Synthesis And Performance Of Non-platinum Catalysts Toward Direct Ascorbic Acid Fuel Cells

Ascorbic acid（AA）exists widely in nature,has good biocompatibility,convenient storage and transportation,and low permeability to polymer electrolyte membrane.It can be used as biomass fuel to drive polymer electrolyte membrane fuel cell,which is promising in the field of portable power supply and implantable medical equipment.However,the kinetically slow AA oxidation reaction（AAOR）on the anode side,lacks efficient catalyst at present,and the oxygen reduction reaction（ORR）on the cathode side is strongly dependent on precious metal Pt.Therefore,anode and cathode catalysts with high performance and low cost are a focus of direct AA fuel cell（DAAFC）.In this paper,two kinds of anode catalysts（oxygen-containing group modified carbon black and CeO₂ modified carbon black）and two kinds of cathode catalysts（carbon supported PdO-CeO₂ and CeO₂ modified Fe-N-C）were designed and prepared.The main progress is as follows:（1）A series of carbon black modified with oxygen-containing groups were obtained by thermal nitric acid oxidation,and the activation energy of its catalytic oxidation of AA was explored.Density function theory（DFT）calculation further shows that carboxyl and carbonyl groups could adsorb AA,which improves the activity of AAOR.In addition,the presence of oxygen-containing groups improves the hydrophilicity of carbon black and contributes to the mass transfer of AA in the catalytic layer.DAAFC single cell was assembled using the carbon black as anode catalyst,and the peak power density was 31.0 m W cm^-2,which is 1.7 times of the highest value in the literature.（2）Ce（OH）₃ was uniformly supported on the surface of carbon black modified by oxygen-containing groups by chemical deposition.After heat treatment,CeO₂ nanoparticles（3.9±1.1nm）modified carbon black was obtained.The effects of different CeO₂ loadings on the properties of AAOR were investigated.It was found that 2 wt%CeO₂/C had the best AAOR activity.DFT calculation shows that CeO₂ promotes AAOR by combining lattice oxygen with hydroxyl hydrogen on AA dienol.In addition,according to X-ray photoelectron spectroscopy,the oxygen vacancy of CeO₂ helps to improve the activity of AAOR.CeO₂/C was used as anode catalyst to assemble DAAFC single cell,and the peak power density was 41.3 m W cm^-2,which is 2.2 times of the highest value in the literature.（3）Pd and Ce precursors in aqueous phase were transferred to organic phase by dodecylamine.After Na BH₄ reduction,loading on carbon and low-temperature（250 ^oC）heat treatment,carbon supported PdO nanoparticles modified by amorphous CeO₂（4.3±0.8 nm）was obtained.Compared with the traditional preparation method of PdO,the temperature of heat treatment was effectively reduced.The CeO₂/CeO_2-x electron pair is able to oxidize Pd nanoparticles to PdO at 250 ^oC.The mass activity（MA）of PdO-CeO₂/C in alkaline medium is1103 m A mg _Pd^-1（0.9 V vs.RHE）,which is 5.9 times higher than that of commercial Pt/C.DFT calculation suggests that the high ORR activity arise from an appropriate oxygen adsorption strength at the interface of PdO-CeO₂.The single cell of DAAFC was assembled with PdO-CeO₂/C and CeO₂/C as anode and cathode catalysts,and the peak power density was 29.2 m W cm^-2.（4）A rice-panicles like dual MOFs material was synthesized by self assembling of hemin,imidazole,zinc（II）nitrate on preformed Ce-BTC nanorods.After pyrolysis,the morphology of nano rice panicle was retained,and CeO₂ nanoparticles and rich Fe-N-C sites were evenly distributed in carbonized dual MOFs.Fe-N-C-CeO₂ exhibits a high ORR activity in terms of a half wave potential of 0.8 V（vs.RHE）,approaching to that of commercial Pt/C.The high ORR activity is closely correlated with well dispersion of Fe-N-C on the carbonized Ce-BTC,allowing the exposure of Fe-N₄ sites as evidenced by atomic resolution transmission electron microscope,⁵⁷Fe M（?）ssbauer spectroscopy,and X-ray absorption fine structure.The high Ce³⁺content in CeO₂ is helpful to remove the ORR intermediate H₂O₂,so as to significantly improve the durability of Fe-N-C catalyst.The single cell of DAAFC was assembled with Fe-N-C-CeO₂and CeO₂/C as anode and cathode catalysts,and the peak power density was 20.0 m W cm^-2.