Rigorous and reliable operations for electrocatalytic nitrogen reduction

https://doi.org/10.1016/j.apcatb.2020.119325Get rights and content

Highlights

  • Several critical concerns in electrocatalytic N2 reduction are investigated.

  • The interferences affecting the accuracy of the analysis method are confirmed.

  • The presence and levels of various contamination sources are identified.

  • The specific operating parameters are systematically explored.

  • A useful flow chart containing crucial recommendations is provided.

Abstract

Electrocatalytic nitrogen reduction in ambient condition has received enormous attention for sustainable ammonia production. Apart from catalyst development, this promising route is confronted with considerable technical and scientific challenges, e.g., the ubiquity of ammonia contaminations, and the reliability of analysis methods for products. Herein, using indophenol blue method as a paradigm, its accuracy and reproducibility challenged by several interference factors are firstly scrutinized. The presence and levels of various contamination sources are then deliberately identified. Also, the specific experimental operations with the regulation of gas flow, stirring and temperature are systematically practised. Based on the experimental explorations together with recent advances in the field, we propose a flow chart that provides some critical recommendations to conduct rigorous and consistent operations in electrocatalytic nitrogen reduction. This may help the beginners avoid common pitfalls and urge the community to pay more attention to other significant aspects (e.g., catalyst design and reaction kinetics).

Introduction

As the basic chemical in the agriculture world and chemical industry, ammonia is mainly produced via the Haber-Bosch reaction, requiring purified nitrogen, hydrogen, and critically, high temperature (400−500 °C) and pressure (100−300 bar) [1]. This energy-intensive process necessitates 1∼2 % of global energy supply, accompanying with the emission of a large quantity of greenhouse gases. Given its energy and environment constraint, it is highly anticipated to develop alternative routes to realize nitrogen reduction at ambient conditions [2,3].

Recently, an emerging, electrocatalytically driven nitrogen reduction reaction (NRR) has received enormous interests, especially in the past three years (Fig. 1A). This impressive process has exhibited several merits, such as simple experimental setups (electrochemical cells), mild reaction conditions (room temperature and pressure), sustainable energy supplies (power supplied by wind turbines or solar cells), and less pollutant emissions (water as the proton donor) [4,5]. Playing critical roles in the electrocatalytic NRR, plenty of innovative catalysts, mainly containing metal-free catalysts [[6], [7], [8], [9]], metal catalysts [10,11], metal-oxide catalysts [[12], [13], [14]], and transition-metal carbides/nitrides or dichalcogenides [15,16], have been widely reported recently.

In addition to the design and synthesis of a functional electrocatalyst, choosing a proper electrolyte is also critical to assess its catalytic performance. Various acid (e.g., 0.1 M HCl and 0.05 M H2SO4) and alkaline (e.g., 0.1 M and 1.0 M KOH) electrolytes have been extensively adopted in electrocatalytic NRR [[17], [18], [19], [20], [21], [22]]. Compared with the harsh acid/alkaline media, the use of neutral electrolytes (e.g., 0.1 M Na2SO4 and 0.1 M PBS) is a more preferable option, owing to the low corrosion and operating risks, as well as the controllable production costs. To date, numerous NRR researches have been performed in such solutions, utilizing multifarious metal/metal-oxide/metal-sulphide based electrocatalysts [[23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35], [36], [37], [38], [39], [40], [41]]. In these studies, the acquired ammonia yield rates range from 0.19 to 9.42 μg h−1 cm−2 or 4.5 to 36.6 μg h−1 mg−1, with the Faradaic efficiency varying from 0.94 to 16.9 % (Fig. 1B). Currently, ammonia yield rate (rNH3, per mass) and Faradaic efficiency (FE) are two key metrics to evaluate the NRR performance, which are calculated by Eqs. 1 and 2.rNH3=CNH3×Vt×mcat.FE=3×F×CNH3×V17×Qwhere CNH3 is the ammonia concentration, F is the Faraday constant, V is the electrolyte volume, t is the reduction time, mcat. is the loading mass of catalyst, and Q is the quantity of electric charge.

According to these equations, the reliability of reported rNH3 and FEs heavily relies on the accurate determination of CNH3. In electrocatalytic NRR, the produced ammonia could be measured via several methods, e.g., spectrophotometric assay, ammonia ion-selective electrode, ion chromatography, and 1H NMR spectroscopy [42]. Amongst these techniques, spectrophotometric approaches, including the Nessler’s reagent and indophenol blue methods, have been extensively applied in quantitative ammonia assay on account of their low cost, high sensitivity and facile operation. Nevertheless, the Nessler’s reagent contains a highly toxic [HgI4]2− and has a short lifetime of few weeks. In comparison, the required reagents and operating conditions are more accessible and convenient with the indophenol blue method, making it a popular approach for the efficient ammonia determination.

Despite the rapid progress in this research hotspot, especially the prosperous development of fascinating electrocatalysts, and well-established methods for the determination of reaction products, a series of concerned issues on the experimental operations and puzzles towards the reliability of test results have plagued most researchers [43]. On the one hand, although possessing a high accuracy and wide linear range, the variations in the pH or concentration of electrolytes, as well as the presence of interferents (for instance, metal ions due to the leaching of catalysts), may adversely affect the reliability of spectrophotometric methods. On the other hand, ascribed to the ubiquity of ammonia sources in aqueous solutions and many surfaces, and even from the N-containing catalysts, any negligent operation would result in the misleading data in NRR, sometimes with vastly overestimated FEs due to these exogenous contaminations. Apart from these concerns, increasing attention has been paid to some crucial experimental operations, for instance, gas flow rate, which may also contribute to significant variations on the observed FEs [32].

To unveil the underlying factors related to aforementioned issues in electrocatalytic NRR, continuous efforts have been conducted by several groups recently from different perspectives. For example, Zhang et al. systematically investigated and compared the merits and limitations of several approaches commonly used for ammonia determination [44]. Meanwhile, Qiu and Yu et al. demonstrated that the ammonia could pass through and be adsorbed on the Nafion membranes, leading to the measurement errors and acceleration of the membrane degradation [45]. Moreover, Qiao et al. explored the effects of trace nitrate/nitrite in the lithium salts toward the reliable electrocatalytic NRR [46]. Even existing at ultralow levels, these impurities could generate remarkable false positives with deceptive reproducibility to mislead researchers. More importantly, to realize a reliable quantification of electrocatalytic nitrogen reduction to ammonia, Chorkendorff et al. proposed rigorous isotope measurements with cycling of 15N2 gas, and showed how to eliminate labile N-containing compounds from the nitrogen stream [47].

In spite of these preliminary efforts and achieved results, considerable research efforts are still urgently demanded to thoroughly uncover the negative factors, which are of great importance for researchers to achieve persuasive results in electrocatalytic NRR. Meanwhile, a series of standardized control/blank experiments widely accepted by the community are required to be established. Moreover, to guide future research in this interesting field more insightfully and efficiently, it is imperative to introduce the benchmarking protocol for related operations in electrocatalytic processes and accurate measurements of reaction products. For this purpose, taking one of the most used electrolytes of 0.1 M Na2SO4 as an example, the applicability of the indophenol blue method, the presence and levels of contamination sources, as well as some critical operations are systematically explored. Finally, a flow chart is proposed to provide beneficial recommendations to perform rigorous and consistent operations in electrocatalytic NRR.

Section snippets

Reagents and materials

2-Methylimidazole (2-MIM), zinc acetate dihydrate (ZnAc2·2H2O), urea, melamine, d-glucose, l-cysteine, sodium sulfate (Na2SO4), sodium hydroxide (NaOH), sodium hypochlorite (NaClO, ρCl = 4–4.99 %), sodium sallicylate (C7H5O3Na), sodium nitroprusside dihydrate (Na2[Fe(CN)5NO]·2H2O) and 5 wt % Nafion were obtained from Sigma-Aldrich. Nafion 211 and 117 membranes were bought from Dupont. Nitrogen gas (99.99 % and 99.999 %), and argon gas (99.999 %) were obtained from BOC gas, Australia. Supplied

Accuracy and reproducibility of indophenol blue method

Belonging to a characteristic absorbance of the generated indicator, a wavelength at ∼660 nm is observed in UV–vis spectroscopy (Fig. S1), and it increases linearly with ammonia concentrations from 0 to 1.0 μg mL−1 (R2 = 0.9992). Given the instability of the oxidizing reagent especially under the light irradiation (Fig. S2), careful attention should be paid to the reproducibility of this method (Fig. S3). Even applying reagents prepared in the same batch, obvious variations in the slopes of

Conclusions

To avoid false negatives/positives in the reporting data, several highly concerned issues related to the electrocatalytic NRR using the electrolyte of 0.1 M Na2SO4 have been deliberately investigated. Firstly, the underlying factors that affect the accuracy of indophenol blue method are confirmed. Meanwhile, ubiquitous ammonia sources, as well as N-containing catalysts themselves, and their contamination levels are evaluated. Moreover, several operating parameters that play crucial roles in NRR

CRediT authorship contribution statement

Lei Shi: Conceptualization, Investigation, Data curation, Writing - original draft. Yu Yin: Methodology, Formal analysis. Shuaijun Wang: Data curation, Formal analysis. Xinyuan Xu: Investigation, Resources. Hong Wu: Formal analysis, Resources. Jinqiang Zhang: Resources. Shaobin Wang: Conceptualization, Writing - review & editing. Hongqi Sun: Conceptualization, Writing - review & editing, Supervision, Project administration, Funding acquisition.

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.

Acknowledgements

The author (Sun) would like to express his thanks for the support from Vice-Chancellor's Professorial Research Fellowship. The work was partially supported by Australian Research Council Discovery Projects (DP170104264 and DP190103548). The authors acknowledge the technical supports from the Centre for Microscopy, Characterization and Analysis, The University of Western Australia; Electron Microscope Facility, Curtin University; as well as the X-ray Surface Analysis Facility, Curtin University,

References (65)

  • X.H. Yang et al.

    Insights into the role of cation vacancy for significantly enhanced electrochemical nitrogen reduction

    Appl. Catal. B-Environ.

    (2020)
  • H. Wang et al.

    Exfoliated metallic niobium disulfate nanosheets for enhanced electrochemical ammonia synthesis and Zn-N2 battery

    Appl. Catal. B-Environ.

    (2020)
  • X. Xu et al.

    1 T-phase molybdenum sulfide nanodots enable efficient electrocatalytic nitrogen fixation under ambient conditions

    Appl. Catal. B-Environ.

    (2020)
  • K. Chu et al.

    Multi-functional Mo-doping in MnO2 nanoflowers toward efficient and robust electrocatalytic nitrogen fixation

    Appl. Catal. B-Environ.

    (2020)
  • J.F. Su et al.

    Fine rhodium phosphides nanoparticles embedded in N, P dual-doped carbon film: new efficient electrocatalysts for ambient nitrogen fixation

    Appl. Catal. B-Environ.

    (2020)
  • M. Ma et al.

    Tuning electronic structure of PdZn nanocatalyst via acid-etching strategy for highly selective and stable electrolytic nitrogen fixation under ambient conditions

    Appl. Catal. B-Environ.

    (2020)
  • Y. Liu et al.

    Electrocatalytic production of ammonia: biomimetic electrode–electrolyte design for efficient electrocatalytic nitrogen fixation under ambient conditions

    Appl. Catal. B-Environ.

    (2020)
  • X.B. Dong et al.

    Sulfur and nitrogen co-doped mesoporous carbon with enhanced performance for acetylene hydrochlorination

    J. Catal.

    (2018)
  • X.Z. Chen et al.

    Photocatalytic fixation of nitrogen to ammonia: state-of-the-art advancements and future prospects

    Mater. Horiz.

    (2018)
  • L. Shi et al.

    Rational catalyst design for N2 reduction under ambient conditions: strategies towards enhanced conversion efficiency

    ACS Catal.

    (2020)
  • W.B. Qiu et al.

    High-performance artificial nitrogen fixation at ambient conditions using a metal-free electrocatalyst

    Nat. Commun.

    (2018)
  • X.J. Zhu et al.

    Ambient electrohydrogenation of N2 for NH3 synthesis on non-metal boron phosphide nanoparticles: the critical role of P in boosting the catalytic activity

    J. Mater. Chem. A Mater. Energy Sustain.

    (2019)
  • Y.-C. Hao et al.

    Promoting nitrogen electroreduction to ammonia with bismuth nanocrystals and potassium cations in water

    Nat. Catal.

    (2019)
  • K. Chu et al.

    Fe-doping induced morphological changes, oxygen vacancies and Ce3+–Ce3+ pairs in CeO2 for promoting electrocatalytic nitrogen fixation

    J. Mater. Chem. A Mater. Energy Sustain.

    (2020)
  • Y.Y. Liu et al.

    Dramatically enhanced ambient ammonia electrosynthesis performance by in-operando created Li-S interactions on MoS2 electrocatalyst

    Adv. Energy Mater.

    (2019)
  • Y.-X. Lin et al.

    Boosting selective nitrogen reduction to ammonia on electron-deficient copper nanoparticles

    Nat. Commun.

    (2019)
  • N. Cao et al.

    Doping strain induced bi-Ti3+ pairs for efficient N2 activation and electrocatalytic fixation

    Nat. Commun.

    (2019)
  • X.H. Li et al.

    Boosted electrocatalytic N2 reduction to NH3 by defect-rich MoS2 nanoflower

    Adv. Energy Mater.

    (2018)
  • Y. Zhang et al.

    High-performance electrohydrogenation of N2 to NH3 catalyzed by multishelled hollow Cr2O3 microspheres under ambient conditions

    ACS Catal.

    (2018)
  • H.J. Yu et al.

    Bimetallic Ag3Cu porous networks for ambient electrolysis of nitrogen to ammonia

    J. Mater. Chem. A Mater. Energy Sustain.

    (2019)
  • K. Chu et al.

    Efficient electrocatalytic N2 reduction on CoO quantum dots

    J. Mater. Chem. A Mater. Energy Sustain.

    (2019)
  • X.J. Xiang et al.

    Ammonia synthesis from electrocatalytic N2 reduction under ambient conditions by Fe2O3 nanorods

    ChemCatChem

    (2018)
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