative (−COO
–
) chemical surface groups to understand and optimize the electrostatic
effect of protein association and orientation on CO
2
reduction.
Immobilization of FDH on positively charged CNT electrodes results
in efficient and reversible electrochemical CO
2
reduction
via direct electron transfer with >90% Faradaic efficiency and
−250
μA cm
–2
at −0.6 V vs SHE (pH 6.7 and
25 °C) for formate production. In contrast, negatively charged
CNTs only result in marginal currents with immobilized FDH. Quartz
crystal microbalance analysis and attenuated total reflection infrared
spectroscopy confirm the high binding affinity of active FDH to CNTs.
FDH has subsequently been coupled to
a
-CDs, where
the benefits of the positive charge (−NHMe
2
+
-terminated
a
-CDs) were translated to a functional
CD-FDH hybrid photocatalyst. High rates of photocatalytic CO
2
reduction (turnover frequency: 3.5 × 10
3
h
–1
; AM 1.5G) with
dl
-dithiothreitol as the sacrificial electron
donor were obtained after 6 h, providing benchmark rates for homogeneous
photocatalytic CO
2
reduction with metal-free light absorbers.
This work provides a rational basis to understand interfacial surface/enzyme
interactions at electrodes and photosensitizers to guide improvements
with catalytic biohybrid materials.