金属Ni和Co氧化物与N掺杂的碳球协同作用用于电解水析氧反应

Synergistic effect of metallic nickel and cobalt oxides with nitrogen-doped carbon nanospheres for highly efficient oxygen evolution

电解水制氢被认为是理想的清洁制氢技术,然而,阴阳两极过电位的存在,特别是阳极析氧反应涉及复杂的四电子转移过程,阳极过电位较高,严重制约了电解水制氢效率的提高.因此,研究发现高效且成本低的析氧反应电催化剂具有重要意义.因此,本课题组利用温和的浸渍-热解策略合成了具有高效析氧活性的Ni/Co3O4与N掺杂的碳复合电催化剂(Ni/Co3O4@NC).其中,第一步浸渍过程将无机Ni与Co源浸渍到聚合物纳米球结构中,可以有效地保证热解过程中产生均匀分散的极细Ni和Co3O4纳米颗粒;此外,高温热解过程形成的N掺杂的碳基质可以有效地阻止活性组分的团聚和交联,调控其电子结构.由于Ni, Co3O4和NC的协同作用,制备得到的Ni/Co3O4@NC电催化剂在碱性溶液中均展现了比由单一金属组成的Co3O4@NC和Ni@NC更优异的析氧活性和稳定性,当电流密度为10 m Acm-2时,所需要的过电位仅为350 m V, Tafel斜率低至52.27 m Vdec-1,电荷转移阻抗极小,双电层电容高达25.53 m Fcm-2.本文采用扫描电镜(SEM),高分辨透射电镜(HRTEM), X射线衍射(XRD)和X射线光电子能谱(XPS)等手段研究了Ni/Co3O4@NC电催化剂的微观结构、元素组成和价态,分析了复合Ni/Co3O4@NC电催化剂具有优异析氧性能的原因.SEM结果表明, Ni/Co3O4@NC电催化剂在经历浸渍-热解过程后,完整地继承了聚合物的纳米球状形貌,只是平均粒径由100 nm缩聚为90 nm左右, SEM mapping显示各元素均匀分散在每一个纳米球结构中.HRTEM结果显示,紧密耦合的Ni和Co3O4超细纳米颗粒均匀分散于纳米球结构中,且碳基质有效地限制了这些颗粒的过度生长及团聚.XRD和XPS结果再次印证Ni/Co3O4@NC是由Ni和Co3O4两种晶体结构构成,此外, XPS结果显示N原子成功掺杂到碳基质中,富电子的N原子掺杂到碳基质中可以有效地调控Ni/Co3O4@NC的电子结构,提高本征电催化活性.相应的催化反应结果表明, Ni/Co3O4@NC相较于单一的Co3O4@NC和Ni@NC析氧活性更高,这是由于Ni与Co3O4协同作用,使Co3O4的Co2+/3+氧化还原峰向阳极偏移,从而增加了Co3O4的本征活性;此外高导电性的Ni掺杂到Ni/Co3O4@NC复合物结构中,提高了催化剂的导电性,加快了电子传输能力;杂原子N的掺杂有效地调控了催化剂的电子结构,提高了催化剂的本征活性.总之, Ni、Co3O4和NC的协同作用使Ni/Co3O4@NC复合物在碱性溶液中具有高效催化析氧性能.该策略为制备杂原子掺杂的负载于高导电碳载体的氧化物基催化剂提供了有益参考.

The most energy-inefficient step in the oxygen evolution reaction(OER), which involves a complicated four-electron transfer process, limits the efficiency of the electrochemical water splitting. Here, well-defined Ni/Co3O4 nanoparticles coupled with N-doped carbon hybrids(Ni/Co3O4@NC) were synthesized via a facile impregnation-calcination method as efficient electrocatalysts for OER in alkaline media. Notably, the impregnation of the polymer with Ni and Co ions in the first step ensured the homogeneous distribution of metals, thus guaranteeing the subsequent in situ calcination reaction, which produced well-dispersed Ni and Co3O4 nanoparticles. Moreover, the N-doped carbon matrix formed at high temperatures could effectively prevent the aggregation and coalescence, and regulate the electronic configuration of active species. Benefiting from the synergistic effect between the Ni, Co3O4, and NC species, the obtained Ni/Co3O4@NC hybrids exhibited enhanced OER activities and remarkable stability in an alkaline solution with a smaller overpotential of 350 m V to afford 10 m A cm-2, lower Tafel slope of 52.27 m V dec-1, smaller charge-transfer resistance, and higher double-layer capacitance of 25.53 m F cm-2 compared to those of unary Co3O4@NC or Ni@NC metal hybrids. Therefore, this paper presents a facile strategy for designing other heteroatom-doped oxides coupled with ideal carbon materials as electrocatalysts for the OER.