together with new insights on the basic mechanisms that underlie electromethanogenesis has allowed during the past years to enhance productivity up to
30 LCH4Ád
−1
Ám
−2 with efficiencies near 100% [122, 123]. Here, it is important to
mention that electromethanogenesis is beginning to be competitive with other
methane-producing technologies that also use CO 2 as a substrate (as those that
involve the use of H 2 as a reductive agent) at least in terms of energy usage [87].
3.3 Challenges and Future Perspectives. Towards Practical
Application
MES is a novel technology that has not yet left the laboratory bench. Developing
appropriate upscaling approaches, together with the need to find suitable electrode
and membrane materials are perhaps the most important challenges that it needs to
face on its way to real field application [124]. Indeed, conceiving high efficiency
and cost-effective large-scale MES requires that several critical issues such as
catholyte flow regimes, reactor architecture or stacking approaches (to name but a
few) should be carefully considered [124]. Most of these issues are shared in
common with other BES such microbial fuel cells and microbial electrolysis cells
and so it seems reasonable to think that the scale-up methodologies developed for
the latter [95, 125, 126] can also be applicable to MES.
Electrodes and ion exchange membranes (IEM) are two key elements that have a
notable effect on the overall performance of MES. As mentioned before, the
interaction between microbes and electrodes lies at the core of MES technology.
Here, efforts have been made to facilitate the electron exchange by modifying the
electrode surface using, for instance, carbon nanotubes, graphene or metal
nanoparticles, and by pre-treating electrode surface with chemicals such as chitosan
or cyanuric chloride [127–129]. The IEM are also crucial in MES. On the one hand
IEM are relatively expensive and their cost may represent a significant part of the
overall capital cost of MES [126]. On the other hand, IEM prevents the mixing of
the electrolytes while allowing the circulation of ionic currents between electrodes
(thus having an impact on the overall performance and efficiency of MES). However, IES are prone to undergo biofouling which can negatively affect their ability
to exchange ions between electrolytes. That means that real-life application of MES
would require periodic maintenance to prevent the excessive deterioration of these
elements.
Feeding CO 2 to the biocathode is another challenging issue. Typical stationary
point sources of CO 2 (e.g. off gas of combustion or fermentative process) may
contain products (such as O 2 ) that can negatively impact on the metabolic processes
of the electrotrophs. The diffusion and dispersion of those CO 2 -rich gas streams on
the cathodic media also deserve some attention to optimize CO 2 availability to the
biofilm. Here, researchers have tried to address this question by adding excesses of
inorganic carbon or recirculating the unreacted CO 2 gas [100, 130, 131]. Nevertheless, both of these approaches present significant drawbacks and more investigation is required on this topic.
Carbon Dioxide Utilization—Bioelectrochemical Approaches
97
30 LCH4Ád
−1
Ám
−2 with efficiencies near 100% [122, 123]. Here, it is important to
mention that electromethanogenesis is beginning to be competitive with other
methane-producing technologies that also use CO 2 as a substrate (as those that
involve the use of H 2 as a reductive agent) at least in terms of energy usage [87].
3.3 Challenges and Future Perspectives. Towards Practical
Application
MES is a novel technology that has not yet left the laboratory bench. Developing
appropriate upscaling approaches, together with the need to find suitable electrode
and membrane materials are perhaps the most important challenges that it needs to
face on its way to real field application [124]. Indeed, conceiving high efficiency
and cost-effective large-scale MES requires that several critical issues such as
catholyte flow regimes, reactor architecture or stacking approaches (to name but a
few) should be carefully considered [124]. Most of these issues are shared in
common with other BES such microbial fuel cells and microbial electrolysis cells
and so it seems reasonable to think that the scale-up methodologies developed for
the latter [95, 125, 126] can also be applicable to MES.
Electrodes and ion exchange membranes (IEM) are two key elements that have a
notable effect on the overall performance of MES. As mentioned before, the
interaction between microbes and electrodes lies at the core of MES technology.
Here, efforts have been made to facilitate the electron exchange by modifying the
electrode surface using, for instance, carbon nanotubes, graphene or metal
nanoparticles, and by pre-treating electrode surface with chemicals such as chitosan
or cyanuric chloride [127–129]. The IEM are also crucial in MES. On the one hand
IEM are relatively expensive and their cost may represent a significant part of the
overall capital cost of MES [126]. On the other hand, IEM prevents the mixing of
the electrolytes while allowing the circulation of ionic currents between electrodes
(thus having an impact on the overall performance and efficiency of MES). However, IES are prone to undergo biofouling which can negatively affect their ability
to exchange ions between electrolytes. That means that real-life application of MES
would require periodic maintenance to prevent the excessive deterioration of these
elements.
Feeding CO 2 to the biocathode is another challenging issue. Typical stationary
point sources of CO 2 (e.g. off gas of combustion or fermentative process) may
contain products (such as O 2 ) that can negatively impact on the metabolic processes
of the electrotrophs. The diffusion and dispersion of those CO 2 -rich gas streams on
the cathodic media also deserve some attention to optimize CO 2 availability to the
biofilm. Here, researchers have tried to address this question by adding excesses of
inorganic carbon or recirculating the unreacted CO 2 gas [100, 130, 131]. Nevertheless, both of these approaches present significant drawbacks and more investigation is required on this topic.
Carbon Dioxide Utilization—Bioelectrochemical Approaches
97