It turns out that voltage and current flow are interrelated. Lowering the voltage for electrolysis, which will increase the energy efficiency of the process, is an important area for research. A number of countries are interested in using hydrogen to fuel aircraft. The Russians developed and tested a plane fueled by liquid hydrogen some years ago. NASA is currently studying the use of hydrogen as aviation fuel.
Slush hydrogen has a lower temperature and a higher density than liquid hydrogen.
Most of the hydrogen produced today is consumed on site, such as at an oil refinery, and is not sold on the market. When hydrogen is sold on the market, the cost of liquefying the hydrogen and transporting it to the user must be added to the production cost. Photoelectrochemistry is the study of semiconductors in contact with a liquid solution. Light shining on a semiconductor can initiate a chemical reaction with species in solution. With respect to solar energy, the interest in photoelectrochemistry stems from the fact that this system can split water into hydrogen and oxygen.