Two Dimensional Materials and Their Heterostructures for Ampere-Level Direct Seawater Splitting

Green hydrogen has emerged as the potential replacement for fossil fuels; however, more than 20 billion m³ of water would be required to meet the future demand of 2.3 Gt of hydrogen per year, which is almost equal to the water needs of a country with a population of more than 60 million. Currently, most of the current industrial processes require purified freshwater for the electrolysis process, which is a limited resource, and utilizing freshwater to meet increasing hydrogen demands can lead to a drinking water crisis. Seawater, an almost unlimited resource, can be a potential feedstock for water electrolysis. However, the complex composition and presence of chloride anions in the seawater can interfere with the electrolysis process. Although desalination has been suggested for seawater purification, the economics of scale do not justify the set-up of the plant. Therefore, synthesizing materials that can withstand complex seawater composition and deliver enhanced performance and long-term stability in direct seawater is crucial.<br/>In my previous work, I developed porous sheets of 2-dimensional (2D) N-NiMo₃P, which required an overpotential of only 35 mV and 346 mV to acquire 10 mA cm-² for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in alkaline seawater, respectively. The presence of Ni and N resulted in enhanced electrical conductivity by assisting in the redistribution of the charge states and the metal-non-metal bond lengths while the pores catered for faster mass transport and increasing active sites. Furthermore, the presence of polyanions (phosphate, nitrate) on the surface safeguarded the catalyst against chlorine chemistry, preventing chlorine evolution reaction at the anode. As a result, the catalyst was stable for over 100 hours of continuous operation. Although the HER performance of the catalyst was exceptional, the long-term operation and anodic performance still needed to be improved to realize the commercial applications.<br/>Learning from my previous work, I have developed a 2D heterostructure catalyst for ampere-level anodic reaction. The as-synthesized catalyst possesses the advantages of its constituent 2D materials while alleviating their disadvantages. The materials are bonded via unique bonds at the heterointerface, which helps in optimizing the surface geometries for selective reactions while also modulating the hydroxyl anions at the interface, which play a crucial role in averting the chlorine chemistry at the anode by repelling the chloride anions. As a result of the unique bonded heterointerface, the material can deliver approximately 10x better performance while also being 65% more economical than commercial IrO₂ for anodic reaction. Furthermore, the catalyst can operate stably for over 1000 h without any current losses. To further confirm the exceptional stability, the working electrode was left idle in the electrochemical system for 5 days and then tested again, where it retained more than 90% of its initial current density after 3 days of continuous operation, making it integrable with renewable power sources. Hence, the interfacial engineering method to obtain a unique bonded heterointerface can be a potential strategy to synthesize a 2D heterostructure catalyst for industrial direct seawater splitting.