http://pubs.acs.org/journal/aelccp
Rational Bottom-Up Engineering of Electrocatalysts by Atomic Layer Deposition: A Case Study of FexCo1−xSy‑Based Catalysts for Electrochemical Hydrogen Evolution Wei Xiong, Zheng Guo, Hao Li, Ran Zhao, and Xinwei Wang* School of Advanced Materials, Shenzhen Graduate School, Peking University, Shenzhen 518055, China S Supporting Information *
ABSTRACT: A rational bottom-up engineering strategy for efficient electrocatalysts based on atomic layer deposition (ALD) is reported. The strategy involves compositional optimization of surface catalyst material by ALD for high specific activity and geometric optimization of electrode structure for high surface area. The two optimizations are decoupled herein, because the conformal ALD ensures that the coating of the catalyst does not depend on the substrate geometry. To demonstrate this strategy, we choose ternary FexCo1−xSy compound as the catalyst for electrochemical hydrogen evolution reaction (HER). Compositional optimization shows that Fe0.54Co0.46S0.92 is the best composition for high specific HER activity, and it is therefore chosen to be conformally coated by ALD on a high-surface-area CNTs/CC (carbon nanotubes on carbon cloth) electrode. The synthesized Fe0.54Co0.46S0.92/CNTs/CC electrode exhibits a fairly low HER overpotential of −70 mV for achieving −10 mA/cm2 in current density in alkaline solution, which demonstrates the effectiveness of this ALD-based engineering strategy. metal oxides,11,12 sulfides,13−25 selenides,26−28 and phosphides29−32 with various elemental compositions, crystal structures, and nanoshapes (e.g., nanosheets, nanowires, nanotubes). Although these synthesized nanomaterials have shown promising activities in electrocatalysis, further improvements in their activities and the associated synthesis techniques are still largely in need.4 Theoretically, for an efficient electrocatalyst material, the bonding strengths of the reaction intermediates on its surface should be neither too high nor too low,4,33 because insufficient bonding strengths would cause difficulty in adsorbing the intermediates, whereas overstrong bonding strengths would lead to the intermediates being reluctant to leave and ultimately poisoning the catalyst surface. Therefore, the catalyst specific activity follows a volcano-shaped relation with respect to the free energy of intermediate adsorption. Pt, for instance, is naturally at the top of the volcano plot for HER4 and thus has been conventionally chosen as the HER catalyst. A different strategy to reach the top activity in the volcano plot is to alloy two compounds which are separately located at two sides of the
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eveloping low-cost efficient electrocatalysts is a core task in many electrochemical energy conversion and storage systems, because a large portion of the electrochemical reactions therein suffer from considerable kinetic barriers that require efficient catalysts to facilitate the reactions.1−4 A typical example is the electrochemical process of water splitting (electrolysis), which is highly useful in practice to produce H2, a clean and eco-friendly fuel in modern energy technology.5 The electrolysis of water involves two half-cell reactions, i.e., the cathodic hydrogen evolution reaction (HER) and the anodic oxygen evolution reaction (OER), and both of the reactions are kinetically sluggish and thus need catalysts.4,6−8 Conventional catalysts for these reactions are noble metals and their compounds (e.g., Pt, IrO2, and RuO2),6−8 but their high cost is a significant limitation for their large-scale commercialization. Therefore, recent attention has been diverted toward the catalysts based on nonprecious metals, such as Fe, Co, Ni, Mo, W, etc.6,9 On the other hand, because an electrocatalytic process is mostly a surface process, and from application viewpoint, the catalytic activity is often gauged with normalization to the geometric area of the electrode, nanostructuring of the catalyst materials to create a microscopically rough surface is an effective strategy to boost the overall catalytic activity.2,10 Therefore, great efforts have been made toward the synthesis of nanostructured nonprecious-metal-based catalysts, such as (binary/ternary) © XXXX American Chemical Society
Received: October 26, 2017 Accepted: November 7, 2017
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DOI: 10.1021/acsenergylett.7b01056 ACS Energy Lett. 2017, 2, 2778−2785
Letter
Cite This: ACS Energy Lett. 2017, 2, 2778-2785
Letter
ACS Energy Letters Scheme 1. Schematic Illustration of the ALD-Based Bottom-Up Engineering Strategy for an Electrocatalysta
a The strategy involves two separate optimization steps, i.e., the compositional optimization of ternary FexCo1−xSy by ALD for high specific activity and the geometric optimization of the electrode structure via nanostructuring for high surface area.
geometric engineering of the support aims for large surface area, and it can be achieved by synthesizing a multiscale rough or porous structure (e.g., creating nanostructures on a mesoscopically rough electrode). The compositional engineering of the surface catalyst, on the other hand, aims for high specific activity, and it can be well accomplished by ALD, as ALD allows for convenient atomic-level engineering of material composition by straightforwardly using the ALD precursors for the target elements. In particular, many ternary (or even quaternary) compounds have been successfully synthesized by ALD with tunable elemental compositions.41,44−47 Also, the process temperature for ALD is generally low (usually
