Toward The Rational Design Of Nonprecious Transition Metal Oxides For Oxygen Electrocatalysis

Non-precious transition metal oxides are promising alternatives to replace novel metal catalysts for catalyzing the oxygen reduction reaction（ORR）and oxygen evolution reaction（OER）,which is of great importance to develop sustainable energy conversion and storage devices.The combination of theory and experiment has proven to be a successful strategy in this respect,which often involves scaling relations among reactive intermediates（O*,OH*and OOH*）.Under this guidance,we have studied the oxygen reduction performance of mullite SmMn₂O₅,the oxygen evolution activity of surface decorated NiFe layered double hydroxides（LDHs）and the bifunctional properties of spinel NiCo₂O₄ supported on graphene with different types of N doping.The adsorption of the relevant species obeys the scaling relationship over SmMn₂O₅and can’t bypass the theoretical limit of the overpotential.NiFe LDHs which hybrid with single atom Au（^sAu/NiFe LDH）or NiO nanoparticles（NiO/NiFe LDH）change the adsorptions of the oxygen-containing species and circumvent the scaling restrict by doping or creating interfacial structures,respectively.Co-N bond of NiCo₂O₄ and N in pyridinic-N doped graphene helps adjust the adsorption of intermediates during OR（E）R and efficiently improves the activity.Our work opens up opportunities and approaches to develop higher-performance electrocatalysts.The main progress is summarized as follows.（1）In 2012,a family of complex oxides,mullites（with SmMn₂O₅ as a prototypical representative）were found to exhibit excellent catalytic activeties for NO oxidation as a diesel engine oxidation catalyst.The moderate p-d hybridization strength during chemical reactions plays an important role in governing the catalytic process.Thus we propose that SmMn₂O₅ may be a promising candidate for a cathode catalyst.Theoretical calculations are performed to investigate the bulk phase diagram,as well as the stability and electrocatalytic activity of the ORR under alkaline conditions for SmMn₂O₅（001）surfaces.The adsorptions of relevant ORR species tend to compensate the coordination of manganese atoms to form Mn-centered octahedral or pyramidal crystal fields,and the corresponding binding energies fulfill a linear relationship.An overpotential of 0.43 V is obtained at the O*binding energy around 3.4 eV in the volcano curve,which is close to the experimental observation（0.413 V）in this work.SmMn₂O₅ nanorod exhibits favorable ORR activity and superior stability with only 5%decay in activity over 20 000 s of chronoamperometric operationin contrast to 15%decrease of Pt/C,making it a promising candidate for a cathode catalyst.（2）NiFe LDHs have attracted great attention due to the excellent performance for water oxidation.The electrochemical activity can be further enhanced by supporting single-atom Au atom.Density functional theory with the Hubbard correction（DFT+U）calculations on representative ^sAu/NiFe LDH catalyst are used to predict and investigate the origin of OER activity of LDH system and significant OER enhancement by ^sAu-decoration.The excellent OER performance of ^sAu/NiFe LDH results from the active Fe atoms in surface NiFe oxyhydroxide in situ transformed from LDHs and the favorable impact of single-atom Au on changing the electron distribution of the surrounding atoms of active sites.^sAu/NiFe LDH possesses a 6-fold OER activity enhancement by 0.4 wt%^sAu decoration with the overpotential of 0.21 V in contrast to the calculated result（0.18 V）.（3）For NiO/NiFe LDHs,reaction intermediates are able to adsorb at the intersection of the two materials（NiO and LDH）and form additional chemical bonds to the surface.Comprehensive characterizations and DFT+U calculations reveal that the intermediates are co-adsorbed by interfacial Ni⁴⁺and surface ligands,besides,the geometry of tridimensional adsorption varies with different intermediates.The adsorption energy of each intermediate can be adjusted independently so as to bypass the scaling relationship with the calculated overpotential of 0.2 V.Eventually,OER overpotential is reduced to 205 mV at current density of 30 mA cm^-2,which is among the best performance achieved by state-of-the-art OER catalysts.Our findings demonstrate that the scaling relationship can be overstepped by engineering interfacial structure.（4）Transition metal species in general demonstrate high OER activity,but often lack of ORR capabilities.Meanwhile,nitrogen doped graphene is proved to be effective for ORR,but is susceptible to thermodynamic instability under the oxidative OER conditions.Hybrid materials can combine these two types of materials,which show synergistically enhanced OR（E）R activities.Inspired by this idea,the bifunctional activity of spinel NiCo₂O₄ supported on graphene with different types of N doping is investigated.For NiCo₂O₄ with pyridinic N,it is noticed that surface Co,pyridinic N as well as the neighboring Ni synergistically promote the efficiency of both ORR and OER processes.For pyrrolic N,the negligible electron transfer number and long bond length of N and Co signify the very weak bonding strength.Thus,pyrrolic N shows no advantage to accelerate the ORR and OER performance.In the graphitic configuration,the excess valence electrons of N are delocalized,thus leading to a n-type doping effect and increasing the difficulty to form bond with Co of NiCo₂O₄ due to the electron repulsion.An innovative strategy based on laser irradiation to controllably tune the relative concentrations of pyridinic and pyrrolic nitrogen dopants in the hybrid catalyst is developed.Strong chemical bonding is verified to form between pyridinic nitrogen dopants and Co ions at the hybrid interface,which predominantly contributes to the synergistic catalytic effect and leads to substantially enhanced ORR and OER activities.