(Digital Presentation) Nickel-Iron Electrocatalysts Modified with Group 11 Metals Achieving 1 A cm−2 of Oxygen Evolution in Buffered Near-Neutral pH Electrolyte
Authors: Nishimoto Takeshi , Shinagawa Tatsuya , Takanabe Kazuhiro 
Published: July 2022

Electrocatalytic processes driven by the renewable electricity will play a pivotal role to achieve sustainable in our society, whereby the thermodynamically stable chemicals are converted into value-added products or energy carriers. For instance, the water electrolysis produces green hydrogen, and the carbon dioxide electrolysis yields commodity chemicals such as ethylene or carbon monoxide.[1] These processes commonly share an anodic half-reaction of oxygen evolution reaction (OER) that requires large overpotentials due to its slow kinetics, leading to the significant loss of overall energy efficiency.[2] This is particularly the case at near-neutral pH,[3] which however is likely the desired condition for electrocatalytic CO2 reduction due to the lessened loss of carbon dioxide via carbonate formation that prevails in alkaline conditions.[4] Toward the large-scale operation of these technologies, it is highly desired to develop an active, stable, and earth-abundant metal based electrocatalyst that catalyzes the OER at near-neutral pH and high current densities.

The present study reports on our discovery of the transition metal-based electrocatalysts that efficiently catalyze OER in carbonate buffer electrolyte at near-neutral pH. Firstly, a variety of electrodes were fabricated by electro-deposition of transition metals (manganese, iron, cobalt, copper) on electrochemically activated Ni (ECA-Ni) substrates[5] with nanostructured surface. Their electrocatalytic testing revealed that iron oxide (Fe-O) modified ECA-Ni achieved a current density of 100 mA cm−2 at an overpotential of ca. 280 mV in dense electrolyte of 1.5 mol kg−1 K-carbonate solution and 353 K, whose pH was adjusted to pH 10.5 at 298 K prior to the testing. This pH level was essential to achieve stable operation using the Ni-Fe electrode. Subsequently, group 11 metals of copper, silver, or gold were introduced into Fe-O/ECA-Ni via co-electrodeposition to tailor the nature of active site for improved OER. Remarkably, electrodes of Fe-Cu-O/ECA-Ni and Fe-Au-O/ECA-Ni catalyzed the OER at a rate of 1 A cm−2 and an overpotential of ca. 330 mV, whose figure is comparable to those in extremely alkaline conditions (Figure 1). Long-term and on-off stability testing revealed that the developed electrodes maintained its performance. Our characterization on double-layer capacitance indicated the enlarged surface area of Fe-Cu-O and Fe-Au-O electrodes with respect to the pristine Fe-O counterparts, which partly contributed to the improved OER performance. In addition, ex situ X-ray photoelectron spectroscopy and in situ X-ray absorption spectroscopy concurrently pointed to the presence of stable Fe(III) species for Fe-Cu-O/ECA-Ni, plausibly FeOOH. The present study discovered transition metal based electrocatalysts for the OER at near-neutral pH and high current densities, achieving comparable performance to those in alkaline conditions, which is significant given the merits of near-neutral pH condition for CO2 reduction. These findings represent the potentiality of near-neutral pH electrochemical system on industrial scale, which can help to construct a sustainable society.

Reference

[1] S. Chu, A. Majumdar, Nature
2012, 488, 294.

[2] T. Reier, H. N. Nong, D. Teschner, R. Schlögl, P. Strasser, Adv. Energy Mater.
2017, 7, 1601275.

[3] T. Nishimoto, T. Shinagawa, T. Naito, K. Takanabe, ChemSusChem
2021, 14, 1554.

[4] J. A. Rabinowitz, M. W. Kanan, Nat. Commun.
2020, 11, 5231.

[5] T. Shinagawa, M. T.-K. Ng, K. Takanabe, Angew. Chem. Int. Ed.
2017, 56, 5061.

Figure 1