Hydrogen fuel from floating solar fuels rig

Materials World magazine
,
1 Feb 2018

Columbia engineers have developed a floating solar fuels rig that could produce hydrogen via seawater electrolysis. Kathryn Allen reports. 

Solar-powered water electrolysis could produce hydrogen for use as a fuel, without creating carbon dioxide – a by-product of methods currently used to obtain hydrogen from natural gas. 

Researchers at Columbia University, USA, have taken advantage of this technique, developing a stand-alone photovoltaic-powered electrolysis device that floats on water – known as a solar fuels rig. Using solar power and water, the rig can produce clean fuel, without consuming fresh water or taking up land. Photovoltaic cells on the rig convert solar energy into electricity to power the electrolyser. 

The diameter of the device used is around 20.3cm, with the electrodes measuring 3cm2 in area. There are two sets of electrodes in each rig. Hundreds to thousands of square miles of these rigs would be required to make a significant contribution to the UK’s energy needs. The team believes the device can be scaled up in theory, but considerations regarding manufacturing – as 3D printing was used to make the device – must be taken into account. Describing how this might look, Esposito, Assistant Professor of Chemical Engineering at Columbia University, USA, referenced Sungrow Power Supply Company’s large-scale floating solar power plant in China. 

Key to the development of this device is the absence of conventionally used expensive membranes in the electrolyser. These membranes separate the two gases. However, in this device, they are separated and collected using electrode configuration and the buoyancy of the bubbles. The absence of membranes reduces the device’s cost and increases its durability – membranes require a high purity water source as impurities, such as those in seawater, can cause damage. 

Esposito told Materials World, ‘With buoyancy-induced separation, our device can produce up to 99% pure hydrogen and oxygen products from water electrolysis without membranes or pumps.’ 

Asymmetric electrodes 

The device comprises porous mesh electrodes coated, on one side only, with a platinum electrocatalyst. Esposito explained, coating only the outer surface of the electrodes increases the purity of the collected gases. ‘The two electrodes – one called the anode where the oxygen evolution occurs and the other called the cathode where hydrogen evolution occurs – are oriented so the catalyst-coated sides of the mesh face outwards from each other. During operation, the oxygen and hydrogen products evolve from the catalytic outer surfaces, and when the individual gas bubbles become large enough, their buoyancy causes them to float directly upward into separate collection chambers.’ 

Esposito continued, ‘When symmetric electrodes containing catalysts on both sides of the mesh electrode are employed, significantly lower product purity is observed because hydrogen and oxygen bubbles formed on the inner surfaces of the electrodes can more easily cross over to opposing collection chambers.’ The team monitored this by including windows in the devices and using a high-speed camera. 

Crossover rates are higher for the device than for conventional electrolysers, Esposito points out. He and the team are exploring various device modifications to reduce these rates. The majority of the hydrogen that crosses is dissolved, rather than individual hydrogen bubbles. 

Scaling up

Large-scale sea rigs pose challenges. Esposito acknowledged the need to assess any potential interference with marine life, but also points out that there could be beneficial side effects, such as oxygenation of the water. 

Challenges also arise from the seawater. Esposito said, ‘Chloride ions are present in high concentrations in seawater and can create a corrosive environment. It will be important for real-world devices to employ electrocatalyst and device materials that are resistant to corrosion in this environment. Similarly, there is concern for biofouling on the electrodes, whereby microorganisms present in unfiltered seawater might attach to the electrodes and create biofilms if the device is left dormant over time.’ 

A second issue related to the presence of chloride ions is the evolution of toxic chlorine gas, which can form at the anode. Some catalytic materials have high reaction selectivity for producing oxygen rather than chlorine gas, but further research is needed. 

Once generated, the hydrogen needs to be stored – typically this is done either by compressing or liquefying it. Esposito explained that compression is preferable and that it could be beneficial to store some hydrogen on the rig to help it float. ‘If this technology were to become commercially viable, I think it would likely make the most sense to locate the rig relatively close to shore so that hydrogen could be easily sent to shore through pipelines,’ he added.

To read Floating membraneless PV-electrolyzer based on buoyancy-driven product separation, visit bit.ly/2A5rMBa