Cathode for microbial fuel cell (MFC) and process for preparing same

Abstract

The invention relates to a cathode for a microbial fuel cell (MFC), as well as to methods of preparing the cathode. In embodiments, the cathode of the present invention can be interlocked with an anode to form a compact anode / cathode pair, which electrode pair can be configured to be inserted into a holder board or tank. The invention further provides methods of treating and monitoring wastewater using the cathode.

Description

[0001] Cathode for Microbial Fuel Cell (MFC) and Process for Preparing Same Technical Field

[0002] The present invention relates to a cathode fora microbial fuel cell (MFC), as well as to methods of preparing the cathode. In embodiments, the invention relates to an electrode assembly comprising the cathode.

[0003] Background

[0004] Microbial fuel cells (MFCs) have attracted great attention over the last 20 years due to their potential to harvest electrical energy from organics in wastewater influent, while simultaneously removing pollutants. However, MFCs are yet to be effectively commercialised due to the difficulties associated with their scale-up. This arises from a variety of causes, including the decrease in cathode specific surface area (CSSA) as conventional MFC reactor sizes are increased; the high cost of MFC fabrication and assembly, in particular associated with the MFC cathodes; and the low compatibility of MFCs with existing wastewater treatment plants (WWTPs).

[0005] The activated sludge process is a widely used method for treating domestic wastewater. In this process, air is pumped into an aeration tank to promote microbial growth. The microbes then feed on the organic material in the wastewater, forming flocs, which can be settled out. This process is inefficient, however, and the aeration tank consumes, and wastes, a significant amount of energy, directly contributing to the operating expense of WWTPs. MFCs were originally envisaged as a replacement for aeration tanks in traditional WWTPs which use an activated sludge method. However, MFCs suffer from poor compatibility with these existing WWTPs, which has limited their adoption. For instance, conventional MFC cathodes have been shown to be waterproof (i.e., the cathode does not break down when submerged), only to around one metre water head in a short-time test (Yang et al., Environ. Sci. Technol. Lett.

[0006] 2014, 1, 416-420). However, the recommended liquid depth for an aeration tank should be at least three metres. Additionally, the typical configuration of conventional MFC cathodes generally requires them to be positioned at the top of the aeration tank for retrofiting into existing WWTPs. In this configuration, only the wastewater at the top of the tank is effectively treated, with poor performance being observed for deep wastewater (Li et al., Journal of Water Process Engineering 40, 101984 (2021)).While more complex retrofitting configurations have been attempted, for instance, using modular MFCs or rebuilding the tanks, these generally result in significantly increased costs, and increased complexity in terms of plumbing and power management.

[0007] In addition to the difficulties associated with retrofiting MFCs to existing WWTPs, conventional MFCs suffer from a number of other drawbacks which limit their widespread use. One of these drawbacks is the low cathode specific surface area (CSSA) of conventional MFCs, which limits the energy output of the cells and results in insufficient performance to support widespread adoption. Further issues with adopting MFCs on a large scale arise from the high cost of MFC fabrication and assembly, with the cathode typically accounting for over 50% of the reactor cost. Cathodes for MFCs are typically fabricated from a flat carbon cloth substrate, platinum catalyst, and a diffusion layer (typically PTFE), and use labour-intensive manual brush methods in their preparation. The use of these expensive materials and the time-consuming methods needed to prepare these conventional MFCs, are prohibitive from a cost perspective and further limit widespread adoption.

[0008] It is an object of the present invention to obviate or mitigate one or more of these problems. It is an aim of the invention to provide a cathode for a microbial fuel cell which has one or more of: improved compatibility with existing wastewater treatment plants; improved energy output; improved ease and / or reduced cost of production; improved submersion properties; and improved treatment efficiency.

[0009] Summary

[0010] According to an aspect of the present invention there is provided a cathode comprising an elongate hollow member, the elongate hollow member having a helical configuration which is arranged to coil around an anode to form an electrode assembly, wherein at least a portion of the external surface of the elongate hollow member is coated with a catalytic formulation. The inventors have advantageously established that a cathode comprising a helical configuration provides an increased cathode specific surface area, providing more reaction sites for oxidant reduction and improving the cathodic reaction rate.

[0011] The power output of conventional MFCs is largely constrained by the cathode specific surface area (CSSA). Cathodes for MFCs are generally planar in shape, and are positioned in use atthe surface of the chamber to allow oxygen to diffuse into the MFC. Such a typical configuration is shown in Figure 1, where a flat, or planar, cathode is assembled with a conventional graphite brush anode in a cylindrical chamber. The CSSA for such an assembly can be calculated using the equation:

[0012] V — TI D2L

[0013] MFC volume (V):

[0014]

[0015] — 2

[0016] Cathode Surface Area (S) : S = i > D “

[0017] ■i

[0018] C=1

[0019] Cathode Specific Surface Area (CSSA): = — -

[0020]

[0021] where D represents the diameter of the cathode in m, and L the length of the chamber. The CSSA is therefore dependent on the chamber length. In practice, this means that while effective CSSAs can be achieved for MFCs on the scale of several litres or smaller, at a cubic metre scale or above, the CSSA reduces by about 3 orders of magnitude, affecting performance of the MFC and reducing its treatment efficiency.

[0022] Cylindrical cathodes have been proposed as an alternative configuration to address this problem. However, the CSSA of a cylindrical cathode still decreases substantially as the cubic diameter is increased, according to the equation:

[0023] CSSA: = s = 4

[0024] V D

[0025] (where the Cathode Surface Area of the cylindrical cathode is based on S = nDL).

[0026] These low CSSAs, with the resulting low power output, have constrained the scale up of MFCs for widespread commercial use.The inventors have advantageously determined that a cathode having a helical configuration can allow the CSSA to be maintained, and even increased, when the cathode is scaled-up. A helical cathode according to the invention is shown in Figures 2A and2B. With a hollow elongate member, oxygen in the air can flow through the inner void of the hollow elongate member, and diffuse through the walls of the member to contact the catalytic material on the external surface (Figure 2A). The helical structure also allows an anode to be positioned or inserted inside the coils of the helix (Figure 2B), to form a compact assembly structure.

[0027] Theoretically, the CSSA of a helical cathode can be one order of magnitude higher than either a cylindrical one or a conventional flat cathode. As an example, for a typical MFC with diameter of the MFC reactor D=10 cm, L=20cm, and diameter of the helical cathode d=2.5cm, the CSSA of a helical cathode can reach 600 m2 / m3, which is 14.8 and 24.0 times that of a comparable MFC with a cylindrical cathode (40 m2 / m3) and a flat cathode (25 m2 / m3(2)), respectively. This increase is crucial because it not only contributes to increased power generation for the MFC, but also facilitates increase oxygen diffusion, allowing partial simultaneous nitrification / denitrification (SND) for nitrogen pollutant removal.

[0028] The invention relates to a cathode comprising an elongate hollow member which is helical in shape, and arranged to coil around an anode. At least a portion of the external surface of the elongate hollow member is coated with a catalytic formulation.

[0029] In some embodiments, the elongate hollow member is at least 50 cm, at least 60 cm, at least 70 cm, at least 80 cm, at least 90 cm, at last 100 cm, at least 110 cm, at least 120 cm, at least 130 cm, at least 140 cm, at least 150 cm, at least 160 cm, at least 170 cm, at least 180 cm, at least 190 cm, or at least 200 cm in length.

[0030] In an embodiment, the elongate hollow member is a tubular member, i.e., it has a substantially circular cross-section. In use, air can flow through the inner void of the elongate hollow member and diffuse through the walls of the elongate hollow member to come into contact with the catalytic material on the external surface.

[0031] The interior of the elongate hollow member may be in gaseous communication with the atmosphere, or may be connected to a gaseous control system (e.g. a vessel comprising a defined gas mix, a valve, and / or control system for delivering the defined gas mix to the interior of the elongate hollow member). For example, the interior of the elongate hollowmember may be connected to a gaseous control system comprising a defined gas mix which is oxygen-enriched (i.e. oxygen may be present at > 21% of the defined gas mix). Therefore, also provided is a gaseous control system for use in combination with the cathode of the invention.

[0032] In an embodiment, the catalytic formulation comprises a carbon-based catalytic material. Advantageously, the use of carbon-based catalytic materials offers a significant cost-saving versus the use of metallic catalyst materials such as platinum, which is conventionally used in MFCs.

[0033] Carbon-based catalytic materials which may be suitable for use in the invention include, but are not limited to, activated carbon (AC), carbon black (CB), carbon fibres, carbon nanotubes (CNTs), graphene, and graphite.

[0034] In an embodiment, the catalytic formulation comprises one or more of activated carbon, carbon black , carbon fibres, carbon nanotubes (CNTs), graphene, and graphite.

[0035] In an embodiment, the catalytic formulation comprises one or more of activated carbon and carbon black. In an embodiment, the catalytic formulation comprises both activated carbon and carbon black.

[0036] In an embodiment, the at least a portion of the external surface of the elongate hollow member comprises copper particles. The inclusion of copper particles inhibits the growth of undesired bacteria and facilitates nitrogen removal and electricity production. The copper particles may be nanoparticles or microparticles.

[0037] The copper particles may be included in the catalytic formulation that is coated on at least a portion of the external surface of the elongate hollow member. Alternatively, the copper particles may be applied to the at least a portion of the external surface of the elongate hollow member as a separate layer or coating contiguous with the catalyst formulation.

[0038] In an embodiment, the catalytic formulation comprises copper powder.

[0039] Alternatively, copper particles can be applied separately to the external surface of the elongate hollow member, for example by electroplating. The catalytic formulation can then be applied to the electroplated surface.In some embodiments, the catalytic formulation comprises a carrier. Suitable carriers include, but are not limited to, polyvinylidene fluoride (PVDF) and Polytrol. The carrier can be multifunctional, and can serve, for instance, to promote mixing of the active catalytic materials in the formulation. Advantageously, carriers such as PVDF and Polytrol also serve to enhance the porosity of the catalytic formulation on the external surface of the elongate hollow member, which facilitates increased diffusion of oxygen into the cathode, and thereby the electrode assembly.

[0040] In some embodiments, the catalytic formulation comprises a solvent. Suitable solvents include, but are not limited to, dichloromethane (DCM), N,N-dimethylacetamide (DMA), and propanol.

[0041] In an embodiment, the catalytic formulation comprises at least one of activated carbon and carbon black.

[0042] The activated carbon and / or carbon black may be included in the catalytic formulation in an amount of 15 - 35 wt.%. The catalytic formulation may comprise no less than 15 wt.% of carbon and / or carbon black. The catalytic formulation may comprise at most 35 wt.% of carbon and / or carbon black.

[0043] In an embodiment, the catalytic formulation comprises activated carbon, carbon black, and a solvent. In an embodiment, the catalytic formulation comprises activated carbon, carbon black, a carrier, and a solvent.

[0044] In an embodiment, the catalytic formulation comprises from 65 to 85 wt.% of solvent. The catalytic formulation may comprise no less than 65 wt.% of solvent. The catalytic formulation may comprise at most 85 wt.% of solvent.

[0045] In an embodiment, the catalytic formulation comprises activated carbon, carbon black and dichloromethane.

[0046] In an embodiment, the catalytic formulation comprises activated carbon, carbon black, polyvinylidene fluoride (PVDF) and a solvent. The solvent may be N,N-dimethylacetamide. However, this is not limited thereto, and a person skilled in the art would appreciate that other solvents could be used.In an embodiment, the catalytic formulation comprises activated carbon, carbon black, graphene nanoplatelets, Polytrol™ (Owatrol, France) and a solvent.

[0047] In an embodiment, the elongate hollow member is formed of a polymeric material.

[0048] The use of a polymeric material to form the elongate hollow member has a number of significant advantages compared with known cathode structures for MFCs. For instance, the polymeric material can be 3-D printed, facilitating ease of manufacture and resulting in a smoother external surface which can withstand greater head depths when submerged. The use of polymeric materials can also generate significant cost-savings when compared with the use of carbon cloth, which is commonly used as a substrate for MFC cathodes. Further advantages of polymeric materials include that they are lightweight, and / or easily modified, and / or suitable for use in scaled-up methodologies.

[0049] Suitable polymers for use in the invention include, but are not limited to, polyethylene (PE), polypropylene (PP), rubber, and polylactic acid (PLA). In an embodiment, the polymeric material comprises polylactic acid.

[0050] Polylactic acid (PLA) is particularly suitable for use in the present invention because it has a high oxygen diffusion coefficient conferring enhanced oxygen diffusion characteristics on the resultant cathode. PLA is also particularly suitable for use when 3-D printing the cathode, due to its low cost, and compatibility with 3-D printing techniques.

[0051] In an embodiment, the elongate hollow member is fabricated by 3-D printing.

[0052] The inventors have advantageously determined that when the elongate hollow member is 3-D printed, a more uniform surface is obtained, with no fine pores on the surface visible under Scanning Electron Microscopy (SEM) imaging. This uniform surface means that air bubbles do not penetrate the cathode surface as the water pressure is increased, and allows the cathodes of the invention to be immersed to greater depths than conventional MFU cathodes. Electrode assemblies according to the invention can therefore be inserted into existing wastewater treatment tanks and have been demonstrated to withstand depths of over 5 metres without suffering deformation or water leakage. This allows the cathode and electrode assemblies containing the cathodes to be retrofitted into current wastewater treatment systems without requiring substantial modification of the existing systems.Alternatively, as a skilled person would appreciate, the cathode of the invention may be fabricated using conventional moulding techniques, including, but not limited to injection moulding, blow moulding, rotational moulding, or by extrusion, vacuum casting, thermoforming and other suitable polymer processing methods. This may be suitable for cathodes which are not intended for immersion at significant depth.

[0053] According to an aspect of the present invention there is provided an electrode assembly comprising the cathode described above and an anode. The cathode may be configured to be used with a screw to facilitate its insertion and fixing into a holder board or tank.

[0054] In embodiments, the cathode of the present invention can be interlocked with an anode to form a compact anode / cathode pair. The anode can be inserted or positioned within the coils of the cathode helix and fixed, for example using pins or threads. The electrode pair may be configured to be inserted, for example plugged or screwed, into a holder board or tank. The electrode pair can be provided with a screw (which in some embodiments may be integral with the cathode) to allow it to be plugged or screwed into an existing holder board or tank. The configuration according to the invention in which the anode is positioned within the coils of the cathode has further advantages. For instance, in 'flow-through' MFC systems (such as the prior art system shown in Figure 1), the cathodes are inserted in the system perpendicular to the flow of water. This means that the cathode is subjected to the force of the flow in use, as the water flows across the cathode. However, in a helical configuration, the external surface of the cathode is subjected to a lower shear force than a cylindrical one under the same flow conditions, as confirmed by a modelling result. This is likely due to the fact that water can pass through gaps between the coils of the helical MFC, leading to minor changes in the flow direction. This has advantages in terms of the stability of the cathode and the duration / life-cycle of the catalytic formulation on the cathode surface. In addition, water dead zones can form behind cylindrical electrodes. For the helical configuration, wastewater can easily flow through the electrodes with full contact with the cathode surface and good hydraulic properties, which assists in the efficient removal of pollutants from wastewater. In an embodiment, the anode is a graphite fibre brush (GFB) anode.

[0055] Graphite fibre brush anodes are known in the art. Advantageously, conventional GFB anodes can be inserted into the coils of the helical cathode, forming a compact anode / cathode pairThis allows the cathode of the invention to be readily combined with existing anode technologies.

[0056] In an embodiment, the cathode of the invention can be interlocked with an anode, such as a graphite brush anode. In this embodiment, the anode and cathode form a compact structure with a very short electrode distance. The paired electrode can be fixed, for example by a screw, as shown in Figure 3A and screwed into an electrode's holder board with a set electric circuit. To facilitate this, in some embodiments, the cathode may be formed with an integral screw or fixing element, or with a mechanism for facilitating attachment to a screw or fixing device. Figure 3 shows an electric holder board (Figure 3B) and an MFC reactor (Figure 3C) comprising multiple electrode pairs according to the invention. In use, the electric holder board can then be put on the top of a water tank to form a microbial fuel cell or array, depending on the number of paired electrodes and the WWTP requirements. This assembly substantially simplifies the MFC, and allows it to be retrofitted into existing wastewater treatment systems with minimal modification. This lowers both the capital and operational costs, and facilitates scale-up.

[0057] In an embodiment, there is provided a method of preparing a cathode for use in a microbial fuel cell, the method comprising:

[0058] preparing an elongate hollow member having a helical configuration, from a polymeric material; and

[0059] immersing at least a portion of the elongate hollow member in a catalytic formulation, wherein the catalytic formulation is as previously described, to form a helical cathode.

[0060] The catalytic formulation may be that previously described herein.

[0061] As a skilled person would appreciate, the elongate hollow member can be fully immersed in the catalytic formulation. Alternatively, a portion of the elongate hollow member can be immersed in the formulation, e.g. by holding or positioning the elongate hollow member such that a portion thereof is immersed in the catalytic formulation. This process could also be carried out using a dipping machine.In an embodiment, the method further comprises etching the exterior surface, or portion thereof, of the elongate hollow member before immersing the elongate hollow member, or portion thereof, in the catalytic formulation.

[0062] Etching the elongate hollow member before immersing the elongate hollow member in the catalytic formulation facilitates attachment of the catalyst formulation to the surface of the elongate hollow member, and in use, enhances oxygen permeability and benefits oxygen diffusion due to the narrower thickness of membrane after etching. The etching step can be carried out by immersing the elongate hollow member in a solvent. The elongate hollow member can be immersed for a period of time, and then allowed to dry. The immersing can be carried out for from 5 seconds to 30, or from 5 seconds to 20 seconds, or preferably from 5 seconds to 10 seconds. The immersing may be carried out for at least 5 seconds, at least 10 seconds, at least 15 seconds, at least 20 seconds, or at last 25 seconds. The immersing may be carried out for at most 10 seconds, at most 15 seconds, at most 20 seconds, at most 25 seconds, or at most 30 seconds. These immersion times allow for the beneficial results of the etching step to be achieved, without diminishing the structural integrity of the elongate hollow member. The drying may be carried out in air.

[0063] The solvent can be any suitable solvent which can dissolve the surface of the elongate hollow member. Suitable solvents would be known to one skilled in the art and include, for example, dichloromethane, chloroform, tetra hydrofuran or acetone.

[0064] The drying step can be carried out at room temperature. The drying step can be carried out for from 6 hours to 24 hours. The drying step may be less than 24 hours, less than 18 hours, less than 12 hours, or less than 9 hours. The drying step may be at most 6 hours, at most 9 hours, at most 12 hours, or at most 18 hours.

[0065] While in some embodiments an etching step can be included to increase the adhesion of the catalytic formulation to the surface of the elongate hollow member, this step is optional, and in some methods, no etching is carried out.

[0066] In an embodiment, the step of preparing an elongate hollow member from a polymeric material comprises 3-D printing the elongate hollow member.In an embodiment, the polymeric material comprises polylactic acid (PLA). As noted above, PLA is particularly suitable for use when 3-D printing the elongate hollow member, due to factors such as its cost, availability, high oxygen diffusion characteristics, and known compatibility with 3-D printing techniques.

[0067] In an embodiment, the method comprises preparing a polymeric composite material and then preparing the elongate hollow member from the polymeric composite material. The elongate hollow member may be prepared from the polymeric composite material by 3-D printing. The polymeric composite material can be prepared by mixing polylactic acid and one or more catalytic materials in a solvent. The one or more catalytic materials may be carbon-based catalytic materials. In an embodiment, the polymeric composite material is prepared by mixing polylactic acid, graphene nanoplatelets and activated carbon. The solvent may be dichloromethane. However, a skilled person would appreciate that alternative solvents could be used.

[0068] In an embodiment, the polymeric composite material comprises polylactic acid, graphene nanoplatelets, activated carbon, and a solvent.

[0069] Alternatively, the polymeric material used to prepare the elongate hollow member may not comprise the catalytic materials. In embodiments, the polymeric material is not a composite material, and can be, for instance, a polymeric material without additional components. In an embodiment, for instance, the polymeric material can be polylactic acid.

[0070] In some aspects the term microbial fuel cell (MFC) may be used interchangeably with the term electrode assembly as used herein. In some cases, an MFC may further comprise a membrane disposed between the cathode and the anode.

[0071] In some embodiments, the MFC comprises more than one electrode assembly (the electrode assembly comprising at least a cathode and an anode described herein) described herein. For example, the MFC may comprise at least two, at least three, at least four, at least five, at least six, at least seven, at least eight, at least nine, at least ten, at least 15, at least 20, at least 25, at least 30, at least 35, at least 40, at least 45, or at least 50 electrical assemblies. In embodiments where the MFC comprises more than one electrode assembly, the electrical assemblies may be electrically connected to one another.The invention further provides methods of treating wastewater using a cathode, electrode assembly, or MFC as described herein, the method comprising placing the cathode, electrode assembly or MFC described herein into wastewater and removing organic carbon and / or nitrogen from the wastewater, thereby treating the wastewater. In some cases, the cathode, electrode assembly, or MFC may be placed in wastewater at a depth of at least 0.5 metres, at least 1.0 metre, at least 1.5 metres, at least 2.0 metres, at least 2.5 metres, at least 3.0 metres, at least 3.5 metres, at least 4.0 metres, at least 4.5 metres, at least 5.0 metres, at least 5.5 metres, at least 6.0 metres, at least 6.5 metres, at least 7.0 metres, at least 7.5 metres, at least 8.0 metres, at least 8.5 metres, at least 9.0 metres, at least 9.5 metres, or at least 10.0 metres. The depth may be measured as the shortest straight-line distance between the base of the cathode, electrode assembly, or MFC and the surface or highest point (e.g. if the wastewater fills a closed vessel, the highest point is the top of the vessel) of the wastewater. The cathode, electrode assembly, or MFC may be placed in wastewater for at least 1 week, at least 2 weeks, at least 3 weeks, at least 1 month, at least 2 months, at least 3 months, at least 4 months, at least 5 months, at least 6 months, at least 9 months, or at least 1 year. The invention therefore also provides methods of (a) removing organic carbon from wastewater using the electrode assembly of the invention; (b) removing nitrogen from wastewater using the electrode assembly of the invention; (c) decolouring wastewater using the electrode assembly of the invention; and / or (d) removing chromatic particles from wastewater using the electrode assembly or MFC of the invention.

[0072] Wastewater may be domestic wastewater, river water, sewage water, landfill leachate, and / or high chemical oxygen demand wastewater. Wastewater may be derived from domestic, industrial, brewery, and / or dairy sites. Wastewater typically contains organic carbon and / or nitrogen.

[0073] The invention also provides methods of monitoring wastewater conditions, the method comprising placing an electrode assembly described herein into wastewater and collecting an electrical readout. The electrical readout may be one or more of voltage, current, current density, and power. The electrical readout may be compared to standard readings to determine a change in wastewater conditions. For instance, the invention is suitable for monitoring the concentration of organic pollutants in, the temperature of, and / or the toxicity of the wastewater.Brief description of the drawings

[0074] The invention will now be more fully described with reference to the following figures.

[0075] Figure 1 illustrates a conventional configuration with a planar cathode positioned at the end of a cylindrical chamber.

[0076] Figure 2 shows a helical cathode (Figure 2A) according to the invention, and the insertion of a GFB anode (Figure 2B).

[0077] Figure 3 shows a paired electrode configured with a screw (Figure 3A), an electric holder board for the screws (Figure 3B); and an MFC reactor with multiple paired electrodes (Figure 3C).

[0078] Figure 4 shows Electrochemical Impedance Spectroscopy (EIS) results for a flat (planar) cathode versus a helical cathode according to the invention.

[0079] Figure 5 shows the power generated from a flat cathode versus a helical cathode.

[0080] Figure 6 shows the results of wastewater treatment using an MFC according to the invention detailed in Example 1.6. Figure 6A depicts total organic carbon (TOC) removal from wastewater; Figure 6B depicts total nitrogen (TN) removal from wastewater; and Figure 6C is a photograph of influent wastewater (before treatment), left, alongside effluent wastewater (after treatment with an MFC), right.

[0081] Figure 7 depicts the current density for one MFC module between weeks 11 and 15 of a wastewater treatment experiment detailed in Example 1.6.

[0082] Detailed description

[0083] A conventional MFC reactor configuration is shown in Figure 1. In this configuration, the cathode (11) has a planar shape, and is positioned at the end of the MFC reactor (12).

[0084] A cathode according to the present invention is shown in Figure 2A. The cathode comprises a elongate hollow member, in this case a tubular structure (23), formed of a polymeric material and coated on its external surface with a catalytic formulation. The tubular structure (23) has a helical configuration which coils around a longitudinal axis (shown), such that in use, an elongate anode can be positioned along the longitudinal axis and inside the coils of the helix. The hollow nature of the tubular structure (23) allows air to move through theinterior of the tube (as shown) and diffuse through the walls of the tubular structure to come into contact with the catalytic formulation and wastewater on the external surface.

[0085] Figure 2B shows the insertion of a GFB anode (24) into the coils of the helical cathode. The complementary shape of the anode allows it to be inserted inside the coils of the helical cathode, so that it sits substantially inside the cathode, resulting in a compact paired structure.

[0086] Figures 3A shows an electrode assembly according to the invention formed of the helical cathode (31) paired with an anode (37) and fixed in place with a screw (38). The screw can be fitted into an electrode holder board (39) with a pre-set electric circuit (40) and holes for fiting the electrodes (41), such as shown in Figure 3B. The electrode holder board can then be positioned at the top of a water tank to form an MFC reactor or array, as shown in Figure 3C. Depending on the requirements of the intended application, the electrode specific surface area (ESSA) can be adjusted by changing the dimensions of the electrodes, and / or the density of the electrode pairs in the electrode carrier board, regardless of the MFC reactor volume. The distance between the anode and the cathode can be very small and fixed, irrespective of the size of the MFC reactors.

[0087] The cathode, electrode assembly, and MFC of the invention are suitable for multiple applications, including the treatment of wastewater. The cathode, electrode assembly, and MFC of the invention may be used to remove organic carbon and / or nitrogen from wastewater. In addition, the cathode, electrode assembly, and MFC of the invention may be used to decolour wastewater in order to produce a substantially clear treated water product.

[0088] Examples:

[0089] The invention will now be described by way of example only.

[0090] Materials:

[0091] Polylactic acid pellets (PLA) were purchased from Smartfil, Spain. Activated Carbon (AC) and Carbon Black (CB) were purchased from Cabot Corporation (AC, Norit SX 2; CB, Vulcan XC-72; Cabot Corporation, USA). Polyvinylidene fluoride (PVDF); graphene nanoplatelets (GNPs), Copper microparticles (10-40 mesh), Graphite powder (<20 pm), and N,N-Dimethylacetamide (>99%) were purchased from Sigma Aldrich, USA. Polytrol was purchased from Owatrol,France. Dichloromethane (DCM) and propan-2-ol, both HPLC grade, were purchased from Fisher Scientific. All reagents were used without further purification.

[0092] Example 1.1: Preparation of Conductive Filament for 3-D Printing

[0093] 1.1.1 Preparation of Composite Material

[0094] To prepare the conductive composite, PLA was first dissolved in DCM at a concentration of 10% w / v. The solution was stirred for 24 hours at room temperature to ensure complete dissolution of the PLA. GNPs and AC were added to the solution and stirred for 12 hours at room temperature to obtain a homogenous dispersion, comprising PLA: GNPs:AC in a weight ratio of 75:20:15. The resulting composite mixture was then air-dried for 24 hours to remove any residual DCM. The composite was then dried in an oven at 90°C for 2 hours to ensure the complete removal of any residual moisture. Finally, the dried composite was ground into pellets and sieved to obtain particles of uniform size (around 2 mm).

[0095] 1.1.2 Fabrication of Conductive Filament

[0096] A conductive filament was then fabricated from the composite material using a simple single screw extruder. The series heaters were set as 175 °C - 190 °C - 195 °C - 185 °C with the rotor at 5 RPM. The diameter of the filament was set at 2.85 mm.

[0097] 1.1.3 Characterisation of Conductive Filament

[0098] Optical and Raman imaging confirmed that the extruded filament had a smooth surface and uniform material distribution.

[0099] Linear Sweep Voltammetry (LSV) showed that the conductive filament had improved electrochemical properties versus a commercial natural PLA filament (2.85 mm, Radionics, Netherlands).

[0100] Example 1.2: Preparation of Catalyst Formulation

[0101] Two catalyst formulations were made, as follows:

[0102] 1.2.1 PVDF-based Catalyst Formulation

[0103] A viscous catalytic formulation was made by mixing 1 g of copper particles, 3 g activated carbon, 0.1 g carbon black and 1 g graphite with 15 ml 10 wt.% PVDF solution (dissolved inN,N-dimethylacetamide). Imaging showed that the catalyst materials were evenly dispersed in the formulation.

[0104] 1.2.2 Polytrol-based Catalyst Formulation

[0105] A viscous catalytic formulation was made by mixing 1.25 g activated carbon, 3.75 g graphene nanoplatelets, 0.25 g carbon black, 9 ml of Polytrol ™, and 4.5 ml of propanol.

[0106] Example 1.3: Preparation of 3-D Printed Elongate Hollow Member

[0107] The geometries of cathodes were designed in Solidworks software (Dassault Systemes, French), and exported as 3D Manufacturing format (3mf) files. The design was then used to convert digital objects into a series of g-codes files that instructed the printer to print out the desired geometry layer by layer (Cure software (Ultimaker, Netherlands).

[0108] 1.3.13-D Printing Parameters

[0109] A fused filament fabrication (FFF) process was used to 3-D print the elongate hollow member, using an Ultimaker S5+ 3D printer. The elongate hollow member was printed using the conductive filament prepared in Example 1.1.2 above, with Poly(vinyl alcohol, PVA) filament (RS, Netherland) used to print the supporting material which would be removed after printing. The 3-D printing setings are set out in Table 1 below for the helical cathode, and Table 2 for the comparative / flat cathode:

[0110] 3D printing settings

[0111] Layer Height 0.2 mm

[0112] Line Width 0.8 mm

[0113] Infill Density 100%

[0114] Printing Temperature 210°C

[0115] Extrusion Cool Down Speed Modifier 0.7 °C / s

[0116] Build Plate Temperature 60 °C

[0117] Print Speed 45.0 mm / s

[0118] Support Speed 35 mm / s

[0119] Enable Retraction YES

[0120] Retraction Distance 5.0 mm

[0121]

[0122] Retraction Speed 25.0 mm / s

[0123] Enable Print Cooling YES

[0124] Generate Support Based on the cathode design

[0125] Support Placement Touching Buildplate / Everywhere based on design Support Overhang Angle 45°

[0126] Support Pattern Grid

[0127] Support Density 45.0%

[0128] Support Interface Pattern Concentric

[0129] Enable Prime Tower YES, if Generate Support is YES, otherwise, NO Union Overlapping Volumes YES

[0130] Remove Mesh Intersection YES

[0131] Alternate Mesh Removal YES

[0132]

[0133] Table 1: Settings for 3-D printing of helical cathode

[0134] 1.3.2 Printing parameters for planar (comparative) cathode

[0135] 3D printing settings

[0136] Layer Height 0.2 mm

[0137] Line Width 0.8 mm

[0138] Infill Density 100%

[0139] Printing Temperature 210°C

[0140] Extrusion Cool Down Speed Modifier 0.7 °C / s

[0141] Build Plate Temperature 60 °C

[0142] Print Speed 45.0 mm / s

[0143] Enable Retraction YES

[0144] Retraction Distance 5.0 mm

[0145] Retraction Speed 25.0 mm / s

[0146] Enable Print Cooling YES

[0147] Union Overlapping Volumes YES

[0148] Remove Mesh Intersection YES

[0149]

[0150] Alternate Mesh Removal YES

[0151]

[0152] Table 2: Setings for 3-D printing of comparative / flat cathode

[0153] Example 1.4: Coating of Elongate Hollow Member with Catalytic Formulation

[0154] 1.4.1 Pre-treatment / etching

[0155] Following fabrication, the 3-D printed PLA elongate hollow member was pre-treated by soaking in dichloromethane for 7 seconds and then being left to air dry at room temperature for 24 hours. This pre-treatment generated an etched surface, confirmed by SEM imaging, to facilitate atachment of the catalyst formulation to the cathode surface.

[0156] 1.4.2 Coating with PVDF Formulation

[0157] Following the etching step carried out in Example 1.4.1 above, the cathode was soaked in the PVDF-based catalytic formulation prepared in Example 1.2.1 for 30 seconds. The cathode was then removed from the formulation, and soaked in water for 10 minutes. The cathode was then left to air-dry overnight to remove any remaining N,N-Dimethylacetamide.

[0158] 1.4.3 Coating with Polytrol-based formulation

[0159] Following the etching step carried out in Example 1.4.1 above, the cathode was then soaked in the Polytrol-based catalytic formulation prepared in Example 1.2.2 for 2 minutes. The cathode was then left to air dry for 24 hours before being heated at 90°C for 2 hours.

[0160] Example 1.5: Characterisation of Helical Cathodes

[0161] Electrochemical Impedance Spectroscopy (EIS) measurements were conducted using a potentiostat / ga Iva nostat (Interface 1010 E, Gamry, US) in a two-electrode configuration. For each EIS test, a singular MFC anode (composed of carbon fibre and a titanium rod) was utilized. The working electrode of the potentiostat / ga Iva nostat was connected to MFC's cathode, while both the counter and reference electrode were connected to MFC's anode. Table 3 shows the EIS measurement setings.

[0162] Initial Frequency (Hz) 100000

[0163] Final Frequency (Hz) 0.01

[0164] Points / decade 10

[0165] AC Current (A rms) lx 10’

[0166]

[0167] 5DC Current (A) 0

[0168] Drift Correction Off

[0169]

[0170] Table 3: EIS measurement conditions

[0171] The results of the EIS measurements are shown in Figure 4. The intersection point of the curve with the real axis in the high-frequency domain represents the total internal resistance of the MFCs. Given that the resistance of the MFC anode is minimal and the same anode was used for both tests, the results effectively display the reduction in resistance achieved using a helical cathode versus a flat cathode, with the resistance of the helical cathodes being only around 10% of that of 2D flat cathode

[0172] The external resistance connected to the MFCs was varied between 2000 and 20 ohms, and the voltage monitored using a digital handheld multimeter (USB6000, National Instruments, USA). The current and power were calculated using standard electrical calculations (e.g., Ohm's law). The results are shown in Figure 5 and demonstrate a significant increase in power for the helical cathode versus the flat cathode, i.e., the power generation for the helical cathode was 250% higher than that of the flat cathode. The maximum power of the helical cathode according to the invention was observed to be around 0.3 mW (Figure 5), which is comparable to that of platinum-based cathodes (Zhang et al., Electrochem. Comm. 2009, 11(11), 2177-2179). These results suggest that non-platinum-based cathodes can represent an efficient alternative to platinum-based cathodes at significantly lower cost.

[0173] Example 1.6: Wastewater Treatment Pilot

[0174] A series of one metre helical air cathode MFCs were installed into a 820 L wastewater treatment vessel in Limerick Bunlicky wastewater treatment plant, Ireland. A pilot wastewater treatment experiment ran for six months.

[0175] 1.6.1. Design

[0176] The air-cathode was designed in SolidWorks software (Dassault Systemes, France). The aircathode was designed with as 50 mm diameter and 1100 mm height, but due to size limitations of the 3D printer (Ultimaker S5+, Netherlands), the air-cathode was designed in four parts and printed separately by natural PLA filament. After printing, the four parts were assembled together through thread and silicone sealant (Tec7, Ireland). Then the cathode waswrapped by copper wire as a current collector. For the coating process, briefly, the air-cathode was coated by a mixture of activated charcoal (Nacorit SX2, Sigma-Aldrich, USA), carbon black (Vulcan XC-72, Cabot Corporation, USA) and poly(vinylidene fluoride) PVDF (Sigma-Aldrich, USA). DMAc (2399%, Sigma-Aldrich, USA ) was used as the solvent treatment and binder. The air-cathodes were soaked in water for at least 10 minutes and air-dried before using. Six aircathodes were set as a group and bundled together. The total area of the final air-cathode assembly was 15.07 m2. Graphite fibre brushes were used as anodes (10 cm diameter, 1 m length). The anodes were heat-treated under 450 °C for 30 minutes for activation before use.

[0177] 1.6.2 Wastewater treatment

[0178] The Limerick Bunlicky wastewater treatment plant primarily processes domestic wastewater, but it also intermittently receives leachate from several landfills via tanker trucks. As a result, the organic matter concentration in the influent to the 3D-printed air-cathode MFC reactor was typically low, yet subject to significant fluctuations. These were not ideal experimental conditions, but still proved useful for demonstrating the potential of the MFC reactor.

[0179] The total organic carbon (TOC) removal by the MFC between weeks 9 and 14 of the pilot experiment is shown in Figure 6A. The TOC concentration had very high volatility. The influent TOC concentration exhibited substantial variability, with the highest recorded at 372 mg / L-C and the lowest at 62 mg / L-C (average 162 mg / L-C). In contrast, the effluentTOC concentration was relatively stable, ranging from a maximum of 102 mg / L-C to a minimum of 21 mg / L-C (average 67 mg / L-C), highlighting the ability of the MFC reactor to remove organic carbon from the wastewater. The maximum removal efficiency reached 81%, reducing TOC from 372 to 70 mg / L-C (at week 9 of the pilot experiment), while the minimum efficiency was only 17%, reducing TOC from 123 to 102 mg / L-C, with average removal efficiency of 52% in 12 hours. Periodic landfill-leachate inputs likely reduced influent biodegradability and increased potential toxicity, thereby contributing to fluctuations in TOC removal. Conversely, the subsequent increase in efficiency can be attributed to the high recovery capability of MFCs. Figure 6B demonstrates the total nitrogen (TN) removal by the MFC during weeks 9 to 14 of the pilot experiment. TN removal was relatively stable across the course of the pilot experiment. The influent TN concentration ranged from a high of 53 mg / L-N to a low of 36 mg / L-N. The average TN removal rate was 34%. Figure 6C compares the colour andtransparency of the influent wastewater (prior to treatment by the MFC reactor) and effluent water samples (following treatment by the MFC reactor), collected at Week 3. The effluent was noticeably clearer than the influent, demonstrating the MFC's capability to significantly reduce chromaticity and improve wastewater transparency.

[0180] No leakage was observed during the course of the pilot experiment, indicating the MFCs were suitable for submersion in wastewater for extended periods of time.

[0181] 1.6.3 Electrical output

[0182] The MFC began producing power one week after inoculation, initially generating approximately 83.08 pA / m2(0.65 pW / m2). Although the initial power output was low, this early production indicated that the MFCs were beginning to adapt to the actual wastewater from the wastewater treatment plant. Throughout the operational period, the average power density was 1.70 pW / m2(130.48 pA / m2). After three months of operation, the maximum power density reached 389.27 pW / m2(2035 pA / m2), which is comparable to the average power density observed when a single module was tested under laboratory conditions for proof-of-concept. The observed fluctuations in power generation suggest the potential for using MFC electricity output as a monitoring tool for wastewater conditions.

[0183] While this invention has been described with respect to microbial fuel cells, one skilled in the art would appreciate that the cathode and associated methodology have more broad utility across a variety of biofuel cell technologies. For instance, the invention could be developed as a COD / BOD sensor to monitor deep water quality in a water tank or in water bodies.

[0184] All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and / or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and / or steps are mutually exclusive. Each feature disclosed in this specification (including any accompanying claims, abstract and drawings) may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features. The invention is not restricted to the details of the foregoing embodiment(s). The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract anddrawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.

[0185] With respect to the use of substantially any plural and / or singular terms herein, those having skill in the art can translate from the plural to the singular and / or from the singular to the plural as is appropriate to the context and / or application. The various singular / plural permutations may be expressly set forth herein for sake of clarity.

[0186] It will be understood by those within the art that, in general, terms used herein, and especially in the appended claims are generally intended as "open" terms (e.g., the term "including" should be interpreted as "including but not limited to," the term "having" should be interpreted as "having at least," the term "includes" should be interpreted as "includes but is not limited to," etc.). It will be further understood by those within the art that if a specific number of an introduced claim recitation is intended, such an intent will be explicitly recited in the claim, and in the absence of such recitation no such intent is present. For example, as an aid to understanding, the following appended claims may contain usage of the introductory phrases "at least one" and "one or more" to introduce claim recitations. However, the use of such phrases should not be construed to imply that the introduction of a claim recitation by the indefinite articles "a" or "an" limits any particular claim containing such introduced claim recitation to embodiments containing only one such recitation, even when the same claim includes the introductory phrases "one or more" or "at least one" and indefinite articles such as "a" or "an" (e.g., "a" and / or "an" should be interpreted to mean "at least one" or "one or more"); the same holds true for the use of definite articles used to introduce claim recitations. In addition, even if a specific number of an introduced claim recitation is explicitly recited, those skilled in the art will recognize that such recitation should be interpreted to mean at least the recited number (e.g., the bare recitation of "two recitations," without other modifiers, means at least two recitations, or two or more recitations).

[0187] It will be appreciated that various embodiments of the present disclosure have been described herein for purposes of illustration, and that various modifications may be made without departing from the scope of the present disclosure. Accordingly, the various embodiments disclosed herein are not intended to be limiting, with the true scope being indicated by the following claims.

Claims

Claims:

1. A cathode comprising an elongate hollow member, the elongate hollow member having a helical configuration which is arranged to coil around an anode to form an electrode assembly, wherein at least a portion of the external surface of the elongate hollow member is coated with a catalytic formulation.

2. A cathode as claimed in claim 1, wherein the elongate hollow member is a tubular member.

3. A cathode as claimed in claim 1 or claim 2, wherein the catalytic formulation comprises a carbon-based catalytic material.

4. A cathode as claimed in claim 3, wherein the carbon-based catalytic material is selected from one or more of activated carbon, carbon black, carbon fibres, carbon nanotubes, graphene, and graphite.

5. A cathode as claimed in any preceding claim, wherein the at least a portion of the external surface of the elongate hollow member comprises copper particles.

6. A cathode as claimed in any preceding claim, wherein the elongate hollow member is formed of a polymeric material.

7. A cathode as claimed in claim 6, wherein the polymeric material is a composite material comprising a polymer and one or more catalytic components.

8. A cathode as claimed in claim 6 or claim 7, wherein the polymeric material is polylactic acid.

9. A cathode as claimed in any preceding claim, wherein the cathode is 3-D printed.

10. An electrode assembly comprising the cathode of any of claims 1 to 9 and an anode.

11. An electrode assembly as claimed in claim 10, wherein the anode is a graphite brush anode.

12. A microbial fuel cell (MFC) comprising the cathode of claims 1 to 9, or the electrode assembly of claims 10 or 11.2313. A method of preparing a cathode for use in a microbial fuel cell, the method comprising:- preparing an elongate hollow member having a helical configuration, from a polymeric material; and- immersing at least a portion of the elongate hollow member in a catalytic formulation to form a helical cathode.

14. A method of preparing a cathode as claimed in claim 13, wherein the catalytic formulation comprises one or both of activated carbon and carbon black.

15. A method of preparing a cathode as claimed in claim 13 or claim 14, wherein the method comprises etching the external surface or portion thereof of the elongate hollow member before immersing the elongate hollow member in the catalytic formulation.

16. A method of preparing a cathode as claimed in claim 15, wherein the etching step comprises immersing the elongate hollow member or a portion thereof in a solvent.

17. A method of preparing a cathode as claimed in any of claims 13 to 16, wherein the step of preparing a helical member from a polymeric material comprises 3-D printing the member.

18. A method of preparing a cathode as claimed in any of claims 13 to 17, wherein the polymeric material comprises polylactic acid, optionally in combination with one or more catalytic components.

19. A cathode prepared by the method of any of claims 13 to 18.

20. A method of treating wastewater, the method comprising the steps of(a) placing the cathode of claims 1 to 9, the electrode assembly of claims 10 or 11, or the MFC of claim 12, into wastewater containing organic carbon and / or nitrogen; and(b) removing organic carbon and / or nitrogen from the wastewater;thereby treating the wastewater.

21. The method of claim 20, wherein placing the cathode, electrode assembly, or MFC into wastewater also decolours the wastewater.

22. A method of monitoring wastewater conditions, the method comprising the steps of (a) placing the cathode of claims 1 to 9, the electrode assembly of claims 10 or 11, or the MFC of claim 12 into wastewater containing organic carbon and / or nitrogen;(b) collecting electrical readouts from the cathode or electrode assembly; and (c) comparing the readouts to a standard to determine wastewater conditions;Wherein the wastewater conditions are one or more selected from the concentration of organic pollutants in, the temperature of, and the toxicity of the wastewater.

23. The method of any one of claims 19 to 22, wherein the cathode, electrode assembly, or MFC is placed in the wastewater at a depth of at least 2 metres.

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