Researchers at IISER Pune have developed new crystalline organic polymers that when used with electrocatalysts can tilt the kinetics favorably to split water and produce hydrogen and oxygen.
Separating water into its constituent molecules requires one to overcome the forces holding the molecules together. This can be achieved by applying an electric current through water, a simpler method to produce hydrogen and oxygen in comparison to the prevalent energy-intensive methods—cryo distillation and steam reformation.
“The stakes are high because the released molecules have vital industrial applications: hydrogen can potentially be used as an alternative to the widely used hydrocarbon fuels and oxygen can be used as a reagent in many industrial processes,” explains R. Vaidhyanathan, who, along with his group at IISER Pune, is working on developing optimal materials and conditions for use in the industry as valuable gases and for clean fuel and alternate sources of energy.
Along with applying electricity, use of certain catalysts (termed electrocatalysts) can aid activation of water splitting. Using electrocatalysts as nanoparticles can give a further boost and can take one closer to the goal; certain caveats persist, however. These nanoparticles are usually grown by using capping agents, which, in spite of their value, can impede accessibility to such particles and thereby the kinetics of the reaction.
In two recently published papers, Vaidhyanathan’s group, in collaboration with researchers at NCL, Pune and University of Ottawa, Canada, has addressed this problem by using covalent organic frameworks (COFs), which are porous organic polymers, as a support on which nanoparticles of metal-based electrocatalysts can be grown, free of capping agents. The team was relying on two particularly helpful characteristics of COFs: their orderly structure (can ensure nanoparticles are well dispersed) and their porous nature (should allow greater accessibility and therefore favourable kinetics).
By including a nitrogen centre into the building unit of a COF, the team developed their first COF support: a highly crystalline but flexible COF that they named IISERP-COF2. The next step was to assess if the COF support can in fact aid water splitting by nickel-hydroxide-based electrocatalyst nanoparticles.
The assessment is done by using a parameter called overpotential, which is defined as the excess electric potential that needs to be applied over the thermodynamic requirement of the water splitting reaction. In recent literature, an overpotential of 300 mV at a current density of 10 mA cm-2 has been termed as a benchmark for a good electrocatalyst. The challenge for the team was to split water at a lower overpotential.
The team found that IISERP-COF2 composite can oxidize alkaline water with a low overpotential of 250 mV @ 10 mA cm−2. While this was a significantly desirable outcome, they set out to improvise on this for better practical applications.
To this end, they developed an alternate COF: a conjugated benzimidazole-based framework (IISERP-COF3) that is organic and can conduct electrons. For the electrocatalyst, they used nickel nitride (Ni3N), which like Ni(OH)2, has a hexagonal sheet structure but is metallic in nature.
The team envisaged that a combination of a conducting COF and a metallic electrocatalyst could favour a higher redox activity and thereby reduce the overpotential. As anticipated, this composite could oxidize alkaline water with a further lower overpotential of 230 mV @ 10 mA cm−2 and released O2 at one of the highest levels (230 mmol h−1 g−1) reported so far upon electrocatalysis.
Speaking about the future prospects of this work, Vaidhyanathan says, “This work reveals the emergence of COF as supports for electrocatalysts. Lot of optimizations are possible, and for practical applications, these electrocatalysts and the water-splitting cells need to be scaled-up. Another important aspect to work on is how to keep the released oxygen and hydrogen separate in large scale water-splitting cells.”
– Reported by Shanti Kalipatnapu with inputs from R. Vaidhyanathan