Numerous electrochemical processes rely upon or are drastically improved by the use of electrodes having porous, 3D structures rather than flat, 2D structures.
This is particularly true for the many electrochemical processes that require the simultaneous presence of different phases of matter—liquid, gas, and solid phases—at an interface. For example, hydrogen-oxygen fuel cells typically utilize the transformation of gaseous oxygen and hydrogen into liquid water at solid-phase, electrically-connected catalysts, like platinum metal. To facilitate this reaction, electrodes capable of providing a three-phase, solid-liquid-gas boundary are required. Such electrodes must contain an electrically-connected solid phase to which both the gases and liquids have access.
3D electrodes are also used in industrial electrochemical processes where slow reaction kinetics or low, mass-transfer-limited current densities (typically less than 100 mA/cm2) necessitate the application of large electrochemically active surface areas.
Several different classes of 3D electrodes have been developed. Common forms include:                (1) Fixed bed electrodes, such as                    i. Reticulated electrodes;            ii. Felt or textile electrodes;            iii. Particulate electrodes.                        (2) Layered, porous electrodes.        
Reticulated electrodes typically resemble an electrically conductive net or a network. Examples include conductive foams through which liquids and/or gases may move such as Reticulated Vitreous Carbon (RVC).
Felt electrodes typically comprise of a textile-like conductive network, often comprising of conductive carbon textile, such as a carbon cloth electrode.
Particulate fixed bed electrodes typically comprise of a porous mass of conductive particles, such as carbon black particles, fused by compression or sintering, with hydrophobic particles like PTFE particles (PTFE=polytetrafluoroethylene, or Teflon™). The key variable in this fabrication process is the relative quantity of PTFE that must be included. PTFE particles are needed to hold the structure together and create the required porosity. However, the quantity of PTFE must be such as to impart an intermediate and not an overwhelming hydrophobicity on the structure. That is, the hydrophobicity of the electrode should allow partial, but not complete water ingress, in order to thereby allow for the creation of a three-phase solid-liquid-gas boundary within the electrode. Consequently, optimisation of the properties of conventional particulate fixed bed electrodes typically involves optimisation of the relative quantity of PTFE particles included in the solid-state mixture used to create the electrode. That is, conventional particulate fixed bed electrodes are typically optimised by manipulating their hydrophobicity to thereby promote the formation of a three-way solid-liquid-gas boundary within the electrode.
An example of a particulate fixed bed reactor of this type is the “Trickle-Bed Reactor” (TBR) in which a liquid and a gas are simultaneously moved over a packed bed of catalyst particles. To act as an electrode, the materials on the surface of the particles in a Trickle-Bed Reactor need to be electrically connected to each other, and collectively comprise either the anode or the cathode in the reactor. The hydrophobicity of the trickle-bed reactor is typically intermediate, allowing the partial ingress of both water and gas, to thereby create a three-phase solid-liquid-gas boundary within the bed. A three-phase solid-liquid-gas boundary refers to a reaction boundary involving gaseous material, liquid electrolyte, and solid matter such as from an electrode or a catalyst. The hydrophobicity is made optimum by adjusting the relative quantity of PTFE particles in the bed.
Layered, porous, particulate fixed beds of this type are also commonly used in the Gas Diffusion Electrodes (GDEs) employed in proton exchange membrane fuel cells. GDEs of this type typically comprise of porous layers of conductive carbon particles of different size fused with PTFE particles of various sizes. The outermost layers typically contain fused carbon black and PTFE particles of the smallest dimensions. The inner-most layers typically contain the largest particles. There may be multiple intermediate layers of intermediate particle size.
The intention of this gradation in particle size within GDEs, from largest in the center to smallest on the outer sides, is to create and control a three-phase solid-liquid-gas boundary within the electrode. This boundary should have the largest possible surface area. The creation of such a boundary is achieved, effectively, by controlling the average pore sizes between the particles, ensuring that the smallest pore sizes are at the edges and the largest are in the center. Since the pores are typically relatively hydrophobic (due to the PTFE binder), the small pore sizes at the edges (e g. 30 microns pore size) act to hinder and limit the ingress of liquid water into the GDE. That is, water can penetrate only a relatively short distance into the GDE, where the electrochemically active surface area per unit volume, is largest. By contrast, the larger pores in the centre of the GDE (e.g. 150 microns pore size), allow for ready gas transmission at low pressure along the length of the GDE, with the gas then forming a three-way solid-liquid-gas boundary with the liquid water at the edges of the GDE, where the electrochemically active surface area per unit volume is the largest.
Layered porous electrode structures are presently the industry standard for:                (1) conventional free-standing GDEs (for example, of the type used in hydrogen-oxygen PEM fuel cells); and        (2) hybrid GDEs, where a GDE layer has been incorporated within an electrode, typically between a current collector and the gas zone.        
GDEs of this type often display significant technical problems during operation. These largely derive from the difficulty of creating a seamlessly homogeneous particulate bed, with uniform pore sizes and distributions, and uniform hydrophobicity (imparted by the hydrophobic PTFE binder within the GDE). Because of the resulting relative lack of uniformity in the GDE structure, the three-phase solid-liquid-gas boundary created within the GDE may be:                Unstable and fluctuating. The location of the boundary within the GDE may be subject to changing conditions during reaction which cause the boundary to constantly re-distribute itself to new locations within the GDE during operation.        Inhomogeneous. The boundary may be located at widely and unpredictably divergent depths within the GDE as one traverses the length of the GDE.        Inconsistent and ill-defined. At certain points within the GDE, there may be multiple and not a single solid-liquid-gas boundary.        Prone to failure. The boundary may fail at certain points within the GDE during operation, causing a halt to the desired chemical reaction. For example, a common failure mode is that the GDE becomes completely filled with the liquid phase, thereby destroying the three-phase boundary; this is known in the industry as “flooding” Flooding is a particular problem in fuel cells, such as hydrogen-oxygen fuel cells, that require the feedstock gases to be humidified. Flooding may be caused by water ingress into the gas diffusion electrode via systematic, incremental percolation through the non-homogeneous pores of the electrode, or it may be caused by spontaneous condensation of the water vapour in the feedstock gas stream. In all cases, flooding induces a decline in the voltage output and power generation of such fuel cells.        
Problems of this type are not conducive to optimum operations and may result in uneven, low-yielding, incomplete or incorrect reactions, amongst others.
The phenomenon of flooding described above, is often caused by water ingress into the gas diffusion electrode when the water is subject to any sort of external pressure. For example, in an industrial electrolytic cell of 1 meter height, the water at the bottom of the cell is pressurised at 0.1 bar due to the hydraulic head of water. If a GDE were used at this depth, the GDE would typically be immediately flooded by water ingress because modern-day GDEs have very low “wetting pressures” (also known as the “water entry pressure”), that are typically less than 0.1 bar (although GDEs with wetting pressures of 0.2 bar have recently been reported in WO2013037902). GDEs are, additionally, relatively expensive.
Conventional 3D Particulate Fixed-Bed Electrodes and GDEs
At the present time, 3D particulate fixed bed electrodes and gas diffusion electrodes (GDEs) are conventionally fabricated by mixing carbon black and PTFE powders and then compressing the solid mixture into a bulk, porous electrode.
The pore size of the resulting structure may be very roughly controlled by managing the particle size of the particulates used. However, it is difficult to achieve a uniform pore size throughout the electrode using this approach because particles, especially “sticky” particles like PTFE, often do not flow evenly and distribute themselves uniformly when compressed. A wide range of pore sizes are therefore typically obtained. It is, moreover, generally not possible to create structures with uniformly small pore sizes, such as 0.05 μm-0.5 μm in size.
The hydrophobicity of the structure is typically controlled by managing the relative quantity of PTFE incorporated into the structure. The PTFE holds the structure together and creates the required porosity. However, its quantity must be carefully controlled so as to impart the electrode with an appropriately intermediate hydrophobicity. An intermediate hydrophobicity is needed to ensure partial, but not complete water ingress. In the case of GDEs, this is needed to thereby create a solid-liquid-gas boundary within the carbon black matrix that makes up the electrode.
This method of constructing 3D particulate fixed bed electrodes and gas diffusion electrodes creates some significant practical problems when operating such electrodes in industrial electrochemical cells, particularly in electrosynthetic and electroenergy applications. These problems include the formation of three-way solid-liquid-gas boundaries that are: ill-defined, inconsistent, unstable, fluctuating, inhomogeneous, and prone to failures like flooding.
Problems of this type largely arise from the intrinsic lack of control in the fabrication process, which attempts to create all of the inherent properties of the electrode—including porosity, hydrophobicity, and conductivity—in a single step. Moreover, the fabrication method seeks to simultaneously optimise all of these properties within a single structure. This is often not practically possible since the properties are inter-related, meaning that optimising one may degrade another.
Despite these drawbacks, the approach of combining particulate carbon black and PTFE into a compressed or sintered fixed bed remains the standard method of fabricating GDEs for industrial electrochemistry. This approach is used to fabricate, for example, free-standing GDEs of the type used in hydrogen-oxygen PEM fuel cells. Even where only a GDE component is required within an electrode, the standard method of fabricating that GDE component is to form it as a compressed, porous layer of particulate carbon black and PTFE.
For the above and other reasons, the conventional method of making GDEs and the properties of conventional GDEs are open to improvement.
FIG. 1 (prior art) depicts in a schematic form, a conventional 3D particulate fixed bed electrode or a gas diffusion electrode (GDE) 110, as widely used in industry at present.
In a conventional 3D particulate fixed bed electrode or GDE 110, a conductive element (e.g. carbon particles) is typically combined (using compression/sintering) with a non-conductive, hydrophobic element (e.g. polytetrafluoroethylene (PTFE) Teflon™ particles) and catalyst into a single, fixed-bed structure 110. The fixed-bed structure 110 has intermediate hydrophobicity, good but not the best available conductivity, and a pore structure that is non-uniform and poorly defined over a single region 113. When the 3D particulate fixed bed electrode or GDE 110 is then contacted on one side by a liquid electrolyte and on the other side by a gaseous substance, these physical features bring about the formation of an irregularly-distributed three-phase solid-liquid-gas boundary within the body of the electrode 110, below its outer surface 112 and within single region 113, as illustrated in the magnified view presented in FIG. 1. At the three-phase boundary, electrically connected catalyst (solid phase) is in simultaneous contact with the reactants (in either the liquid or the gas phase) and the products (in the other one of the liquid or gas phase). The solid-liquid-gas boundary within the GDE 110 therefore provides a boundary at which electrochemical liquid-to-gas or gas-to-liquid reactions may be facilitated by, for example, the application of a particular electrical voltage. The macroscopic width of the three-phase solid-liquid-gas boundary is comparable or similar in dimension to the width of the conventional GDE. The thickness of the three-phase solid-liquid-gas boundary in a conventional GDE is typically in the range of from 0.4 mm to 0.8 mm in fuel cell GDEs up to, higher thicknesses, such as several millimeters, in industrial electrochemical GDEs.
Because of the practical and commercial importance of 3D electrodes, new 3D electrodes and practical methods of fabricating 3D electrodes are always of interest. This is especially true for GDEs, upon whose effective operation many industrial electrochemical reactions rely.
Efforts have therefore been made to develop new 3D electrodes and fabrication processes therefor. By way of example only, U.S. Pat. No. 7,229,944 B2 teaches the use of a new technique known as “electrospinning”, to generate a novel, conductive “nano-fibrous” 3D electrode comprising conductive carbon fibres decorated with catalyst materials.
There exists a need for new types of practically useful three-dimensional (3D) electrodes, preferably for industrial scale electro-energy or electro-synthetic applications, cells or devices including one or more of the 3D electrodes, and/or methods of fabrication of 3D electrodes. Of particular interest are 3D electrodes that can act as Gas Diffusion Electrodes (GDEs).
The reference in this specification to any prior publication (or information derived from it), or to any matter which is known, is not, and should not be taken as an acknowledgment or admission or any form of suggestion that the prior publication (or information derived from it) or known matter forms part of the common general knowledge in the field of endeavour to which this specification relates.