Improved catalytic performance and corrosion resistance of selective laser melted 316L SS in a direct methanol fuel cell by surface anodization

https://doi.org/10.1016/j.surfcoat.2020.126172Get rights and content

Highlights

  • Anodized selective laser melted 316L SS has high corrosion resistance due to Mo and Cr enrichment at cellular boundary.

  • The Fe/Cr oxides formed by the cellular boundary after anodization increase the catalytic oxidation of methanol.

  • Selective laser melted 316L SS, oxidized under 15 V and loaded with Pt, exhibited the best catalytic performance.

Abstract

The cellular microstructure is a special phenomenon in many additively manufactured metals and alloys. This work provided a new way to achieve both high catalytic performance and superior corrosion resistance for additively manufactured 316L stainless steel by surface engineering. Results show that a well-organized cellular structure array is formed along the sub-grain boundary during the anodization process and an oxidation voltage of 15 V afforded the selective laser melted 316L (SLM 316L-A15) with the largest specific surface area. Compared with the traditional 316L plate, the SLM 316L-A15 plate deserves a superior corrosion resistance due to the enrichment of molybdenum and chromium at the sub-grain boundary. The SLM 316L-A15 plate surface loaded with Pt nanoparticles exhibited excellent catalytic effect, which is ascribed to the increased electrochemical surface area and the beneficial synergistic effect between iron/chromium oxide and platinum.

Introduction

Proton exchange membrane fuel cells (PEMFCs) have the advantages of simple structure, high theoretical specific energy, and lack of a need for external auxiliary equipment compared with other types of fuel cells [1,2]. Among them, because methanol has the advantages of large-scale production, low price, and convenient storage, proton exchange membrane fuel cells that directly use methanol as fuel (DMFCs) are more advantageous. The bipolar plate is a key component that affects the performance of DMFCs, and the selection of the plate material, the subsequent surface manufacturing method has attracted much attention [[3], [4], [5]]. Bipolar plates made of graphite, a traditional material, have a large volume and a low power density, but with little ductility [6]. In addition, some new oxide and sulfide materials have also attracted widespread attention as sensor and supercapacitor electrode materials [7,8]. In comparison, metal materials are considered more suitable because of their advantages of superior mechanical properties, airtightness, ease of press formation, and low cost [9]. Currently, traditional 316L stainless steel has good scope for development in microbial fuel cells [10], sensors [11] and solid oxide fuel cells [12]. However, there are still some unresolved problems. The pH in the working environment of fuel cells is generally between 3 and 6, and the working temperature is 60–80 °C [13,14]. Most austenitic stainless steels, including 316L stainless steel, are still corroded in the working environment of DMFCs, which limits their further application [15,16].

To overcome the corrosion of traditional 316L in the acidic, 60–80 °C environment of DMFCs, the corrosion resistance of 316L can be improved by a new advanced preparation process—additive manufacturing (AM) technology. As a new molding technology, AM has attracted widespread attention because it can make complex parts without being constrained to traditional manufacturing designs. Our previous work reviewed the relationship between the microstructure and the corresponding corrosion behavior of several metallic alloys fabricated by AM technology [17]. Ambrosi et al. used selective laser melting technology to manufacture a spiral steel electrode that was electrochemically modified with an IrO2 film and exhibited excellent performance in capacitors, pH sensors and oxygen evolution catalysts [18]. Yan et al. found the rapid solidification in AM to refine the size of oxide inclusions, converting generally harmful oxide inclusions into beneficial nanoscale oxide inclusion particles, a new type of particle that refines and strengthens the grains [19]. Our another previous work noted that the SLM 316L stainless steel has stronger corrosion resistance and excellent resistance to hydrogen damage in fuel cells than conventional 316L stainless steel [20,21]. Meanwhile, we also found that the SLM 316L was subjected to the solution annealing treatment, and the precipitation of oxide and sulphide particles at the grain boundaries prevented the formation of a continuous passivation film, which was not conducive to improving its corrosion resistance [22]. In addition, by optimizing the printing laser power, we fabricated the SLM 316L stainless steel with both high corrosion resistance and the passivation film of SLM 316L was thicker than the quenched 316L [23]. Meanwhile, in recent years, various surface modification methods have been widely used to improve the working efficiency of stainless steel in PEMFCs. Tian et al. found that the passivation current density of high chromium and nickel (HCN) austenitic stainless steel is lower than 316L SS (after passivation) and the passivation film caused an increase in interfacial contact resistance between carbon paper and HCN [24]. Bi and other researchers synthesized a new zirconium carbon/amorphous carbon coating on the surface of 316L stainless steel by magnetron sputtering technology, and the modified 316L bipolar plate showed good corrosion resistance and high interface conductivity [25]. Pugal Mani et al. confirmed that nitridation greatly improves the corrosion resistance of 316L SS bipolar plates [26]. Chromium-rich oxide film was formed on the surface of 316L stainless steel by the anodizing process, and the porous structure formed on the surface is widely used in the fields of magnetic, electrical, optical, sensing equipment and biological materials [27]. In our work, we used the anodic oxidation technique to form a layer of porous cellular structure, we predicted that SLM 316L after anodic oxidation would exhibit high corrosion resistance and good catalytic performance, even in high-temperature environments.

In this work, SLM 316L SS plates were anodized and then loaded with platinum nanoparticles by electroplating. The cellular structure array on the surface of the steel plate was observed by field emission secondary electron microscope (FE-SEM) and atomic force microscope (AFM), including cellular structure diameter and depth. The chemical composition of cellular structure array was obtained by X-ray photoelectron spectroscopy (XPS) analysis. The corrosion resistance of SLM 316L-A15 and the catalytic performance of SLM 316L-A15-Pt were analyzed by electrochemical tests. In summary, a series of modified or corrosion-resistant materials were applied to the surface of SLM 316L SS to develop a high-efficiency composite coating material for SLM 316L SS electrode plates in order to achieve excellent corrosion resistance and enhanced catalytic performance.

Section snippets

Synthesis of SLM 316L-A15-Pt

The morphology of the powders used to prepare the SLM 316L stainless steel was shown in Fig. 1, and the diameter of the powder ranged from 10 to 60 μm, with a good apparent density in our work. The SLM 316L was printed by EOS M290 machine (Germany) in an argon atmosphere and the detailed printing parameters in this work were as follows: the laser power was 195 W with a wavelength of 1070 nm, the scanning speed was 1083 mm s−1, and the size of the laser spot was 80 μm. The printing machine was

Surface morphology characterization of SLM 316L by anodization

Fig. 2 shows the TEM micrograph of as-prepared SLM 316L. The surface of SLM 316L is found to have a sub-grain structure. According to TEM-EDS analysis test, the contents of Cr and Mo element at the sub-grain interior are 16.9 wt% and 1.7 wt%, however, the element contents at the sub-grain boundary has reached 19.3 wt% and 2.9 wt%. The content of the Fe element at the sub-grain interior is 69.8 wt%, whereas 66.0% at the sub-grain boundary. The contents of Cr and Mo are higher at the boundaries

Conclusions

This is the first time that we studied the promotion effect of a cellular structure array formed by anodization and selective laser melting processes on catalytic methanol oxidation and we demonstrated that the SLM 316L-A15-Pt anode plate has better catalytic performance for electrocatalytic methanol oxidation compared with the T316L-A30-Pt plate.

In our work, Pt nanoparticles supported on anodized SLM 316L are prepared by electrodeposition and anodization steps. SLM 316L-A15 is obtained by

CRediT authorship contribution statement

Ruixue Li:Methodology, Investigation, Writing - original draft.Chaofang Dong:Methodology, Writing - review & editing.Decheng Kong:Formal analysis, Writing - review & editing.Xiaoqing Ni:Investigation.Liang Zhang:Investigation.Min Ao:Formal analysis.Xiaogang Li:Methodology, 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.

Acknowledgements

This work was supported by the National Key Research and Development Program of China (No. 2017YFB 0702300), Fundamental Research Funds for the Central Universities (No. FRF-TP-18-002B2), National Natural Science Foundation of China (No. 51871028) and Shanghai Materials Genome Institute No. 5 (No. 16DZ2260605).

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