3D Printed Electrodes: A New Platform for Studies of Water Oxidation
Authors: dos Santos Pamyla Layene , Rowley-Neale Samuel James , Banks Craig Edward , Bonacin Juliano Alves 
Published: February 2020

3D printing technology is becoming an important platform for several applications in electrochemistry. In this sense, we can highlight the use of 3D printed electrodes (3D-pe) in the field of electrochemical sensors, point-of-care devices, energy storage, and energy conversion1–4.
The technique of fused deposition modeling (FDM) is a manufacturing process where the polymeric materials are melted and forced out the heated nozzle at a smaller diameter. These features allow freedom of design, waste minimization and, most importantly, fast prototyping. Considering these advantages, the use of 3D printers in electrochemistry was only possible owing to the cost reduction, mainly by the introduction of FDM technique. In addition, the emergence of conductive filaments such as poly(lactic acid)/graphene allowed the creation of printed electrodes in 3D dimension4 (Figure 1).
Due to the versatility of 3D-pe for various applications, chemical stability and very low cost, we are using this kind of electrodes in studies of energy conversion. In this scenario, the water splitting is an important process in the energy conversion field where it is possible to decompose water in H2 and O2. Producing H2 from water does not require a high overpotential, but the process is limited by the water oxidation.
The reaction of water oxidation is thermodynamically (E0 = 1.229 V vs NHE) and kinetically unfavorable. In that way, catalysts are necessary to overcome the activation energy of this special reaction. The challenges in the development of these catalysts are: i) obtaining materials that work in low over potential; ii) operating in mild conditions; iii) composition based on nature abundant elements; iv) high stability and robustness; v) the reaction mechanism comprehension to development of new catalysts5.
In this work, we have proposed the use of cost-effective Ni-Fe­­(oxy)hydroxides modified 3D-printed graphene electrodes as highly efficient electrocatalysts for the studies of water oxidation.
The 3D-pe were electrochemically pre-treated according to our previous report1. The activation of 3D-pe was performed by the oxidation of the electrode (+1.8 V vs SCE for 900 s) followed by the reduction (from 0.0 V to -1.8 V vs SCE at 50 mV s−1). After this, Ni(OH)2 or Ni1-xFexOOH films were electrochemically deposited on the pre-treated 3D electrodes, applying a cathodic current density of 50 mA cm-2. Electrocatalytic activities of the bare and modified electrodes with respect to water oxidation were investigated in 0.1 mol L-1 KOH.
The onset potentials were determined at a current density of 10 mA cm-2. The bare 3D-pe presented poor activity to water oxidation and the iridium electrode (Ir) was used as a reference to benchmark the OER activity. The onset potentials for water oxidation (V vs. SCE) were 0.83, 1.11 and 1.17 V for Ir, 3D-pe/Fe:Ni(OH)2 (20:80) and 3D-pe/Ni(OH)2, respectively. Tafel analysis were performed on the Faradaic sections of the Linear Voltammograms and the bare 3D-pe electrode presented Tafel slope of ca. 360 mV dec-1. Ir electrode presented the value of 26 mV dec-1 and the Tafel slope for 3D-pe/Fe:Ni(OH)2 (20:80) and 3D-pe/Ni(OH)2 were 40 and 96 mV dec-1, respectively. These results suggest that the presence of iron contributes to decrease the potential onset and increases the rate of water oxidation when compared to the nickel hydroxide. Considering the heterogeneous electron transfer rate constants (k0
) using an inner-sphere mediator [Fe(CN)6]3-/4- we found 1.6 10-4, 9.3 10-5 and 1.6 10-5 cm s-1 for 3D-pe, 3D-pe/Fe:Ni(OH)2 (20:80) and 3D-pe/Ni(OH)2, respectively.
As conclusion, we observed that 3D-pe can be modified with catalysts for studies of water oxidation and results are equivalent to conventional electrodes. The modification of nickel hydroxide with 20% of iron contribute to the decreasing of onset potential for water oxidation and the Tafel slope.

Acknowledgments

FAEPEX-UNICAMP (grant#2145/18), São Paulo Research Foundation, FAPESP (grant#2013/22127-2 and grant#2017/23960-0). CAPES - Finance Code 001.

References

(1) dos Santos, P. L.; Katic, V.; Loureiro, H. C.; dos Santos, M. F.; dos Santos, D. P.; Formiga, A. L. B.; Bonacin, J. A. Sensors and Actuators B: Chemical
2019, 281, 837–848.
(2) Cardoso, R. M.; Mendonça, D. M. H.; Silva, W. P.; Silva, M. N. T.; Nossol, E.; da Silva, R. A. B.; Richter, E. M.; Muñoz, R. A. A. Analytica Chimica Acta
2018.
(3) Foo, C. Y.; Lim, H. N.; Mahdi, M. A.; Wahid, M. H.; Huang, N. M. Scientific Reports
2018, 8 (1).
(4) Foster, C. W.; Down, M. P.; Zhang, Y.; Ji, X.; Rowley-Neale, S. J.; Smith, G. C.; Kelly, P. J.; Banks, C. E. Scientific Reports
2017, 7, 42233.
(5) Galán-Mascarós, J. R. ChemElectroChem
2015, 2 (1), 37–50.

Figure 1