DISCUSSION SECTION 843device described. The apparatus produces hydrogen and oxygen via a photoelectrotytic process involving water, and the authors compute its conversion efficiency as the ratio of the heat of combustion of the hydrogen it produces to the time-integrated insolation at its photoanode array. What is overlooked, however, is that the system does not merely electrolyze water. Since the anodes operate in 1.0M NaOH and the cathodes in 0.5M H2SO4, the electrode reactions are 2H + + 2e-->/-/2 at the cathode, and 2 OH---> H20 q-~ O2 q-2eat the anode, from which one finds that the actual cell reaction is 2H + -~ 2 OH--> H~O + It2 + ~ O2 It is clear, therefore, that 1 mole of water is produced per mole of hydrogen, and that the enthalpy of the neutralization, which is essential for spontaneous operation of the cell, is provided not by the sun, but rather by the power plants of the firms that produce the NaOH and H2SO4.The heat of combustion of hydrogen to water vapor is --57.8 kcal/mole, while the heat of neutralization of H + by O,I-F is approximately --13 kcal/mole. From these values one finds that, at best, the energy conversion efficiency calculated by the authors is overstated by ~25%. This alone is significant, but perhaps more so is the fact that remote energy consumption has not been acknowledged. I feel it is necessary that any new energy production technique be scrutinized to determine to the greatest extent practical not only the amount of energy that is produced, but the amount consumed in hardware fabrication, operation, and maintenance.
A. Fujishima, 44 K. Kohayakawa, 45 and K. Honda: 44We agree with the comments given by Bezman in relation to the contribution of the neutralization energy to the formation of hydrogen. This contribution was already pointed out in our previous paper 46 where the neutralization energy was expressed in the form of the electromotive force due to the difference in pH between the anolyte and the catholyte. As the electromotive force is 59 mV for unit pH difference in comparison with the thermodynamic decomposition voltage of water of 1.23V, the contribution of neutralization energy to the decomposition of water will be about 5% for each one unit pH difference.As the integrated incident solar energy was not measured on the day of experiments in the present paper, the reported value of solar energy in Japan of 3000 kcal/m2/day was referred to and taking into consideration the above-mentioned contribution and the actual weather conditions of the day of experiments, the rough estimate of the conversion efficiency of 0.4% was given for a simple example.
