Enhancing Water Electrolysis through Identification and Optimization of Real Active Sites via Electronic and Structural Modulation of Transition Metals Nanostructure
Developing efficient, selective, and durable catalysts for water electrolysis is critical for sustainable and renewable energy production. In recent years, Ni, Fe, and Co- based trimetallic systems have emerged as promising alternatives to the benchmark Ru and Ir-based catalysts due to their lower cost and relatively high abundance. However, the limited activity, selectivity, and durability of these trimetallic systems have hindered their practical applications. This is primarily due to the uncertainty regarding the identity of the actual active centers on the surfaces of these catalysts. Previous studies have reported conflicting results as to which are true active sites in these catalysts. Some studies identify Fe as the active center while others report Ni or Co or both. In addition, identifying the true active center is further complicated by the complex metal-metal interactions present in these trimetallic systems, which generally results in changing the electronic and surface structures of these catalysts. One of the most important challenges in developing efficient and durable catalysts for water electrolysis remains the identification of the real active sites in these catalysts and the modulation of their electronic and surface structures to expose and maximize the number of active sites on their topmost layers.
This study aims to identify the real active centers in these trimetallic systems and to develop a strategy for electrode assembly that enables the most active sites to be available on the topmost layer for efficient electrocatalysis. To achieve this goal, we introduced a layer-by-layer deposition strategy for the assembly of these electrodes, which allows for the modification of the electronic and surface structure of the resulting catalysts while maintaining control over their heterointerface chemistry. The designed strategy offers several unique advantages.
(1) It allows precise control over the fabrication process to realize any desired structure with predefined composition
(2) the layer-by-layer strategy helps in stabilizing Fe in 4+ valence state, which is crucial for efficient water electrolysis. In this configuration, Fe mimics the electron configuration of Ru in RuO2 .
(3) The well-defined structures that can be realized through this strategy simplify spectrochemical, electrochemical, and theoretical DFT studies, which helped in identifying Fe as a real active center in these trimetallic catalysts. Co and Ni were found to act as promoters that help maintain Fe in its high valence state. This novel strategy is different from the traditional synthetic methods, where the top layer is made up of all three metals in equal proportions, making these electrocatalysts less efficient in utilizing active sites. Lastly, the layer-by-layer strategy, where Fe active sites were anchored on a Co surface followed by Ni nanostructure, offers a very strong interfacial interaction that not only enhanced the activity of these active sites but also helped increase their long-term stability as well. The catalysts realized through this route offer much lower charge transfer resistance at the electrode-electrolyte interface with relatively low input voltage to deliver a commercial-level current density of 500 mAcm2
. The catalysts prepared through this novel strategy sustained a high current density for an extended period of over 90 h without noticeable degradation. Spectroscopic and theoretical investigations demonstrated that this novel assembling process not only enhanced the activity of resulting electrodes but also changed the thermodynamics of reaction pathways during the water electrolysis process. As a result, the electrodes synthesized in this study meet the requirements desired for commercial water electrolysis technology. In addition, the layer-by-layer assembly strategy proposed in this thesis is equally applicable to designing heterogeneous electrodes that require maximum exposure of active sites with minimum metal dosage for different applications, including CO2 reduction, methanol oxidation reaction, etc.
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