Photoelectrochemical (PEC) water splitting is one of the promising renewable energy techniques that can store solar energy into one of the clean and renewable energy resources, hydrogen (H₂). However, facilitating the renewable PEC water splitting system on a global scale is still challenging. Especially, finding suitable p-type photocathodes is critical which have a narrow bandgap, a great solar to hydrogen (STH) efficiency and economical fabrication processes with earth abundant elements. Metal oxides have shown remarkable advantages that satisfy the important requirements to be an excellent photocathode among the suggested candidates.

Copper ferrite (CuFe₂O₄) is a promising but rarely studied p-type photocathode based on the relatively narrow bandgap (~1.7eV) which can utilize a great portion of solar light to realize the maximum STH efficiency of ~30% and photocurrent of ~25mA/cm² theoretically. In addition, CuFe₂O₄ has positive flat band and photocurrent onset potentials (~1.0 VRHE) that are critical to achieve high operating photocurrent with n-type photoanode for an ideally unbiased entire solar water splitting system. Moreover, it consists of earth abundant elements. However, there are still limiting features for the use of CuFe₂O₄ photocathode since it has shown a relatively poor photocurrent and the synthesis process inevitably requires long time annealing at high temperature to produce less defective and better crystalline material for a high charge transport property.

Here, we employed the rapid and high temperature (>980°C) flame as an annealing process for CuFe₂O₄ photocathode synthesis, demonstrating the significantly enhanced photocurrent (-1.82 mA/cm² at 0.4V_RHE in Ar purged 1M NaOH) as a single material photocathode whose photoactivity is greater than that of most studied photocathodes. There are several unique advantages of the flame annealing compared with other conventional annealing methods for the synthesis. For example, the flame does have negligible ramping time to reach high temperature above 980°C due to its short ignition delay time that finally makes the high crystalline CuFe₂O₄ in only 16 minutes. Moreover, the flame is operated in the open atmospheric environment and the amount of oxygen can be easily controlled by adjusting the fuel to oxygen equivalence ratio as a combustion parameter. Oxygen rich environment was employed for CuFe₂O₄ annealing to provide fully oxidized condition for an amorphous Cu and Fe mixture prepared by sol-gel method. Finally, the porosity of CuFe₂O₄ was highly increased by flame annealing, and the physical structural change positively affects the enhancement of the light absorption property and the specific surface area increase.