The cathode of an electrochemical cell plays an essential role during a process known as a microbial electrosynthesis (MES) driven carbon dioxide reduction reaction. MES is a promising biochemical process which allows for an eco-friendly reduction of carbon dioxide into various carbon-based products.
Many efforts to create a suitable cathode with a high efficiency have been tested and a team of Researchers from Denmark and China have now developed a freestanding and flexible graphene paper cathode which outstrips other carbon-based cathodes currently used in MES processes.
Microbial electrosynthesis (MES) is a bioelectrochemical technique which holds a great deal of potential for aiding in the reduction of greenhouse gases, namely carbon dioxide. During the process, the carbon dioxide molecules are reduced into a variety of carbon products using electrons derived from the cathode of an electrochemical reactor.
MES reactions typically facilitate the donated electrons on a biological catalyst which convert the carbon dioxide into one of many carbon molecules. To date, the carbon products generated through MES reactions include methane, acetate, butyrate and 2-oxobutyrate, as well as various biofuels.
MES reactions are powered by electricity surpluses in the power grid, although they are partially driven by the biological oxidation of wastewater at the anode. MES reactors are also a versatile piece of equipment and can be integrated into bio-inorganic artificial photosynthesis devices to reduce carbon dioxide molecules using solar energy– a process which is highly efficient compared to natural photosynthesis.
Many efforts to improve the electron transfer rate and productivity of MES reactors has occurred over the years by Researchers and has commonly taken the form of developments in high-performance cathode materials, microbial catalysts and growth media. There have been many attempts to create efficient cathode materials using freestanding carbon derivatives, including carbon paper, but to improve efficiencies further the Researchers have turned to a freestanding and flexible graphene paper to be used as the all-important cathode.
The Researchers chose reduced graphene oxide (rGO) as the graphene material for the cathode based on its excellent physicochemical properties (large surface area, high mechanical strength and good biocompatibility) as well as its high flexibility, thermal stability, electrical conductivity and low-cost nature.
The Researchers constructed reduced graphene oxide electrodes through a modified Hummers’ method. The reduced graphene oxide sheets were produced from a layered graphene oxide sheet (graphene oxide paper) network that arose from a graphene oxide solution which were vacuum filtered and subsequently layered.
The reactions were tested in a dual chambered bioelectrochemical reactor using an acetogen known as Sporomusa ovata as the microbial catalyst. The reaction(s) proceeded to produce acetate once the carbon dioxide molecules were injected within the reactor. A potentiostat (CH Instruments, Inc) was used to perform the MES, as well as cyclic voltammetry (CV), experiments, whilst the concentration of acetate produced was measured by high-performance liquid chromatography (HPLC) with an anion exchange column (HPX-87H, Bio-Rad Laboratories Inc.).
The electrochemical cell involving the graphene paper was composed of reduced graphene oxide paper cathode, a graphite stick anode separated by a Nafon 115 ion-exchange membrane (Ion Power, Inc).
To characterize the new cathodes themselves, the Researchers used a combination of scanning electron microscopy (Quanta 200, FEI), confocal laser scanning microscopy (CLSM, LSM 5 Pascal microscope, Zeiss), UV-visible spectroscopy (8453 G1103A, Agilent), X-ray photoelectron spectroscopy (XPS, K-Alpha, Thermo Scientific), Raman Spectroscopy (DXR, Thermo Scientific) and Brunauer–Emmett–Teller (BET) methods.
The donation of electrons from the cathode is a pivotal mechanism during MES reactions and the graphene oxide paper was found to enhance the electron transfer between the cathode and the catalytic microbe, which allowed for an enhanced reduction of the carbon dioxide species.
The acetate production during these reactions was found to be 168.5 ± 22.4 mmolm-2 d-1 at -690 mV. The production rate was found to up 8-fold fast than carbon paper electrodes with the same dimension, and possessed an increased current density (2580 ± 540 mAm-2) of around 7-fold. The increase in these factors also led to an enhanced cathodic current response which showed a coulombic efficiency of 90.7 ± 9.3% for the conversion of carbon dioxide to acetate.
In comparison, carbon paper cathodes only show an efficiency of 83.8 ± 4.2%
In addition, the reduced graphene oxide paper has a greater flexibility than previously utilized carbon electrodes. As such, it allowed the cathode to be folded in the MES reactor, increasing its surface area, which lead to an improvement of the electron exchange and cell attachment between the graphene cathode and microbial catalyst.
Further experiments are required to see how the new cathode may be further used to significantly increase the productivity of MES reactions in real-world applications. But, the initial research is very promising and we could see a greater implementation of the cathodes in MES reactors in the not too distant future.
“Freestanding and flexible graphene papers as bioelectrochemical cathode for selective and efficient CO2 conversion”- Aryal N., et al, Scientific Reports, 2017¸ DOI:10.1038/s41598-017-09841-7