Rigorous and reliable operations for electrocatalytic nitrogen reduction
Graphical abstract
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 (, per mass) and Faradaic efficiency (FE) are two key metrics to evaluate the NRR performance, which are calculated by Eqs. 1 and 2.where 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 and FEs heavily relies on the accurate determination of . 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,
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