Various kinds of bacteria can evolve hydrogen from water using the enzyme hydrogenase. The evolution of hydrogen is a key step in bacterial fixation of nitrogen from the atmosphere. Chemists at the Pacific Northwest National Laboratory have developed a synthetic nickel based catalyst for electrolytic hydrogen evolution based on the known structure of hydrogenase. Physorg has published a story about this research and an abstract of a recent publication of the experimental results is available on line. Nickel based catalysts are already used in commercial alkaline water electrolyzers such those produced NEL Hydrogen (formerly a part of Norsk Hydro). The hope of using hydrogenase base catalysts is to speed up the reaction and thus to reduced the amount of electrode material required to produce a given amount of hydrogen. The PNNL group has succeeded in making a fast hydrogen evolution catalyst but the efficiency with which it turns electrical energy into chemical potential energy is low compared to more conventional catalysts. The PNNL group is now focusing on means for making their catalysts more efficient.
Several month’s ago Green Car Congress published a story about the development of water electrolyzer designs which do not require an ion exchange membrane separating the two half cells of the device. Since ion exchange membranes are quite expensive such designs have the potential to reduce the costs of hydrogen producing water electrolyzers. Furthermore, the researchers who published the paper in question argue that the elimination of the ion exchange membrane can relax the design constraints of MEA (Membrane Electrode Assembly) based electrolyzers and can potentially lead to design configurations which have other cost reductions in addition to the membrane manufacturing cost.
As the diagram below indicates this form of electrolyzer does indeed appear to have a very simple design. It would appear that the most costly components would be the titanium mesh electrodes impregnated with the appropriate catalysts.
The researchers used platinum nano-particles attached to the titanium mesh as catalysts. They were able to operate prototype electrolyzers in both acidic and alkaline solutions but obtained the highest electrolysis efficiency (72.5% based on the HHV of H2 at a current density of 100mA/cm2) for with alkaline electrolyte. This efficiency is comparable to that of current commercial alkaline electrolyzers such as those produced by Nel (formerly a division of Norsk Hydro). Alkaline electrolysis is economically attractive because it does not require the use of platinum group metals as catalysts.
In spite of the interesting initial results, one should not get too excited yet about this electrolyzer design entering the market place in the near future. Although the overall electrolytic efficiency of 72.5% is comparable to commercial alkaline electrolyzers an additional hydrogen loss of 10% occurs during the gas collection phase of the separation process reducing the effective efficiency to 65.3%. This loss of efficiency may not be a deal breaker since it is cost/kg that matters. Furthermore the electrolzyer in question is a non-optimized preliminary prototype and substantial optimization may be possible. The electrode current density of 100mA/cm2 is a factor of three lower than the current density of commercial alkaline electrolyzers although again similar comments to that just made about efficiency apply.
Another problem is that some amount (probably more than 1%) oxygen crosses over to the hydrogen side to the electrolysis cell and contaminates the hydrogen gas at the collection point. PEM fuel cells require less than 5 ppm V of oxygen content. Although further optimization may reduce the amount of crossover, orders of magnitude improvement seem unlikely. Nel claims that their electrolyzer systems produce H2 with less than 2ppm of O2 after purification. Whether or not a fraction of a percent O2 contamination represents a significant economic barrier to the production high purity H2 is not clear from any information that I have located so far.
Chemistry World recently published an article about a new technique which achieves atomic level dispersion of palladium on a titanium oxide surface. The dispersed catalyst was 55 times more active than other palladium catalysts for a specific hydrogenation reaction. The researchers at Xiamen University in China who developed this technique are working to extent it to other noble metal catalysts (e.g. platinum). It is not immediately obvious that this development has direct implications for energy production/storage but since PEM fuel cells/electrolyzers use noble metal catalysts useful application in this field might eventually be found.