High-performance anion exchange membrane water electrolyzer enabled by highly active oxygen evolution reaction electrocatalysts: Synergistic effect of doping and heterostructure

Highlights

• The Ce doping improves the OER activity of CoFe LDH by modifying its electronic structure.

• The CeO₂/CoFeCe-LDH improves the OER activity by creating a new active interface between CeO₂ and CoFeCe-LDH.

• The AEM electrolyzer catalyzed by CeO₂/CoFeCe-LDH outperformed the electrolyzer with an IrO₂ benchmark electrocatalyst.

Abstract

Anion exchange membrane water electrolyzers (AEM electrolyzers) have been demonstrated for efficient green hydrogen production. The primary advantage of AEM electrolyzers is replacing the noble metal catalysts with low-cost platinum group metal (PGM)-free catalysts. Despite some astonishing demonstrations of high-performance AEM electrolyzers [1], [2], [3], [4], most of them suffer from lower energy efficiency due to the poor catalytic activity of PGM-free catalysts. Herein, to improve the performance of AEM electrolyzers, we developed a CeO₂/CoFeCe-layered double hydroxide (LDH) heterostructure as the OER catalyst. The Ce doping modulates the electronic structures of CoFe-LDH while creating a CeO₂/CoFeCe LDH interface, which dramatically improves the OER activity. The Ce doping synergizes with the CeO₂/CoFeCe LDH interface to enable a high-performance AEM electrolyzer, achieving an exceptional current density of 2.7 A/cm² at 1.8 V_cell. This work validates that AEM electrolyzers equipped with PGM-free OER catalysts can achieve comparable energy efficiency with proton exchange membrane water electrolyzers (PEM electrolyzers).

Introduction

Hydrogen is attracting attention as it is completely carbon-free and exhibits high gravimetric energy density. Among all hydrogen production methods, water electrolysis using renewable electricity is one of the most promising technologies for clean hydrogen production [2], [5], [6].

The traditional water electrolysis technology is the alkaline water electrolyzer (AWE) [2], [5], [7], [8]. AWEs typically operate at ~80 °C using a concentrated basic liquid electrolyte (KOH or NaOH solution). However, this highly corrosive liquid electrolyte poses extensive challenges, including potential leakage, high capital cost, and costly maintenance. Furthermore, AWEs are efficient when operated at a current density ranging from 100 to 400 mA/cm², limiting the hydrogen yield and leading to high hydrogen production costs. To overcome these challenges, proton exchange membrane water electrolyzers (PEM electrolyzers) that employ a locally acidic solid ionomer membrane as the electrolyte have been developed and commercialized [2], [5], [9]. Utilizing a thin and solid ionomer membrane significantly lowers the ohmic loss, enabling a current density over 2.0 A/cm² at 1.8 V_cell [10]. However, the acidic aqueous media necessitates the use of noble metal-based electrocatalysts. Additionally, the components that constitute the full PEM electrolyzers, such as bipolar plates and endplates, should possess high corrosion resistance [11].

Fortunately, anion exchange membrane water electrolyzers (AEM electrolyzers) combine the advantages of both AWEs and PEM electrolyzers [2], [5], [12], [13]. AEM electrolyzers employ a locally basic solid-state membrane as the electrolyte, which greatly reduces the Ohmic resistance and enhances the electrolysis current density. Additionally, AEM electrolyzers operate in a diluted alkaline solution (e.g., 1 M KOH solution), enabling the use of PGM-free electrocatalysts [12], [14], [15]. Although the capital cost of AEM electrolyzers can be reduced by using PGM-free electrocatalysts, the performance of AEM electrolyzers is still much lower than that of PEM electrolyzers, which is due to the poor electrocatalytic activity at the anode. To facilitate the scaleup and commercialization of AEM electrolyzers, the performance of AEM electrolyzers must be improved. It is expected that AEM electrolyzers, which can deliver a current density of > 2.5 A/cm² at 1.8 V, will achieve comparable energy efficiency with PEM electrolyzers.

Ohmic loss of AEM electrolyzers has been primarily addressed by developing a highly conductive anion exchange membrane. The electrode losses, especially anode losses, dominate the electrode performance due to the sluggish kinetics of oxygen evolution reaction (OER, 4OH^- → O₂ + 2H₂O + 4e^-) [16]. Therefore, improvement of the AEM electrolyzer performance hinges on a high-performing anode. At the AEM electrolyzer single-cell level, the OER electrocatalytic activity, electronic conductivity, and ionic conductivity of the anode are essential for AEM water electrolysis performance [17], [18]. Improving the OER electrocatalytic activity reduces the activation loss of the AEM electrolyzer attributed to the anode [3], [19].

Moreover, an enhanced electronic and ionic conductivity of the anode catalyst layer accelerates the charge transfer at the anode to further reduce the anode loss while reducing the ohmic loss. Prior research on OER electrocatalysts has mainly focused on studying the OER activity using a three-electrode setup without considering its electrical conductivity. For example, the OER electrocatalyst is generally mixed with carbon to enhance the conductivity, which is subsequently coated on glassy carbon or carbon cloth to evaluate its performance. Unfortunately, a composite of OER electrocatalyst and carbon cannot be used as the anode for AEM electrolyzers as carbon can be easily oxidized. Therefore, the electrical conductivity of the OER electrocatalyst itself is essential for high-performance AEM electrolyzers. Similarly, enhancing the ionic conductivity of the OER electrocatalyst also reduces the ohmic loss while simultaneously facilitating the OER kinetics [2], [17].

The transition metal-based layered double hydroxide (LDH) is one of the best OER electrocatalysts [20]. The performance of AEM electrolyzers primarily depends on their ohmic losses, electrode activation losses, and mass transport losses. The electrode activation losses, especially anode activation loss, typically dominate the electrode performance, which can be minimized by employing highly active OER electrocatalysts. Consequently, the AEM electrolyzer energy efficiency can be improved [3], [19]. Recently, numerous high-performance OER electrocatalysts for alkaline water electrolysis have been developed. In particular, CoFe-LDH and NiFe-LDH display exceptional OER activities compared to other electrocatalysts, which are attributed to their unique coordination environment and synergistic interaction between metal ions [21]; thus, the anode losses of AEM electrolyzers can be dramatically reduced if CoFe-LDH or NiFe-LDH is applied as the anode of AEM electrolyzers. However, the LDH OER electrocatalysts have relatively low electronic conductivity, a chronic problem of LDH electrocatalysts, which exacerbates the ohmic loss and limits the OER activity [22], [23], [24]. That said, the AEM electrolyzer performance could be further improved if the conductivity of LDH can be enhanced. The challenge associated with the low electronic conductivity of LDH can be tackled by modifying its electronic structure. The most straightforward method is to dope the LDH matrix with a third element [25], [26]. For example, it has been recognized that cerium (Ce)-doping can enhance electrical conductivity because it exhibits two oxidation states (Ce³⁺/ Ce⁴⁺) [27]. It is expected the Ce³⁺/ Ce⁴⁺ redox couple in LDH lattice can also improve the conductivity of LDH [28], [29], [30], [31]. Additionally, cerium oxide (CeO₂) allows reversible surface oxygen ion exchange. We hypothesize that developing a composite of LDH and CeO₂ will form an interface between these two phases, which could alter the RDS (i.e., rate-determining step) of OER, thereby improving the intrinsic catalytic activity [30], [32].

This work has developed a series of Ce-doped LDH (CoFeCe-LDH) and CeO₂/CoFeCe-LDH (CoFeCe^0.5) heterostructures for OER on the anodes of AEM electrolyzers. Ternary CoFeCe-LDH (CoFeCe^0.1) where Ce is doped into CoFe-LDH lattice displays enhanced OER activity via electronic structure modifications. An excessive Ce doping leads to the self-assembled heterostructured CeO₂/CoFeCe-LDH (CoFeCe^0.5) electrocatalyst, which generates an active interface between CeO₂ and CoFeCe-LDH, further enhancing the OER activity and improving turnover frequency. The AEM electrolyzers equipped with CoFeCe^0.5 OER electrocatalyst show both reduced ohmic losses and activation losses and achieve an outstanding current density of 2.7 A/cm² at 1.8 V_cell, which outperforms the AEM electrolyzers with IrO₂ OER electrocatalyst and presents comparable performance with PEM electrolyzers. Thus, the remarkable water electrolysis performance of AEM electrolyzers with PGM-free OER electrocatalysts, > 150 h of stability testing, and this low-cost and scalable OER electrocatalysts synthesis approach reported here represent an unprecedented contribution to the field and convincingly establishes the promise of this technology and its potential for clean hydrogen production at the commercial scale.