I will present my group work in the past 10 years in designing nanomaterials and devices for applications in water. Examples include: 1) We developed three-dimensional nanostructured electrodes to interface with microbes effectively for high efficiency microbial fuel cells and invented microbial batteries where air cathodes are replaced by solid-state battery cathodes, resulting in multiple times of efficiency improvements. 2) We invented a low-cost disinfection technology based on nanoscale electroporation and tunable bandgap of MoS2 layered materials for photoelectrochemical disinfection. 3) We also originated a concept of seawater battery which can do both desalination and electricity generation. 4) We developed new electrochemical devices for uranium extraction from seawater and heavy metal extraction.