Salinity differences (e.g. at river mouths where fresh water mixes with salty ocean water) are a potential source of energy, although no practical method of converting the chemical potential differences to electricity have ever been developed. Chemistry World recently published an article about the development of MoS2 osmotic nano generators which set record performance levels in this obscure branch of renewable energy generation. However, even though this electricity generation method may find niche applications, it appears that the costs will still be too high to allow it to become a significant contributor to the global supply of energy.
A decade and half ago a group of scientests Calilfornia Institute of Technology (CIT) in vented a new type of fuel cell in which the proton conduction membrane of PEM fuel cells is replaced by a substance called a superprotonic solid acid. The result of this replacement is a new type of fuel cell called a solid acid fuel cell (SAF). SAF fuel cells operate at at temperature of 250C which is intermediate between PEM fuel cells (<120C) and solid oxide fuel cells (>800C). Unlike PEM fuel cells SAF cells do not require humidification and they are relatively tolerant of impurities (e.g. S and CO) which allows them to be operated using reformed hydrocarbon fuels. The 250 degree operating temperature also potentially allows the electrodes to use much lower loadings of platinum group metals in the electrodes thus leading to lower costs. On the other hand avoiding the high temperature of solid oxide fuel cells avoids some of the high costs associated with using exotic materials which can withstand high temperature operation.
However, as discussed at length in this paper the performance of state of the art SAF cells is still not good enough to allow them to complete with PEM fuel cells in most markets. SAFcell is a commercial spinoff from the CIT group, and they currently have plans to manufacture low power (100 watts or less) SAF cells for remote power and mobile power (i.e. military backpacks) applications. Apparently this is a performance niche in which the ability of the SAF cells run off reformed hydrocarbon fuels rather than highly purified hydrogen gives them an advantage over the more mature PEM fuel cells. Whether or not further advancements in this fuel cell technology will open other market niches remains to be seen.
Fossil hydrocarbons (e.g oil, natural gas, coal, etc) are used as feed stock for synthesizing a wide variety of useful compounds. Since humanity is tearing through the available feed stocks at far higher than the natural replacement rates an alternative feed stock will some day be required. One possibility is to use CO2 captured from the atmosphere. However, in order to convert CO2 into useful products it must first be reduced to carbon monoxide (CO) which is an energy intensive process. Some amount of research effort has been directed at using sunlight to drive photo catalytic reduction of CO2 to CO. Rhenium based catalysts can accomplish this feat using ultraviolet photons. However, ultraviolet light is a small component of incident sunlight so that a practical solar driven process requires a catalyst than can use the lower frequency visible components of sunlight. Chemistry World recently published an article about an Chinese/French collaboration that has succeeded in altering rhenium based catalysts so that they can catalyze the reduction of CO2 when radiated with visible light with a quantum efficiency similar to the utra-violet driven version of the same complex. Whether or not this particular organometallic complex is good enough (e.g. cheap enough, long-lived enough, efficient enough) to from the basis of a practical industrial process is not yet clear.