Two things that have caught my attention recently.

The first concerns trapping solar energy.  One way to do this is to convert it directly into electricity with a solar panel, but one with much wider application would be to split water into oxygen and hydrogen, which can then be stored and transported.

Chemistry World article Sun rises on new solar route to hydrogen points to research in China and Israel where the water is first oxidised to hydrogen peroxide, which is then decomposed by a cheap chemical catalyst.  The composite catalyst containing C3N4 (which I assume refers to the graphitic form of carbon nitride — there is also a diamond-like form) in a composite containing carbon nanodots.  The C3N4 photocatalyst splits water into hydrogen and hydrogen peroxide, which would normally stick to the surface of the catalyst poisoning it.  However, the carbon nanodots act as a further chemical catalyst that decomposes the hydrogen peroxide into water and oxygen. The nanodots also allow the catalyst to absorb more light.

The new catalyst has a solar-to-hydrogen conversion efficiency of 2%. The best water-splitting photocatalyst to date is nanocrystalline cobalt oxide, which has a conversion efficiency of around 5%.  However, this began to lose its activity within 1 hour.  The current photocatalyst, however, showed no degradation after 200 days.

The quick decomposition of the hydrogen peroxide prevents it from poisoning the first catalyst.  However, if a way of removing it quickly and recovering it were found, it too would have a use.  It is a very good general purpose disinfectant, but because it decomposes on storage it is not widely available for domestic use (though in the UK Sainsburys did for a while market it as Greencare: perhaps it went off too quickly in the bottle, but it did not catch on.)  However, unlike acid or bleach based toilet cleaners, it does not corrode the fittings, so I liked it.

The researchers calculated that if they could optimize their photocatalyst to a 5% conversion rate this would lower the cost of hydrogen production to $2.30/kg — well below the US Department of Energy target of $4/kg.  Even at 2%, though, the number obtained is as low as $6 — not bad going, methinks.

One great point about this catalyst is that it only employs two of the most earth abundant elements.  Even cobalt, though not horribly scarce, is somewhat limited.  With even rarer elements, whether they are truly rare, or like many of the rare earths, hard to find in workable deposits, one wants to limit their use or find a way of recovering them with a process that is not more expensive that the elements themselves.

Another one from Chemistry World: Fish sperm spawns rare earth metal recycling idea.  Salmon milt (sperm) is a delicacy in Japan, and not too costly.  A Japanese team team created a milt powder which when added to a solution containing neodymium, dysprosium and trivalent iron — the main metals used in neodymium magnets — they discovered that metal ions had a high affinity for phosphate in the powder.  The rare earths were subsequently recovered from the milt powder using acid and centrifugation.

Y Takahashi et al, PLoS One, 2014, DOI: 10.1371/journal.pone.0114858

The lead author acknowledges that since recovery of rare earths from scrap magnets is not (yet) big business, the salmon-milt process might be better suited for extracting and recycling other elements on a large scale.  The team has not yet shown the process works with other elements, but Takahashi is confident that it will because of salmon milt’s high ion-exchange capacity.