Development Of Nanoporous Metal Ultrathin Electrocatalysts For Direct Formic Acid Fuel Cell

The direct formic acid fuel cell（DFAFC）is widely regarded as having enormous potential in the field of portable small-scale power sources due to its simple structure,high energy density,high theoretical open circuit voltage,wide availability of fuel sources and ease of transportation and storage.However,the commercialization of DFAFC is constrained by the lack of electrode electrocatalyst activity and high cost.Therefore,the design and optimization of catalysts play a pivotal role in achieving optimal catalyst utilization efficiency and cost-effectiveness.Compared to traditional supported nanoparticle catalysts,self-supported nanoporous metal ultrathin catalysts possess many unique structural advantages.Firstly,its large specific surface area provides abundant catalytic active sites and mass transfer channels,accelerating the transport and reaction rate of substances.Secondly,the strong metal bonds make the catalyst highly corrosion-resistant.In addition,the open and tunable porous structure not only helps to construct efficient three-phase interfaces,but also promotes the mass transport dynamics in the membrane electrode level,thereby achieving high cell performance.Based on this,this paper focuses on the application of DFAFC and develops highly active DFAFC anode and cathode catalysts through systematic research on the surface design,structural design,and mass transfer control of nanoporous metal catalysts.These catalysts have been successfully applied to both DFAFC single cell and full cell,and the fuel cell performance has been significantly improved through effective mass transfer regulation.The specific research contents are as follows:（1）Construction of anodic formic acid electrocatalysts based on Pd modified nanoporous gold（NPG-Pd）.The first principles calculation based on density functional theory（DFT）reveals that the high coordination environment can effectively modulate the electronic structure of Pd and optimize the adsorption strength of the active site with formic acid electrooxidation（FAOR）descriptors（CO and OH）,thus adjusting its position in the formic acid electrooxidation volcano map and FAOR activity.In experiment,NPG-Pd formic acid electrocatalysts with abundant highly coordinated Pd on the surface were prepared through copper under potential deposition（Cu-UPD）and a subsequent galvanic replacement reaction（GRR）.The catalyst NPG-Pd-Au with a higher proportion of highly coordinated Pd was prepared by further modification of Au clusters,and its FAOR intrinsic activity（8.62 m A/cm²）and mass activity（27.25 A/mg）were 7.9 and 85.2 times higher than those of commercial Pd/C,respectively.The residual current density（0.31 A/mg）of NPG-Pd-Au after 2.5 hours of stability testing was higher than that of commercial Pd/C catalyst after 1 hour of stability testing（0.01A/mg）.In order to meet the needs of practical fuel cell applications,the highly coordinated NPG-Pd catalysts with Pd decorated on the surface of NPG were prepared by differential normal pulse voltammetry in controllable and uniform manner.The FAOR intrinsic and mass activity of catalyst NPG-Pd30 are 5.12 and 10.28 times higher than commercial Pd/C,respectively.Although the FAOR activity of the highly coordinated NPG-Pd is slightly lower than that obtained by the Cu-UPD-GRR method,this method enables the repetitive batch preparation of catalytic layers in membrane electrodes.This work provides a theoretical and experimental basis for the design and development of new high-performance and stable FAOR catalysts.（2）Construction of cathodic oxygen reduction electrocatalyst based on PdPt co-modified nanoporous gold（NPG-PdPt）.High oxygen reduction reaction（ORR）active electrocatalysts were prepared by theoretically guided experiments.The NPG-PdPt electrocatalysts with Pt-rich surface were prepared by functionalizing the NPG surface.The modulation of Pt electronic structure by Au and Pd can maximize the atomic utilization of noble metal Pt,thereby enhancing the ORR activity of Pt-based catalysts.The mass activity（MA）and specific activity（SA）of NPG-Pd₇Pt₁₃ were as high as 0.68A/mg _Pt+Pd and 0.741 m A/cm²,which were 4.72 and 3.55 times higher than those of commercial Pt/C,respectively.The SA of the NPG-Pd₇Pt₁₃ decayed by only 5.2%after5000 cycles.The ORR intrinsic activity of NPG-Pd₇Pt₁₃(0.207 A/mg _Pt+Pd and 0.226m A/cm²)was still higher than the initial activity of commercial Pt/C(0.144 A/mg _Pt and0.209 m A/cm²)at the maximum formic acid permeation concentration disturbance in the DFAFC test environment.This core-shell structure NPG-Pd₇Pt₁₃ combines high ORR activity,high stability and formic acid resistance,which effectively solves the bottleneck problem of Pt-based catalysts in DFAFC cathode ORR and provides valuable technical guidance to promote the design/manufacture of high-performance Pt-based catalysts for DFAFC and other energy technologies.（3）DFAFC single cell mass transfer modulation based on nanoporous metal ultrathin catalytic layers.The newly developed nanoporous metal ultrathin electrocatalysts NPG-Pd and NPG-PdPt as the anode and cathode catalysts for DFAFC,and they were assembled into single cells.By regulating the fuel flow rate in the membrane electrode to adjust the residence time of formic acid in the electrode,the diffusion rate and concentration distribution of the product can be controlled,which in turn regulates the reaction rate and reaction efficiency and realizes the full play of the electrocatalyst activity at the membrane electrode level.In the DFAFC with NPG-Pd as the anode catalyst,the maximum power densities before and after mass transfer modulation were 147 and 207 m W/cm² and mass power densities were 4.9 and 6.9W/mg _Pd,respectively.However,the cell performance of commercial Pd/C assembled single cell was basically unimproved after mass transfer conditioning.When NPG-Pd is compared with commercial Pd/C catalysts,the single cell power density and mass power density are increased by a factor of 2.95 and 98.15,respectively,even when the Pd loading is reduced by a factor of 33.The cell power density normalized to the total noble metal（Au+Pd）loading is still at least one order of magnitude higher than that of commercial Pd/C.The single cell with NPG-PdPt as the cathode catalyst,with a total Pd and Pt loading of 20μg/cm²,has a power density about 47%higher than that of a commercial Pt/C with a loading of 2 mg _Pt/cm².In addition,the nanoporous metal ultrathin catalytic layer with integrated structure and function improves the stability of the membrane electrode.（4）Study on the performance of DFAFC full cell based on nanoporous metal ultrathin catalytic layers.The newly developed NPG-Pd and NPG-PdPt were matched as anode and cathode catalysts for DFAFC,and ultra-low noble metal loading ultrathin film electrodes were assembled with cathode and anode catalytic layer thickness below500 nm and total noble metal loading of Pt and Pd of 50μg _Pd+Pt/cm².By adjusting the humidity of the fuel in the cathode of the full cell,the water content inside the membrane electrode can be regulated,which affects the proton transport rate and further adjusts the performance of the cell.After mass transfer modulation,the nanoporous metal-based DFAFC full cell achieves a maximum power density of 139 m W/cm² at a temperature of 60°C and a mass power density of up to 2.78 W/mg _Pd+Pt,which is much higher than the cell performance（53.8 m W/cm²）and the utilization of noble metals(0.018 W/mg _Pd+Pt)of commercial catalysts under the same test conditions.The new nanoporous metal ultrathin catalytic layer constructed full cell achieves efficient expression of catalyst performance at low temperatures,which is expected to promote the application of DFAFC in the field of portable small power devices.