# Corrosion Behavior and Conductivity of TiNb and TiNb Coated Steel for Metallic Bipolar Plates
## 1 Introduction  
The proton exchange membrane fuel cell (PEMFC) is a kind of energy conversion device, which transforms chemical energy into electricity. Owing to its high power-density and zero-emission features, PEMFC has been considered as one of the most prospective power suppliers for vehicles, portable devices and distributed generation [1]. PEMFC is composed of a proton exchange membrane, catalysts, gas diffusion layers, and bipolar plates. The lifetime of PEMFC depends on the durability of individual components. One of the reasons for failure is corrosion of the bipolar plates. This may not only destroy the bipolar plate itself, it also produces ions, which can be detrimental to both membrane [2] and catalyst. Therefore, the corrosion resistance of the bipolar plates is one of the decisive factors influencing the lifetime of a PEMFC. The American Department of Energy (DOE) has established a series of required properties for bipolar plates and one crucial target is to improve corrosion resistance and conductivity [3; 4]. Earlier studies concentrated on graphite materials and graphite-based composite bipolar plates [5; 6; 7], considering the high corrosion resistance of graphite. However, graphite is brittle and in order to prevent breaking, it is usually machined into quite thick bipolar plate, which increases weight and manufacturing costs of the PEMFC stack [8]. Along with the development of metal forming and welding techniques, metallic bipolar plates have become increasingly relevant for PEMFC due tothe good mechanical strength. This is especially for automotive stacks where weight and volume are critical parameters. In order to reduce cost, the plates should preferably be manufactured as coated inexpensive substrates [9; 10; 11]. Austenitic stainless steel (SS), especially 316L, is prone to passivation and can be a suitable bipolar plate material [12; 13]. As for coatings, noble metals like Au have been investigated widely [14; 15], but although most noble metallic bipolar plates established great corrosion resistance and conductivity, high prices have confined their applications. Compared with noble metals, the low-cost transition metals and transition metal nitrides [16] have showed good performances in PEMFC surroundings attracting significant interest. The niobized 304 SS was reported to a decrease of alloy's corrosion current density, meeting the DOE demand (<1 uA\(\cdot\)cm\({}^{-2}\)) [17]. In acid solution, the corrosion resistance of Ti still needs to be further improved [13]. Metal nitrides such as TiN and NbN have been deposited on different substrates establishing low interfacial contact resistance (ICR). However, after a transient polarization test, the TiN coated sample achieved higher current density than the base SS substrate material [18]. Similarly, the NbN coating's corrosion resistance at high potential also needs to be improved [19]. Considering the embarrassing situation that it is hard to obtain high conductivity and great corrosion resistance at the same time, the idea of mixing Ti, Nb, TiN and NbN properly to combine the best of each of the advantages is proposed. The alloyed TiNb and TiNbN have acceptable prices when applied as films. Additionally, a TiNbN coating can be expected to have attractive mechanical property and has been used in biomedical or mechanical areas [20; 21]. TiNb alloys have also been studied earlier for their unique shape memory effect [22]. [PERSON] et al. [23] evaluated the chemical durability and surface resistances of Ti alloys with 3 at.% Nb and confirmed that the oxidized Ti-3 Nb alloys had lower resistances than Ti oxides. However, to the best of our knowledge, there is still a lack of adequate literature investigating the corrosion behavior and conductivity of TiNb or TiNbN coated SS with low Ti content under simulated PEMFC working conditions. As for the deposition method, physical vapor deposition (PVD) provides firm, uniform, and dense coatings compared with electroplating [24].