Topics in Current Chemistry (2018) 376:43
1 3
Saccharides boast high theoretical energy densities that are extracted in vivo by
a series of enzymes that catalyze the oxidation of sugars to CO 2 while utilizing
ATP as a means of exchanging chemical energy. Such high energy densities make
saccharides and other biologically derived metabolic intermediates attractive fuel
sources for electrical energy generation. However, the full utilization of these
biofuels requires multiple sequential oxidative steps to extract all of the electrons
from a single molecule of fuel. While cellular metabolism offers a roadmap of
possible reaction pathways for the complete oxidation of biofuels, methods to
convert ATP hydrolysis to electrical energy remain in their infancy [1, 2]. Consequently, alternative catalysts must be selected to convert chemical bonds into
electrons at each step of the metabolic pathway in the form of electrochemical
oxidation.
This review aims to provide an overview of recent developments in research to
utilize enzymatically inspired catalytic cascades in the context of electrochemical energy conversion. Of particular interest is work to bias the advantages that
catalytic cascades afford with respect to overcoming diffusional and kinetic limitations that arise when multiple catalysts are used to maximize the depth of substrate oxidation. While many of the electrocatalytic oxidation cascades described
herein are direct electrochemical adaptations of a natural metabolic pathway, we
will also provide an update on recent work being done to design artificial electrocatalytic oxidation pathways, herein referred to as electrometabolic pathways.
This term is meant to describe a newly emerging field of artificial cascades, which
can consist of (1) native metabolic pathways, in which one or more enzyme is
replaced with an electrochemically interfaced enzyme or non-enzymatic electrocatalyst, (2) sequential non-natural electrochemical oxidation reactions in which
more than one electrocatalyst (enzymatic or otherwise) is employed, or (3) native
metabolic pathways, in which one or more pairs of sequential enzymes has/have
been modified (translationally, post-translationally, or synthetically) to manipulate spatial orientation of corresponding enzymes.
Biofuels, broadly speaking, contain a diverse array of chemical functionalities, and therefore their complete electrochemical oxidation offers a unique challenge. Specifically, there has been a growing effort to design molecular architectures containing promiscuous electrocatalysts capable of deep biofuel oxidation
[3–8]. The earliest attempts to facilitate multiple electrocatalytic oxidations for
energy conversion were in the context of a direct methanol fuel cell [9]. Direct
liquid fuel cells (DLFCs) employ transition metals (usually Pt and Pt alloys) to
electrochemically reduce O 2 to water at the cathode while oxidizing a carbonbased fuel to CO 2 at the anode. While research into DLFCs remains active, these
electrocatalytic reactions tend to exhibit relatively low efficiencies, and rely on a
single catalyst for multiple reactions [10–13]. As the focus of the current review
is aimed at the simultaneous utilization of multiple electrocatalysts for cascade
reactions, DLFCs are outside the scope of this work. However, several reviews
exist on recent research and advancements in DLFC technologies [11]. In contrast
to the heterogeneous transition metal electrocatalysts found in DLFCs, enzymes
operate under mild conditions at ambient temperatures and near neutral pH. It is
this ubiquitous reaction environment that enables combinations of redox enzymes
Reprinted from the journal
150
1 3
Saccharides boast high theoretical energy densities that are extracted in vivo by
a series of enzymes that catalyze the oxidation of sugars to CO 2 while utilizing
ATP as a means of exchanging chemical energy. Such high energy densities make
saccharides and other biologically derived metabolic intermediates attractive fuel
sources for electrical energy generation. However, the full utilization of these
biofuels requires multiple sequential oxidative steps to extract all of the electrons
from a single molecule of fuel. While cellular metabolism offers a roadmap of
possible reaction pathways for the complete oxidation of biofuels, methods to
convert ATP hydrolysis to electrical energy remain in their infancy [1, 2]. Consequently, alternative catalysts must be selected to convert chemical bonds into
electrons at each step of the metabolic pathway in the form of electrochemical
oxidation.
This review aims to provide an overview of recent developments in research to
utilize enzymatically inspired catalytic cascades in the context of electrochemical energy conversion. Of particular interest is work to bias the advantages that
catalytic cascades afford with respect to overcoming diffusional and kinetic limitations that arise when multiple catalysts are used to maximize the depth of substrate oxidation. While many of the electrocatalytic oxidation cascades described
herein are direct electrochemical adaptations of a natural metabolic pathway, we
will also provide an update on recent work being done to design artificial electrocatalytic oxidation pathways, herein referred to as electrometabolic pathways.
This term is meant to describe a newly emerging field of artificial cascades, which
can consist of (1) native metabolic pathways, in which one or more enzyme is
replaced with an electrochemically interfaced enzyme or non-enzymatic electrocatalyst, (2) sequential non-natural electrochemical oxidation reactions in which
more than one electrocatalyst (enzymatic or otherwise) is employed, or (3) native
metabolic pathways, in which one or more pairs of sequential enzymes has/have
been modified (translationally, post-translationally, or synthetically) to manipulate spatial orientation of corresponding enzymes.
Biofuels, broadly speaking, contain a diverse array of chemical functionalities, and therefore their complete electrochemical oxidation offers a unique challenge. Specifically, there has been a growing effort to design molecular architectures containing promiscuous electrocatalysts capable of deep biofuel oxidation
[3–8]. The earliest attempts to facilitate multiple electrocatalytic oxidations for
energy conversion were in the context of a direct methanol fuel cell [9]. Direct
liquid fuel cells (DLFCs) employ transition metals (usually Pt and Pt alloys) to
electrochemically reduce O 2 to water at the cathode while oxidizing a carbonbased fuel to CO 2 at the anode. While research into DLFCs remains active, these
electrocatalytic reactions tend to exhibit relatively low efficiencies, and rely on a
single catalyst for multiple reactions [10–13]. As the focus of the current review
is aimed at the simultaneous utilization of multiple electrocatalysts for cascade
reactions, DLFCs are outside the scope of this work. However, several reviews
exist on recent research and advancements in DLFC technologies [11]. In contrast
to the heterogeneous transition metal electrocatalysts found in DLFCs, enzymes
operate under mild conditions at ambient temperatures and near neutral pH. It is
this ubiquitous reaction environment that enables combinations of redox enzymes
Reprinted from the journal
150