Prussian Blue Analogues-derived Composite Nanomaterials As Active Electrocatalysts For Water Splitting In Alkaline Media

Today,environmental deterioration and energy crisis are the major problems that our world is facing.Hydrogen,as a clean and renewable energy,has attracted growing attention for its high gravimetric energy density,zero-emission and earth-abundance.The scalable production of hydrogen could conveniently be realized by alkaline water electrolysis.Unfortunately,water splitting is kinetically not favored and hence requires highly active electrocatalysts to be expedited.The catalysts could change the kinetical process and hence reduce the overpotential,contributing to lowering the cost of hydrogen.Importantly,electrochemical water splitting is a clean and non-fossil fuel-based technology for producting hydrogen by consumes electricity which could be generated via chansfer and storage of other unstable clean energy resources.Prussian blue analogues,discovered in 1704,were the oldest and simplest metal organic frameworks,which have attracted extensive interest and been intensely investigated due to its application in gas adsorption,energy storage,catalysis,drug loading and so on.Prussian blue analogues were created from supramolecular assembly of transition metal ion centers as metallic nodes and cyanide group as organic linkers.There are two different kinds of position coordinated respectively with carbon and nitrogen for transition metal ions in the framework of Prussian blue analogues.So,both two different position in the framework can be filled with different metal ion centers such as Fe,Co,Ni,Zn,Cu and so on.Besides,some transition metal ions can be replaced by noble metal ions,which will maintain the initial framework structure of Prussian blue analogues.Thus,Prussian blue analogues can have rich species of metal elements for the formation of various alloy including transition metal,transition metal alloy and transition metal/noble metal alloy,which can adjust the lattice and bond length to alter the adsorption energies toward optimal electrocatalytic activity.Additionally,CN-group linkers in Prussian blue analogues can serve as both nitrogen and carbon sources for the formation of N-doped graphene layers covering the alloy particles,which is beneficial for the electron transfer from the alloy to the graphene and subsequently improving electrocatalytic activity and stability.Therefore,we design a series of various Prussian blue analogues precursors as promising templates to facilely fabricate transition metal-based electrocatalysts via thermolysis.The electrocatalytic HER or OER performances of our obtained prepared nanoparticles were investigated in basic media.The details contains the following aspects:1.Currently,the major challenge confronting hydrogen evolution reaction(HER)is lacking inexpensive alternatives to platinum-based electrocatalysts.We use Ruthenium doped cobalt hexacyanocobaltate as template to prepare a high-efficient and stable electrocatalyst composed of Ruthenium and Cobalt bimetallic nanoalloy encapsulated in nitrogen doped graphene layers(RuCo@NC).The catalysts display remarkable performance with low overpotentials of only 28 mV and 218 mV at 10 mA cm-2 and 100 mA cm-2,respectively,and excellent stability of 10 000 cycles.Ruthenium is the cheapest Platinum group metal and its amount in the catalyst is only 3.58 wt.%,showing the catalyst high activity at a very competive price.Density functional theory calculations reveal that the introduction of Ruthenium atoms into Cobalt core can improve the efficiency of electron transfer from alloy core to graphene shell,beneficial for enhancing Carbon-Hydrogen bond,thereby lowing?GH*of hydrogen evolution reaction.2.Oxygen evolution reaction(OER)is a key half reaction involved in electrochemical water splitting.However,it is kinetically not favored and hence requires highly active electrocatalysts.RuO2 or IrO2 is state-of-the-art catalysts for OER,but its widespread practical application is limited by high cost and scarcity.Transition metals and their alloy have high theoretical activity and low cost,with great potential to replace precious catalysts.Herein,we adopt a nickel hexacyanoferrate assisted strategy to fabricate FeNi nanoalloys encapsulated in N-doped graphene layers(FeNi@NC).The catalyst shows a low overpotential of only 299 mV at 10 tA cm-2 in alkaline solutions and possesses a high catalytic durability after 5 000 cycles,indicating both the electrocatalytic activity and long cycling stability of our obtained catalyst much exceed the state-of-the-art commercial RuO2 catalysts.Therefore,as-prepared catalyst shows great potential to be a substitute for precious metal based catalysts for OER.3.Oxygen evolution reaction(OER),as the cathode reaction involved in electrochemical water splitting,plays a key role in hydrogen production.Moreover,OER coupled with its reverse reaction(oxygen reduction reaction(ORR)),is at the heart of unitized regenerative fuel cells(URFCs)and metal-air batteries.However,it requires highly active electrocatalysts to reduce the energy wastage by reducing electrocatalytic overpotential due to its unfavorable reaction kinetics.Non-precious metal based catalysts have emerged as the most promising materials for the replacement of the state-of-the-art noble metal catalysts.However,more progress is still in need to improve activity and stability of these catalysts.Herein,we synthesized Co3ZnC/Co nanojunctions encapsulated in highly nitrogen doped graphene layers(ZnCo3C/Co@NC)by one-step annealing of Zinc hexacyanocobaltate.The catalyst shows a low overpotential of only 366 mV at 10 tA cm-2,which is superior to the state-of-the-art commercial RuO2 catalysts.Besides,it exhibits a better performance compared with similar metallic carbide-based catalysts towards ORR with Onset Potential and Cathodic peak potential at 0.912V and 0.814V respectively.The metallic Co NPs in the Co3ZnC/Co interfaces,as electron donors,become more electrophilic,promoting the nucleophilic reaction by OH-with the intermediates and thus accelerating OER,while Co3ZnC NPs the Co3ZnC/Co interfaces,as electron acceptors,become more nucleophilic,facilitating the electrophilic reaction by the intermediates with OH-and thus accelerating ORR.