TiN Anode for Electrolytic Reduction of UO₂ in Pyroprocessing

TiN 양극을 이용한 파이로프로세싱 UO₂ 전해환원

Abstract

Developing novel anode materials to replace the Pt anode currently used in electrolytic reduction is an important issue on pyroprocessing. In this study, the electrochemical behavior of TiN was investigated as the conductive ceramic anode which evolves O₂ gas during the reaction. The feasibility and stability of the TiN anode was examined during the electrolytic reduction of UO₂. The TiN anode could electrochemically convert UO₂ to metallic U in a LiCl–Li₂O molten salt electrolyte. No oxidation of TiN was observed during the reaction; however, the formation of voids in the bulk section appeared to limit the lifetime of the TiN anode.

파이로프로세싱 전해환원 공정에서 현재 사용 중인 Pt 양극을 대체하기 위한 소재 개발은 매우 중요하다. 이 연구에서는 전기화학 반응시 산소를 발생시키는 전도성 세라믹 양극으로서 TiN의 전기화학적 거동을 알아보았다. UO₂의 전해환원이 일어나는 동안 TiN 양극의 적합성과 안정성에 대한 평가를 진행하였다. LiCl-Li₂O 용융염에서 TiN 양극을 이용하여 UO₂를 전기화학적으로 금속 U로 변환시킬 수 있었다. 반응 도중 TiN의 산화 반응은 관찰되지 않았다. 하지만 TiN 내부에서 공공이 생기는 것을 확인하였으며, 이에 따라 소재 수명에 제한이 있을 것으로 판단된다.

1. Introduction

Pyroprocessing is currently under development to recycle and manage the spent oxide fuels generated from nuclear reactors [1-3]. Electrolytic reduction is the first electrochemical process in pyroprocessing, in which oxide fuels are converted to the metallic states. Then the reusable nuclear species (e.g., U, Pu) are recovered during subsequent electrorefining and electrowinning processes for use as metal fuels in next-generation fast reactors [1-3].

Li2O-containing LiCl molten salt at 650℃ is commonly used as an electrolyte for electrolytic reduction in pyroprocessing [4-10]. During the electrochemical reaction, metal oxides (e.g., UO2) are reduced to metals (e.g., U) at the cathode, while O2 gas is formed at the anode [4-5]. The anode is thus exposed to a highly oxidizing environment because of the high reaction temperature. Although Pt is the most widely used anode material because of its resistance against oxidation [4-7], a Pt anode would be damaged during the electrochemical reaction owing to the formation of Li2PtO3 and anodic dissolution [5]. In our previous papers, we have suggested various materials as alternatives to expensive Pt anodes [8-10]. Carbon-based compounds and liquid Sb anodes have shown low current efficiency, however, because of side reaction cycles during the reaction [8-9]. While B-doped diamond was able to reduce UO2 in a KCl–LiCl–Li2O electrolyte at a low temperature of 550℃, it had poor current density [10].

In this study, we studied the feasibility of a conductive nitride, TiN, as the anode material. TiN thin films have been widely investigated as an electrode material in semiconductors because of their thermal stability in an oxidizing atmosphere [11-12]. During the electrochemical reaction, TiN would be sacrificially oxidized to form oxides and N2 gas (2TiN + 2O2(g) = 2TiO2 + N2(g), ΔG = −1052.15 kJ at 650℃), which is not corrosive. Alternatively, it may remain stable during the reaction because the oxidation kinetics is significantly affected by the O2 partial pressure. Indeed, the oxidation behavior of TiN varies under different oxidation conditions [13-14]. Herein, we investigated the electrochemical properties of the TiN anode in a 650℃ LiCl–Li2O electrolyte that was used to reduce UO2 to metallic U.

 

2. Experimental

A disk-shaped bulk TiN of 50.8 mm in diameter and 6.35 mm in thickness (Alfa Aesar, USA) was cut into adequate dimension (1/4 position from edge) and then connected to stainless steel lead for the test (Fig. 1(a)). An electrolytic cell was installed inside an Ar-filled glove box. Detailed descriptions of the experiments are provided in the literature [4-7]. Prior to the electrochemical study, the TiN anode was immersed in a 700 g of LiCl–Li2O (1wt% Li2O) electrolyte at 650℃ to examine its stability in the molten salt. The Li2O electrolysis test was done with a stainless steel cathode (rod-type) and the TiN anode. A stainless steel basket containing 10 g of UO2 was used as the cathode for the electrolytic reduction test. A constant voltage was applied using a power supply (E3633A, Agilent, USA) during the reaction and, at the same time, the potential difference between the cathode and the Li–Pb reference electrode was monitored using a digital multimeter (34401A, Agilent, USA).

Fig. 1.Photographs of the TiN anode (a) before and (b) after immersion in the molten salt electrolyte. (c) Electrochemical behavior of the TiN anode during voltage scan for Li2O electrolysis (inset: photograph of metallic Li formed on the cathode surface).

The crystallinity was determined by X-ray diffraction (XRD; D8 Advance, Bruker, Germany). Thermogravimtery (TG; TGA/DSC1 Mettler Toledo, Switzerland) was used to evaluate the conversion rate of UO2 to metallic U, based on the re-oxidation of U (or un-reduced UO2) to U3O8 in O2 atmosphere (3U + 4O2(g) = U3O8 or 3UO2 + O2(g) = U3O8). Prior to the TG measurement, the reaction product was rinsed with distilled water to remove residual LiCl-Li2O. Elemental analysis was carried out by energy-dispersive X-ray spectroscopy (EDS; X-MAX, Horiba, Japan) coupled with scanning electron microscopy (SU-8010, Hitachi, Japan).

 

3. Results and discussion

Before the electrolytic reduction test, a preliminary test on the stability inside the molten salt and capability of Li2O electrolysis was carried out, as shown in Fig. 1. The TiN anode sustained its shape after immersion in the molten salt electrolyte (Fig. 1(a) and (b)), demonstrating the stability of TiN. No breakdown induced by thermal shock, which is frequently observed in ceramic compounds, was seen in the TiN anode. Fig. 1(c) shows the Li2O electrolysis behavior of the TiN. The decomposition of Li2O is an important process in the electrolytic reduction owing to the catalytic behavior of Li2O [4-7]. Once Li2O dissociates to form metallic Li on the cathode and O2 gas on the anode (2Li2O = 4Li + O2(g)), the metallic Li readily reacts with UO2 to form metallic U and new Li2O (UO2 + 4Li = U + 2Li2O) [4-7]. The stainless steel rod without UO2 was used as the cathode here. The cell voltage between the anode and the cathode was controlled to vary from 0.5 to 3.25 V to determine adequate reaction voltage range. When the cell voltage was above 3.0 V, the cell current increased drastically, which means that the electrochemical reaction was triggered between 2.5 and 3.0 V. High reaction voltage compared the decomposition voltage of Li2O (~2.46 V) might be due to the surface overpotential, induced by O2 bubble, electric double layer, or some other reasons. The cathode potential (i.e., potential difference between the cathode and the reference electrode) at this range was approximately −0.73 and −0.56 V vs. Li–Pb in the closed-circuit and open-circuit configuration, respectively. The open-circuit potential of the cathode is comparable to that of the Li+/Li0 redox couple (approximately −0.54 V vs. Li/Pb) [4], demonstrating the formation of metallic Li on the cathode surface. The more negative value of the closed-circuit potential was due to the polarization applied to the cathode. The metallic Li deposited on the cathode rod surface can be clearly seen in the inset of Fig. 1(c).

Fig. 2 shows the electrolytic reduction behavior of the TiN anode with the UO2-containing stainless steel cathode basket. The cell voltage was fixed at 3.25 V to induce the electrolysis of Li2O, as shown in Fig. 2(a). A value equivalent to 150% of the theoretical charge to convert all UO2 to U was supplied to the electrolytic cell. The open-circuit potential is comparable to that shown in Fig. 1(c), indicating the formation of Li. The lower cell current compared to that observed in the Li2O electrolysis test was due to the reduced cathode area. After the reaction, the color of the sample turned from dark brown of the as-prepared UO2 to shiny silver of the reduced UO2 (Fig. 2(b)), suggesting that the electrochemical conversion of UO2 to the metallic state took place. We carried out XRD analysis to identify the reaction product, and the result is shown in Fig. 2(c). The diffraction peaks are well matched to the metallic U phase with small UO2 peaks. The conversion rate was estimated to be 87.11–100.58% by TG analysis at different locations of the reduced UO2 sample. These results obviously confirm that the TiN anode was capable of electrochemically reducing metal oxides to their metallic forms.

Fig. 2.(a) Electrochemical behavior of the TiN anode during the electrolytic reduction of UO2 at 3.25 V, (b) photograph images of UO2 sample before and after the reaction, and (c) XRD pattern of reduced UO2 sample after the reaction.

After the reaction, no oxidation of the TiN anode was observed in the XRD pattern in Fig. 3(a). On the other hand, a number of voids were formed in the immersed area of TiN, while no noticeable change was seen in the non-immersed area (Fig. 3(b) and (c)), which emphasize the electrochemical instability of the TiN anode after prolonged reaction duration. A shiny layer also can be seen on the surface of the TiN anode. This is due to the corrosion of the stainless steel lead, originated from the surface climbing of the molten salt through the electrode surface. The degradation mechanism of TiN remains unclear at this moment, but it is speculated that the electrolyte-soluble phase, somehow, was formed during the reaction. Indeed, EDS analysis of the electrolyte byproduct after the reaction shows the dissolution of Ti into the electrolyte (Fig. 3(d)). Despite the void formation, a dominant reaction pathway is believed to be the O2 evolution reaction. Fig. 3(e) shows the bottom plate of the electrolytic reducer flange made with stainless steel. The stainless steel flange became significantly rusty, implying the formation of a large amount of O2 during the reaction.

Fig. 3.(a) XRD patterns of the TiN anode before and after the reaction. Photographs of the TiN anode after the reaction in (b) immersed and (c) non-immersed areas. (d) EDS analysis of the electrolyte byproduct after the reaction. (e) Photograph of the bottom plate of the flange after the reaction.

 

4. Conclusion

The suitability of the conductive nitride TiN as anode material for the electrolytic reduction in pyroprocessing was investigated. The TiN anode was shown to electrochemically reduce UO2 to metallic U in a molten salt electrolyte. While no noticeable oxidation of the TiN anode was observed, the dissolution of TiN gradually occurred during the reaction. Hence, it is apparent that TiN has a limited lifetime as the anode material in the electrolytic reduction and its use should be limited in small-scale or short-term experiments.