Designing Oxide Catalysts for Oxygen Electrocatalysis: Insights from Mechanism to Application

The electrochemical oxygen reduction reaction(ORR) and oxygen evolution reaction(OER) are fundamental processes in a range of energy conversion devices such as fuel cells and metal–air batteries. ORR and OER both have significant activation barriers, which severely limit the overall performance of energy conversion devices that utilize ORR/OER. Meanwhile, ORR is another very important electrochemical reaction involving oxygen that has been widely investigated. ORR occurs in aqueous solutions via two pathways: the direct 4-electron reduction or 2-electron reduction pathways from O_(2) to water(H_2O) or from O_(2) to hydrogen peroxide(H_2O_(2)). Noble metal electrocatalysts are often used to catalyze OER and ORR, despite the fact that noble metal electrocatalysts have certain intrinsic limitations, such as low storage. Thus, it is urgent to develop more active and stable low-cost electrocatalysts, especially for severe environments(e.g., acidic media). Theoretically, an ideal oxygen electrocatalyst should provide adequate binding to oxygen species. Transition metals not belonging to the platinum group metal-based oxides are a low-cost substance that could give a d orbital for oxygen species binding. As a result, transition metal oxides are regarded as a substitute for typical precious metal oxygen electrocatalysts. However, the development of oxide catalysts for oxygen reduction and oxygen evolution reactions still faces significant challenges, e.g., catalytic activity, stability, cost, and reaction mechanism. We discuss the fundamental principles underlying the design of oxide catalysts, including the influence of crystal structure, and electronic structure on their performance. We also discuss the challenges associated with developing oxide catalysts and the potential strategies to overcome these challenges.

Boosting Lean Electrolyte Lithium-Sulfur Battery Performance with Transition Metals: A Comprehensive Review

Lithium–sulfur(Li–S) batteries have received widespread attention, and lean electrolyte Li–S batteries have attracted additional interest because of their higher energy densities. This review systematically analyzes the effect of the electrolyte-to-sulfur(E/S) ratios on battery energy density and the challenges for sulfur reduction reactions(SRR) under lean electrolyte conditions. Accordingly, we review the use of various polar transition metal sulfur hosts as corresponding solutions to facilitate SRR kinetics at low E/S ratios(< 10 μL mg~(-1)), and the strengths and limitations of different transition metal compounds are presented and discussed from a fundamental perspective. Subsequently, three promising strategies for sulfur hosts that act as anchors and catalysts are proposed to boost lean electrolyte Li–S battery performance. Finally, an outlook is provided to guide future research on high energy density Li–S batteries.

Sulfur-modified nickel selenide as an efficient electrocatalyst for the oxygen evolution reaction

The sluggish four-electron transfer of the oxygen evolution reaction(OER)limits the performance of water electrolyzers.Hence,OER electrocatalysts based on earth-abundant elements are urgently needed.Heteroatom doping has been an efficient approach to boost the intrinsic OER activity of the active sites by modifying the electronic structure.Here,a simple anion substitution strategy is reported that increases the OER activity of nickel selenides via a one-step hydrothermal treatment of a metal–organic framework precursor.The resulting S-substituted Ni_(3)Se_(4) nanoparticles display distortion of their crystal lattice.As expected,the sulfur substitution modifies the electronic structure of Ni_(3)Se_(4) and leads to outstanding electrocatalytic activity.All the S-substituted Ni_(3)Se_(4) catalysts exhibit higher OER activities than the original Ni_(3)Se_(4).The optimized catalyst achieves a current density of 10 mA cm^(−2) at an overpotential of 275 mV with a Tafel slope of 64 mV dec^(−1) in 1.0 M KOH.In addition to its electrochemical activity,the S-Ni_(3)Se_(4)-2 catalyst also exhibits good stability with only a 7.5%increase in overpotential at 50 mA cm^(−2) after 100 hours.This work demonstrates one strategy to modify the electronic structure of transition metal compounds by anion regulation.

金属有机框架衍生的阳离子调控金属硫化物增强析氧反应活性

析氧反应(OER)在电化学能源存储与转化技术(例如,电解水与金属-空气电池)中扮演着至关重要的角色.OER涉及四个电子的连续转移,动力学较为缓慢,因此需要较高的过电位来驱动反应进行,这严重限制了其在电化学储能和转换系统中的应用.IrO_(2)和RuO_(2)等贵金属基催化剂资源稀缺、价格高昂,因此,开发高活性、高稳定性及低成本的OER电催化剂显得尤为重要,并且极具挑战.杂原子掺杂是一种有效提升过渡金属化合物OER电催化剂活性的策略,但是当前对其本征活性位点的识别及活性提升机制的研究仍然不足.本文提出了一种阳离子掺杂策略,通过引入金属阳离子调控多金属组分的电子结构,优化OER中间体吸附能,进而提升OER活性.通过简单的一步热解硫化钴镍双金属有机框架材料前驱体,成功制备了Ni掺杂CoS/氮掺杂介孔碳(Ni-CoS/NC)复合结构电催化剂;并采用循环伏安法研究了其电化学行为与OER性能,结合谱学研究结果与密度泛函理论(DFT)计算,从原子层面揭示了OER条件下真实活性位点及掺杂型电催化剂的活性提升机制.电化学研究结果表明,所制备Ni-CoS/NC催化剂在1.0 mol L^(-1)KOH溶液中表现出较好的OER反应活性,其在10 mA cm^(-2)电流密度下的过电位为270mV,Tafel斜率为37 mV dec^(-1).采用X射线光电子能谱、高角度环形暗场扫描透射电子显微镜与电子能量损失谱等表征方法分析了OER前后催化剂的结构变化;结果表明,在OER电位下Ni-CoS/NC催化剂由金属硫化物转变为羟基氧化物Co_(x)Ni_(1–x)OOH,Co_(x)Ni_(1-x)OOH才是OER反应的活性位点.理论结算结果表明,在Co_(x)Ni_(1–x)OOH材料中,Ni部分取代Co位点,使两种金属之间产生强烈的电子相互作用,导致Co位点带有更多的负电荷,Ni位点带有更多的正电荷,从而增强了对中间物种OOH*的吸附,提升OER活性.综上,金属阳离子掺杂调变金属活性中心的电子结构,是提高电催化剂OER活性的有效策略.过渡金属硫化物电催化剂在OER电位下发生了重构,由晶态的金属硫化物转变为非晶态的金属羟基氧化物,作为OER催化剂的活性位点.本文为低成本、高性能电催化剂设计提供了一种可行的阳离子调控策略,并且加深了对本征活性位点及活性提升机制的认识,可为电化学能源存储与转化材料的开发提供借鉴.

Mathematical Modeling of the Amperometric Response to Glucose of Glucose Oxidase Films Deposited by AC-Electrophoresis

Previous work illustrated that glucose oxidase (GOx) could be deposited on conducting substrates using asymmetrical alternating current electrophoretic deposition (AC-EPD) to form thick enzyme layers suitable for the manufacturing of highly active biosensors. Here, we modeled the amperometric response of GOx layers to glucose as a function of the thickness of the enzyme layer. The model is based on reaction-diffusion equations with irreversible first-order catalytic reactions. The numerical results displayed qualitative and reasonable quantitative agreement with the experimental data obtained for oxidation currents due to glucose, which increase with the enzyme thickness.

Cobalt-embedded 3D conductive honeycomb architecture to enable high-sulphur-loading Li-S batteries under lean electrolyte conditions

High sulphur loading and lean electrolyte conditions are important to achieve the high theoretical energy density of lithiumsulphur(Li-S)batteries.However,serious problems such as low sulphur utilization and fast capacity fade are typically experienced under low electrolyte/sulphur(E/S)ratios and high sulphur loading conditions.To address these issues,a cobaltcontaining three-dimensional conductive honeycomb(Co@N-HPC)is proposed in this work as a material for sulphur cathodes.The good electrical conductivity and high density of catalytic sites of(Co@N-HPC)allow fast redox kinetics of lithium polysulfide(LiPS)in high-sulphur-loading electrodes.In addition,the hierarchical structure and good wettability by the electrolyte of Co@NHPC facilitates electrolyte penetration and LiPS conversion,leading to a high utilization of sulphur under lean electrolyte conditions.Therefore,at a current density of 0.2 C,a volumetric capacity of 1,410 mAh·cm^(−3)was attained with a sulphur loading of 5.1 mg·cm^(−2)and an E/S ratio of 5μL·mg^(−1).This work provides ideas for the development of lean electrolyte Li-S batteries with a high sulphur loading.

Metal-organic framework-derived CoP_(2)@N-doped carbon hollow spheres for efficient oxygen evolution reaction

1.Introduction.Hydrogen is considered as a viable alternative to fossil fuels[1,2].Large-scale hydrogen production by electrochemical water splitting is regarded as an important approach.It consists of two half-reactions:hydrogen evolution reaction(HER)at the cathode and oxygen evolution reaction(OER)at the anode[3].Compared to the HER,the OER has sluggish kinetic and large overpotentials,re-sulting from the complex oxidation pathway[4,5].Although RuO_(2) and IrO_(2) have excellent catalytic OER performance[6],their large-scale deployment is unfeasible due to the scarcity and high cost of these noble metal elements.Thus,cheap OER electrocatalysts have been actively investigated[7].

Rational design of Ruddlesden-Popper perovskite electrocatalyst for oxygen reduction to hydrogen peroxide

Although the oxygen reduction process to hydrogen peroxide(H_(2)O_(2))is a green option for H_(2)O_(2)generation,the low activity and selectivity hindered the industry's process.In recent years,the electrochemical synthesis of H_(2)O_(2)through a 2e-transfer method of oxygen reduction reaction(ORR)has piqued the interest of both academics and industry.Metal oxide catalysts have emerged as a novel family of electrochemical catalysts due to their unusual physical,chemical,and electrical characteristics.In this work,we first developed a Ruddlesden-Popper perovskite oxide(Pr_(2)NiO_(4+δ))as a highly selective and active catalyst for 2e-ORR to produce H_(2)O_(2).Molybdenum was introduced here to adjust the oxidation states of these transition metals with successful substitution into Ni-site to prepare Pr_(2)Ni1-xMoxO_(4+δ),and the molybdenum substitution improves the H_(2)O_(2)selectivity during the ORR process,in 0.1 M KOH,from 60%of Pr_(2)NiO_(4+δ)to 79%of Pr_(2)Ni_(0.8)Mo_(0.2)O_(4+δ)at 0.55 V versus RHE.A limiting H_(2)O_(2)concentration of_0.24 mM for Pr_(2)NiO_(4+δ)and 0.42 mM for Pr_(2)Ni_(0.8)Mo_(0.2)O_(4+δ)was obtained at a constant current of 10 mA/cm2 using a flow-cell reactor using a gas-diffusion electrode.