Continued concerns about global warming have boosted work on alternative fuel sources that reduce emissions. Hydrogen is an appealing, clean-burning fuel. Currently, most hydrogen comes from the processing of fossil fuels, which produces carbon dioxide as a byproduct. However, the electrolysis of water produces hydrogen without the release of greenhouse gases—provided the electricity used in the process comes from renewable energy.

Currently, the favored method for producing hydrogen involves what are called proton exchange membrane electrolyzers (PEMEs). These use a polymer membrane that allows the movement of protons between solutions of varied charge while separating the negatively charged cathode and positively charged anode. Since the two gases, hydrogen and oxygen, are produced at different electrodes, the membranes separate them as well, which allows for the easy harvesting of hydrogen.

Unfortunately, PEMEs are expensive because they require precious metal catalysts. Although higher power loads offset the price of these catalysts to some extent, these loads can lead to the simultaneous presence of hydrogen, oxygen, and catalytic particles, resulting in the production of reactive oxygen species (ROS) that degrade the membranes. Low power loads are not as effective because the rates of oxygen and hydrogen production are similar to the rates at which these gases diffuse through the membrane. As a result, rather than pure hydrogen, you get a hazardous mix of the two gases.

Recently, researchers have developed a system that limits the mixing of gases and uses precious metal catalysts more efficiently. The scientist introduced what they call an “electron-coupled proton buffer” (ECPB), which produces hydrogen and oxygen gases in separate compartments. To separate the production of these gases, they relied on a chemical that can be loaded with hydrogen via loss of two electrons and gain of two protons at an electrode. Later, the chemical can be transferred to a separate compartment to be oxidized via gain of two electrons, resulting in the spontaneous release of hydrogen.

This compartmentalized approach has many benefits: it enables the production of hydrogen at atmospheric pressure at an increased rate, while limiting the production of hydrogen within the electrolytic cell and minimizing the production of oxygen overall, which reduces membrane degradation.

The chemical mediator they used is called silicotungstic acid (H 4 [SiW 12 O 40 ]). Water is split at the anode into oxygen and protons; the protons remain in solution, where they can move to the cathode. There, electrons and protons are transferred to the mediator, which can then be moved to a separate chamber where it is oxidized, releasing hydrogen gas.

Loading the mediator with hydrogen was tested in an airtight electrolysis cell containing either a Pt or carbon felt anode and a carbon cathode. The researchers found that the reduced form of the mediator could easily be transferred into a sealed reaction flask where metal foils can catalyze hydrogen evolution.

Carbon-based precious metal catalysts produced the greatest rate of hydrogen production with Pt/C leading to a rate of hydrogen evolution 30 times greater than current membrane-based systems. In addition, the efficiency of the mediated electrochemical process, which is measured by a comparison of the input and output of energy, was found to be 16 percent more efficient than when the process was tested without the mediator. It had an overall efficiency of 63 percent, which is on par with the current membrane-based systems. The mediator was also found to be stable to over nine cycles. Taken together, this suggests that this system could be more cost-effective than the current PEMEs.

This system also produces hydrogen samples with a high purity; the levels of oxygen in the hydrogen samples were found to be below detectable limits. Moreover, when 10 percent oxygen was purposely introduced to the compartment, it was completely removed—it reacted with the reduced mediator, resulting in the production of water and the reoxidized mediator. This system is not only economical, but also has enhanced safety because it ensures that a dangerous mixing of the gases does not occur.

Science, 2014. DOI: 10.1126/science.1257443 (About DOIs).