Processing of Advanced Foam Scaffolds for Iron-Air Battery Applications

Novel iron-air batteries under current development utilize iron powder beds which are subjected to redox cycles with steam and hydrogen, but sinter during cycling and thus lose their capacity. We propose to develop iron scaffolds that can withstand multiple redox cycles to generate/consume hydrogen during the operation of iron-air batteries. Our hypothesis is that iron scaffolds, because of their high surface/volume ratio and connectivity, allow for volume changes occurring during redox cycling without damage or sintering, unlike powder beds. Here, we propose a fundamental study to test this hypothesis by developing a new manufacturing route relying on (a) directional freeze-casting of Fe2O3 nano-powder slurries to create powder scaffolds (a method established for other oxides), followed by (b) reduction and sintering (the novel aspect in this proposal) of the Fe2O3 to Fe. These iron scaffolds with aligned walls and high surface area are expected to show good gas permeability and to withstand redox cycles without sintering or pulverizing because of the continuous architecture and the high surface area allowing for mismatch-free volume changes. We will further modify the Fe scaffolds by incorporating, within their walls, (i) Ni in solid solution, by freeze-casting a slurry of mixed NiO and Fe2O3 powders and sintering and reducing, to increase resistance to spallation; (ii) micron-sized equiaxed pores created by temporary space-holders (such as SrF2), to increase the surface area of the scaffold and further reduce mismatch strains during cycling, and (iii) inert reinforcement (non-reduced ceramic particles, dispersoids or whiskers) to increase the strength and fracture resistance of the scaffolds during redox cycling. Architecture and microstructure of the metal and oxide scaffolds will be studied at various stages of reduction/oxidation within and across chemical cycles. Finite-element modeling based on experimental tomographic data will help predict the architectures most stable during multiple redox cycles. Intellectual merit — Iron powders have attracted attention as an inexpensive and environmentally benign option to generate/store hydrogen through the Fe/FeO redox reaction: (i) the hydrogen created during oxidation of iron by steam is consumed in a fuel cell to generate electricity, and (ii) the hydrogen produced by operating the fuel cell in reverse is consumed by reducing the oxide to metal. This combination of hydrogen generator/storage and reversible fuel cell functions overall as an iron-air rechargeable battery. Currently, a bed of iron powders is used in these experimental batteries, but the powders sinter rapidly during the redox cycles, reducing the surface area of the iron and thus the efficiency of the battery. Our approach — creating a continuous iron scaffold capable of accommodating volume changes during redox cycles without damage or sintering — will enable the development and large-scale deployment of inexpensive, environmentally-benign, scalable iron-air batteries for applications such as electricity storage from intermittent renewable energy sources, backup storage, load balancing and peak shaving. Broader impacts — The ability to use hydrogen to generate/store electricity is critical to increasing the share of renewable but intermittent electricity sources such a wind or solar. Current hydrogen storage materials contain expensive, scarce and toxic elements; by contrast, our proposed starting material, Fe2O3, is both inexpensive and non-toxic. If verified, our hypothesis – that iron scaffolds accommodate