| Hydrogen production by water electrolysis,as a foundation in an energy system with a core of hydrogen energy,can be contributed to the global realization of carbon neutrality.Proton exchange membrane water electrolysis(PEMWE)has been widely concerned,because of its high current density and strong flexibility.However,the cost,efficiency and system stability are still the obstacles on industrialization road.Starting with building the model of proton membrane exchange electrolysis cell(PEMEC),energy and exergy analysis of water electrolysis system,catalyst synthesizing and single cell experiment,the complicated heat and mass transfer in the PEMEC,system efficiency and flow control,the catalyst with low-loading Iridium and the thermal response characteristics of the PEMEC are investigated to optimize the performance of PEMWE.A three-dimensional,non-isothermal,two-phase model for PEMEC is established in this study.An effective connection between two-phase transport and performance in the PEMECs is built through coupling the liquid water saturation and temperature in the charge conservation equation.The distributions of liquid water and temperature with different operating(voltage,temperature,inlet velocity)and physical(contact angle and porosity of anode gas diffusion layer)parameters are examined and discussed in detail.The results show that the water and temperature distributions,which are affected by the operating and physical parameters,have a combined effect on the cell performance.The effects of various parameters on the PEMEC are of interaction and restricted mutually.As the voltage increases,the priority factor caused by the change of inlet water velocity changes from the liquid water saturation increase to the temperature drop in the anode catalyst layer.While the priority influence factor caused by the contact angle and porosity of anode gas diffusion layer is the liquid water saturation.Decreasing the contact angle or/and increasing the porosity can improve the PEMEC performance especially at the high voltage.The effects of different temperature boundary conditions and water supply methods on the performance of PEMEC are analyzed.It can be seen that compared with the adiabatic condition,the constant temperature condition can effectively reduce the uneven distribution of memebrane temperature and avoid the degradation because of the overheat.Counter-flow water supply makes the temperature distribution more uniform in the flow direction and the average temperature larger,but the temperature gradient in z axis direction increases obviously.The temperature distribution of three PEMECs connected in series is further considered.Regardless of the outermost cell,the cell performance and temperature distribution gradually increase from the outermost cathode side to the outermost anode side.The dynamic response characteristics of the cell are analyzed,the stability process of the temperature inside the cell is the main factor that promotes the current density to the stable state.The dynamic response process caused by a sudden change of voltage is mainly that the change of electrochemical performance caused by voltage change leads to the change of liquid water saturation and temperature,and finally the cell reaches the steady state.The dynamic response process caused by a sudden change of flow rate is mainly that the change of liquid water saturation and temperature caused by flow rate change leads to the change of electrochemical performance,and finally the cell reaches the steady state.A new PEMEC system without integrated cooling channels inside the PEMEC stack is analyzed in this paper.An external energy source is integrated into the heat exchanger to control the stable operation of the system.Based on the balance of charge,mass and energy,the models of both the stack and the system are established.The influences of operating parameters,such as inlet water flow rate,inlet water temperature and current density,on the PEMEC stack performance are investigated,and the energy efficiency and exergy efficiency of the PEMEC system under both the heat dissipation condition and the adiabatic condition are discussed.The efficiencies under the adiabatic condition are superior to that under the heat dissipation condition,especially at low current densities.And it is beneficial to energy conservation under the adiabatic condition when the current density is within the scope of this paper through comparing the consumed electric energy between the two working conditions.Novel Pt-IrO2 nanofiber catalysts for oxygen evolution reaction(OER)were successfully prepared using the electrospinning method with a high-concentration precursor solution.The composition and structure of catalysts were characterized,and the electrocatalytic performance of the nanofiber catalysts in water electrolysis was investigated from various aspects such as overpotential,charge transfer resistance,and exchange current density.Special attention was paid on the effect of Pt/Ir ratio on the electrocatalytic activity of the electrospun nanofibers.The results showed that the Pt-IrO2 nanofiber catalysts with average fiber diameters ranging from 10 to 100 nm can be successfully obtained by the electrospinning method with 0.17 mol/L precursor solution,and the average diameter decreased with the increase of Pt content.Since a bigger amount of IrO2 content and a thinner fiber structure are all beneficial to the electrocatalytic performance,the Pt-IrO2 catalysts with Pt:Ir=1:9 and Pt:Ir=9:1 both showed better catalytic performance for OER than the commercial IrO2,which was also demonstrated by the lower charge transfer resistance and higher exchange current density.As a result,the overpotentials of Pt-IrO2 catalysts with Pt:Ir=1:9 and Pt:Ir=9:1 are 50 mV and 32 mV lower than the overpotential of the commercial IrO2 at 30 mA cm-2 in 0.5 M H2SO4,respectively.The electrospun Pt-IrO2 nanofiber catalyst with proper Pt content seems to be very promising since it can not only improve the catalytic performance and the safety of PEM electrolysis cells,but also reduce the amount of more expensive Ir in catalysts.A single proton exchange membrane electrolysis cell experiment platform was built,the inlet water temperature and flow rate were controlled by water bath and peristaltic pump.The effect of flow rate,inlet temperature and current density on the performance of water electrolysis was investigated through monitoring the inlet and outlet temperature of anode and the temperature of anode and cathode.The temperature variation of several temperature measuring points under different conditions was emphatically studied.Depending on whether the heat generated by the electrolytic cell itself is greater than the heat loss,the liquid water is distinguished as a cold source or a heat source.As different source terms,increasing the flow has different effects on the performance of the electrolytic cell.The heat absorption reaction of liquid water was observed by monitoring the outlet temperature,and the heat absorption increased with the increase of temperature,which also corresponded to the theoretical energy demand of water electrolysis. |
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