Water splitting to generate O2 and H2 fuel has been a major focus of (photo)electrochemical energy storage and conversion efforts, but many challenges remain. In this talk, I will begin by showing our recent efforts to elucidate the catalytically active phase and OER mechanism on NiFe layered double hydroxides by combining electrochemical measurements, operando experiments, DFT calculations, and ab initio molecular dynamics simulations. Next, for HER, I will introduce the methodologies we have recently developed towards the highly accurate prediction of Pourbaix diagram of transition metal (oxy)hydroxides. Subsequently, using monolayer Ni (oxy)hydroxide films as an example, I will describe a simple scheme to study the structures and the stability of these films on precious metal surfaces. I will show how the ultrathin films can be dramatically stabilized with respect to the corresponding bulk analogs. Then, using the hydrogen evolution reaction as an example, I will demonstrate how these techniques can be applied to understand the steady state, the active phases, and the catalytic mechanism of bi-functional interfaces. I will then demonstrate the extension of the present understanding to real-world catalysts, i.e. precious metal nanoparticles supported on ultrathin transition metal (oxy)hydroxide films. Finally, I will show this understanding can be used to design new bi-functional catalysts with improved performances. If time permits, I will also show our recent work on tunable intrinsic strain in two-dimensional transition metal electrocatalysts for the oxygen reduction reaction.