text: Water-splitting photoelectrochemical (PEC) cells use light energy to decompose water into hydrogen and oxygen within a two-electrode cell. In theory, three arrangements of photo-electrodes in the assembly of PECs exist:
  * photo-anode made of a n-type semiconductor and a metal cathode
  * photo-anode made of a n-type semiconductor and a photo-cathode made of a p-type semiconductor
  * photo-cathode made of a p-type semiconductor and a metal anode
  There are several requirements for photoelectrode materials in PEC  production:
  * light absorbance: determined by band gap and appropriate for solar irradiation spectrum
  * charge transport: photoelectrodes must be conductive (or semi-conductive) to minimize resistive losses
  * suitable band structure: large enough band gap to split water (1.23V) and appropriate positions relative to redox potentials for  and 
  * catalytic activity: high catalytic activity increases efficiency of the water-splitting reaction
  * stability: materials must be stable to prevent decomposition and loss of function
  In addition to these requirements, materials must be low-cost and earth abundant for the widespread adoption of PEC water splitting to be feasible.
  While the listed requirements can be applied generally, photoanodes and photocathodes have slightly different needs. A good photocathode will have early onset of the oxygen evolution reaction (low overpotential), a large photocurrent at saturation, and rapid growth of photocurrent upon onset. Good photoanodes, on the other hand, will have early onset of the hydrogen evolution reaction in addition to high current and rapid photocurrent growth. To maximize current, anode and cathode materials need to be matched together; the best anode for one cathode material may not be the best for another.