Hemoglobin-derived Fe-Nx-S species supported by bamboo-shaped carbon nanotubes as efficient electrocatalysts for the oxygen evolution reaction

doi:10.1016/j.carbon.2020.06.064

Carbon

Volume 168, 30 October 2020, Pages 588-596

https://doi.org/10.1016/j.carbon.2020.06.064 Get rights and content

Highlights

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Fe-Nx-S species were formed by a direct pyrolysis of hemoglobin and carbon nanotubes.
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The resulting sample showed outstanding performance for the OER in alkaline solutions.
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The sample gave a current density of 83.6 mA cm⁻² at 380 mV, which is 2.5 times that of IrO₂.
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Density functional theory calculations were done to explore the role of the sulfur dopant.

Abstract

Herein, we report a facile route to synthesize isolated single iron atoms on nitrogen-sulfur-codoped carbon matrix via a direct pyrolysis process in which hemoglobin, a by-product of the meat industry, was utilized as a precursor for iron, nitrogen and sulfur while bamboo-shaped carbon nanotubes served as a support owing to their excellent conductivity and numerous defects. The resulting metal-nitrogen complexed carbon showed outstanding catalytic performance for the oxygen evolution reaction (OER) in alkaline solutions. At an overpotential of 380 mV, the optimal sample yielded a current density of 83.6 mA cm⁻², which is 2.5 times that of benchmark IrO₂ (32.8 mA cm⁻²), rendering it as one of the best OER catalysts reported so far. It also showed negligible activity decay in alkaline solutions during long-term durability tests. Control experiments and X-ray absorption fine structure analyses revealed that Fe-N_x species in the samples are the active sites for OER. Further density functional theory calculations indicated that the presence of sulfur in the carbon matrix modified the electronic structures of active species, thereby leading to the superior activity of the sample.

Graphical abstract

Introduction

Molecular hydrogen is an ideal candidate for replacing fossil fuels because of its environmental-friendliness and high energy efficiency [1]. Electrochemical water splitting via the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) represents a promising approach for hydrogen production [[2], [3], [4], [5]]. Owing to the sluggish kinetics and high overpotentials, the OER is considered to be the bottleneck for water splitting. It necessitates the utilization of high-performance electrocatalysts to maximize energy efficiency. Currently, IrO₂ and RuO₂ are regarded as state-of-the-art OER electrocatalysts. Unfortunately, the scarcity and cost of these precious metal oxides greatly prevent their large-scale application in OER. Searching for high-performance and cost-effective electrocatalysts is thus of great significance for the widespread application of water-splitting techniques [[6], [7], [8], [9]].

Recently, single-atom catalysts (SAC) have triggered enormous interest owing to their maximum atom efficiency, outstanding activity and selectivity [[10], [11], [12]], and tunable coordination structures [13,14]. Among the reported SAC, metal-nitrogen complexed carbon (MNC) materials are of particular interest for electrocatalysis because of their excellent conductivity [[15], [16], [17], [18], [19]]. So far, MNC has been used in electrochemical processes, such as the oxygen reduction reaction [20,21], HER [22,23], OER [[24], [25], [26]], and CO₂ reduction reaction [27]. The cost-effective and large-scale production of MNC hold the key to their practical applications. Unfortunately, the extremely high surface energy of single metal atoms always leads to natural aggregation into nanoparticles which are catalytically less active than SAC, and this represents a grand challenge for the synthesis of MNC. Currently, pyrolysis of metal salts with nitrogen/carbon precursors has been extensively employed for the synthesis of MNC [[28], [29], [30]]. This synthesis protocol involves high pyrolysis temperatures to yield conductive carbonaceous supports, which unfortunately leads to metal aggregation due to the weak interaction of the metal cations with N/C precursors, greatly reducing the number of single atoms. Such a trade-off relationship between conductivity and the number of catalytically active single atoms greatly affects the performance of the resulting MNC. Alternatively, metal organic framework (MOF) consisting of metal sites coordinated by N-containing ligands is a better precursor for MNC synthesis [[31], [32], [33], [34]]. However, to facilitate a strong interaction of metals with surrounding ligands in MOF, the choices of metal type and ligands are limited, not to mention that the synthesis of MOF is costly and time-consuming, which significantly prohibits the scalable production of MNC [35]. Thus, the development of cost-effective synthesis methods is still highly desirable to realize the large-scale production of SAC.

The catalytic performance of MNC is closely related to the local geometric and electronic structures of M-N_x sites. It has been demonstrated that metal atoms in MNC catalysts always exhibit unsatisfactory adsorption-desorption behaviors toward intermediates owing to the electron depletion arising from the interaction of neighboring N atoms, thus increasing the potential barriers of the catalytic reactions [[36], [37], [38], [39]]. The electronic structures of the metals can be further tuned by introducing proper foreign atoms such as sulfur in the carbonaceous matrix [[34], [35], [36]]. Inspired by these results, herein, we report a simple route for the synthesis of MNC via pyrolysis of a mixture of hemoglobin (Hb) and bamboo-shaped carbon nanotubes (BCNT). Being the major content of animal blood which can be obtained as a by-product of the meat industry [40], Hb is composed of globin and heme. The former has sulfur elements while the latter contains Fe–N₅ moieties. Such abundant availability and structural features render Hb to be an ideal precursor for the synthesis of MNC. BCNT was utilized as a catalyst support owing to their excellent conductivity and abundant defects [41]. The contents of iron and nitrogen were readily optimized by varying the Hb content in the feedstock and pyrolysis temperature. High-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) confirmed the presence of abundant single iron atoms. X-ray absorption fine structure (XAFS) analyses revealed that the iron atoms were coordinated with nitrogen atoms. The optimal sample showed remarkable OER activity in alkaline solutions and outperformed precious IrO₂ at high overpotentials. Density functional theory (DFT) calculations were conducted to investigate the critical role of sulfur dopant. The superior OER performance was mainly attributed to the abundant Fe-N_x sites and the promoting role of sulfur.

Section snippets

Materials

Nafion solution (5 wt%) was purchased from the Sunlaite Co. Ltd. (Kunshan, China). Isopropanol and ethylene glycol were provided by the Qiangsheng Chemical Reagent Co. Ltd. (Jiangsu, China) and Fuyu Chemical Reagent Co. Ltd. (Tianjin, China), respectively. Hemoglobin was purchased from the Yuanye Biotechnological Co. Ltd. (Shanghai, China). All the chemicals were of analytic grade and were used as received without further purification.

Synthesis of BCNT

BCNT was obtained from a typical process of thermocatalytic

Structural characterization

As schematically illustrated in Fig. 1A, the sample is synthesized via pyrolysis of a mixture of Hb-adsorbed BCNT in inert atmosphere. The synthesis process is simple and does not involve any sophisticated instrumentation and fabrication, which is beneficial for large-scale production. Herein, acid-treated BCNT (see Fig. S1) was utilized as a catalyst support because of the numerous defects and surface groups, and large surface areas, which afford prominent advantages for adsorbing Hb

Conclusion

In summary, we reported a facile synthetic protocol to prepare single isolated iron atoms anchored to nitrogen-sulfur-codoped carbon matrix via pyrolyzing the mixture of Hb and BCNT. The composition of iron and nitrogen can be readily tuned by varying the Hb content in the feedstock and pyrolysis temperature. Among the samples, the Hb/BCNT-1000 sample possessed the best catalytic performance, which outperformed the benchmark of IrO₂ at an overpotential of 380 mV. The X-ray absorption fine

CRediT authorship contribution statement

Weiming Chen: Formal analysis. Xuanli Luo: Formal analysis. Sanliang Ling: Formal analysis, Writing - review & editing. Yongfang Zhou: Validation. Bihan Shen: Validation. Thomas J.A. Slater: Formal analysis. Jesum Alves Fernandes: Formal analysis, Writing - review & editing. Tingting Lin: Visualization. Jianshe Wang: Writing - review & editing. Yi Shen: Writing - original draft, Supervision.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgement

The project was financially supported by the National Natural Science Foundation of China (Grant No. 21706081) and 111 Project (B17018). The authors thank Diamond Light Source for the support from the electron Physical Science Imaging Centre for the use of Instrument E01 under proposal number MG23723. The authors thanks to Professors Alan Chadwick and Giannantonio Cibin for the XAS measurements, and the Diamond Light Source for provision of beam time through the Block Allocation Group (BAG) for

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¹: The two authors contributed equally to this work.

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Hemoglobin-derived Fe-Nx-S species supported by bamboo-shaped carbon nanotubes as efficient electrocatalysts for the oxygen evolution reaction

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Materials

Synthesis of BCNT

Structural characterization

Conclusion

CRediT authorship contribution statement

Declaration of competing interest

Acknowledgement

Joule

Carbon

Carbon

Appl. Catal. B Environ.

Carbon

Inside Chem.

Appl. Catal. B Environ.

Appl. Catal. B Environ.

J. Power Sources

Nat. Commun.

Comput. Mater. Sci.

Combining theory and experiment in electrocatalysis: insights into materials design

Science

Metallic single-unit-cell orthorhombic cobalt diselenide atomic layers: robust water-electrolysis catalysts

Angew. Chem. Int. Ed.

Single atoms and clusters based nanomaterials for hydrogen evolution, oxygen evolution reactions, and full water splitting

Adv. Energy Mater.

Transition metal oxides as electrocatalysts for the oxygen evolution reaction in alkaline solutions: an application-inspired renaissance

J. Am. Chem. Soc.

Single-atom Au/NiFe layered double hydroxide electrocatalyst: probing the origin of activity for oxygen evolution reaction

J. Am. Chem. Soc.

General synthesis and definitive structural identification of MN4C4 single-atom catalysts with tunable electrocatalytic activities

Nat. Catal.

Catalysis by supported single metal atoms

ACS Catal.

A stable single-site palladium catalyst for hydrogenations

Angew. Chem. Int. Ed.

Catalysis by clusters with precise numbers of atoms

Nat. Nanotechnol.

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Nat. Catal.

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Nat. Catal.

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Nano Res.

Coordination structure dominated performance of single-atomic Pt catalyst for anti-Markovnikov hydroboration of alkenes

Sci. China Mater.

Atomic Fe dispersed on N-doped carbon hollow nanospheres for high-efficiency electrocatalytic oxygen reduction

Adv. Mater.

single-atom electrocatalysts

Angew. Chem. Int. Ed.

Earth-abundant catalysts for electrochemical and photoelectrochemical water splitting

Nat. Rev. Chem.

Supported single atom and pseudo-single atom of metals as sustainable heterogeneous nanocatalysts

ChemCatChem

Efficient oxygen reduction reaction (ORR) catalysts based on single iron atoms dispersed on a hierarchically structured porous carbon framew

Angew. Chem. Int. Ed.

Dynamic traction of lattice-confined platinum atoms into mesoporous carbon matrix for hydrogen evolution reaction

Sci. Adv.

Ruthenium atomically dispersed in carbon outperforms platinum toward hydrogen evolution in alkaline media

Nat. Commun.

Hemoglobin-derived Fe-N_x-S species supported by bamboo-shaped carbon nanotubes as efficient electrocatalysts for the oxygen evolution reaction

General synthesis and definitive structural identification of MN₄C₄ single-atom catalysts with tunable electrocatalytic activities