Mass Transfer Enhancement For Ions Within Porous Electrodes Of Redox Flow Batteries Via Flow Field Designs

With their decoupled energy and power capacities,quick response,and long lifecycle,redox flow batteries(RFBs)are expected to play an important future role in energy systems such as renewable energy integration,and grid peak load shifting.In RFB systems,as the electrolytes are stored outside of the stacks,the mass transfer of reactive ions from the tanks to the carbon fiber surfaces of the porous electrodes,is operationally important.The mass transfer process includes the electrolyte flow in the flow field,the electrolyte flow in the porous electrode,and the ion diffusion and migration in porous electrode.When the electrolyte flows from the inlet to the cell outlet,the flow field engraved on the bipolar plate impacts the electrolyte velocity distribution along with the ion concentration distribution in the porous electrode,therefore affecting the RFB system energy efficiency.Based on experimental and simulative approaches,this paper investigates the effects of flow fields on ion mass transfer in porous electrodes and offers improved RFB flow field design.RFB flow fields follow fuel cell designs such as parallel,interdigitated,and serpentine flow fields.Previous studies have proved the effectiveness of interdigitated and serpentine flow fields on improving ion distribution in porous electrodes and reducing the electrolyte pressure drop in RFBs.However,several problems still exist:(i)increased electrode thickness can increase the numbers of reaction sites in the porous electrode,but poor mass transfer in the through-plane direction limits the flow cell performance;(ii)with flow cells using a conventional serpentine flow field,the electrolyte penetration distribution between the two adjacent channels is not uniform,which causes ion starvation in the local porous electrode;(iii)existing flow field research is mostly based on lab-scale flow cells,and research on application-scale designs is lacking;(iv)most flow field research is based on experimental and numerical methods,and related analytical model is scarce.To try to resolve these problems,the contents of this paper are as follows:To improve ion mass transfer in the electrode through-plane direction,a three-dimensional detached serpentine flow field was realized.The flow field included two serpentine parts,one engraved on the bipolar plate,and the other engraved on the near-membrane side of the porous electrode,which extends the flow field from a 2-D to 3-D distribution.Experimental results show that this novel flow field design could significantly improve the limiting current density(60%),charge-discharge capacity(40%),and energy efficiency(4%)of the flow cell.Simulation results based on a 3-D multi-physical numerical model of the flow cell show that the new flow field design could significantly enhance electrolyte convection in the electrode through-plane direction,which is beneficial to ion concentration distribution uniformity.To modify the electrolyte penetration distribution in the flow cell using conventional serpentine flow fields,rib width changing serpentine flow fields(sloped serpentine,partially sloped serpentine,stepwise serpentine)are proposed.Through changing the rib width distribution between the adjacent channels of the serpentine flow field,the under-the-rib flow resistance is modified to match the local pressure difference distribution.Experimental results show that the new flow field design could improve the limiting current density(50%),charge-discharge capacity(25%),and energy efficiency(4%)of the flow cell.Furthermore,the simulation results show that the new design could enhance electrolyte penetration uniformity in the in-plane direction of the porous electrode,and therefore improve electrolyte velocity and ion concentration distribution.To facilitate the application of flow field design in large-scale flow cells,different scale-up approaches(geometric similarity design,constant cross-section and rib width design,constant pressure drop design,split flow design)are proposed and discussed.The simulation results show that the electrolyte velocity,ion concentration distribution,and pressure drop in differently designed flow cells vary.The split flow design performed best at improving the mass transfer uniformity in the porous electrode.However,the pressure drop in this design was significant,which reduces the system energy efficiency by 3%-5%.Therefore,a split flow design with two inlets and a broader channel width was proposed to further decrease the flow cell pressure.Finally,to investigate ion concentration distribution in the flow cell porous electrode,a 1-D analytical model was derived,and non-dimensionalized,based on the Nernst-Planck equation and mass transfer conservation.Furthermore,analytical models with different flow path types(U-shaped and L-shaped)were also established.Based on this,the effects of electrode thickness,channel width,electrolyte flow rate,and applied current density on ion concentration distribution were studied.The results show that increased electrode thickness and channel width decreases the porous electrode local Pe number,and therefore decreases ion concentration.In addition,a decreased electrolyte flow rate and increased applied current density raises the ratio of the amount of ion consumption to the amount of supply(the Iq number),further reducing porous electrode ion concentration.