Preparation And Electrocatalytic Performance Of Single-atom Catalysts Based On Sodium Alginate

Single atom catalysts?SACs?have unique properties,such as high atom ultization,high catalytic activity,catalytic selectivity and catalytic stability.SACs can largely reduce the usage of metals,thereby significantly reducing the cost of the electrochemical energy conversion system and promoting its development and practical application.However,the fabrication of SACs is a big challenge,such as complicated synthesis,expensive equipment,low yield,expensive precursor,and high cost et al.Therefore,it is urgently needed to develop a simple,scalable and cost-effective synthesis for SACs.Sodium alginate is a natural polysaccharide that extracted from brown algae.It is a linear unbranched polymer consisting of?-D-mannuronic acid?M segment?and?-L-guluronic acid?G segment?monomers.The unique property of sodium alginate is the ability to form nanoconfined"egg box"structure with multivalent metal ions where each metal ion is coordinated with four G blocks from two adjacent chains.The unique"egg box"in alginate makes it easy to combine inorganic metal ions with natural biomass polysaccharides.Ideal metal-biopolysaccharide precursors can be obtained via regulating the metal ion species,ratios and contents.After simple pyrolysis and acid leaching,SACs can be successfully prepared.In this work,we have developed a general strategy for the SACs preparation using sodium alginate as precursors,and studied their electrocatalytic properties,including controllable synthesis and electrochemical properties of various STMACs?Co,Ni,Cu single atoms/carbon aerogels?,synthesis and electrochemical properties of various SPMACs?Pt,Au,Ru single atoms/carbon aerogels?,and synthesis and electrocatalytic activity for nitrogen reduction reaction of Pt single atom/ZrO₂.This work is of important theoretical and practical significance for the low-cost synthesis of SACs,the practical application of SACs and the development of electrochemical energy conversion technology.The specific research contents are as follows:?1?We have developed a general strategy for the low-cost synthesis of STMACs.The STMACs is obtained after high temperature carbonization and acid leaching of the transition metal-alginate precursors which is formed by the gelation of sodium alginate with Zn²⁺and other metal ions(such as Co²⁺,Ni²⁺,Cu²⁺,etc.).Various STMACs are synthesized,including Co,Ni,Cu single atoms/carbon aerogels.Especially,single Co atom electrocatalyts show excellent catalytic activity for oxygen reduction reaction?ORR?with the half-wave potential 15 mV more positive than the commercial 20%Pt/C.When applied in zinc-air batteries?ZABs?,it can work robustly for more than 250 h with refueling the Zn anode and KOH electrolyte periodically.?2?We have developed a general strategy for the low-cost synthesis of SNMACs.Using sodium alginate as reducing agent and stabilizer,the noble metal precursors are reduced to noble metals via thermal reduction,forming noble metal/sodium alginate solution.The STMACs is synthesized after high temperature carbonization and acid leaching of the noble metal/zinc alginate aerogel formed by the gelation of noble metal/sodium alginate solution with Zn²⁺.Various SNMACs are synthesized,including Pt,Au,Ru single atoms/carbon aerogels.After futher S-doping treatment,the obtained S-doped single Pt atom electrocatalyts exhibit excellent ORR activity with the half-wave potential 32 mV more positive than the commercial 20%Pt/C with the same loading.When applied in ZABs,it can continuously work robustly for more than 140 h.This electrocatalyst large reduces the usage of Pt,thus redcuing the cost of electrocatalyst,facilitating its commercial application.?3?Preparation of Pt single atom/ZrO₂ carbon aerogel electrocatalyst for nitrogen reduction reaction.Besides ORR,Pt also has the ability to catalyze the reduction of nitrogen?NRR?.However,its NRR performances are limited by its high hydrogen evolution reaction activities?HER?.In order to improve the NRR performance,it is necessary to suppress HER during the reaction,such as combining with the metals that can supress the HER.Using sodium alginate as precursors,we can combine Pt single atom with ZrO₂ which will inhibit hydrogen evolution,forming Pt single atom/ZrO₂carbon aerogel.This electrocatalysts can significantly improve the ammonia production rate and the faraday efficiency.Especially,its faraday efficiency is two orders of magnitude higher than the 20%Pt/C.