In the search for new large-scale battery technologies that are cheap, safe and durable, in this research it is found that the state of the art semi-solid redox flow battery (“SSRFB”) technology shows perspective. Therefore, a novel, cheap and safe aqueous SSRFB design is introduced based on iron- and manganese hydroxide materials. This design was subsequently tested in the lab through which it was concluded that it is not possible to effectively make a SSRFB based on FeOOH and Mn(OH)2 semi-solid electrode (“SSE”) suspensions. Systematic proof is provided on how this can be attributed to the specific combination of the active materials in the highly alkaline environment, but that in itself, both the tested SSRFB configuration and SSE suspensions are functional. On the one hand, a complex chemical system of the active species consisting of multiple thermodynamic equilibria had the consequence that the open circuit voltage (“OCV”) had a very low value of 0.2 V instead of the expected 0.63 V. On the other hand, it was shown that both the FeOOH and the Mn(OH)2 SSE suspension could be separately cycled versus respectively a solid zinc and copper electrode and that a cheap microfiltration membrane suffices in the design of a SSRFB (on the condition that ionic active species are not apparent in the system). In sum, this research demonstrates that the SSRFB by its very nature is a suitable battery technology in which cheap and safe materials can be used for large-scale energy storage. However, that the combination and optimization of the right active materials in the right environment proves to be fundamental for proper operation and is in fact a complex balance between electric and ionic conductivity, colloidal stability, viscosity and (electro)chemical stability.