A lot of the high-profile developments in renewable energy have focused on improving the efficiency of technologies that are destined for the industrialized world, which has the infrastructure to support expensive, centralized solutions. Powering the developing world will have a distinct set of problems, as equipment will have to be cheap, rugged, and capable of operating in situations where there may not be an electric grid. Cobalt phosphate catalysts, which form spontaneously and split water (releasing hydrogen) when provided with an electric current, may satisfy many of these requirements, and a paper released by the Proceedings of the National Academies of Science described a way to simplify cobalt-based systems even further.

We've been tracking the cobalt catalysts since they first appeared in the scientific literature. The constituent materials are cheap and readily soluble in water. Run a current through the solution, and an amorphous form of cobalt phosphate will form on the electrodes. Once in place, further current will catalyze the breakdown of water into hydrogen and oxygen, which can be stored for later use and recombined in a fuel cell, producing usable electricity.

The process isn't necessarily efficient, but it doesn't have to be, according to its chief proponent, MIT's Daniel Nocera. Since it's cheap, robust, and can operate with just about any source of water available, it's perfect for the developing world (where the pure water that comes out of a fuel cell is a big plus).

The problem with widespread deployment, of course, is that the process doesn't only depend on a cheap catalyst. The current to split the water has to be supplied, presumably by photovoltaic systems that aren't currently cheap enough for mass deployment. And storage and fuel cells need to be made cheap and robust, as well. So the development of the catalyst was just one step in solving a system-level problem.

The PNAS paper suggests one potential route to simplifying things down: it might be possible to get rid of the photovoltaics entirely. The paper describes how the cobalt phosphate catalyst can be made to form directly on the surface of a photovoltaic material, where it harvests the charge differences directly, rather than requiring that they be piped in as current from an external source.

The research was done with a rather unusual photovoltaic material: zinc oxide, better known as a component of some sunblocks. In bulk form, ZnO makes for a lousy photovoltaic, but it's a semiconductor with a large bandgap and, when appropriately structured on the nanometer scale, it can function as part of a photovoltaic device. Incident photons can separate electrons from its surface, creating positively charged holes.

In a normal photovoltaic device, the electrons are captured and used to generate current before being recombined with these holes. The authors reasoned that the whole process of harvesting the charge separation, with its associated equipment and cost, could be done without—if the cobalt catalyst was placed in close enough proximity to the zinc oxide, it could directly harvest the charge differential from the surface to split water.

The materials and methods section of the paper suggests that arranging this couldn't be simpler: zinc nanorods were dumped in a petri dish containing a cobalt phosphate solution and hit with UV light. That appears to be all that's needed to get the catalyst to form. The end result are small spheres of catalyst scattered across the surface of the nanorods. According to the authors, the spacing is sparse enough that the majority of the zinc oxide surface is still available to absorb light. The authors suggest that the cobalt phosphate is likely to precipitate where charge holes are most likely to appear on the surface, which should also enhance the activity of the resulting material.

The authors demonstrate that the presence of the cobalt catalyst dramatically changes the electrical properties of the zinc oxide, which indicates it's putting the charge to work. Unfortunately, the paper doesn't include any information about gasses being evolved from the sample, which would have been a nice confirmation that everything was acting as expected.

Still, the paper shows that it may be possible to radically simplify the approach to hydrogen generation using the cobalt phosphate catalyst and, even if this particular implementation isn't great, there are a tremendous number of options for improving it: different photovoltaic materials, different solution conditions, etc. There are obviously contexts where having an actual solar panel array might be more flexible, but if the goal is simple mass production, it's hard to beat dumping some pellets into a solution and putting them in the sun.

PNAS, 2009. DOI: 10.1073/pnas.0910203106