揭示硫掺杂的碳材料在水电氧化过程中活性苯醌基团及惰性硫残留物的形成

doi:10.1016/S1872-2067(23)64394-3

摘要/Abstract

摘要：

杂原子掺杂的碳材料具有成本低、导电性高、耐酸碱性强等优点, 被直接或作为载体材料广泛应用于各类电催化反应. 由于杂原子和碳原子之间的电负性差异很大, 人们通常将杂原子掺杂的碳材料析氧(OER)活性提高的原因归于杂原子诱导的碳原子上电荷的重新分布. 然而, 硫(2.58)与碳(2.55)的电负性几乎相同, 说明硫掺杂不会导致碳上显著的电荷重新分布. 因此, 硫掺杂碳材料的活性来源可能与其他元素掺杂的碳材料不同. 目前, 部分研究表明, 硫掺杂是通过改变碳基体的自旋密度而非电荷密度来优化反应活性. 还有一些研究将活性的增强归因于硫掺杂的碳材料中存在的硫杂环结构. 然而, 上述结论都是基于硫掺杂碳材料本身可以在电催化中保持稳定这一前提下提出的, 材料本身电催化过程中可能发生的结构和组分转变, 尤其是在强氧化OER条件下发生的转变被忽略了. 硫杂环是一种具有还原性的硫物种, 在强氧化的OER条件下应当同样发生氧化. 因此, 硫掺杂的碳材料及其作为金属材料的载体在催化OER时的活性起源和催化机制仍然不明确.

本文研究了硫掺杂的石墨薄片(S-GP)在碱性OER过程中的活化以及相应的结构演化过程. 连续的电化学线性扫描结果表明, S-GP可以在OER过程中发生活化. 将线性扫描曲线用电化学活性面积归一化后活化现象依然明显, 说明活性的增强来源于本征催化活性的提高, 而不是活性面积的增加. 此外, 活化后S-GP还表现出更快的反应动力学, 更小的电荷转移电阻, 以及较好的稳定性, 即当电流密度达到100 mA cm^-2时至少可以稳定运行30 h. X射线能谱(EDS)、X射线光电子能谱(XPS)、软X射线吸收光谱(XAS)以及拉曼光谱结果表明, S-GP在催化OER过程中发生了氧化, 形成了大量含氧官能团, 并且其中部分掺杂的硫也发生氧化并以SO₄^2-的形式溶出. 通过添加额外的硫酸盐以及重新更新电解液以除去其中的SO₄^2-并不能使OER性能发生改变, 表明溶解的SO₄^2-对碳材料的OER不起作用. 为了分辨电催化活性究竟是来源于残留的硫物种还是新产生的含氧官能团, 对样品采用高温H₂煅烧以除去其中的硫和氧. 尽管煅烧后的样品OER活性很差, 但是连续的线扫能够使它的催化活性快速恢复到煅烧前的水平, 表明残留的硫物种对OER是惰性的, 而在线扫过程中重新生成的含氧官能团才是真正的OER活性物种. 为进一步确定哪一种含氧官能团具有OER活性, 结合XPS、soft XAS、电位依赖的原位衰减全反射-傅里叶变换红外光谱(ATR-FTIR)、循环伏安法(CV)和C=O基团选择性阻断实验, 最终证实苯醌基团是真正的OER活性位点. 还发现当硫掺杂的碳材料作为基底与Ni(OH)₂复合时, 同样会发生类似的转化, 并且其中溶出的SO₄^2-还能对所负载的Ni(OH)₂的OER活性起到促进作用. 综上, 本文不仅阐明了硫掺杂的碳自身及其作为金属材料的载体在电催化析氧反应中的催化机制和活性起源, 而且能够推动研究者重新考虑硫掺杂碳材料在其它电催化反应中的反应机制.

关键词: 析氧反应, 掺杂碳材料, 结构演化, 硫, 苯醌

Abstract:

Doping carbon materials with heteroatoms is an effective strategy to improve the catalytic performance of carbon materials through charge redistribution. Furthermore, heteroatom-doped carbon materials have been proven to be unstable and can be completely removed from the electrode via the electrochemical oxygen evolution reaction (OER). However, since S has a electronegativity similar to that of C, the behavior of S-doped carbon materials under OER conditions might differ and thus deserves special attention. In this study, we investigated the structural evolution of S-doped carbon materials during the alkaline OER. It was observed that the S-doped graphite flake (S-GP) underwent oxidization. Notably, only partial S dopants dissolved in the form of sulfates, resulting in the emergence of new forms of S- and O-containing groups on the electrode. The results from well-designed experiments demonstrated that despite remaining on the electrode, the S-containing groups had no effect on the OER activity, and the high OER activity was attributed to the derived benzoquinone moiety. The dissolved sulfates further promoted OER activity when S-doped carbon materials were used as substrates for the Ni(OH)₂ anode. Our work reveals the real activity origin of S-doped materials towards OER, motivating researchers to reconsider the catalytic mechanism of the S-doped carbon materials and their supported composites for other reactions.

Key words: Oxygen evolution, Doped carbon material, Structural evolution, Sulfur, Quinone

张志普, 卢珊珊, 张兵, 史艳梅. 揭示硫掺杂的碳材料在水电氧化过程中活性苯醌基团及惰性硫残留物的形成[J]. 催化学报, 2023, 47: 129-137.

Zhipu Zhang, Shanshan Lu, Bin Zhang, Yanmei Shi. Unveiling inactive sulfur residue and benzoquinone moiety formation in sulfur-doped carbon for water electrooxidation[J]. Chinese Journal of Catalysis, 2023, 47: 129-137.

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链接本文: https://www.cjcatal.com/CN/10.1016/S1872-2067(23)64394-3

https://www.cjcatal.com/CN/Y2023/V47/I4/129

图/表 6

Fig. 1. Comparison of the OER performances of the initial and activated S-GPs. LSV curves (a), Tafel slopes (b), and Nyquist plots (c) of initial and activated S-GPs. The corresponding oxygen Faradaic efficiencies of activated S-GP at the specific potentials are labeled in (a). (d) The durability of S-GP at a current density of 100 mA cm-2.

Fig. 2. Structural evolution of the S-containing groups of S-GP. (a) The atomic ratio of S and O in the initial and activated S-GPs based on the EDS analysis. S 2p XPS spectra (b), S K-edge XAS spectra (c), and Raman spectra (d) of initial and activated S-GPs. (e) Potential-dependent in situ Raman spectra of S-GP during the OER process.

Fig. 3. Structural evolution of the O-containing groups of S-GP. O 1s XPS (a) and C K-edge XAS (b) spectra of the initial and activated S-GPs. (c) Potential-dependent in situ ATR-FTIR spectra of S-GP during OER. (d) CV voltammogram of the activated S-GP.

Fig. 4. Illustration of the sulfidation of GP to form S-GP and the activation process of S-GP towards alkaline OER.

Fig. 5. Effect of hydrogen treatment on the activated S-GP. (a) Activity comparison of S-GP-H, S-GP-HA and activated S-GP; S 2p (b), O 1s XPS spectra (c), and O K-edge XAS spectra (d) of the S-GP-H and S-GP-HA.

Fig. 6. Investigating the OER mechanism of the Ni(OH)2/S-CP composite. (a) LSV curves of the Ni(OH)2/S-CP, Ni(OH)2/CP, and S-CP before and after activation; (b) LSV curves of the activated Ni(OH)2/S-CP and the same sample tested in renewed KOH.

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