| The anode CO poisoning problem is one of the important factors restricting the cost reduction and long life of proton exchange membrane fuel cell(PEMFC).As an integral part of the hydrogen economy,the purification or direct use of the H2/CO mixture from steam reforming is an important research direction to reduce costs and improve the performance of anode catalysts.At present,the Rh-based porphyrin structure is the most excellent active site for CO electrooxidation.Based on the laws of CO*and OH*adsorption,by adjusting the geometric structure and electronic structure of Rh-based single-atom catalysts,the adsorption properties of reactants,intermediate species and products in the CO electrooxidation process were optimized,and then the catalytic properties of various single-atom catalysts were explored to achieve the improvement of catalytic activity.The specific research contents are as follows:1.Carbon monoxide powered fuel cell towards H2-onboard purificationA single-atom Rh-based catalyst was synthesized by utilizing the characteristics of nitrogen-rich carbon support and easily coordination with metal atoms.By XAFS fitting verification,the coordination number of Rh single-atom catalyst is 4.The generation energy of RhN4 structure is much larger than that of RhNxC4-x(x=0,1,2,3)structure.Combined with XAFS simulation,it can be confirmed that the prepared RhN-C catalyst is RhN4 structure.The Rh-N-C catalyst exhibits strong electro-oxidative properties for CO.AN30(30%Nafion ionomer content)is the best level to effectively extend three-phase interface and improve fuel cell performance.The maximum current density of PEMFC driven by CO as anode gas can reach 970 mA cm-2,and the maximum power is as high as 236 mW cm-2.The TOF value on the active site of the catalyst is 64.65 s-1,which is about 1-2 orders of magnitude higher than that of the currently reported PROX catalysts.Both the three-electrode and DEMS test show that the Rh-N-C catalyst can oxidize CO even at 0 V.In addition,Rh-N-C catalyst can also selectively remove CO gas from CO/H2 gas mixture,and has the ability of continuous purification.Adjacent RhN4 sites in Rh-N-C catalysts can activate H2O molecules and CO molecules,which is an important factor for the unique high COOR activity.2.The decisive role of adsorbed OH*in low potential CO electrooxidation on singleatom catalytic sitesThe electro-oxidation of CO is mainly realized by the reaction between CO*and OH*on the catalyst.In order to explore the effect of the adsorption properties of CO*and OH*on the Rh-based single-atom active center for CO electrooxidation performance,the local coordination structure was adjusted to achieve the two different active centers,which are the RhN4 active site and the RhC4 active site.ToF-SIMS characterization shows that the RhN4 structural fragments are derived from RhN4Cy fragments.CO-TPD and DFT calculations can prove that the CO*adsorption strengths on RhN4 structure and RhC4 structure are almost identical.However,electrochemical tests show that the Rh/N/C catalyst exhibits excellent CO electro-oxidation performance and can express a discharge power density of 189 mW cm-2 even in a pure CO-driven fuel cell.While Rh/C catalysts have similar single-atom dispersion properties and CO adsorption properties,the COOR performance is much worse.Both direct spectroscopic results and DFT calculations indicate that OH*on RhN4 sites is consumed faster than that on RhC4 sites.The more active feature of OH*on the RhN4 site can be attributed to the significantly weaker adsorption strength on the RhN4 site than that on the RhC4 site.3.First constructing volcano plot for carbon-supported single-atom catalysts for COORA series of single-atom catalysts of group Ⅷ elements were synthesized with the help of ZIF-8 carrier confinement,in order to explore the influencing factors and laws of CO electro-oxidation process.It can be seen from the HAADF-STEM images combined with the XAFS characterization that the prepared eight catalysts are all single-atom catalysts,namely Fe-N-C,Co-N-C,Ni-N-C,Ru-N-C,Rh-N-C,Pd-N-C,Ir-N-C,Pt-N-C.The catalyst structure was identified as the MN4 structure and used for DFT calculations.Electrochemical tests show that the Rh-N-C catalyst has the best performance,followed by the Ir-N-C catalyst.The Co-N-C catalyst has weak performance,and the other catalysts almost have not performance.The DFT thermodynamic analysis shows that the activity of single-atom catalysts for CO electrooxidation is closely related to the CO*and OH*adsorbed species,and the best performance is achieved when both are located at or near the apex of the fire curve.COHP and DOS analysis indicated that the activation ability of CO*and OH*on MN4 site is also one of the important indicators of CO electrooxidation activity. |
