Rapid Outgassing of Hydrophilic TiO2 Electrodes Achieves Long-Term Stability of Anion Exchange Membrane Water Electrolyzers

Abstract

The state-of-the-art anion-exchange membrane water electrolyzers (AEMWEs) require highly stable electrodes for prolonged operation. The stability of the electrode is closely linked to the effective evacuation of H2 or O2 gas generated from electrode surface during the electrolysis. In this study, we prepared a super-hydrophilic electrode by depositing porous nickel–iron nanoparticles on annealed TiO2 nanotubes (NiFe/ATNT) for rapid outgassing of such nonpolar gases. The super-hydrophilic NiFe/ATNT electrode exhibited an overpotential of 235 mV at 10 mA cm−2 for oxygen evolution reaction in 1.0 M KOH solution, and was utilized as the anode in the AEMWE to achieve a current density of 1.67 A cm−2 at 1.80 V. In addition, the AEMWE with NiFe/ATNT electrode, which enables effective outgassing, showed record stability for 1500 h at 0.50 A cm−2 under harsh temperature conditions of 80 ± 3 °C.

1. Introduction

Water electrolyzer is a well-established device that utilizes renewable energy sources to produce green H2 [1–7]. Conventional devices include alkaline water electrolyzer (AWE) and proton-exchange membrane water electrolyzer (PEMWE), but AWE suffers from low current density and use of about 40% concentrated KOH solutions, while PEMWE has barriers to megawatt-scale production of H2 due to the high cost of electrocatalysts and anti-corrosive components [8, 9]. In recent years, researchers have developed an anion-exchange membrane water electrolyzer (AEMWE) to balance performance and cost [1, 3, 10, 11]. However, achieving high efficiency of hydrogen evolution reaction at the cathode and oxygen evolution reaction (OER) at the anode remain challenges, thus the development of cost-saving electrocatalysts has been extensively investigated [3, 12–20].

Meanwhile, electrocatalyst development alone is limited to improving the performance of electrodes in water electrolyzers [21]. One of the major obstacles in electrode technology that reduces the electrolyzer performance is the accumulation of nonpolar gas bubbles, such as H2 and O2, on the electrode surface, which blocks the active sites of the catalysts [22–25]. To reduce the bubble adhesion to the electrode surface, electrocatalysts with super-hydrophilic or super-aerophobic surface have been utilized [3, 24–26]. For instance, Wang and coworkers utilized a super-hydrophilic Fe–Ni–P–S catalyst loaded on nickel foam (NF) support as the OER electrode in AEMWE to accelerate the release of O2 bubbles from the electrode surface [24]. However, since the generally utilized electrode materials, such as NF, titanium felt (TF), and carbon paper, were hydrophobic, gas bubbles accumulated on the electrode, resulting in particle detachment/agglomeration during the reaction [1]. Therefore, electrode development strategies that combine the advantages of hydrophilicity and effective gas evacuation are essential to advance the long-term operation of electrolyzers. To achieve this, recent researchers have investigated on hydrophilic electrodes with distinct structures, like nanotubes, rather than flat surfaces [27, 28]. The curvature and vertical orientation of nanotubes can reduce the bubble contact area with the electrode, promoting smaller bubble formation and quicker detachment.

This work aims to present super-hydrophilic electrode surface structures that allow O2 bubbles to escape efficiently, and to improve OER performances by including electrode materials with high surface area, and enhanced stability. To offer all these advantages, we prepared an annealed TiO2 nanotubes (ATNT) with a super-hydrophilic and rough surface structure as a catalyst support by anodizing and annealing of T