Controlled Preparation Of Nanoporous Platinum-Based Alloys And Catalytic Electro-Oxidation Of Ammonia

Achieving carbon neutrality by 2050 is an urgent challenge to address the climate change for our universe.Countries around the world are currently focusing on search for renewable,carbon-free,and green energy sources to replace traditional fossil fuels.Ammonia,as one of the most promising hydrogen storage media,is considered to be an ideal carbon-free energy carrier.In addition,ammonia shows great advantages in long-distance transportation and storage,which can compensate for the lack of hydrogen transportation.In recent years,the direct ammonia fuel cell（DAFC）,which is made by directly feeding ammonia gas to the anode of a fuel cell,has attracted increasing attentions from researchers.It is a clean energy conversion device that can directly convert chemical energy into electricity and whose oxidation products are only nitrogen and water.However,this extremely attractive direct ammonia fuel cell technology is limited by the anodic ammonia oxidation reaction（AOR）,a sluggish kinetics with six-electron transfer process.Therefore,the development of efficient and stable ammonia oxidation catalysts for the anode is in high demand,needing to be addressed.Based on these issues,this work has successfully prepared several Pt-based alloy catalysts with high catalytic activity and stability by using the modulation strategies of morphology and structure along with chemical component.The details of the structural composition trends of the optimal catalysts before and after stability testing were also explored in detail.The specific research conducted in this paper is as follows:（1）PtIralloy aerogel catalysts with three-dimensional porous network structure were prepared by self-assembly of nanoparticles（NPs）using NaBH₄as the reducing agent.This type of structure provided abundant open interconnected proton transport channels and additional catalytically active sites that contribute to the deprotonation process of NH₃ molecules in ammonia electrocatalytic oxidation.The Ptand Ir substance ratio of 80/20 exhibited the optimal catalytic activity for ammonia oxidation(onset potential:0.368 V;peak mass activity:86.3 A g^-1).It was found that the AOR performance of Pt₈₀Ir₂₀ alloy aerogel catalyst was enhanced with the increasing of ammonia concentration or operating temperature,and its activation energy of ammonia oxidation was estimated to be 40.69 k J mol^-1,which was lower than commercial Pt/C(50.12 k J mol^-1).This catalyst also exhibited good stability with a peak mass activity loss of～50.6%after 2000 cycles of cyclic voltametric（CV）cycling,which is lower than Pt/C（～74.9%）.（2）Based on the above studies,we then prepared a ternary PtIrNi alloy nanospheres with porous architecture using a shape-induced synthesis method.The introduction of non-precious metal Ni could effectively regulate the electronic structure of Pt,reduce the excessive adsorption of Pton ammonia oxidation intermediates,and improve the catalytic activity and anti-poisoning ability of the ternary alloy for ammonia oxidation.Because in relative to nanowire aerogels,the porous nanospheres are less prone to the agglomeration within different particles.In particular,their open and well-defined pore channels together with rough surface/interface structures can substantially improve the utilization of metal active sites.Moreover,the unique structure of porous nanospheres is more favorable for the uptake/desorption of ammonia oxidation intermediates,thus resulting in an enhanced catalytic activity and stability.In particular,the optimal Pt₆₆Ir₁₇Ni₁₇ nanospheres have a low AOR onset potential of 0.361 V,a peak mass activity as high as 186.2 A g^-1,and a reduced activation energy of 39.41 k J mol^-1.In addition,it also exhibited a high catalytic activity for hydrazine hydrate（N₂H₄）,a key intermediate during the AOR oxidation.These results indicate that the ternary PtIrNi alloy nanospheres have high intrinsic activity for the AOR.（3）Based on ternary PtIrNi alloy nanospheres,we further performed a systematic study of catalyst stability evaluation.The stability experiments for catalyst materials showed that the optimal Pt₆₆Ir₁₇Ni₁₇ nanospheres after 2000 cycles of CV cycling have retained a peak mass activity of 117.7 A g^-1,which was approximately two times higher compared to Pt₈₀Ir₂₀nanospheres(58.7 A g^-1).Meanwhile,the durability experiments for catalyst’s ammonia oxidation electrocatalysis showed that the optimal Pt₆₆Ir₁₇Ni₁₇ nanospheres after 18000s of chronoamperogram durability test have retained a mass activity of 3.23 A g^-1,which was higher than Pt₈₀Ir₂₀(2.29 A g^-1).The comparisons of morphological structures,chemical compositions and elemental valence states for the catalyst before and after the reaction can be obtained.Results indicate that the ternary PtIrNi alloy nanospheres have displayed superior structural stability,while its chemical composition and elemental valence change a little bit.In particular,partial Ptin the alloy has showed an increased valence state after the stability tests,whereas Ir in the alloy occurs certain dissolutions.This phenomenon is the reason why a reduced stability is observed on the ternary PtIrNi alloys.Additionally,the NO_x species generated by peroxidation of ammonia could also induce catalyst deactivation.