原子级铱掺杂增强γ-MnO2析氧活性和稳定性的机理研究

doi:10.1016/S1872-2067(24)60201-9

催化学报 ›› 2025, Vol. 69: 99-110.DOI: 10.1016/S1872-2067(24)60201-9

原子级铱掺杂增强γ-MnO₂析氧活性和稳定性的机理研究

孙艺萌^a^,^b^,¹, 陈军^a^,^b^,¹, 刘琳^a^,^b, 池海波^a^,^c, 韩洪宪^a^,^b^,^*()

^a中国科学院大连化学物理研究所, 大连洁净能源国家实验室, 催化基础国家重点实验室, 辽宁大连 116023
^b中国科学院大学, 北京 100049
^c中国科学技术大学化学与材料科学学院, 安徽合肥 230026

收稿日期:2024-08-18 接受日期:2024-10-30 出版日期:2025-02-18 发布日期:2025-02-10
通讯作者: *电子信箱: hxhan@dicp.ac.cn (韩洪宪).
作者简介:第一联系人：
¹共同第一作者.
基金资助:
国家重点研发项目(2017YFA0204804);国家自然科学基金人工光合作用基础研究中心(22088102)

The mechanism of OER activity and stability enhancement in acid by atomically doped iridium in γ-MnO₂

Yimeng Sun^a^,^b^,¹, Jun Chen^a^,^b^,¹, Lin Liu^a^,^b, Haibo Chi^a^,^c, Hongxian Han^a^,^b^,^*()

^aState Key Laboratory of Catalysis, Dalian National Laboratory for Clean Energy, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, Liaoning, China
^bUniversity of Chinese Academy of Sciences, Beijing 100049, China
^cSchool of Chemistry and Materials Science, University of Science and Technology of China, Hefei 230026, Anhui, China

Received:2024-08-18 Accepted:2024-10-30 Online:2025-02-18 Published:2025-02-10
Contact: *E-mail: hxhan@dicp.ac.cn (H. Han).
About author:First author contact:
¹ Contributed equally to this work.
Supported by:
National Key R&D Program "Nanotechnology"(2017YFA0204804);National Natural Science Foundation of China(22088102)

摘要/Abstract

摘要：

随着可持续能源和绿色氢能的迅速发展, 电解水作为一种能够高效生产高纯、清洁氢气的技术逐渐得到广泛关注. 目前, 酸性环境下的质子交换膜(PEM)水电解槽, 阳极电催化剂只能依赖稀缺且昂贵的铱基(Ir)催化剂, 极大地限制了水电解的商业化. 为开发低Ir基氧析出反应(OER)电催化剂, 通常的策略是在酸性稳定的载体上构建Ir基活性位点. 近期报道了在酸性稳定的γ-MnO₂晶格中进行原子Ir掺杂的突破性进展, 该催化剂实现较高催化性能的同时, 相比于商业PEM电解槽Ir载量减少了95%以上. 然而, 对于γ-MnO₂中Ir掺杂对活性和稳定性提升的原因仍然缺乏清晰的认知. 因此, 进一步探究Ir掺杂如何提高MnO₂电催化性能的内在机理是本文的主要研究思路.

本文通过基于非贵金属硝酸盐和贵金属前驱物的熔盐热分解法, 不断优化热分解温度和Ir基前驱体, 实现了Ir在γ-MnO₂基底中的掺杂, 定量分析显示Ir掺杂量最高可达1.37 atom%(标记为Ir-MnO₂). X射线衍射(XRD)和高角环形暗场扫描透射电镜等结果表明, Ir在γ-MnO₂基底中呈现高分散的原子级掺杂. XRD测试显示, Ir-MnO₂的(021)衍射峰向低角度移动约0.26°, 归因于更大的Ir原子取代Mn引起的晶格参数变化. 同时, 与γ-MnO₂结构中两相交互生长的相对比例相对应的(110)和(130)衍射峰间距的增大表明Ir掺杂造成γ-MnO₂自身结构缺陷的变化, 即组分中β-MnO₂结构单元相对含量的减少, De Wolf无序度的略微下降. X射线光电子能谱(XPS)结果表明, 在Ir-MnO₂表面形成了高价态的Ir⁵⁺, 同时伴随Mn³⁺和表面吸附氧(O_surf)相对比例的增加, 进而推断Ir掺杂是通过形成Ir⁵⁺-O(_H)-Mn³⁺结构来增加O_surf的含量. 与单组分催化剂对比的结果表明, 在1.70 V vs. RHE时Ir-MnO₂催化剂在0.5 mol L^-1 H₂SO₄中表现出最高的酸性OER电催化活性, 达到243 mA cm^-2的电流密度, 是MnO₂催化剂的23倍, 是商业IrO₂催化剂的5倍. 得益于原子级分散的掺杂, 最优Ir-MnO₂催化剂在过电位为300 mV时Ir的转换频率(TOF)达到0.655 s⁻¹. 将MnO₂和Ir-MnO₂负载于氟掺杂氧化锡(FTO)基底, 在三电极体系中进行稳定性测试, 结果表明, Ir-MnO₂催化剂表现出较高的稳定性, 在Ir载量低至0.091 mg cm^-2时在100 mA cm^-2下持续工作24 h且没有出现明显的衰减. 通过对反应后电解液中Mn和Ir含量分析以及表面XPS等结果的研究, 进一步验证Ir-MnO₂催化剂在酸性OER过程中的稳定性. 理论计算结果表明, Ir优先掺杂到γ-MnO₂结构中的β相结构单元并取代原有的Mn. 理想(100)晶面上析氧反应过程的计算结果显示, 相比于Mn位点, 掺杂后的Ir位点更可能是OER的活性位点. OER性能的显著提高是由于Ir掺杂后反应决速步从MnO₂催化剂表面的O*的形成步骤改变为*OOH的形成步, 对应于实验中Ir掺杂对OER性能的提升. 此外(021)面的补充计算结果与上述结论保持一致, 进一步证实该机理的有效性.

综上, 本工作不仅证明在载体表面实现原子级Ir掺杂从而大幅减少Ir使用量的可能性, 更重要的是揭示了Ir掺杂后同时提高Mn基氧化物催化活性和稳定性的机理. 这些重要发现也为后续开发更高效、稳定和具有经济效益的低Ir基酸性析氧电催化剂提供有价值的参考.

关键词: 酸性析氧反应, 低铱, 锰基氧化物, 质子交换膜电解水, 绿氢

Abstract:

Construction of iridium (Ir) based active sites on certain acid stable supports now is a general strategy for the development of low-Ir OER catalysts. Atomically doped Ir in the lattice of acid stable γ-MnO₂ has been recently achieved, which shows high activity and stability though Ir usage was reduced more than 95% than that in current commercial proton exchange membrane water electrolyzer (PEMWE). However, the activity and stability enhancement by Ir doping in γ-MnO₂ still remains elusive. Herein, high dispersion of iridium (up to 1.37 atom%) doping in the lattice of γ-MnO₂ has been achieved by optimizing the thermal decomposition of the iridium precursors. Benefiting from atomic dispersive doping of Ir, the optimized Ir-MnO₂ catalyst shows high OER activity, as it has turnover frequency of 0.655 s⁻¹ at an overpotential of 300 mV in 0.5 mol L^-1 H₂SO₄. The catalyst also shows high stability, as it can sustainably work at 100 mA cm^-2 for 24 h. Experimental and theoretical studies reveal that Ir is preferentially doped into β phase rather than R phase, and the Ir site is the active site for OER. The OER active site is postulated to be Ir⁵⁺-O(H)-Mn³⁺ unit structure on the surface. Furthermore, Ir doping changes the potential determining step from the formation of O* to the formation of *OOH, emphasizing the promoting effect toward OER derived from Ir sites. This work not only demonstrates the possibility of achieving atomic-level doping of Ir on the surface of a support to dramatically reduce Ir usage, but also, more importantly, reveals the mechanism behind accounting for the stability and activity enhancement by Ir doping. These important findings may serve as valuable guidance for further development of more efficient, stable and cost-effective low Ir-based OER catalysts for PEMWE.

Key words: Acidic oxygen evolution reaction, Low-iridium, Mn-based oxides, Proton exchange membrane water, electrolysis, Green hydrogen

孙艺萌, 陈军, 刘琳, 池海波, 韩洪宪. 原子级铱掺杂增强γ-MnO₂析氧活性和稳定性的机理研究[J]. 催化学报, 2025, 69: 99-110.

Yimeng Sun, Jun Chen, Lin Liu, Haibo Chi, Hongxian Han. The mechanism of OER activity and stability enhancement in acid by atomically doped iridium in γ-MnO₂[J]. Chinese Journal of Catalysis, 2025, 69: 99-110.

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图/表 7

Table 1 The Ir contents in precursors and the final working electrodes by ICP analysis.

Catalyst	C₁₄H₂₇Ir₃O₁₈ and Mn(NO₃)₂ Ir/Mn molar ratio	Final product
Catalyst	C₁₄H₂₇Ir₃O₁₈ and Mn(NO₃)₂ Ir/Mn molar ratio	Composition formula	Ir/Mn molar ratio	Ir loading mg cm^-2
0.1%Ir-MnO₂	0.1%	Ir_0.001Mn_0.999O₂	0.10%	0.007
0.5%Ir-MnO₂	0.5%	Ir_0.005Mn_0.995O₂	0.50%	0.033
1%Ir-MnO₂	1%	Ir_0.009Mn_0.991O₂	0.89%	0.059
2%Ir-MnO₂	2%	Ir_0.014Mn_0.986O₂	1.37%	0.091
3%Ir-MnO₂	3%	Ir_0.016Mn_0.984O₂	1.59%	0.103
4%Ir-MnO₂	4%	Ir_0.020Mn_0.980O₂	2.05%	0.128

Fig. 1. (a) Schematic crystal structure of γ-MnO2. (b) XRD patterns of γ-MnO2 and Ir-MnO2 with varying iridium contents (the orange squares represent characteristic peaks of Mn2O3).

Fig. 2. (a) TEM image and statistical particle size distribution of as-synthesized 2%Ir-MnO2. (b,c) Structural characteristic of the intergrowth of two phases as shown in the HR-TEM images. (d) Representative high-magnification HAADF-STEM image of 2%Ir-MnO2, with bright spots highlighted by red circles ascribed to Ir atoms. (e) The corresponding element mapping of O, Mn, and Ir in 2%Ir-MnO2. (f) Enlarged XRD patterns of γ-MnO2 and 2%Ir-MnO2.

Fig. 3. High-resolution XPS spectra. (a) Ir 4f of 2% Ir-MnO2 and commercial IrO2. (b) Fitting the valence state of Ir. Mn 2p (c) and O 1s (d) of γ-MnO2 and 2% Ir-MnO2.

Fig. 4. OER performance of catalysts in a three-electrode test with catalysts loaded on GC working electrodes. (a) Representative LSV curves of γ-MnO2, Ir-MnO2 with different Ir doping levels, and commercial IrO2 in 0.5 mol L-1 H2SO4. (b) The overpotential values of the catalysts at 1.70 VRHE. (c) Tafel slope derived from (a). (d) EIS curves and the fitting results of γ-MnO2, Ir-MnO2, and commercial IrO2 at 1.55 VRHE. The insert in (d) shows the equivalent circuit.

Fig. 5. Stability test by chronopotentiometry of γ-MnO2 and 2%Ir-MnO2 grown on FTO at 10 mA cm-2 (a) and 100 mA cm-2 (b). Ir 4f (c) and Mn 2p (d) XPS spectra of the pristine and utilized 2%Ir-MnO2 catalyst (in 0.5 mol L-1 H2SO4 solution for 24 h at 100 mA cm?2).

Fig. 6. DFT calculations. (a) Structures of Ir-MnO2 with Ir atoms doped in different Mn sites (β phase and R phase site) of γ-MnO2. The red, purple, and yellow spheres are O, Mn, and Ir atoms, respectively. The diagram below illustrates normal COHP and ICOHP value of O(2p)-Ir(4d) on different Ir-MnO2 models. (b) Mn 3d and Ir 5d pDOS spectra of 2%Ir-MnO2. The inset is a magnified view at the Fermi level. (c) Gibbs free energy diagrams for OER on Ir site and Mn site on Ir-γ-MnO2 surface and Mn site on γ-MnO2 (100) surface. (d) Structures of OER intermediates on Ir site on the Ir-γ-MnO2 (100) slab.

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原子级铱掺杂增强γ-MnO₂析氧活性和稳定性的机理研究

The mechanism of OER activity and stability enhancement in acid by atomically doped iridium in γ-MnO₂

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