Nanoporous Non-noble Intermetallic Compound Hybrid Electrocatalysts For Efficient Water Splitting

In view of serious energy crisis and environmental pollution all over the world,developing clean and low-carbon energy is an inevitable choice to cope with global warming,ensure national energy security and promote economic development healthily.Hydrogen energy is a clean and efficient secondary energy,which can be used as an energy interconnection medium.Electrocatalytic water splitting reaction driven by electricity from intermittent renewable energy such as solar and wind energy is the most promising approach for large-scale production of hydrogen,while the conversion and utilization of electric energy and chemical energy can be realized through the zero-carbon water cycle.In view of hydrogen evolution reaction(HER)and oxygen evolution reaction(OER)taking place at the solid-iquid-gaseous interface of electrode materials,the reaction rate depends on several critical steps:mass transport of electrolyte and gas in electrode channels,electron transfer between collectors and electroactive sites,and electrochemical redox reactions at electroactive site/electrolyte interface.Noble-metal Pt and Ir O₂/Ru O₂ are the benchmark HER and OER electrocatalysts with the highest intrinsic activities,respectively.Although nano-structure can improve their utilization,the low abundance and high cost substantially hinder their widespread application for industrial electrolyzer.As a consequence,it is desirable to design and develop novel low-cost electrocatalytic materials with simple preparation process,stable electrode structure,high surface area,abundant high active sites,electron transport and mass transfer ability for boosting water splitting.Most nanocatalysts usually have to be immobilized on a conductive substrate with insulative polymer binders,inevitably leading to supplementary interfaces linked to conductivity issues and low accessibility of active sites.Self-supported three-dimensional nanoporous metals can satisfy multifunctional requirements of electron transport and mass transport in electrode systems due to their high conductivity and large surface area,which could not only form multicomponent alloys directly as catalytic reaction electrodes but also serve as conductive collectors to construct active phases on their surface.Intermetallics with ordered crystal structures possess intrinsic thermodynamic stabilities and unique electrocatalytic activities.Based on the above considerations,we focus on nanoporous non-noble intermetallic compound hybrid electrocatalysts for efficient water splitting.The research contents are as follows:1.Intermetallic Cu₅Zr clusters anchored on hierarchical nanoporous copper as efficient catalysts for hydrogen evolution reactionWith an aim to replace noble metal catalysts,considerable endeavors have been made to develop non-noble metal electrocatalysts for alkaline environment.However,HER kinetics in alkalilne electrolyte is approximately 2 or 3 orders of magnitude slower than that in acidic electrolytes.As a result,it is urgent to develop efficient alkaline HER electrocatalysts.Here,we report nonprecious intermetallic Cu₅Zr clusters that are in situ anchored on hierarchical nanoporous Cu(Cu₅Zr/Cu)by facile alloying and dealloying strategies for efficient hydrogen evolution in alkaline medium.It is thermodynamically favorable to adsorb HO*on the surface Zr atoms in the HER potential range,and hydroxygenated Zr atoms activate their nearby Cu-Cu bridge sites with appropriate hydrogen binding energy to improve reaction kinetics.The steady and bicontinuous nanoporous Cu skeleton facilitates electron transfer and electrolyte accessibility,and the seamless interfacial structure between Cu₅Zr clusters and Cu skeleton reduces contact resistance and enhances electrochemical stability.Associated with unique electrode architecture and high intrinsic catalytic activity,the self-supported monolithic nanoporous Cu₅Zr/Cu electrodes realize ultrahigh current density of-500 m A cm^-2 at the overpotential of?280 m V with a Tafel slope of?68 m V dec^-1,with exceptional stability in 1 M KOH electrolyte.2.Spontaneously separated intermetallic Co₃Mo from nanoporous copper as versatile electrocatalysts for highly efficient water splittingIn order to further improve the utilization of active sites,we develop that intermetallic Co₃Mo spontaneously separated from hierarchical nanoporous Cu skeleton(Co₃Mo/Cu)as high-performance alkaline HER and OER electrocatalysts via in-situ hydroxylation and electro-oxidation,respectively.The hydroxylated intermetallic Co₃Mo possesses an appropriate HBE to accelerate adsorption/desorption of hydrogen intermediates.Associated with fast electron/ion transport of bicontinuous nanoporous skeleton and high utilization of active sites,Co₃Mo/Cu electrodes exhibit impressive HER catalysis with a low Tafel slope(?40 m V dec^-1)in 1 M KOH electrolyte,realizing current density of-400 m A cm^-2 at overpotential of as low as?96 m V.Owing to highly coherent interface between Co₃Mo nanoparticles and Cu substrate to enhance electrochemical stability,Co₃Mo/Cu electrodes can stably maintain-210 m A cm^-2 for 1000 h at overpotential of 60 m V with excellent durability.When triggered by electro-oxidation,there in-situ forms Mo-doped Co₃O₄nanoflakes on the nanoporous Cu O/Cu skeleton to accelerate OER.The alkaline electrolyzer assembled with nanoporous Co₃Mo/Cu as cathode and their electro-oxidized derivative as anode only takes 1.65 V to achieve?145 m A cm^-2 in a brine electrolyte containing 1 M KOH and 0.5 M Na Cl,along with good stability.These electrochemical properties make them promising candidates for overall water splitting.3.Mo-/Co-N-C hybrid nanosheets oriented on hierarchical nanoporous Cu as versatile electrocatalysts for efficient water splittingSelf-supported three-dimensional nanoporous metals can not only be directly used as electrodes,but also serve as conductive collectors to construct catalytic active phases.Here,nitrogen-doped carbon engrafted Mo₂N/Co N hybrid nanosheets that are seamlessly oriented on hierarchical nanoporous Cu scaffold(Mo-/Co-N-C/Cu),as highly efficient electrocatalysts for water splitting are reported.Mo-/Co-N-C/Cu nanosheets improve HER kinetics with bifunctional electroactive sites,where the constituent Mo₂N and Co N promote water dissociation and adsorption/desorption of hydrogen intermediates,respectively.The nitrogen-doped carbon bridges electron transfers between electroactive sites and interconnective Cu current collectors by making use of Mo-/Co-N-C bonds and intimate C/Cu contacts at interfaces,while nanoporous Cu skeleton facilitates electron transfer and electrolyte accessibility.As a consequence,self-supported nanoporous Mo-/Co-N-C/Cu hybrid electrodes only take overpotential of as low as?230 m V to reach current density of-1000 m A cm^-2 with a low Tafel slope of?47 m V dec^-1 in 1 M KOH electrolyte.When electro-oxidized in alkaline solution,they can serve as OER electrocatalysts with low onset overpotential of?261 m V and Tafel slope of?58 m V dec^-1.The alkaline water electrolyzer employing nanoporous Mo-/Co-N-C/Cu electrode and its electro-oxidized derivative as cathode and anode can achieve 100 m A cm^-2 at?1.62 V,outperforming state-of-the-art devices based on nonprecious bifunctional electrocatalysts reported previously.