Microreaction Synthesis Of Low-Platinum And Non-Platinum Electrocatalysts And Their Microstructures And Electronic Structure Regulation

Fuel cell is a very promising new energy conversion equipment,and its electrochemical reactions such as methanol oxidation at the anode,oxygen reduction at the cathode,and electrocatalytic hydrogen evolution are the focus of current research.The precious metal platinum is currently the most effective electrocatalyst known.Because platinum is scarce and expensive,the cost of fuel cells is high and it is difficult to apply on a large scale.Many solutions have been proposed to solve this problem,such as low-platinum catalysts and non-platinum catalysts.Although many catalysts have been developed,their electrochemical performance needs to be further improved.Exploring new synthesis methods and in-depth study of the relationship between the structure and performance of electrocatalysts are not only of important scientific significance,but also of great practical significance to the application of fuel cells.This article aims to explore the regulation of precious metal and non-precious metal catalysts through microreactions,and study the influence of interface structure,component composition,electronic structure and other factors on the properties of alloy heterostructure nanoparticles.In this paper,micro-reactions are used to control low-platinum PtFeCe catalysts and non-platinum Fe single-atom and FeCo core-shell catalysts,and the relationship between their structure and catalytic performance is studied.The specific content is as follows:1.PtFe nanocrystals hybridized by one rare-earth metal oxide(i.e.,CeOx)(namely PtFe-PtxFeyCe2Oj)with controlled compositions and gradient microstructures are synthesized continuously in a simple programmed microfluidic process.Their electrochemical catalytic performances are evaluated by methanol oxidation reaction(MOR),oxygen reduction reaction(ORR)and hydrogen evolution reaction(HER).TEM,EDS,XRD and XPS measurements are used to characterize their microstructures and compositions.Results suggest that PtFe-PtxFeyCezOj nanohybrids have overall sizes around 2 nm,showing gradient structures composed of PtFe rich inner parts and cerium oxide rich outer layers.They preserve angular shapes and rough surfaces,whose atoms have metallic states and/or multi-oxidation states.The electrochemical catalytic measurements indicate that PtFe-PtxFeyCezOj nanohybrids exhibit excellent electrochemical catalytic performances in MOR,ORR and HER.Particularly,the PtFe-PtxFeyCezOj nanohybrids with the designed atom ratio of Fe:Pt:Ce=1:1:1 show the best MOR performance,with a forward peak current of 1018.6 mA/mgpt,which is 5.6-fold enhancement compared to the commercial Pt/C catalysts(180.8 mA/mgPt),and the best ORR performance,with a specific activity of 3.28 mA/cm2 and mass activity of 0.73 A/mgPt,which are 6.0-and 4.6-fold enhancement compared with the commercial Pt/C catalysts(0.55 mA/cm2 and 0.16 A/mgPt).It also gives HER a much low Tafel slope(19 mV dec-1)comparing with those previously reported.Chronoamperometry tests suggest that PtFeCe1 catalysts can maintain 2-fold activity of commercial Pt/C catalysts at the end of 4000 s’ operation.TEM images of nanohybrids before and after electrochemical catalytic tests confirm that the suitable coating of CeOx prevents the PtFe nanohybrids from intensive etching and dissolving in the strong acidic reaction electrolytes and improves the durability and microstructure stability of these nanohybrids.The greatly improved catalytic performances are fundamentally resulted not only from the synergistic interface effect between the PtFe alloy rich inner parts and the cerium oxide rich outer layers,but also from the electron orbital hybridization effects among Pt,Fe and Ce atoms.This study provides a new methodology in the microstructure design and controlled synthesis of efficient multifunctional nanocatalysts with stable microstructures by hybridizing rare-earth metal oxides to form stable thin outer-layers.These stable nanocatalysts shall preserve valuable applications in fuel cells and hydrogen production by water splitting.2.Mass production of highly efficient.durable,and inexpensive single atomic catalysts is currently the major challenge associated with the oxygen reduction reaction(ORR)for fuel cells.In this study we develop a general strategy that uses a simple ultrasonic atomization coupling with pyrolysis and calcination process to synthesize single atomic FeNC catalysts(FeNC SACs)at large scale.The microstructure characterizations confirm that the active centers root in the single atomic Fe sites chelating to the four-fold pyridinic N atoms.The identified specific Fe active sites with the variable valence states facilitate the transfer of electrons,endowing the FeNC SACs with excellent electrochemical ORR activity,as exemplified by a halfwave potential of E1/2=0.87 V and a kinetic current density of 10.44 mV cm-2,and the substantially enhanced long-term stability(maintaining 93%activity after 36,000 s in the durability test).The FeNC SACs were used as cathode catalysts in a homemade Zn-air battery,giving an open-circuit voltage of 1.43 V,which is substantially higher than that of commercial Pt/C catalysts.This study not only provides a simple approach to the synthesis of single atomic catalysts at large scale but also promises a great potential for industrial application of fuel cells.3.Designing high-efficitent electrocatalysts for oxygen reduction reaction(ORR)is of great importance for realizing practical applications of many significant energy conversion and storage devices.In this study,several heterostructured FeCo@C-N-doped CoFe2O-4 core-shell nanocatalysts were synthesized using our previously invented ultrasonic atomization microreaction coupling post annealing process,whose active sites and the related electronic structures can be well-tuned via the nitrogen species and Fe/Co ratios.Using the Fe-N bonds as a marker,we estthe relationship between the electronic structure of the active sites and ORR catalytic performance was established.The electronic structure of the active sites could be successfully controlled by adjusting the Fe:Co ratio and the doping type of nitrogen.A comparison of the results showed that the electron density around Fe could be increased by increasing the Co content,resulting in a reduced Fe-N binding energy.In addition,the ORR tests confirmed that the optimized electronic structure could greatly improve the electrochemical catalysis performance.The catalysis performance of the best nanocatalysts obtained by optimizing their electronic structures,including the half-wave potential,onset potential,kinetic current density,and limiting current density,far exceeded those of commercial Pt/C catalysts.