3D Printed Electrodes for Improved Gas Reactant Transport for Electrochemical Reactions

Introduction

Additive manufacturing (AM), or three-dimensional printing (3D printing), allows rapid production of complex structures from software made models, using a wide array of materials, ranging from polymers^1–3 to metals^4–7 and even human cells.^8–11 As an alternative to traditional subtractive processes, this offers the ability to produce structures with a complexity not attainable using conventional technologies, in particular the ability to create complex internal structures within 3D systems, and creates new opportunities.^12,13 Within the branch of AM, there has been an increasing focus toward energy-based applications from the many techniques under this manufacturing style: primarily, the creation of metal electrodes for sensing^14–17 and, to a lesser extent, developments into conductive thermoplastic sensory structures. ¹⁸

AM has already begun to bring specific benefit to electrochemistry, with the ability to build very large surface area structures an attractive aspect. ¹⁹ A number of different 3D printed electrodes and cell designs have been reported to date, such as interdigitated ²⁰ and high surface area^21,22 electrodes for energy applications, as well as synthetic reactors for microfluidics. ²³ These recent contributions highlight that the use of such technology for electrochemical applications is still at a nascent stage, with opportunities to use these advances to increase the capabilities of electrochemical reactions and devices as AM decreases the gap between theoretical and experimental. ²⁴ Moreover, it is considered that the advantages brought about through 3D printing have the potential to revolutionize energy materials and their applications in an efficiency impossible through traditional manufacturing. ²⁵

Gas oxidation/reduction reactions are a major field of research^26,27 with wide reaching practical benefit. There is, however, a trade-off between the advantages of high surface area electrodes, that is, an increased number of active sites, and the challenge of getting the reactants to these sites. In such reactions, gas must be delivered into the electrolyte and then to the surface of the electrode to be oxidized/reduced. For practical applications, elegant alternatives for transport of the gas reactants are desired. The ability to control the delivery of reactants or removal of products through judicious 3D electrode design and fabrication offers an enormous promise as this can help overcome limitations that may arise from slow diffusion of reactants into a high surface area electrode. It should be noted that, at the research scale, such as in reaction mechanism studies, diffusion limitations may be overcome by using either rotating disk electrodes (RDEs) or gas diffusion layers (GDLs) loaded with catalyst. Obviously, RDEs are not used in practical, large scale, devices and GDLs are limited to 2D configurations, again not ideal for large-scale reactions.

The hydrogen oxidation reaction (HOR), which takes place at the anode of fuel cells, involves fast electron transfer at the interface of platinum (Pt) electrodes, under acidic conditions. The speed of this means that the reaction is typically severely limited by diffusion of the H₂ molecules to the electrode surface. ²⁸ In this work, we introduce a bespoke 3D printed metal electrode fabrication through selective laser melting (SLM) for electrochemical reactions involving gaseous reactants. This fabrication method has been selected due to its reliability in production of high-resolution prints, as well as production of parts with a very high relative density and low porosity, ²⁹ making it ideal for fabrication of solid complex electrode structures.

The electrode presented here was designed to improve gas delivery to the active catalytic sites of a porous scaffold structure and manufactured using SLM of a titanium alloy (Ti6Al4 V). This was then coated with a Pt catalyst by electrodeposition, and the effect of designed gas delivery system on their HOR response was observed by linear sweep voltammetry (LSV). Other reactions involving gas molecules as reactants such as CO₂ reduction and oxygen reduction/oxidation reactions could also benefit from this new concept.

Results and Discussion

The electrode design (Fig. 1) consists of a cylindrical section comprising a body center cubic based 3D scaffolded structure (beam element diameter 200 μm, lattice spacing 1000 μm), with a total height of 61.1 mm and diameter of 15.5 mm, with the addition to two connections at the top. One of these is for electrical contact and the other to connect a gas supply. Gas is distributed inside the electrode through a four-way gas splitter, with each internal tube having a diameter of 1.5 mm, with the ends of each terminating 2.54 mm above the bottom of the electrode. Detailed design and melting parameters can be found in the experimental section.

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FIG. 1.

(A) Cross-sectional rendering of the printed Ti6AL4 V electrode structure (simplified lattice structure), (B) Photograph of actual printed device, and (C) zoomed section of gas splitter with dimensions.

Before being used for HOR, the printed working electrode was coated with Pt by electrodeposition, according to literature methods, ³⁰ using a two-step process. Supplementary Figure S1a and b (Supplementary Data available online at www.libertpub.com/3dp) shows the current and charge profiles, respectively, as a function of time during the electrodeposition. Cyclic voltammetry of the Pt coated electrode in an acidic electrolyte (Supplementary Fig. S1c) shows hydrogen adsorption and desorption characteristic peaks similar to those using a Pt sheet (Supplementary Fig. S1d).

For the HOR study, a carbon cloth, also with deposited Pt, and Ag/AgCl (NaCl 3 M), were used as counter and reference electrodes, respectively. Hydrogen gas was introduced into the electrolyte through the electrode internal tubes until saturation (determined by the open circuit potential [OCP] plateauing) before LSV measurements (Fig. 2a). H₂ flow was maintained at 10 mL/min during the LSV experiments, which were made over a potential range from 1 mV more negative than the OCP to 600 mV positive of it, at 50 mV/s. Control experiments were done by purging H₂ directly into the electrolyte next to the printed structure, rather than through the tubes (see inset of Fig. 2b).

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FIG. 2.

(A) LSV response during HOR, with H₂ either being purged through the internal tubes (blue) or directly into the electrolyte (orange). Operation is shown schematically in inset of (B), with the pathways for H₂ ingress shown using arrows of corresponding colors. HOR, hydrogen oxidation reaction; LSV, linear sweep voltammetry. Color images available online at www.liebertpub.com/3dp

The results presented in Figure 2a show that H₂ oxidation reaches a higher limiting current (i_L) when the gaseous reactant is purged through the 3D printed electrode internal tubes, directly to the active reaction sites. This verifies the assertion that gas reactant transport is a significant impediment to this reaction, as the internal delivery tubes allow the reactant gas to reach the surface of the electrode more efficiently. The limiting current is related to the diffusion layer according to Equation 1. ³¹ \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\usepackage{upgreek}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*} i_L = nFDC^{0} / _{ \delta} \tag{1} \end{align*} \end{document}

where i_L is the limiting current, n is the number transferred electrons, F is the Faraday constant, D is the diffusion coefficient of the redox specie (cm² s), C⁰ is the concentration of the redox specie (M), and δ is the thickness of the diffusion layer. The diffusion layer thickness is the region at which the concentration of the reactant is depleted due its reduction/oxidation. The less efficient the transport of reactants is to the surface of the electrode, the thicker this layer will be. As such, the limiting current is inversely proportional to the diffusion layer thickness. This relationship is independent of the catalytic properties of the electrode and the applied potential, but rather is dependent only on how efficiently reactants are transported. As such, the higher limiting current for the experiments with gas purging through the internal tubes represents a 40% improvement in terms of diffusion of gas reactants to the electrode.

The normalized logarithm of the current with respect to the logarithm of the limiting current, for each electrode, allows us to evaluate the dependence on diffusion on the reaction. ²⁸ The graphs presented in Figure 2b show that for the system with gas delivered directly at the reaction site through the internal tubes of the 3D printed electrode, the limiting current is reached at a lower overpotential (η). It means that the reaction is less dependent on transport limitation and more on the charge transfer rate than when the purging is done directly in the electrolyte.

Conclusions

This work has demonstrated for the first time, to our knowledge, a high surface area printed electrode with an integrated reactant delivery system. This results in substantial enhancements to the HOR efficiency as compared with gas being delivered external to the electrode structure, where slow diffusion of the gas is limiting. This represents an alternative to other approaches such GDLs, with a greater focus on practicality. This concept may be applied to a range of gas-based reactions and helps to demonstrate the array of possibilities for AM in electrochemistry applications, which are only just starting to be explored.