Efficiency Improvement Strategy of Electrocatalytic Ammonia Synthesis under Ambient Conditions

Author:Chen Gao Feng

Supervisor:wang hai hui


Degree Year:2019





Ammonia is of great significance to the life and development of human beings.At present,the only industrial-scale ammonia synthesis technology is the ammonia synthesis process developed by German chemists-Haber and Bosch in the early 20th century using gas phase N2and H2 as the feeding gases.However,the Haber-Bosch process is subject to harsh conditions,high equipment requirements,high energy consumption(consuming 2%of global energy supply per year),and low conversion rates,which are increasingly inconsistent with economic and social development requirements.Compared to the Haber-Bosch method,electrochemical ammonia synthesis has been proven to be carried out under mild conditions in recent years and is a potential replacement technology for ammonia synthesis.The key problem it currently faces is the extremely low ammonia yield rate and selectivity.Therefore,how to achieve high ammonia yield rate,high current efficiency and low energy consumption,and ultimately achieve low-cost,large-scale production of ammonia will be the focus of future research.In view of the above key scientific issues,this paper starts research on three aspects of catalyst modification,nitrogen source selection and two-electrode anode reaction substitution strategy to improve the efficiency of ammonia synthesis:(1)The field of electrochemical nitrogen-fixing ammonia synthesis is still in the initial research stage.At present,there may be problems such as lack of testing means and pollution introduction in the research process.This research summarizes a complete and strictly implemented control experiment method system.(2)For the first time,a targeted solution to suppress the hydrogen evolution reaction strategy was proposed.The hydrogen evolution reaction is the main competitive reaction in the electrochemical ammonia synthesis,and it is also the cause of the low efficiency(<1%)of electrochemical ammonia synthesis.In this work,a Li+incorporation into poly(N-ethyl-benzene-1,2,4,5-tetracarboxylic diimide)material was designed and prepared.The Li+-coupled C=O site can effectively inhibit hydrogen evolution and provide C=O→Li+nitrogen fixation sites,which effectively enhances ammonia synthesis selectivity(current efficiency of 2.85%).(3)In view of the problem that the N≡N bonds of nitrogen molecules are very stable,the nitrate with relatively weak activation bond energy is used as the nitrogen source for electrocatalytic ammonia synthesis.On the basis of occupying strategy of active site for hydrogen suppressionn,a Cu-doped 3,4,9,10-perylenetetracarboxylic dianhydride catalyst material was prepared,which proved that the material has ultra-high electrocatalytic ammonia synthesis performance by nitrate reduction,and the ammonia synthesis efficiency of the system is remarkable.Under the optimal potential conditions,the ammonia yield rate was 305.7±29.8μg h-1 cm-2,the ammonia yield Faradaic efficiency was 80.0±5.9%,and the total Faradaic efficiency was 96.0±1.6%.(4)The traditional anode iron-based catalyst was modified by fluorine doping to enhance its catalytic activity,and the ethanol oxidation reaction was introduced at the anode instead of the oxygen evolution reaction,so that the anode potential was reduced to 1.207 V vs.RHE(10mA cm-2).In the two-electrode test,hydrogen production from electrolyzed water was used to simulate the two-electrode ammonia synthesis.When using ethanol oxidation instead of OER on the anode,only a low voltage of 1.43 V is required to achieve a current density of 10 mA cm-2,which is much smaller than the cell voltage(>1.8 V)required for conventional electrochemical hydrogen production,effectively improving the reaction efficiency.