Accelerated N2 reduction kinetics in hybrid interfaces of NbTiO4 and nitrogen-doped carbon nanorod via synergistic electronic coupling effect
Graphical Abstract
Introduction
Ever since Fritz Haber’s patent on the “synthesis of ammonia from its elements” it has revolutionized the world’s agricultural productivity through the industrial manufacture of fertilizers along with population explosion [1]. Ammonia (NH3) also plays a central role as the future clean energy carrier due to its high energy density (5.52 KWh Kg−1) amidst environmentally friendly combustion products (N2 & H2O) [2], [3]. The benefits of ammonia in the imminent century were foreseen by Haber. However, he failed to surmise its impact on biodiversity by generous fossil fuel consumption and CO2 emissions as the process operates under harsh conditions (400–500 °C, 10–30 MPa) [4]. Due to the strong chemical stability (941 KJ mol−1) of the triple bond of nitrogen molecule (NN), tremendous activation energy has to be necessarily fed to maintain the desired ammonia production rate [5], [6]. Consequently, it is compelling to innovate the conventional Haber Bosch process or design a new process with a less polluting, less energy-consuming, and clean ammonia production route.
Over the last decades, the inexhaustible chemical energy resources like N2 and H2O in the electrolyzers driven by renewable power sources have attracted concerted attention as alternative ways to produce NH3 gas [2], [3], [7]. Several strategies have been focused on lowering the activation energy to the NN bond break and pushing the selectivity towards NRR from competing for hydrogen evolution reaction (HER) [8]. Hence, the design of effective electrocatalysts dramatically reducing the threshold for N2 activation is essential to resolve the never-ending quest for NH3 production.
In accordance with the conventionally known design principle, the best natural catalyst of nitrogenase comprises an N2 binding catalytic molybdenum-iron activation center and electron-donating iron protein, combined producing NH3 under mild conditions [9]. Therefore, mimicking the “binding-activation” mechanism as in enzymatic N2 fixation was extensively studied to fabricate electrophotocatalysts for N2 reduction [10], [11], [12], [13]. To promote electron-donation into antibonding orbitals of adsorbed N2, various strategies such as defect engineering, substantial doping, and hybridization have been attempted [3], [4], [14], [15], [16], [17]. Some of the approaches indeed were successful for remarkable enhancement of NRR activity and stability.
On the other hand, tuning the properties of nanostructured materials by strong catalyst-support interaction can lower the activation barrier through interfacial charge transfer and stabilize the adsorbed intermediates [18], [19], [20], [21]. The hybrid nanostructures are suitable candidates for tuning the electron-donating function for N2 activation, notwithstanding it is less considered in designing electrocatalysts for NRR. For instance, the strong interfacial bonds between cobalt sulfide nanoparticles and graphene support accelerated the reaction kinetics for N2 reduction, though the study lacks information on binding energy perturbations on the metal centers and the effect of support loading [22]. Likewise, similar studies on various heterojunctions have featured enhanced electronic coupling effect and NRR activity compared to the bare catalyst without support, indicating the significance of strong catalyst-support interaction [15], [16], [17], [23], [24], [25].
Besides, the favorable adsorption of N2 over protons on the electrocatalyst surface is a crucial factor determining the selectivity and faradaic efficiency. Recently, increasing attention has been paid to developing cost-effective and earth-abundant transition metal oxides as NRR catalysts, which can effectively bind the N2 molecule by accepting electrons from the σ orbital and cleaving the triple bond through back donation into antibonding orbitals [25], [26], [27], [28], [29], [30]. Theoretical studies have revealed the outstanding NRR stability of rutile-type metal oxides with (110) surface, and particularly, niobium oxide (NbO2) exhibited favorable N2 adsorption ability comparable to the noble rhenium dioxide (ReO2) [31]. As reported by Huang and associates, NbO2 showed an exceptional faradaic efficiency of 32%, yet a poor NH3 yield rate (11.6 µg h−1mg−1) is a limitation for its practical application [30]. According to our knowledge, the absence of an electron boosting component can be the reason for sluggish NRR kinetics, despite the high N2 selectivity of NbO2. Recently, Hu et al. demonstrated the electron-donating role of Ce through synergistic charge transfer in Ce1/3NbO3, resulting in a five-fold improvement of NH3 yield rate than a pristine Nb2O5, which implies that electron boosting elements should promote the activation of inert N2 molecule [32].
Inspired by these pioneering works, for the first time, we demonstrate the rutile-type niobium titanium oxide nanoparticles on nitrogen-doped carbon nanorods (NbTiO4@NCNR) hybrid as an effective NRR catalyst for electrochemical NH3 synthesis under ambient conditions. Density functional theory (DFT) calculations evidently show the outstanding N2 adsorption ability on metal-terminated Nb-site of NbTiO4 and Ti is the key acting as electron-reservoir during the N2 activation. Moreover, the effect of carbon loading on the NbTiO4@NCNR significantly affects the NRR activity via electron transport from the support. The strong interaction between N2 binding NbTiO4 catalyst center and electron contributing NCNR support imitates the binding-activation mechanism of enzymatic N2 fixation.
Section snippets
Experimental details
Synthesis of NbTiO4@NCNR involves the electrospinning of the polymer composed of Polyacrylonitrile (PAN, 1 g), Niobium oxalate (0.7 mmol), Titanium oxy acetylacetonate (0.7 mmol), and DMF (9 g). First, metal precursors and DMF were sonicated in a glass vial until uniform dispersion, followed by the addition of PAN and overnight stirring at 90 °C before electrospinning. As electrospun matt was stabilized in N2 atmosphere (500 cc/min) at 250 °C for 2 h (Step-I) and carbonizing at 500 °C
Structural characterization
To explore the growing interest in using transition metal oxides as N2 reduction electrocatalysts, we developed unique NbTiO4 nanoparticles supported on carbon nanorods by the electrospinning method as schematically depicted in Fig. 1a. X-ray Diffraction (XRD) patterns reveal the formation of a single-phase rutile structured metal oxide with space group P42/mnm in both NbTiO4@NCNR hybrids and pure NbTiO4 (ICSD 01-081-0911) (Fig. 1b). The broad peak in the hybrid sample around 26° originated
Conclusion
NbTiO4@NCNR hybrid catalysts were successfully fabricated using the electrospinning method, and the hybrid sample achieved a maximum NH3 yield rate of 58.13 ± 8 µg h−1mg−1 with 10.4 ± 1.4% faradaic efficiency. The impact of carbon loading on metal oxide and its influence on NRR activity was further demonstrated by observing the synergistic effect between NCNR and NbTiO4. The interfacial bridging bonds between the catalyst and support accelerated the electron transport to the metal center for
CRediT authorship contribution statement
David Kumar Yesudoss: Conceptualization, Data curation, Formal analysis, Writing – original draft. Hoje Chun: Data curation, Writing – original draft. Byungchan Han: Supervision, Funding acquisition, Writing – review & editing. Sangaraju Shanmugam: Conceptualization, Funding acquisition, Supervision, Writing – review & editing.
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.
Acknowledgments
The authors acknowledge the Brain Korea 21 Program (BK-21) and the National Research Foundation (Project No. 2021R1A2C2009223) of Korea funded by the Ministry of Education, Science for financial support. This work was partly supported by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government (MOTIE) (20214000000090, Fostering human resources training in advanced hydrogen energy industry) and the Global Frontier Program through the Global
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