Preventing climate change is a major challenge of our society today and electrochemical energy storage devices are key elements for rendering our economy energy efficient. Whereas batteries store energy within their electrodes, fuel cells and flow batteries store it externally in an electrochemical energy carrier (a fuel). Both technologies operate reversibly, i.e. can cycle between electrical and chemical energy, yet only the latter can store energy independent from cell size and proportional to the amount of external energy carrying fluid. Energy conversion then depends on fluid flow processes within the electrodes, which requires transporting and/or mixing fuels, bringing reactant species to the electrolyte, and transporting products away. This challenging task has not been tackled sufficiently so far to bring such green energy concepts to broader application. The core component responsible for transport in all such architectures is the electrode, which must fulfil a multitude of conflicting transport requirements. Electrodes must provide pathways for reactants and products in different states of matter and facilitate dissimilar transport mechanisms. They must be chemically inert, corrosion resistant, provide conflicting surface properties for separating the transport of reactants and products while being catalytically active or able to support anchoring sites for catalytic particles. They must combine high mechanical strength with low weight and volume while being gas tight, crack free, and stable towards oxidation as well as compatible with other components of the assembly. In addition, they must operate reliably over a wide range of temperatures, must be cost effective and easily manufacturable.
The design of an electrode needs to balance convection and diffusion, which are physically fundamentally different processes separated by two orders of magnitude in length scales. The former transport mechanism involves the movement of species by bulk motion and the latter by a gradient in concentration. Convection is related to macroscopic (mm to cm scale) flow (inside the flow channels, FC) and diffusion to the movements of atoms and molecules (µm-nm scale). The complex interaction of multiphase and multiphysics flow in electrode architectures has challenged researchers for half a century.
The state-of-the-art electrode is a multi-layer stack comprising a catalytic layer (CL, site of the chemical reaction), a micro-porous layer (MPL, blocks liquids), a gas diffusion layer (GDL, facilitates diffusion-based transport of reactants and products), flow channels (FC, provides convective fuel flow pathways for reactants), and bipolar plates (BP) that compresses the entire Membrane Electrode Assembly (MEA). However, this architecture is a compromise: to allow for these vastly differing feature ranges, our limited manufacturing abilities require us to confine a set of producible geometries and properties in layers separated by sharp boundaries, whereas the transport mechanisms at play are manifold and spatially co-located not having clear and sharp demarcation lines. For GDL and MPL architectures, we are only able to produce heterogeneous architectures with limited control over their geometries. This entails that researchers first need to discern the random, inhomogeneous, and quite complex internal structure and material properties before studies on mass transport mechanisms can start.
To overcome this impasse, we are changing the starting point of future experiments: Rather than tediously studying mechanisms at play in current designs, we aim to hypothesize and validate our assumptions directly by converting a simulated model into a physical design for subsequent testing. We first model individual transport mechanisms and the correlation between individual structural or chemical changes on the computer, derive 3D architectures on the computer and print them on a 3D printer for subsequent evaluation in a fuel cell testing station. The enabling technology is the fabrication of dimensionally stable carbon from patterned photopolymers. We define dimensionally stable carbon as a type of carbon material that is derived from patterned photopolymers and is converted to carbon in a subsequent heat treatment. During patterning, the photopolymer is cured in a predefined 3D shape through interaction with light of a specific wavelength. Both chemical composition (the formulation) and the heat treatment program (carbonization protocol) is being developed in this project. With this approach, we can derive amorphous carbon electrodes from a Computer Aided Design (CAD) with features spanning over multiple orders of magnitude.
This way, we will omit the conventional necessary step of evaluating single features of electrode architectures and allow researchers to focus on what is relevant: finding the right mechanisms at play and derive corresponding strategies to govern the complex flow characteristics and the electrical, thermal and chemical properties required to construct the next generation of clean energy converters.
Ultimately, the knowledge generated in this project shall render electrochemical flow converters (fuel cells, electrolyzers and flow batteries) more attractive and commonplace in our transition towards clean energy concepts.
