Anodic vs cathodic potentiostatic control of a methane producing microbial electrolysis cell aimed at biogas upgrading

Highlights

• CO₂ removal through Microbial electrolysis cell can be used for biogas upgrading.

• CO₂ removal in the MEC cathode is driven by CH₄ generation and CO₂ sorption.

• The anodic fluid dynamics influences the process energy efficiency.

• The shift of the potentiostatic control enhances the process energy efficiency.

• No increase of CO₂ removed is obtained at cathodic potential lower than −0.9 V vs SHE.

Abstract

A fully biological Microbial Electrolysis Cell (MEC) aimed at biogas upgrading has been operated under different operating conditions in order to enhance CO₂ removal from a synthetic biogas. Specifically, CO₂ reduction into CH₄ occurred at the MEC biocathode with the oxidation of organic substrates in the anodic chamber partially sustaining the energy demand of the process. In the cathode chamber, methane formation was the main driver of current generation which, in turn, sustained alkalinity generation and related CO₂ sorption. This study mainly focused on the minimization of the anodic and cathodic overpotentials to maximize the process efficiency. To accomplish this objective, an innovative strategy of MEC polarization was adopted, consisting in the shift of the potentiostatic control of the process from the anode (at +0.2 V vs SHE) to the cathode (at −0.65, −0.90 and −1.00 V vs SHE), along with the control of the fluid dynamic conditions of the anode chamber. An almost complete (99%) energy recovery was obtained by methane production with the cathode potential controlled at −0.65 V vs SHE. Finally, at the MEC cathode, current was utilized to reduce CO₂ into CH₄ (with a cathodic capture efficiency of about 70%) as well as to promote CO₂ sorption into HCO₃⁻. The latter represents the main CO₂ removal mechanism that accounted for 85% of the CO₂ removal.

Introduction

Bioelectrochemical systems are innovative techniques that stand on the utilization of “electroactive” microorganisms as biocatalysts of electrochemical reactions. In particular, the ability of electroactive microorganisms to exchange electrons with a solid state electrode allowed the development of several bioelectrochemical devices with different applications in the environmental field, such as the electricity generation from wastewater treatment [1,2], the removal of target pollutants [[3], [4], [5]], the production of target molecules [6,7] or the desalinization of salty and brackish waters [8,9]. As for the anodic side of bioelectrochemical systems, the ability of exoelectrogens microorganisms on the degradation of organic compounds has been widely explored in the last 20 years [10,11], and the direct electron transfer from microbes to the electrode surface has been identified and characterized by the identification of specific c-type cytochromes or nanowire [[12], [13], [14]]. However, more recently, research on bioelectrochemical systems has been focused on the possibility to drive the cathodic reactions towards the production of target molecules by applying an external electric power [15,16]. As an example, biocathodes can be exploited for the conversion of CO₂ into target molecules such as methane [17,18] and short chain volatile fatty acids by using an appropriate inoculum and an electricity source [19,20]. In this case, the bioelectrochemical system, commonly referred to as Microbial Electrolysis Cells (MEC) [21,22], permits to couple CO₂ reduction to the generation of valuable products and the utilization of the surplus electricity energy production, giving the possibility to store part of the electrical energy into reduced chemical compounds such us hydrogen [23,24], methane [25]or acetate [26]. In this frame, several authors recently focused their attention on the development of methane producing MEC, the so called bioelectromethanogenesis as an innovative strategy to couple CO₂ reutilization and the storage of the surplus electricity produced by renewable source [27,28]. The proposed approached was recently named as Bioelectro Power-to-Gas approach (BPTG) [29,30]. One of the main aspects of BPTG is the availability, for the electromethanogenesis reaction, of concentrated gaseous streams of CO₂ from different industrial fields, and one of the most attractive CO₂-rich stream is represented by the biogas deriving from anaerobic digestion processes [31]. Biogas is a gas mixture mainly composed by methane and CO₂, which is typically utilized for the cogeneration of heat and electric power in stationary applications [32]. CO₂ removal from biogas, also named upgrading process, permits to obtain biomethane, which has the same characteristics of natural compressed gas (CNG) and it can be used in automotive engines or injected into the distribution grid [33,34]. The possibility to use a biological approach [35], such as a methane producing MEC, as an innovative biogas upgrading approach has been proposed in the literature by several authors [[36], [37], [38]]. The integration of the anaerobic digestion with a methane producing MEC, has been widely proposed by several authors [39,40], which proposed both in situ and ex situ combination of the two technology. Recently, the main CO₂ removal mechanisms in a biocathode have been identified, particularly the sorption of CO₂ in the catholyte plays a pivotal role since it that permits the removal of up to 9 mol of CO₂ for each mole of methane produced [41]. Indeed, the sorption mechanism is driven by the alkalinity generation in the cathodic chamber due to the ionic transport of different ionic species for the electroneutrality maintenance [42,43]. In the present work, a fully bio-catalyzed microbial electrolysis cell has been deeply studied testing different previously unexplored operating conditions in order to evaluate its performance in terms of both CO₂ removal efficiency and energy efficiency. More in detail, the most innovative approach hereby proposed consists in the sequential polarization of the anodic and cathodic chamber of the MEC as a strategy to minimize the overall energy consumption [44]. Also, the anodic fluid dynamics conditions have been found to significantly affect the anode overpotentials and, as a consequence, the energy consumption of the overall process. Finally, three different cathodic potentials (i.e., −0.65, −0.90, and −1.0 V vs. the Standard Hydrogen Electrode) have been investigated to identify the best operating conditions which allow to enhance the process performance, in terms of methane production and CO₂ removal, while simultaneously lowering the overall energy consumption.