There is an ever increasing demand for renewable and non-carbonaceous clean energy sources to fulfill massive global energy demand¹. Hydrogen (H₂) as a fuel is clearly the frontrunner due to its non-carbonaceous nature and superior energy density than the carbonaceous energy sources¹. Acid assisted proton exchange membrane (PEM) based water electrolysis is promisingly at the forefront among all other conventional hydrogen production methods. Commercial development of PEM water electrolyzer has been stymied due to requirement of highly expensive and scarce noble metal based electro-catalysts such as Pt, RuO₂, IrO₂; which exhibit excellent oxygen evolution reaction (OER) activity in PEM water electrolysis². Therefore, identification and development of ultra-low noble metal containing electro-catalyst system which can exhibit superior electrochemical performance than state of the art-noble metals based electro-catalysts in highly aggressive acidic media for OER, will indeed lead to significant reduction in capital costs of the water splitting process². In the pursuit of this objective, based on theoretical first principles calculations of the total energies and electronic structures, the present authors have identified fluorine (F) doped transition metal oxide (TMO) based solid solution electro-catalyst with ultra-low noble metal for OER in PEM water electrolysis. The F-doped synthesized TMO in two dimensional (2D) thin film morphology exhibits electro-catalytic performance (i.e. overpotential and electro-catalytic activity) similar to that of IrO₂ thin film electro-catalyst. Therefore, aiming to further improve the reaction kinetics and also achieve superior electro-catalytic activity, tailoring of the 2D material length scale to 1D vertically aligned nanotubular (VANT) architecture has been executed. Over the past few years, electro-catalysts with 1D nanostructured morphologies such as nanowires (NWs) as well as nanotubes (NTs) have garnered significant attention as a potentially effective materials for water splitting due their inherent benefits such as high electro-catalytic surface area, high aspect ratios (length-to-width ratio) and facile electron transport though 1D nanotubular arrays³. Therefore, in order to enhance the electrochemical performance of as prepared 2D thin film electro-catalyst i.e. F-doped TMO for OER, we have explored 1D nanotube (NT) structured-morphology; retaining similar electro-catalyst composition. In this study, a sacrificial template-assisted approach has been utilized to grow VANTs on titanium (Ti) substrate. Fig. 1 displays the SEM micrograph showing top view, cross-sectional view of F-doped TMO NTs. The electrochemical characterization of synthesized electro-catalysts has been carried out in three-electrode configuration using 1N sulfuric acid (H₂SO₄) solution as a proton source as well as the electrolyte, Pt wire as counter electrode and Hg/Hg₂SO₄ as the reference electrode (+0.65 V with respect to normal hydrogen electrode, NHE), with a scan rate of 10 mV/sec and at temperature of 40^oC. 1D VANTs of F-doped TMO grown on Ti substrate showed onset potential of ~1.43 vs NHE, similar to that of IrO₂ (total loading=0.3 mg/cm²) and lower charge transfer resistance (R_ct) than 2D thin films of F-doped TMO. The 1D F-doped TMO VANTs also exhibit remarkable ~2.3 and ~ 2.6 fold higher electro-catalytic OER activity (current density) than that of 2D thin film architectures of F-doped TMO and IrO₂ respectively. In addition, the chronoamperometry test conducted in 1N H₂SO₄ solution at ~1.5 V (vs NHE) for 24 hours shows minimal loss in current density demonstrating good electrochemical stability of the F-doped TMO VANTs, comparable to that of IrO₂2D thin films.

In summary, we have synthesized high performance 1D vertically aligned nanotubes of F-doped TMO OER electro-catalyst. The superior electrocatalytic activity of the F-doped TMO VANTs is attributed to the presence of vertical channels in 1D morphology, exhibiting higher electrochemical surface area (ECSA) than 2D thin film architectures. These encouraging results of 1D-F-doped TMO VANTs with ultra-low noble metal content and superior OER kinetics show potential of these systems for water electrolysis. The results of this study will be presented and discussed.

References:

1. Ball, M.; Wietschel, M. International Journal of Hydrogen Energy 2009, 34, (2), 615-627.

2. Patel, P. P.; Datta, M. K.; Velikokhatnyi, O. I.; Kuruba, R.; Damodaran, K.; Jampani, P.; Gattu, B.; Shanthi, P. M.; Damle, S. S.; Kumta, P. N. Scientific reports 2016, 6.

3. Liu, G.; Xu, J.; Wang, Y.; Wang, X. Journal of Materials Chemistry A 2015, 3, (41), 20791-20800.

Acknowledgements:

Financial support of NSF-CBET grant# 1511390, Edward R. Weidlein Chair Professorship funds and the Center for Complex Engineered Multifunctional Materials (CCEMM) is acknowledged.