the National Natural Science Foundation of China(22078374);the National Key R & D Program of China(2020YFC1807600);the National Ten Thousand Talent Plan;the Key-Area Research and Development Program of Guangdong Province(2019B110209003);the Guangdong Basic and Applied Basic Research Foundation(2019B1515120058);the Scientific and Technological Planning Project of Guangzhou(202206010145)
摘 要：
电解水是一种常用的制氢方法，但高能耗的阳极析氧反应(OER)阻碍了其应用。尿素氧化反应(UOR)具有较低的热力学电势，是最有前景的OER替代反应之一。过渡金属基水滑石具有独特的层状结构和层间阴离子可交换等优点，被认为是性能优异的UOR催化剂，然而目前大多数研究主要聚焦于后过渡金属元素。该研究通过一步法制备了具有前/后过渡金属的CoV-LDHs纳米片。与相同方法制备的Co(OH)2相比，CoV-LDHs纳米片具有以下优点：1)纳米片结构有利于暴露更多的活性位点。2) V的引入增强了CoV-LDHs的亲水性，提高了其本征电催化动力学。3) Co (3d74s2)和V (3d34s2)之间的d-电子补偿效应有利于促进尿素的吸附。因此，CoV-LDHs仅需要1.52 V (vs. RHE) 就可以达到10 mA?cm?2的电流密度，比Co(OH)2低了70 mV，同时CoV-LDHs较低的塔菲尔斜率表明了其较快的反应动力学。此外，CoV-LDHs在连续反应10 h后，驱动电位几乎没有增加，表明其具有良好的稳定性。该研究结果不仅证明了前/后过渡金属之间的d-电子补偿效应可以提高UOR催化性能，还为设计高效的UOR催化剂提供了可行的途径。
1. School of Environmental Science and Engineering, Sun Yat-sen University, Guangzhou 510275, China;2. Mechanical Engineering Department, Faculty of Engineering, Alexandria University, Alexandria 21544, Egypt
Abstract:
Hydrogen is considered as a desirable clean energy source for supporting human life in the future. Electrochemical water splitting is a promising method for generating carbon-free hydrogen. However, the relatively high overpotential of anodic oxygen evolution reaction (OER) is the main obstacle hindering the widespread popularity of water electrocatalysis technology. Recently, urea oxidation reaction (UOR) has gained significant attention as a potential alternative to OER for hydrogen production since the equilibrium potential of UOR is 0.86 V lower than that of OER. Transition metal-based layered double hydroxides (TM-LDHs) have been explored as promising UOR electrocatalysts, with the advantages of diversified metal species, stable two-dimensional layered structure and exchangeability of interlayer anions. To date, most studies have focused on TM-LDHs of late transition metals (e.g., Ni, Co, and Fe). In this work, by combining early and late transition metals, CoV-LDHs nanosheets were fabricated via a simple one-step coprecipitation method as high-performance UOR electrocatalysts. Additionally, cobalt hydroxide (Co(OH)2), with a similar lamellar structure, was synthesized via the same method. When compared with Co(OH)2, CoV-LDHs nanosheets exhibited better UOR performance owing to the following advantages: 1) The nanosheet structure of the as-fabricated CoV-LDHs electrocatalyst exposed a high number of active sites for the electrocatalytic conversion of urea. 2) The introduction of V enhanced the wettability of the CoV-LDHs electrocatalyst; thus, increasing its intrinsic electrocatalytic kinetics. 3) The d-electron compensation effect between Co (3d74s2) and V (3d34s2) was conducive to promoting the adsorption of urea. Therefore, the CoV-LDHs electrocatalyst exhibited a low electrochemical potential (1.52 V vs. the reversible hydrogen electrode, RHE) to achieve a current density of 10 mA?cm?2 in 1 mol?L?1 of potassium hydroxide containing 0.33 mol?L?1 urea, which was 70 mV less than that of Co(OH)2. The Tafel slope value of the CoV-LDHs electrocatalyst (99.9 mV?dec?1) was lower than that of Co(OH)2 (115.9 mV?dec?1), indicating faster UOR kinetics over the CoV-LDHs electrocatalyst. Furthermore, the CoV-LDHs electrocatalyst displayed high stability, with a negligible potential increase after a 10-h chronopotentiometry test by maintaining the current density of 10 mA?cm?2. In conclusion, the present work not only shows that the d-electron compensation effect between early and late transition metals could adjust the local electronic structure of TM-LDHs to improve the UOR efficiency, but also provides a feasible route to design dedicated nanostructured TM-LDHs as high-performance UOR electrocatalysts.
