Split water splitting raises green hydrogen hopes

UK scientists say that they have developed the first widely-useable electrolysis system that splits water and releases hydrogen and oxygen in separate stages. Lee Cronin and Mark Symes from the University of Glasgow used a phosphomolybdate anion buffer to store protons and electrons generated when oxidising water to oxygen. Instead of directly producing hydrogen, as electrolysis normally does, the buffer lets the scientists choose where and when they do the second step. That could aid efforts to store renewable energy as hydrogen fuel made by water electrolysis. 

The polyoxometalate can release the protons and electrons later to produce hydrogen on demand © NPG

Though electrolysis usually produces oxygen and hydrogen together, two interlinked half-reactions actually generate the gases. The first oxidises water into oxygen, protons and electrons. In the second, electrons reduce the protons to give hydrogen. To separate the reactions, Cronin and Symes sought chemicals that could be reduced and protonated, but could later release those electrons and protons again. They reasoned that the necessary electron-coupled-proton buffer (ECPB) properties might lie within the transition metal and oxygen-containing networks ofpolyoxometalate anions.

‘Doing tests where you oxidise water, but make absolutely no hydrogen would seem like failure to most people,’ Cronin says. ‘But this was exactly what we were after, as when you reverse the process you get pure hydrogen and no oxygen.’ Of the polyoxometalates they tried, phosphomolybdate anions, [H2PMo12O40]-, worked best. ‘In one instance, we stored our reduced and protonated ECPB for eight months before we re-oxidised it to liberate hydrogen,’ Cronin notes. But splitting the reaction into two steps imposes an energy penalty, making it 87% as efficient as the one step reaction, at best.

The polyoxometalate acts as a sink, storing the protons and electrons released when water is split © NPG

‘Simultaneous hydrogen and oxygen production is a kind of “elephant in the room” for water splitting,’ Cronin tells Chemistry World. The gases can pass through and degrade expensive Nafion polymer membranes meant to separate them in existing electrolysers, with potentially explosive consequences. ‘We knew that natural photosynthetic systems separated oxygen and hydrogen-equivalent production in time,’ Cronin says. ‘So we thought, “Can we do this, but electrochemically?”’