11 October 2005

Photoelectrochemical Alternatives

The Energy Blog had a post detailing electrochemical photovoltaic cells that are capable of directly producing Hydrogen gas. It has a pair of advantages:

  1. It reduces the electron current on the PV cell by consuming the electrons. This should result in a decrease of the recombination of electron-hole pairs.
  2. The electrolyzer is effectively integrated into the system.

I do not think too highly of this concept. It makes several requirements of photovoltaic systems:

  1. The active semiconductor material must be capable of resisting corrosion from the hydrogen.
  2. The semiconductor's band gap must be well matched to the radiation spectrum of the sun.
  3. The redox potential of water must be well matched to the band edges of the semiconductor.

Simply put, there are no known materials that can meet all three criteria simultaneously. The solution seems to be to produce triple-junction cells to produce mediocre hydrogen conversion efficiencies. I remain unimpressed. Nature simply has not cooperated in providing an appropriate material.

If we take off our hydrogen goggles for a second an alternate solution presents itself. Instead of searching for a suitable semiconductor material perhaps we should search for an appropriate reversible reduction-oxidation reaction. I am thinking specifically of flow battery concepts.

The big name in flow batteries is VRB Power of Vancouver. They build a type of flow battery that uses Vanadium electrolyte to store power. In addition to the Vanadium concept, there are many other redox reactions that have been implemented such as Zinc-Bromine and Sodium Polysulfide Bromine. Such a coupled electrolytic photochemical system would probably still require a membrane to separate the positive and negatively charged electrolytes. Unlike the water reduction, the products won't bubble off as a gas; bromine systems may be an exception.

By introducing another degree of freedom by allowing the redox reaction the number of possible permutations between semiconductor and redox will greatly increase. This increases the likelihood that a natural match will be found to create a solar energy solution coupled to a storage technology.

There's no need for storage coupled to solar energy for on-grid applications yet. On the other hand, off-grid applications do tend to have very large battery packs. The main question is would the extra cost of the flow battery be offset by the superior energy return rate? My gut feeling is no -- off-grid applications are typically very small with electronics designed to minimize power drain.

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