Elsevier

Journal of Power Sources

Volume 556, 1 February 2023, 232484
Journal of Power Sources

La–Sr–Co oxide catalysts for oxygen evolution reaction in anion exchange membrane water electrolyzer: The role of electrode fabrication on performance and durability

https://doi.org/10.1016/j.jpowsour.2022.232484Get rights and content

Highlights

  • La–Sr–Co oxide tested as catalyst for oxygen evolution reaction in alkaline media.

  • Addition of Nafion binder in catalyst ink prevents catalyst detachment in DI water.

  • Ionomer-to catalyst ratio impacts performance and durability in DI water.

  • Lower impact of electrode fabrication on performance with supporting electrolyte.

Abstract

Anion exchange membrane water electrolysis is an attractive technology for low-cost generation of “green” hydrogen by combining the use of noble metal-free catalysts with pure water feed. By thus addressing main drawbacks of the liquid alkaline electrolysis and proton exchange membrane water electrolysis, anion exchange membrane water electrolysis stands an excellent chance of replacing the two technologies. The development of active and stable platinum group metal (PGM)-free catalysts for oxygen evolution reaction (OER) is crucial for making anion exchange membrane water electrolyzers (AEMWEs) practical. Here, we synthesized, characterized and tested two La–Sr–Co oxide-based OER catalysts. First, we characterized the catalysts by XRD, SEM, and N2 physisorption and assessed their OER activity in a three-electrode cell. Next, we focused on electrode fabrication, demonstrating the importance of catalyst-ink application to the porous transport layers (PTLs) and a key role of adding a binder to the catalyst ink to prevent the catalyst detachment from the PTL in pure water. We tested three membrane electrode assemblies prepared using different formulations of the anode catalyst ink. The results show that the optimum ink formulation is essential for the performance on pure-water feed by maximizing OH conductivity of the catalyst layer and catalyst-membrane interface.

Introduction

Limiting greenhouse gas emissions has become one of the most stringent goals for many governments across the world as the key means of mitigating the detrimental impact of climate change [1]. Generating electricity from renewable sources, e.g., wind, solar, is one of the most promising ways to reduce such emissions [2]. However, renewable energy sources are generally intermittent and not available with the same intensity everywhere in the world. This increases the importance of storage for accumulating the energy when not immediately used and its distribution from the generation to the utilization sites [3].

Hydrogen (H2) is an energy vector which becomes strategic in this scenario since it can be generated using the excess electricity, stored, distributed (e.g., using existing natural gas distribution pipeline), and used to produce electricity on demand [4].

H2 is also used by industry for refining petroleum, treating metals, producing fertilizers (ammonia), and processing foods [5]. Currently, a major part of the H2 is produced from steam reforming of natural gas, with its CO2 byproduct released to the atmosphere. Only a minor part is produced via water electrolysis, which does not involve any CO2 emissions if the electricity is generated from renewable sources (“green” H2) [6]. Therefore, to accomplish the goal of reducing the CO2 emissions, a great impulse to H2 generation via water electrolysis is needed [7]. Low-temperature water electrolysis (LTWE) systems are particularly advantageous because they can operate under close-to-ambient conditions, enabling fast start-up and shut-down, and on a relatively small scale, if needed (e.g., residential and portable devices) [4,7,8]. The major LTWE technologies currently on the market are liquid alkaline water electrolyzers (LAWE) and proton exchange membrane water electrolyzers (PEMWE). LAWEs have the advantage of being cheaper, not needing a solid polymer membrane and enabling the use of non-noble catalysts. However, they operate at relatively low current density (ca. 0.5 A cm−2), with highly concentrated alkali solutions (20–40% KOH) and therefore are more subject to components corrosion. On the other hand, PEMWE operate with pure water and can reach higher current densities (>2 A cm−2), but they need expensive platinum group metal (PGM) catalysts at both electrodes [[8], [9], [10], [11]]. In fact, expensive and rare Ir (in the form of IrO2) is used as anode catalyst in PEMWEs, which is currently a major hindrance to their widespread commercialization [[12], [13], [14]].

After years of successful improvements to performance and stability of anion exchange membranes (AEMs) at research and development level, several commercial grades of AEMs and anion exchange ionomers (AEIs) are currently available [15,16]. Although improvements are still needed, the availability of these important components is paving the way for anion exchange membrane water electrolyzer (AEMWE) as a promising alternative to the LAWE and PEMWE, potentially allowing for combining the use of PGM-free catalysts in both electrodes and pure-water operation [8,10,17]. Taking advantage of their higher stability at high pH, several PGM-free transition metals and transition metal oxides have been explored as catalysts for oxygen evolution reaction (OER) in alkaline media. Some of these materials have shown catalytic activity comparable or even higher than IrO2, the benchmark PGM catalyst for OER in acidic media [[18], [19], [20]]. Mixed transition-metal oxides containing Co, Fe, or another 3d transition metal, plus a lanthanide and an alkaline earth metal have been explored as OER catalysts in three-electrode cells, showing promising performance [[21], [22], [23], [24]]. However, works reporting their application in the AEMWE anode are less common, especially with pure water, although a few examples do exist [25,26]. Based on recent AEM water electrolysis publications, it is becoming clear that the electrolyzer performance strongly depends on the electrode fabrication, which involves several variables in addition to catalyst itself. Such variables are for example: ionomer type and content, catalyst ink deposition method, type of porous transport layer (PTL), just to name a few [9,[27], [28], [29], [30], [31]].

In this work, we investigated the performance of a LaxSr1-xCoO3-δ oxide (hereinafter defined as LSC) as the anode AEMWE catalyst. We focused on elucidating the impact that the ink composition and deposition method have on the catalyst layer (CL) stability, i.e., catalyst detachment, and on the performance and durability of the AEMWE, highlighting the differences between the AEMWE operated with pure water versus supporting electrolyte solution.

Section snippets

Materials synthesis

Two different LSC catalysts were investigated in this work. The first one was synthesized at Los Alamos National Laboratory. The precursor metal salts used in the catalyst synthesis were La(NO3)3∙6H2O; Sr(NO3)2 (Sigma Aldrich), and Co(NO3)2∙6H2O (Acros Organics). The salt amount used in the synthesis was to assure x = 0.85 in LaxSr1-xCoO3-δ in the original precursor solution. The metal salts were initially dissolved in deionized (DI) water. After complete dissolution, the water was evaporated

Results and discussion

The two LSC catalysts examined in this work were characterized using different physicochemical techniques to confirm their chemical composition and determine their morphology and porous structure. The crystalline structure of the catalysts was determined by XRD (Fig. 1a). Diffractograms for both LSC-LANL and LSC-PP-OER49B show peaks at the same angles and with similar peak widths, pointing to similar crystal composition and size. Per XRD instrument software, the XRD peak positions correspond to

Conclusions

We have synthesized and characterized La–Sr–Co oxides as OER catalysts for the alkaline electrolyze anode. The OER activity of the catalysts was determined in an RDE cell and in a liquid electrolyzer with three differently fabricated AEMWE anodes. We demonstrated how the method of ink deposition is key to obtaining a uniform catalyst layer coating on the porous transport layer, avoiding catalyst ink seeping, and ensuring an optimal catalyst-membrane interface. We also showed that addition of

CRediT authorship contribution statement

Luigi Osmieri: Writing – original draft, Conceptualization, Methodology, Investigation, Formal analysis, Validation, Visualization. Yanghua He: Investigation, Validation. Hoon T. Chung: Resources. Geoffrey McCool: Resources. Barr Zulevi: Resources. David A. Cullen: Investigation, Formal analysis, Visualization, Funding acquisition. Piotr Zelenay: Writing – review & editing, Conceptualization, Supervision, Project administration, Funding acquisition.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

Funding was provided by US DOE Office of Energy Efficiency and Renewable Energy, Hydrogen and Fuel Cell Technologies Office, under the Electrocatalysis Consortium (ElectroCat), Dr. Dimitrios Papageorgopolous and Dr. David Peterson, Technology Managers. Research presented in this article was also supported by the Laboratory Directed Research and Development program of Los Alamos National Laboratory under project number 20210953PRD3. This work was authored in part by Los Alamos National

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