Scientists of Environmental and Energy Nanomaterials (CEEN), Institute of Solid State Physics, Hefei Institutes of Physical Science, design and develop new-type and energy-saving rechargeable zinc (Zn)-air battery and enhanced electrocatalytic H2 production technology by replacing oxygen evolution reaction (OER) with thermodynamically more favourable urea oxidation reaction (UOR).
These results provide new ideas for the design of new energy-saving metal-air battery devices and the development and utilization of clean hydrogen energy. And the works were published in Electrochimica Acta and Chemical Communications.
The development of new-type, high-efficient and environment-friendly energy techniques and devices, such as metal-air batteries, fuel cells, electrocatalytic water-spitting to generate H2 and O2, is the important research content to realize sustainable clean energy technology.
Generally, electrocatalysts with the activities of oxygen reduction reaction (ORR), oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) are critically important to develop highly efficient clean and renewable energy technologies to replace traditional fossil fuels.
Although O2 production by OER process from water splitting is not very significant compared with ORR and HER, the activities of OER electrocatalysts determine greatly the performance of OER electrocatalysts participated energy devices, for instance, water splitting to generate H2/O2, fuel cells and rechargeable metal-air batteries.
Generally, the electrocatalysts with low OER overpotential are very favourable for improving the H2 generation efficiency on counterpart HER electrode by water splitting.
The research team recently adopts the strategy of utilizing UOR to replace OER for effectively decreasing the overpotential of oxidation half-reaction and thus enhancing the H2 evolution efficiency of reduction half-reaction by water splitting and the charging voltage of metal-air battery.
The conventional one is to develop high OER active electrocatalysts with inherently low overpotential, although great progresses have been achieved on the OER electrocatalysts such as transition metal-based catalysts and carbon-based catalysts, OER is still the bottleneck of overall water splitting and rechargeable metal-air batteries, owing to the currently developed OER catalysts with high overpotential and sluggish kinetics.
The other novel one is to integrate electrocatalytic oxidation of organics to further decrease the overpotential of oxidation half-reaction electrode.
This innovatory strategy means that if ORR/UOR actives nickel-based electrocatalysts are used as air cathode material of rechargeable metal-air battery in the presence of urea, the charging voltage of rechargeable metal-air battery can be effectively decreased with a value approximate to the decreased overpotential between UOR and OER in the charging process of battery;
On the other hand, replacing OER with thermodynamically more favourable UOR for high-efficiency H2 generation and simultaneous urea degradation, should be very promising and attractive from both energy and environmental viewpoints.
Based on this novel strategy, the group members from CEEN, ISSP, CAS, design and prepare the Ni2Mn-Hy/CFC electrode with ORR and UOR actives, which directly as cathode material was assembled into a rechargeable Zn-air battery, exhibiting almost 0.3 V lower charging voltage in the presence of urea compared to the battery without urea, consequently obtaining about 12%~21% energy saving in the charging process.
On the other hand, the Ni2P/CFC electrode with bifunctional electrocatalytic activities of HER and UOR was successfully developed.
On this basis, an efficient two-electrode system for high-efficiency H2 production and simultaneous urea decomposition was constructed using Ni2P/CFC as anode and cathode.
Compared to the pure water-spitting, the urea electrolysis system significantly reduce the cell voltage by about 200 mV to reach benchmark current densities (10 mA cm-2) while degrading the urea in the waste water, which could attribute to the more favourable thermodynamics and kinetics of UOR than OER catalyzed by Ni2P/CFC electrode.
The findings in this work demonstrate an effective means to utilize low-cost and abundant urea as sacrifice agent by high UOR active electrocatalysts for energy saving conversion applications, and also enlighten us that this coupling concept is potentially extendable to combine HER with many other the degradation of organic pollutants or biomass valorization for cost-effective energy conversion applications.
This work was supported by the Natural Science Foundation of China (Grant No. 51672277, 51432009 and 51372248), the CAS Pioneer Hundred Talents Program, and the CAS/SAFEA International Partnership Program for Creative Research Teams of Chinese Academy of Sciences, China.
Link to the papers:¡°Vapour-phase hydrothermal synthesis of Ni2P nanocrystallines on carbon fiber cloth for high-efficiency H2 production and simultaneous urea decomposition¡± and Chemical Communications, entitled with ¡°Highly efficient electrocatalytic oxidation of urea on a Mn-incorporated Ni(OH)2/carbon fiber cloth for energy-saving rechargeable Zn-air batteries¡±.
Figure (1) The schematic diagram of rechargeable Zn-air battery and the charging and discharging curves of the new-type Zn-air battery; (2) The schematic diagram of H2 production and simultaneous urea decomposition, and the LSV curves of Ni2P/CFC electrode in the presence and absence of urea. (Image by ZHANG Xian)
Prof. ZHANG Haimin
Key Laboratory of Materials Physics, Centre for Environmental and Energy Nanomaterials, Anhui Key Laboratory of Nanomaterials and Nanotechnology, Institute of Solid State Physics, Chinese Academy of Sciences
Hefei 230031, China.