(Invited) Electrolyte Engineering for Disruptive Cost Reduction of Electrolyzer: Hydrogen Generation from Dense Carbonate Buffer Solution

Abstract

Water electrolysis using renewable electricity is a key process for generating green hydrogen, a crucial element in the pursuit of a sustainable society. To make substantial strides in lowering the cost of green hydrogen, it is imperative to address not only the expense of the electricity involved but also the cost associated with the electrolyzer itself [1]. The current challenge lies in the materials chosen for electrolyzers operating under extreme acidic or alkaline conditions, which necessitate the use of costly corrosion-resistant components. A viable solution involves considering non-extreme pH water as an electrolyte, allowing for the utilization of affordable and readily available materials, such as iron or stainless steel, in key electrolyzer components like the frame or bipolar plate [2]. Despite the reported inferior performance of non-extreme pH electrolysis compared to extreme pH conditions, primarily due to inevitable pH shifts (resulting in concentration overpotential) and elevated solution resistance, advancements in technology and research hold the potential to overcome these challenges.Our research group has been actively engaged in the exploration of electrolyte engineering, a concept centered on optimizing the diffusion flux of buffer ions to mitigate the previously mentioned challenges in non-extreme pH conditions [3,4]. Our approach involves leveraging a dense buffer solution to maximize the flux of buffer ions, resulting in a significant reduction in concentration overpotential. This strategy becomes even more impactful when the operating temperature is elevated to the commercially viable range of 80-100 °C [5]. The synergistic effect of employing a dense buffer solution, combined with elevated temperatures, proves to be a promising avenue for advancing electrolyte engineering and optimizing the performance of electrolysis systems in non-extreme pH conditions. In this presentation, we will delve into our discoveries concerning non-noble metal electrocatalysts, specifically focusing on their efficacy in both the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER).Our research has led us to identify transition metal-based electrocatalysts that demonstrate exceptional efficiency in catalyzing OER within a carbonate buffer electrolyte at near-neutral pH [6]. Among the diverse materials we explored, the iron-nickel composite, coupled with copper or gold on electrochemically activated Ni (ECA-Ni) substrates [7], stands out as a superior performance. Notably, these electrodes exhibited a remarkable OER catalysis rate of 1 A cm−2 and an overpotential of approximately 330 mV, becoming competitive with highly alkaline counterpart.Next, our research has uncovered noteworthy electrocatalysts based on copper and molybdenum, demonstrating exceptional efficiency in catalyzing the hydrogen evolution reaction (HER) within a carbonate buffer electrolyte at pH 10.5 [8]. The most promising sample achieved a current density of 1 A cm−2 at an overpotential of approximately 0.2 V, a performance comparable to previously reported HER results in highly alkaline conditions. Detailed characterization of these electrocatalysts unveiled that the addition of copper resulted in a rougher surface structure, contributing to the largest double-layer capacitance. These characteristics are indicative of an enlarged active surface area, which is pivotal for enhanced catalytic activity. Furthermore, the introduction of copper element correlated with a decreased apparent Tafel slope, suggesting a tailored nature of the active sites. These insights not only shed light on the structural modifications induced by copper but also underscore its role in optimizing the electrocatalytic performance.Finally, we integrated the previously developed HER and OER catalysts with a carefully selected polyether sulfone (PES) diaphragm [8]. This strategic combination resulted in a notable enhancement of overall cell performance within a 3.0 mol kg− 1 K-carbonate solution at pH 10.5 and a temperature of 353 K. The resultant cell performance, factoring in the series resistance, achieved a 1 A cm− 2 at a voltage of approximately 2.0 V. This achievement stands competitively against the top-class commercial alkaline electrolyzers. These findings not only highlight the feasibility of non-extreme pH electrochemical electrolyzers in industrial applications but also underscore the potential for further advancements and optimizations. The demonstrated success serves as a compelling indicator of the viability of our approach and opens avenues for refining non-extreme pH electrolysis for even greater efficiency and applicability in industrial settings.Reference[1] M. Chatenet, et al., Chem. Soc. Rev. 2022, 51, 4583.[2] M. Pourbaix, “Atlas of Electrochemical Equilibria in Aqueous solutions”, 1974.[3] T. Shinagawa and K. Takanabe, ChemSusChem 2017, 10, 1318.[4] T. Shinagawa et al., ChemCatChem 2019, 11, 5961.[5] T. Naito, et al., ChemSusChem 2022, 8, e202102294.[6] T. Nishimoto, et al., ChemSusChem 2022, e202201808.[7] T. Shinagawa, et al., Angew. Chem. Int. Ed. 2017, 56, 5061.[8] T. Nishimoto, et al., ACS Catal. 2023, 13, 14725.