Developing Novel Copper Foam Electrodes for Water Splitting Rogine Gomez, Alessandro Pereyra, Dr

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Developing Novel Copper Foam Electrodes for Water Splitting Rogine Gomez, Alessandro Pereyra, Dr. Vilupanur Ravi Department of Chemical and Materials Engineering, Cal Poly Pomona

ABSTRACT The proton exchange membrane electrolyzer cell (PEMEC) offers a potential solution to reduce the carbon footprint for hydrogen production via water splitting, but its feasibility and longevity are limited by the lack of functional materials. The demanding environment of the PEMEC requires a porous electrode with high mechanical strength, corrosion resistance, and surface area; however, none of the current materials in use are optimal for this application. A viable option to mitigate this problem is the implementation of open-cell metallic foam. Open-cell metal foam is an ideal material because of its customizable porous structure (e.g. pore size, porosity) and high conductivity that allow for optimized mass and charge transfer through the cell. However, current manufacturing routes (e.g. investment casting, gas injection) are limited in their ability to meet the growing demands for specialized applications while keeping fabrication costs low. In this project, a porous copper electrode was designed and fabricated using a novel electroless plating process. The electroless plating procedure follows a three-step treatment process: (1) etching of 3D printed PLA foam surface using KOH, (2) a catalytic process involving the redox reaction between CuSO4 (oxidizer) and NaBH4 (reducer), and (3) electrodeposition of copper in NaOH environment. The surface morphology and composition of metallized surface was characterized using scanning electron microscopy and energy dispersive spectroscopy.

INTRODUCTION Hydrogen gas is of great value in the chemical synthesis and metallurgical processing industries. More recently, there is a growing demand for hydrogen because of its usefulness in power generation using fuel cells. However, current practices for hydrogen production, such as steam- methane reforming, have a large carbon footprint and pose environmental challenges.1 The proton exchange Figure 1: A Typical Schematic for PEMEC membrane electrolyzer cell (PEMEC) is a potential solution to reduce carbon footprint for pure Despite its high potential, the feasibility and longevity of hydrogen/oxygen production via water splitting.2 PEMECs are limited by the lack of durable and affordable materials. One of the critical materials challenges As seen in Figure 1, a typical PEMEC setup consists of associated with the PEMEC is the demanding service bipolar plates, cathodic and anodic sites, liquid-gas condition at the liquid/gas diffusion layer (LGDL). This diffusion layers, and the proton exchange membrane. layer is acidic, and with the presence of dissolved oxygen Water in the cell undergoes a reduction-oxidation reaction it can serve as a strong oxidizer for metals.4 Currently, through electrolysis that effectively splits it into hydrogen carbon paper and titanium mesh are being explored as and oxygen ions. 2 The oxygen ions undergo oxidation, potential candidate materials for the LGDL, but the producing pure oxygen gas. The hydrogen ions pass former has been reported to poison the expensive Pt through the proton exchange membrane and undergo catalyst during service, while the latter is high cost.4,5 reduction, producing pure hydrogen gas for energy Apart from corrosion and cost, materials for LGDP also applications.2 PEMEC offers high potential for a need to be porous enough for effective reactant and sustainable and efficient energy source that can be utilized product transports and mechanically strong to withstand at a global scale.3 Currently, PEMEC systems are being the operating pressure.6 investigated to be implemented in hydrogen-powered vehicles as an alternative to vehicles run by fossil fuels.3 The demanding environment of the PEMEC requires a porous electrode with high mechanical strength, corrosion resistance, and high surface area.6 A viable option to

mitigate this problem is the implementation of open-cell implementation as an LGDL component for PEMEC metallic foams (OCAF) as electrodes in PEMEC applications. applications.7 OCAF is an ideal material for LGDL because of their lightweight and porous structure as well OBJECTIVE as their high conductivity that allow for optimized mass The objective of this project is to develop a novel and electron transfer through the cell.7 The geometry (i.e. fabrication method for metallic foams through electroless pore size, pore distribution, etc.) of the foam can be deposition of copper on 3D printed PLA foam and customized according to the specifications required by the evaluate its feasibility in PEMEC environments. given applications.8 However, current manufacturing routes (e.g. investment casting, gas injection, etc.) are MATERIALS AND METHODS limited in their ability to create complex geometries for Electrode Substrate Fabrication 8 specialized applications at low fabrication costs. With the PLA foams were 3D printed using the Craftbot 3D Printer growing demand for metallic foams for specialized (Craft Unique Inc.) located at the Bronco Make Studio at applications, there is a need for a manufacturing process the Cal Poly Pomona University Library as indicated in that allows for their fabrication at a more cost- and time- Figure 2. The bulk dimension of as-printed foams samples effective manner compared to current methods. is 1” x 0.5” x 0.5” with 40% infill. A rectangular bar with

the dimension 1” x 0.375” x 0.125” was also printed with 3D printing has gained popularity due to its accessibility 40% infill to be used as the baseline for performance and affordability to fabricate complex designs using a characterization. The 3D printed samples utilized for the material of choice at set specifications. 3D printing experiments can be seen in Figure 3. polymer materials have been widely used in creating components for the biomedical, aerospace, and electronic industry.9 However, in applications where a conductive surface is required, 3D printed polymer objects can undergo a metallization process.10 One form of metallization is electroless plating, which involves metallic ions being deposited and plated on the polymer substrate without the use of an external power source or plating electrodes.10 This is ideal for polymer substrates because of its non-destructive nature.11 From this process, the polymer substrate would obtain the conductive and protective properties that a metal coating can provide.10 Using the combined techniques of 3D printing and metallization, a porous copper foam can be designed, fabricated, characterized, and optimized for PEMEC application.

In this project, PLA was selected as the polymer foam substrate and copper as the metallic coating. PLA is known for its easy printability and accessibility, which makes it the ideal material to print complex foam geometries.9 Copper was chosen as the coating material because of its high conductivity, excellent mechanical properties, low cost, and corrosion resistance in acidic Figure 2: (Top to Bottom) Bronco Maker Studio at the CPP University 11 Library; Craftbot Pro used to 3D Print PLA Samples; 3D Printing of environments. The optimization of the fabrication of PLA Foam in Progress copper foams can lead to further expansion and

temperature and duration to generate chemically active sites on the PLA substrate to catalyze metal deposition.14

For the electroplating process, surface catalyzed PLA coupons were immersed in a plating solution consisting

of copper sulfate pentahydrate (CuSO4●5H2O),

triethylamine (C6H15N), formaldehyde (CH2O), and

ethylenediaminetetraacetic acid or EDTA (C10H16N2O8). The composition of the plating solution is described by 12 Bernasconi et al. Upon electrodeposition, the coupons Figure 3: 3D Printed PLA Flat (left) and Foam (right) Samples were dried in an oven at 30 °C.

Electroless Plating Process Microstructure & Film Characterization The metallization process of polymer substrate involves a The microstructure and surface morphology of the series of surface treatment and chemical reaction in the metallized PLA substrate were characterized using following order: (1) degreasing, (2) etching, (3) surface scanning electron microscopy (SEM). Chemical analysis catalyzation, (4) electrodeposition, and (5) drying. of the copper film was conducted using energy dispersive Prior to the metallization process, as-printed materials spectroscopy (EDS). Macrophotographs were taken at underwent neutral degreasing to remove impurities, e.g. each step of the metallization process to its respective oil, debris, resulted from the 3D printing process. This is local changes. To determine the thickness of the thin film, achieved via ultrasonication in deionized water, sodium as-metallized foam was mounted in epoxy resin and carbonate-containing degreasing solution, and then ground to a 1200 µm surface finish using silicon carbide deionized water for 3 minutes each.12 To roughen the PLA grinding paper. substrate to create sites for copper deposition, 3D printed PLA coupons was etched in potassium hydroxide (KOH) RESULTS AND DISCUSSIONS 12 solution. Following etching, PLA coupons were 3D printed PLA foams were successfully coated with immersed into anhydrous copper sulfate (CuSO4) and copper through electroless plating. sodium borohydride (NaBH4) solutions for a controlled

Figure 4: Schematic of the Metallization of 3D Printed PLA Foam through Electroless Plating

Figure 4 shows the macro-images of the 3D-printed PLA a a higher atomic number, whose surface will be shown foam and the schematic that shows the reaction as brighter. It can be shown that the foam strut exhibited mechanism at each step of metallization. At the etching a uniform coating with the exception of the crevices on process, the OH- molecule hydrolyzes the PLA substrate. the strut. These crevices were created by the different This leads to the local dissociation of the PLA substrate, strands of PLA ligaments combining during 3D printing. creating roughened surface, i.e. more effective sites, for The EDS point analysis shows that the copper thin film Cu2+ particles to adhere to as seen on Step 2.14 This also consists of 93 at% of copper with the remaining removes the trace filament strands still present on the PLA components being oxygen and carbon, which can be substrate, such as that shown in Step 1, after the 3D sources of impurities. printing process. During catalysis and reduction processes (Steps 3 and 4), the Cu particulates are deposited to the 11 roughened surface via a reduction reaction with NaBH4. It is notable that no observable changes occurred on the surface of PLA foam after etching (Step 2) and catalysis (Step 3), and the surface revealed a light reddish brown texture after reduction, which is indicative of the deposition of Cu particles. Lastly, upon immersion in the electrodeposition bath solution, it is evident that the redox reaction between formaldehyde and copper led to the formation of a reddish copper film on the PLA substrate (Step 5).12

Examining the cross section of the copper metallized

foam in Figure 5, it was determined that the thin film Figure 6: Backscattered Electron Micrograph of Copper Metallized thickness is about 3±1 μm. Foam with Elemental Composition

SUMMARY AND CONCLUSIONS In conclusion, a novel metallization process based on electroless plating was developed and utilized to deposit thin copper film on 3D printed polylactic acid (PLA) foam for PEMEC water splitting application. This process provides a low-cost alternative due to the use of a palladium-free catalyst. The morphology and composition of the thin film were characterized using scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). These results show that high purity copper had been successfully deposited on the PLA foam substrate with a thickness of 3±1 μm.

FUTURE WORK Figure 5: Optical Micrograph of the Copper Metallized PLA Foam To ensure the feasibility of the fabricated foam samples Cross Section Mounted in Epoxy Resin in PEMEC applications, further experimentation is Figure 6 shows the backscattered electron micrographs required. Further characterization of the metallic film (BEI) of the as-metallized PLA foam strut. The BEI will give a better understanding of its properties like imaging technique in the SEM provides image contrast conductivity, adhesion, etc. based on the atomic number, i.e., higher atomic number portions of the image will appear brighter phase will have 4

and the lower atomic number areas will be darker. cells. Fuel and Energy Abstracts, 43(4), 260. Additionally, its corrosion behavior in simulated doi: 10.1016/s0140-6701(02)86280-4 PEMEC environments need to be studied to determine 5. Steen, S. M., Mo, J., Kang, Z., Yang, G., & Zhang, F.-Y. (2016). Investigation of titanium the film’s stability in acidic conditions. liquid/gas diffusion layers in proton exchange membrane electrolyzer cells. International Mechanical testing can also be conducted to compare the Journal of Green Energy, 14(2), 162-170. strength of the fabricated foams and current materials. doi:10.1080/15435075.2016.1253582 6. Wang, Y., Wang, C.-Y., & Chen, K. (2007). ACKNOWLEDGMENTS Elucidating differences between carbon paper The authors would like to thank Dr. Vilupanur Ravi for and carbon cloth in polymer electrolyte fuel all his unequivocal support and guidance as our faculty cells. Electrochimica Acta, 52(12), 3965–3975. advisor. We would also like to acknowledge everybody doi: 10.1016/j.electacta.2006.11.012 who has helped contribute to the progress of this project: 7. Toghyani, S., Afshari, E., & Baniasadi, E. Ho Lun Chan, Kevin Guo, Harjot Singh, Eduardo (2018). Metal foams as flow distributors in Perez, Anan S. Hamdan, Joey Tulpinski, and Ulus comparison with serpentine and parallel flow Ekerman (Cal Poly Pomona). 3D printing capabilities fields in proton exchange membrane electrolyzer were made possible by the Bronco Maker Space and CPP cells. Electrochimica Acta, 290, 506–519. doi: Innovation Lab. Financial support from Ms. Sylvia Hall, 10.1016/j.electacta.2018.09.106 Drs. George and Mei Lai, the LA section of NACE 8. Ashby, & Ashby, M. F. (2000). Metal foams: A International, Western States Corrosion Seminar, design guide. Boston: Butterworth-Heinemann. Western Area of NACE International, the NACE 9. Ortiz-Acosta, D., & Moore, T. (2019). Foundation, California Steel Industries Inc., Southern Functional 3D Printed Polymeric California Chapter of the Association for Iron & Steel Materials. Functional Materials. doi: Technology, Achieve Scholars Program, and Louis 10.5772/intechopen.80686 Stokes Alliances for Minority Participation are gratefully 10. Bindra, P., & White, J.R. (2002). Chapter 12 acknowledged. The SEM images and EDS analysis were Fundamental Aspects of Electroless Copper made possible through a NSF MRI grant DMR-1429674. Plating. 11. Langner, M., Agarwal, S., Baudler, A., Schröder, U., & Greiner, A. (2015). Large WORKS CITED Multipurpose Exceptionally Conductive Polymer Sponges Obtained by Efficient Wet- 1. Dufour, Javier, et al. (2012). Life Cycle Chemical Metallization. Advanced Functional Assessment of Alternatives for Hydrogen Materials, 25(39), 6182–6188. doi: Production from Renewable and Fossil 10.1002/adfm.201502636 Sources. International Journal of Hydrogen 12. Bernasconi, R., et al. (2016). Electroless Plating Energy, 37(2), 1173–1183., of NiP and Cu on Polylactic Acid and doi:10.1016/j.ijhydene.2011.09.135. Polyethylene Terephthalate Glycol-Modified for 2. Maric, Radenka, and Haoran Yu. (2019). Proton 3D Printed Flexible Substrates. Journal of The Exchange Membrane Water Electrolysis as a Electrochemical Society, 163(9). Promising Technology for Hydrogen Production doi:10.1149/2.1201609jes. and Energy Storage. Nanostructures in Energy 13. Xu, Wang, et al. “Environmentally Friendly Generation, Transmission and Storage, Copper Metallization of ABS by Cu-Catalysed doi:10.5772/intechopen.78339. Electroless Process.” Rare Metal Materials and 3. Han, B., Steen, S. M., Mo, J., & Zhang, F.-Y. Engineering, 45(7), 1709–1713. (2015). Electrochemical performance modeling doi:10.1016/s1875-5372(16)30145-x. of a proton exchange membrane electrolyzer cell 14. Tham, C., Hamid, Z. A. A., Ahmad, Z., & for hydrogen energy. International Journal of Ismail, H. (2013). Surface Engineered Hydrogen Energy, 40(22), 7006–7016. doi: Poly(lactic acid) (PLA) Microspheres by 10.1016/j.ijhydene.2015.03.164 Chemical Treatment for Drug Delivery 4. Baschuk, J., & Li, X. (2002). Carbon monoxide System. Key Engineering Materials, 594-595, poisoning of proton exchange membrane fuel 214–218. doi: 10.4028/www.scientific.net/kem.594-595.214

Reflective Essay

My career as an undergraduate researcher was catalyzed by my acceptance into the Achieve Scholars Program (ASP) at Cal Poly Pomona. It is a program under the Office of Undergraduate Research that provides opportunities for students to learn more about the resources available on campus to aid them with starting a research career as an undergraduate student. In the program, I had met aspiring researchers who had led me to meet my faculty advisor, Dr. Vilupanur Ravi. The program had not only expanded my knowledge on resources and services that the campus provides but also shaped an integral part of my college experience.

Learning about the Library Resources

Through ASP, I took LIB 150, instructed by CPP librarians Paul Hottinger and Sally Romero, where I learned about the different types of resources the library offers to students, from unlimited access to online databases to interlibrary loans. This newfound skill had been very helpful with writing research papers for classes, but it proved to be an essential skill in being successful in my research endeavors. In Spring 2018, I started attending Dr. Ravi’s research meetings. The first few weeks were admittedly brutal in a since I had no idea what was going on, but I was persistent in securing a position in the group because I knew I wanted to be a part of a community of determined, goal-oriented, and hardworking students. For the rest of the quarter, I sat in the meetings and listened patiently to all the weekly updates, slowly familiarizing myself to words like “corrosion,” “microstructures,” and “Tafel Plots.” I was applying what I had learned from LIB 150 and looked for introductory material on the library website to gain a basic understanding of the topics I will be learning about when I start my own project. I was effectively applying what I had learned in the class to the start my undergraduate research career and beyond.

Learning and Sharing my Knowledge

My success in LIB 150 had also made me qualified to work for the library initially as a Document Delivery Assistant and currently as a Knowledge Center Consultant. As I become more familiar with the resources and services the library offer, I find myself teaching my fellow

students how they can access them for their class assignments, research projects, and general interest. Outside of my employment, I was answering library questions for students in my research group. I had assisted multiple students in my group how to access ASTM standards for their experiments, how to order books through CSU+ and Document Delivery, and how to navigate the online databases. It was fulfilling to see that this skillset that I have acquired through the years was being utilized to help other people.

Applying the Knowledge

This school year, I started working on our senior project with Dr. Ravi. We have been given the opportunity to conduct research on metallic foams, a novel material whose potential is still being explored and tested. As the interest for metallic foams is relatively recent, the documentation is quite limited, and the literature gap is broad. To aid me with developing a research topic, I have communicated with the engineering librarian, Paul Hottinger, about how to organize topics and keywords to see which areas do not have a lot of papers written but enough to provide us an understanding to pursue forward. With this literature review, we decided to explore manufacturing routes for metallic foams and develop a cost- and time-effective method.

Making Stuff in the Library!

This objective was heavily supported by the addition of the Bronco Maker Studio at the library, which provided students free access and supplies for projects involving 3D printing, laser cutting, vinyl printing etc. Through literature research, we learned that a method to fabricate metallic foams to 3D print structures out of polymers and metallizing it with the metal of choice. Given the availability and accessibility of the resources the campus offers, especially the library, we decided to pick PLA as the polymer substrate to be coated with a thin copper film. Using the Maker Space at the library, we 3D printed our samples with controlled specifications and conducted our experiments on them. This eliminated the need for external sources to obtain our materials, which would have cost more given that we have specific requirements. The easy accessibility and availability of this equipment gave us flexibility to change properties and observe how it affects the coatings. Additionally, we were also able to design and fabricate equipment like the working electrode holder to hold our samples during electrochemical testing.

This had allowed us to be creative and innovative in our methods as making our own equipment no longer poses a problem for us.

Figure 1: (left) Solidworks Design for Working Electrode Sample Holder; (right) Assembled Working Electrode Sample Holder with 3D Printed Part

Thanks to the Library

From the beginning of my career as an undergraduate researcher, the library has played a significant role in my growth and accomplishments. Without a doubt, this project would not have run as smoothly as it did. From reading numerous articles and ordering multiple books through interlibrary loans, the foundation and continued success of this project is enabled by the services and resources that the library offers. I have also had the opportunity to share this knowledge to my fellow students by teaching them how to navigate these resources. Now more than ever, with the pandemic occurring, the library continues to be a huge part of the research process. My team and I have been using the online database to understand the fundamentals and the complexities of our projects further. One thing we have learned during the research process is that we never stop having questions about our project, and as long as the library resources are available, we will be able to accomplish this whatever the state of the world may be.