Production of a monolithic fuel cell stack with high power density

Abstract

The transportation sector is undergoing a technology shift from internal combustion engines to electric motors powered by secondary Li-based batteries. However, the limited range and long charging times of Li-ion batteries still hinder widespread adoption. This aspect is particularly true in the case of heavy freight and long-range transportation, where solid oxide fuel cells (SOFCs) offer an attractive alternative as they can provide high-efficiency and flexible fuel choices. However, the SOFC technology is mainly used for stationary applications owing to the high operating temperature, low volumetric power density and specific power, and poor robustness towards thermal cycling and mechanical vibrations of conventional ceramic-based cells. Here, we present a metal-based monolithic fuel cell design to overcome these issues. Cost-effective and scalable manufacturing processes are employed for fabrication, and only a single heat treatment is required, as opposed to multiple thermal treatments in conventional SOFC production. The design is optimised through three-dimensional multiphysics modelling, nanoparticle infiltration, and corrosion-mitigating treatments. The monolithic fuel cell stack shows a power density of 5.6 kW/L, thus, demonstrating the potential of SOFC technology for transport applications.

Fig. 1. Metal-based monolithic stack design.

a Illustrations of conventional and monolith fuel cell stacks with five repeat units and a single repeating unit (SRU) monolith stack. The numbers 1, 2, and 3 correspond to the interconnects, fuel cells (electrolyte and electrodes), and cell supports, respectively. b Photograph of three modules of SRU monolith. c Illustration of an exploded view of a SRU monolith. The numbers 4, 5, 6, and 7 correspond to interconnects and gas distribution channels (400 µm thick including 250 µm high gas channels), electrodes (10 µm), electrolyte (10 µm), and sealing (10 µm), respectively. d Enlarged cross-sectional view of a SRU monolith (for clarity, the scale of the layers thickness is not respected).

Fig. 2. Effect of pore-forming materials on built-up pressure during the debinding step.

The graphs display the predictions of a multiphysics model on the pressure built-up inside the SRU monoliths (black lines/symbols) during the debinding step (red lines). Below the graphs, the photographs show the integrity of the corresponding SRU monoliths after heat treatment (debinding and sintering steps). a, Case of a SRU monolith manufactured using only graphite as sacrificial material to form the gas channels, b, Case of a SRU monolith manufactured using a 50–50 vol.% mixture of graphite–PMMA as sacrificial material to form the gas channels, and c, Case of a SRU monolith manufactured using only PMMA as sacrificial material to form the gas channels. The SRU presented in Fig. 2c reveals large cracks after debinding/sintering steps which is in good accordance with the model which predicted that SRU monolith manufactured using 100% PMMA would lead to the highest pressure (14 mbar around 200 °C) among the three pore-forming agents investigated, and therefore will be the most likely to fracture. Note that both PMMA and graphite are also contained the electrode tapes which explains why a pressure peak corresponding to graphite removal can also be found in the case of Fig. 2c, for example.

Fig. 3. Microstructural analyses of the monolith.

a Cross-sectional SEM image showing the SRU monolith with a gas channel enlarged. The annotations E, El, and IC correspond to electrode, electrolyte, and interconnect, respectively. b High-magnification cross-sectional SEM images of the cell layers. c Cross-sectional SEM images of the SRU monoliths after oxidation in air or 80% H₂O–20% H₂ for 100 h at 650 °C. Note the scale difference.

Fig. 4. Electrochemical performance and comparison with the SoA SOFCs.

a i–V curve (full symbol) and power density (empty symbol) of SRU monolith cell with an active cell area of ∼18 cm² measured at 780 °C, dry H₂ (25 l/h) to the fuel electrode, and O₂ to the oxygen electrode. b, Comparison of initial electrochemical performance of the SRU monolith cell (blue full symbol) with that of SoA commercially available Ni/YSZ anode supported SOFCs (green empty symbol). The power density in terms of power per volume is based on a SRU height of 830 µm for the monolith and in the range of 1400–4000 µm for the SoA anode supported SOFCs design.