The generation of clean hydrogen as an energy carrier via water electrolysis is becoming central for the modern renewable-energy economy. Of the water electrolysis operated at low temperatures, two main techniques, namely, alkaline water electrolysis (AWE) and proton exchange membrane (PEM) electrolyzers, are involved in today's hydrogen market. Their unique solid-ionomer membrane property enables PEM to operate at a high current density (> 2 A/cm 2 ), while the locally acidic conditions of PEM require high-yield catalysts based on expensive platinum group metals (PGM), which drives up the capital expenses. Compared to PEM, AWE operates in liquid-alkaline electrolytes that allow the use of inexpensive non-PGM materials. However, the sluggish gas crossover through the membrane and the need for highly efficient catalysts on the OER side limit the high H2 yield of AWE. It is also noteworthy that both AWE and PEM require two separate electrodes that need to be operated under the same conditions for the generation of O2 (anode) and H2 (cathode) simultaneously. To further lower the costs, rational design of earth-abundant catalysts that show excellent catalytic activities for overall water splitting is therefore crucial.