Application of Interface Engineering in Electrochemical Hydrogen Evolution Reaction and Oxygen Evolution Reaction

Author:Xiong Jie

Supervisor:cheng han song cai wei wei


Degree Year:2019





The globally increasing consumption of fossil fuels has caused severe environmental contamination and potential energy crisis.Thus,the urgent demand of clean and renewable energy is driving the exploration of effective strategies to produce environmentally friendly new energy carriers.Hydrogen,which possesses the highest gravimetric energy density compared with other fuels and yields only non-polluted water as the oxidation or combustion product,is proposed as the potential promising candidate for the future energy supply.Consequently,sustainable hydrogen production becomes an essential prerequisite for the future hydrogen economy.Water electrolysis driven by renewable resource-derived electricity is deemed to a promising pathway for sustainable hydrogen generation.However,water splitting technique depends mainly on highly active noble metals,i.e.,Pt as the hydrogen evolution reaction(HER)catalyst at the cathode and iridium or ruthenium oxides as the oxygen evolution reaction(OER)catalyst at the anode,to accomplish electrolysis of water.Unfortunately,the high price and low earth-abundance of these precious metals have impeded their large-scale application in the near future.Thus,it seems to be necessary to develop novel HER and OER electrocatalysts to generate hydrogen with low cost and high efficiency as alternatives to Group VIII Pt-based noble metals,rendering the water electrolysis process more energy-efficient and economical.Very recently,cost-effective transition metal compounds,such as carbides,nitrides,sulfides,phosphates,borides,and oxides(including perovskites and spinels)with various nanostructures have been widely designed for efficient water splitting HER or OER catalysis,which have also been demonstrated to be high performance electrocatalysts with high activity,great durability,and approaching 100%Faradic efficiency in not only harsh acidic electrolytes,but also in strong alkaline and neutral media for electrochemical water splitting.However,there is remain gap needs to be filled between the transition metal compounds and the Pt-group catalysts towards both HER and OER,and the HER and OER performances of transition metal compounds are still away from the requirement of commercialization of water electrolysis technique.In order to further enhance the hydrogen evolution reaction and oxygen evolution reaction electrocatalytic activity and stability of transition metal compounds electrocatalysts,several modified strategies based on interfacial engineerings,including constructing defect structure,designing heterojunction,phase engineering,creating hierarchical structure and doping heteroatoms,etc.were employed and studied comprehensively.The research contents of this thesis are shown as the following:1.Salt-templated Synthesis of Defect-rich MoN Nanosheets for Boosted Hydrogen Evolution ReactionExposing maximally the catalytic active sites of electrocatalysts is a prerequisite for efficient hydrogen evolution reaction.Here,two-dimensional defect-rich molybdenum nitride(dr-MoN)nanosheets were successfully prepared via a NaCl template-directed synthesis route followed by an incomplete ammoniation of MoO3 nanosheets.With additional edge defects arising from the etched MoO3 compared to those in the intact MoN nanosheets,dr-MoN was capable of efficiently electro-catalyzing the hydrogen evolution reaction under both acidic and alkaline conditions with impressive activity and durability.The dr-MoN-0 catalyst possessed an ultrasmall onset overpotential of approximately 10 mV in 0.5 M H2SO4 electrolyte,which is competitive with that of the Pt/C catalyst.Overpotentials of only 125 mV and 139 mV are required to deliver a current density of 10 mA cm-2 for the dr-MoN-0 catalyst in 0.5 M H2SO4and 1 M KOH,respectively.More importantly,the dr-MoN-0 maintained a prominent amperometric(I-t)durability during a 20 h test and also a superior cycling stability with negligible overpotential loss,all of which are among the best results for current MoN based HER catalysts.The exceptional performance was attributed to the defect-abundant structure which resulted in the formation of tiny cracks on the surface of the nanosheets and caused the additional exposure of active edge sites.These findings highlight the prospective potential of dr-MoN with additional active edge sites as highly efficient and stable platinum-free electrocatalysts towards the HER.2.In Situ Engineering of Double-Phase Interface in Mo/Mo2C Heteronanosheets for Boosted Hydrogen Evolution ReactionThe charge transfer ability and electron configuration of the electrocatalyst also play decisive roles on the HER catalytic activity.Here,two-dimensional Mo/Mo2C heteronanosheets(Mo/Mo2C-HNS)were successfully prepared via a NaCl template-assisted synthesis route followed by a controllable simultaneous reduction and carbonization of MoO3nanosheets for efficient hydrogen evolution reaction catalysis under both acidic and alkaline conditions.The Mo species in the atomically thin Mo/Mo2C-HNS not only guarantees the rapid transport of electrons but also optimizes the electronic configuration ofβ-Mo2C.Besides,the abundant Mo/β-Mo2C heterointerfaces in nano-dimension afford large numbers of additional heterogeneous catalytic sites.HER electrocatalytic performance with overpotential of merely 89 mV to drive a current density of 10 mA cm-2 in 0.5 M H2SO4 is therefore achieved.Strikingly,stable chronoamperometric electrolysis for 20 h and also an impressive cycling stability with negligible overpotential decay over 4000 sweeps demonstrate its considerable durability in an acidic environment.These findings highlight the promising potential of Mo/Mo2C-HNS catalyst as an efficient and stable noble metal-free electrocatalyst toward the HER.3.Metallic 1T-MoS2 Nanosheets In-situ Entrenched on N,P,S-codoped Hierarchical Carbon Microflower as an Efficient and Robust Electro-catalyst for Hydrogen EvolutionPhase engineering,heteroatomic doping and constructing hierarchical structure have profound effect on both the activity and stability of 2D HER electrocatalysts.Inspired by this,phosphorus(P)contained metallic 1T-MoS2 produced from in-situ ammonium intercalation induced surface sulfur atom distortion of MoS2 rooted in N,P,S-codoped hierarchical flowerlike carbon(HCMF)hybrids were successfully prepared via a dopamine self-polymerization together with molybdate process followed by a hydrothermal reduction route.On account of the coupling effect of metallic phase,defect-rich character,heteroatomic dopants and highly conductive carbon support,the P-MoS2@HCMF demonstrates prominent performances with an overpotential of merely 86 mV to deliver a current density of 10 mA cm-2 and a small Tafel slope of 42.35 mV dec-1 for hydrogen evolution reaction in 0.5 M H2SO4,which are among the best results for molybdenum disulfide based HER catalysts.Strikingly,benefiting from S vacancies and P dopant functioning as electron donors,as well as strain arisen from the tensile of rigid carbon microflower scaffold to MoS2 nanosheets to overcome agglomeration barriers of nanosheets,P-MoS2@HCMF remained well the pristine1 T phase and exhibited superior cycling stability with indistinguishable overpotential decay over 5000 sweeps and extraordinary HER durability during the 100 h long-term operation with negligible current deterioration.These findings highlight the prospective potential of P-MoS2@HCMF as a highly efficient and stable noble metal-free electrocatalyst towards HER.4.Engineering of Highly Active Oxygen Sites in Perovskite Oxides via Fluorine Substitution for Boosted Oxygen Evolution ReactionHeteroatomic doping induced anion substitution can engineering highly active oxygen sites towards efficient oxygen evolution reaction.Here,perovskite oxides,represented by Ba0.5Sr0.5Co0.8Fe0.2O3-δ(BSCF),with oxygen anion partially substituted by fluorine anion are facilely and successfully prepared.The introduction of fluorine anions in perovskite lattice initiated the transition of Co(III)and Fe(III)species to corresponding lower oxidation states and triggered the surface oxygen anion to be activated to highly oxidative O2?/O?species caused by the rippling effects of charge compensation,the latter of which are deemed to be efficient active sites for oxygen evolution reaction(OER).As a consequence,the F substituted BSCF(F-BSCF)catalyst exhibit outstanding electrocatalytic performance with an overpotential of only 280 mV to deliver a 10 mA cm-2 OER in 1 M KOH electrolyte,which is63 mV lower compared with the pristine BSCF and among the best results for state-of-the-art metal oxide based OER catalysts.Strikingly,a stable chronoamperometric response prolonged for 100 h and an impressive cycling stability with indistinguishable overpotential deterioration over 3000 sweeps demonstrate its prominent durability in a basic environment,which are much superior to the commercial IrO2 catalyst.These findings highlight the promising potential of F substitution as a highly efficient strategy for active site engineering in traditional precious metal-free OER electrocatalysts.In summary,four kinds of novel non-noble metal HER and OER electrocatalysts were meticulously developed via interface engineerings,such as creating defect structure,constructing heterojunction,phase engineering,designing hierarchical structure and heteroatomic doping.As a consequence,electrocatalysts with proliferated active site density,promoted conductive properties and/or modulated electronic structure were achieved,demonstrating not only exceptional electrocatalytic HER or OER activity,but also striking kinetic metrics as well as durable stability,which sheds new light on the designing of highly catalytic active and robust electrocatalysts towards efficient water splitting.