A nanoporous CeO2 nanowire array by acid etching preparation: an efficient electrocatalyst for ambient N2 reduction

In 0.1 M HCl, this catalyst achieves a high faradaic efficiency of 4.7% with a NH3 yield of 38.6 μg h−1 mg−1cat. at −0.3 V vs. reversible hydrogen electrode, outperforming most reported Ce-based NRR electrocatalysts under ambient conditions.

As an important industrial chemical, NH 3 has attracted much attention as potential energy carrier and fertilizer precursor. 1,2 With the increase of population and the decrease of fossil fuels, the large demand for NH 3 has become an urgent social problem, which promotes the in-depth study of artificial NH3 production technology. Due to the need for hydrogen input and energy consumption from fossil fuels, traditional industries for ammonia-producing (350-550 °C and 150-350 atm) is an energy intensive procedure: Haber -Bosch process, which will result in a great deal of carbon dioxide in the process. 3 Therefore, there is a tough importunity for the development of facile and sustainable alternate strategies for NH 3 production.
As a kind of nitrogen reduction reaction (NRR) which can synthesize NH 3 at room temperature only via using high efficiency electrocatalyst, 4,5 the electrocatalytic NRR plays a significant role in attracting the attention of researchers, 6-9 the electrocatalytic NRR plays a significant role in attracting the attention of researchers. Recently, considerable attention has focused on exploring non-noble-free NRR electrocatalysts. [10][11][12][13][14][15][16][17][18][19][20][21][22][23] Porous noble metals are displayed to be effectual electrocatalysts for electrochemical storages and energy conversions, [24][25][26] which are assuring to be investigated for NRR. Over the homogeneous metal surface, the application prospect of forming unsaturated active sites on the surface of phosphide which is beneficial to the bonding of nitrogenrelated intermediates is worth discussing in NRR. Cerium(IV) oxide (CeO 2 ) benefits desire electronic/ionic conductivity, cerium ion group plays a role as intermediate in catalytic reaction and adsorbed gas ， as well as they are exposed. 27 Both element doping 28 and interface energinerring 29 are verified productively to improve the NRR ability of catalysts. Porous nanostructures are of apparent advantages of high surface-area, 30 providing good benefit to improve the electrocatalytic NRR catalysis. It is thus trusted that constructing of porous Ce-based catalysts is a good strategy to enhance the NRR activity of transition metal catalysts.
Herein, we report our finding that CeO 2 nanowire is splendid catalyst for NH 3 synthesis at ambient conditions. The key idea is to selectively generate NP-CeO 2 nanowires with different corrosion stability of oxalic acid on MnO 2 and CeO 2 . CeO 2 achieves a high FE (4.7 %) and NH 3 yield (38.6 µg h -1 mg -1 cat. ) at -0.3 V vs. reversible hydrogen electrode (RHE), which are notably higher than that for MnO 2 -CeO 2 precursor (NH 3 yield:  (Fig. S1). Indicating that the constructing np-CeO 2 /TM maintains nanowire array feature (Fig. 1b). The transmission electron microscopy (TEM) image of etching np-CeO 2 is shown in Figure 1e, which expresses a truth that the High-resolution TEM (HRTEM) supports interplanar distance of 0.313 nm corresponding to (111) plane of CeO 2 (Fig. 1c).
The Brunauer-Emmett-Teller (BET) pore-size distribution curves of np-CeO 2 (Fig. 1e) exhibits a extensive peak centering at 8.6 nm, associating excellent with TEM data. Meanwhile, the energy-dispersive X-ray (EDX) elemental mapping images of CeO 2 clearly show that Ce and O elements evenly distribute on the surface. All these measurements absolutely approbate the convincing formation of MnO 2 -CeO 2 resulted high surface area nanoporous CeO 2 nanowires under the condition of etching via acid.  X-ray photoelectron spectroscopy (XPS) was used to investigate the elemental composition and chemical valence states of porous CeO 2 . As shown in Fig. 2b. high-resolution Ce 1s spectra (Fig. 2a) displays binding energies about 882.6 and 901.2 eV matching to Ce 3d 5/2 and Ce 3d 3/2 , accordingly. 31 For O1s, we can attribute it to three characteristic peaks. The two peaks at 530.1 and 531.7 eV correspond well to the ordered lattice oxygen ions of CeO 2 , as well as the oxygen vacancy. For the peak at 533.3 eV, it can be defined to the absorbed hydroxyl on the surfaces of the CeO 2 from water molecules. 32,33 The difference of peak area at 531.2eV indicated that the oxygen vacancy of CeO 2 increased significantly during hydrogen reduction after acid treatment. 34,35 NRR conventional is a conventional hydrogenation reduction, after N 2 bubbling to cathode's surface, where H + could transfer the electrolyte to product NH 3 by reacting with CeO 2 /N 2 . For our experiment, the NRR tests were conducted in a twochamber cell separately at ambient conditions, which is partitioned by Nafion membrane (115). For our research, the NH 3 obtained from the cathode is formed by the interaction of N 2 and H + by avoiding the oxidation of produced NH 3 through spaced cell can at anode. At moderate temperature and atmospheric pressure, the voltage was corrected by means of a reversible hydrogen electrode (RHE). The NH 3 and N 2 H 4 produced by electrocatalytic reaction were determined via the indophenol blue method, 36 as well as the Watt and Chrisp method. 37 The electrolyte was colored with indophenol indicator after 2 h electrocatalytic NRR reaction at constant potentials for collecting UV-Vis absorption spectra (Fig. S2 and  S3). Np-CeO 2 /GCE (0.3 mg/cm 2 ) demonstrates exceptional selectivity without N 2 H 4 -production (Fig. S4). Fig. 3b exhibits average NH 3 yields, as well as FEs at different potentials. In the study of the effect of load on catalytic activity, it was found that when the load was 0.3 mg, the best NRR activity was shown (Fig. S5). The optimum NRR rate fixes at -0.3 V vs. RHE, causing an average yield of 38.6 µg h -1 mg -1 cat NH 3. And 4.7 % FE. Over most reported NRR catalysts, as a catalyst with good performance, it has a great advantage, including Au nanorod (6.042 μg h -1 mg -1 , 4%), 38 Cu 3 P-rGO (26.38 μg h -1 mg -1 cat. , 1.9%), 39 γ-Fe 2 O 3 (0.212 μg h -1 mg -1 cat. , 1.9%), 40 and N-doped nanocarbon (27.2 μg L -1 h -1 , 1.42%). 41 Detailed comparison is presented in Table S1. Fig. 3a displays that the yield increases with the increase of potential. In view of the surface competitive adsorption between N 2 and H, the catalyst performance is significantly reduced when the voltage transcends -0.3 V. For comparison, we provide hydrogen yield rates for hydrogen evolution reactions (Fig. S5). By comparing the pH test paper of electrolyte solution before and after electrolysis (Fig. S6), it can be concluded that the pH hardly changed in the experiment, which shows that the whole system has not transformed through the reaction. In Fig. 3b, np-CeO 2 /GCE exposits speedy NRR rate than MnO 2 -CeO 2 /GCE (14.3 µg h -1 mg -1 cat. ), demonstrating the N element plays an important role in NRR. Meanwhile, in the whole process, the weak signal value expressed by the blank GCE is completely offset. To confirm the sensed NH 3 is produced through NRR of np-CeO 2 /GCE, a series of controlled experiments are conducted (experiments' conditions: Ar for carrier gas, -0.3 V vs. RHE for open-circuit potential and 20 h for electrochemical reaction). Moreover, in 0.1 M HCl, we tested the NRR performances of the nanoporous CeO 2 nanowires deposited on carbon paper, it also acquires the greatest NH 3 yield of 34.6 µg h -1 mg-1 cat. as well as a high FE of 4.6 % (Fig. S7). For comparison purpose, NH 3 yield and FE of the MnO 2 -CeO 2 were shown in Fig. S8 (Fig. S9). Stability is an additional significant parameter to estimate catalyst behavior. Np-CeO 2 /TM has insignificant changes in NH 3 yield and FE through recycling experiments for 6 times (Fig.  4a). The Fig. 4b displays the long-term electrolysis at a set of potentials, which indicate good stability of np-CeO 2 /TM. Moreover, a slight change occurred after the NRR reaction at -0.3V for 24 h (Fig. 4c). The XRD (Fig. S10) and XPS (Fig. S11) show almost no changes before and after the long test, they also demonstrate high electrochemical stability. The FEs for np-CeO 2 demonstrate slight loss came up to the initial one after long-term testing. Based on the experimental data, it can be concluded that np-CeO 2 is exceptionally stable and durable for the NRR through ambient reaction conditions. The influence of N 2 flow rate on electrocatalytic N 2 reduction was examined concurrently. What is shown in Fig. 4d is that there is inapparent fluctuation in FEs and NH 3 yields following a series of N 2 flow-rates, suggesting that the rate of reduction is impartial of the gas-solid interface. What's more, N 2 is transported toward the cathodic catalyst's surface within the N 2 the electrolyte. In addition, Since the speed of electrocatalytic reaction is independent of N 2 concentration, it can be concluded that the diffusion of N 2 is not the decisive step of the reaction.
In summary, np-CeO 2 nanowire is confirmed as a highefficiency and selective electrocatalyst for qualify leading NH 3synthesis of N 2 to NH 3 in acidic media. The np-CeO 2 nanowire attains a NH 3 yield of 38.6 µg h -1 mg -1 cat. as well as a FE of 4.7% at potential of -0.3 V. Besides, what's surprising is that np-CeO 2 possesses appealing selectivity and long-term stability for electro-hydrogenation under ambient conditions. This investigation is not only the first demonstration of applying np-CeO 2 for efficient and stable NRR electrocatalysis, but would expose a stimulating new path to the advancement of transition metal nitrides as attractive low-cost NRR catalyst materials for implementations.

Conflicts of interest
There are no conflicts to declare.