Research On Nafion Based Room Temperature Gas Sensors For Safety Monitoring

Hydrogen sulfide(H2S)gas exists in environments such as petrochemical and urban underground corridors,methanol(CH3OH)gas is widely available in environments such as fuel cell vehicles and petrochemicals,and hydrogen(H2)is widely used in many fields such as metal smelting,petrochemicals and fuel cells.Excessive emissions or leaks of H2 S,CH3OH,and H2 not only pose fire and explosion risks,but also pose health hazards to personnel.Therefore,it is essential to monitor H2 S,CH3OH and H2 gas.However,H2 S,CH3OH,and H2 are often present in environments where other flammable and explosive gases are also found.In order to quickly and accurately detect the safety of these gases,sensors with high sensitivity,selectivity,and fast response times are required.In addition,these sensors should be intrinsically safe,preferably able to operate at room temperature.Nafion-based gas sensors are highly suitable for detecting H2 S,CH3OH and H2 in areas where flammable and explosive gases are present,such as underground pipelines,petrochemicals and fuel cell vehicles.They are intrinsically safe,operate at room temperature,and provide high sensitivity and fast response without the need for external voltage or heat source.The Nafion based gas sensor includes a gas diffusion cap,a membrane electrode assembly,a gasket,and a water reservoir.The membrane electrode assembly,which includes a sensing electrode,a reference electrode and a Nafion membrane,is the core of the sensor.The gas sensing properties of the sensor are determined by the catalyst and carrier materials used in the sensing electrode.This thesis aims to enhance the sensitivity of the sensor by exploring high-performance catalyst materials,developing new carriers,and optimizing the sensor structure.Characterization methods such as XRD,XPS,SEM,TEM,EIS,and CV were used to clarify the sensitization mechanism of the sensors.The main research content is as follows.(1)In order to solve the current problems of low sensitivity and high cost of H2 S gas sensors,we constructed high performance Nafion-based current-type H2 S gas sensors through two schemes.Scheme I: The pore volume and specific surface area of the carbon carrier material were changed to regulate the catalyst loading and the adsorption performance of the measured gas,and the design guidelines of the carbon material were determined;Scheme II: The catalytic activity of the catalyst was enhanced through the alloying of the catalyst to improve the sensing performance of the sensor and reduce the cost.Ⅰ.The electrode materials Pt(10wt%,20wt% and 30wt%)/CMK-3 were synthesized by chemical reduction and then used to fabricate Nafion-based current-type gas sensors.The optimum mass fraction of Pt in the Pt/C electrode material was determined to be 20wt%.Based on this result,the electrode materials Pt/MC(MC:CMK-3,CMK-8 and NCP-10)were prepared to investigate the effect of carrier morphology on the gas sensing performance of the sensors.It was found that the small pore volume material as the carrier is beneficial to promote the adhesion of catalyst particles on the carrier surface,which in turn improves the response/recovery speed of the sensor;the carrier material with large specific surface area can provide sufficient adhesion sites for the catalyst particles,which is conducive to the maximum utilization of the catalyst.Since the internal pores of the mesoporous carrier material are not conducive to the desorption of H2 S and its intermediates,as the sensor use time increases,it causes the reactants to accumulate and then the response value of the sensor gradually increases.Therefore,the repeatability and long-term stability of the sensors are not good.Ⅱ.Based on the results of the study in Experiment I,the electrode material PtNi/CF was developed to improve the response/recovery rate,sensitivity and stability of the sensor,and the effect of the ratio of Pt and Ni in the electrode material on the catalytic activity of the electrode material was investigated.The electrode material has the best electrochemical catalytic performance when the ratio of Pt and Ni is 1:1.The amount of Pt in the electrode material is reduced from 20wt% to 10wt%,reducing the cost of the sensor.(2)In order to solve the problems that the current-type room-temperature CH3 OH gas sensor is prone to CO poisoning and has a narrow detection range,a high performance Nafion-based current-type CH3 OH gas sensor is constructed by two schemes.Scheme Ⅰ: to improve the anti-poisoning ability of the catalyst by using the higher reaction potential of Pt-Cu alloy catalyzing CH3 OH to generate CO and the easy catalytic reaction of Cu element to generate CO2 and desorption;Scheme Ⅱ: to improve the catalytic ability of the electrode material by using the electron transfer property between the metal oxide carrier and Pt to realize the improvement of the gas-sensing performance of the sensor.Ⅰ.In terms of catalyst,the electrode material Pt-Cu(Pt:Cu = 1:0,2:1,1:1 and1:2)/CF was developed by chemical reduction method,and then a room temperature Nafion-based current-type CH3 OH sensor based on Pt-Cu/CF was constructed in this way,and then room temperature CH3 OH sensors based on Pt-Cu/CF and Nafion were constructed.The effect of the ratio of Pt and Cu in the catalyst alloy on the catalytic activity and selectivity of the electrode material was investigated.When the atomic ratio of Pt to Cu is 1:1,the catalyst particles in the electrode material have the weakest agglomeration phenomenon and the best electrocatalytic activity to CH3 OH oxidation reaction.Ⅱ.In terms of carriers,metal oxide MOx(MOx: Sn O2,In2O3 and Cu O)was used as a carrier to develop electrode materials,and room temperature current-type CH3 OH sensors based on Nafion and Pt/MOx were fabricated.It was experimentally confirmed that the improved sensor performance was mainly due to the electron transfer between Pt and Sn O2 which changed the adsorption strength of the reaction intermediates on Pt,promoting the electrode reaction to proceed continuously and efficiently,and finally realizing the catalyst performance enhancement,which has the lowest detection limit of current-type CH3 OH sensors.(3)In order to solve the problems of insufficient selectivity and low sensitivity of the current room temperature H2 sensors,the Nafion-based current-type H2 sensors with high selectivity and high sensitivity were constructed by two schemes.Scheme Ⅰ: using the gas diffusion layer to filter the macromolecule gas to enhance the selectivity of the sensor;Scheme Ⅱ: using the property of Pd catalyst to easily adsorb and catalyze H2 to improve the catalytic performance of the electrode material,thus enhancing the sensitivity of the sensor.Ⅰ.Based on the original device structure,a nano-Zn O gas diffusion layer was added.The H2 sensor based on the electrode material Zn O-Pt/CF was developed,and the effect of the gas diffusion layer on the gas sensing performance of the sensor was initially investigated.The addition of the gas diffusion layer greatly reduces the interference of other gases to the sensor.The Zn O-Pt/CF-based sensor has almost no response to gases such as CH3 OH and CO.The selectivity of the sensor to H2 is greatly improved and the detection range is expanded,but the response value and sensitivity decreased.Ⅱ.To improve the sensitivity of H2 sensors,Pd(5wt%,10wt% and 20wt%)/CF electrode materials were developed in the second part of the work by using Pd instead of the conventional Pt catalyst.The effect of the content of Pd catalyst in the electrode material on the catalytic activity of the electrode material and the gas sensing performance of the sensor was investigated.When the mass fraction of Pd is 10 wt%,the content and utilization of Pd catalyst reach the best balance and the electrode material has the highest catalytic performance.In summary,this thesis addresses the problems of H2 S,CH3OH and H2 detection in several fields,and finally constructs Nafion-based gas sensors with high sensitivity,selectivity and stability through three schemes of high-performance catalyst materials exploration,novel carriers development and sensor structure improvement,which provides new solutions for room temperature gas detection in complex atmospheres.