Molybdenum‐based Nanocatalysts for CO Oxidation Reactions in Direct Alcohol Fuel Cells: A Critical Review

Abstract

Carbon monoxide (CO) oxidation is crucial in fuel cell anodes. Recent research has focused on electrocatalysts that synergistically enhance CO oxidation alongside alcohol/hydrogen oxidation. High sensitivity and selectivity for CO oxidation at lower onset potentials are the key objectives. Molybdenum (Mo) has emerged as a promising non‐noble transition metal co‐catalyst for CO oxidation. Mo versatility arises from its ability to alloy with Pt and mix with other non‐noble transition metals in various forms (MoOx, MoS2, MoC). Carbon‐supported Mo nanoparticles have shown potential in reducing Pt loading and improving CO tolerance due to Mo oxyphilic properties, effectively oxidizing weakly adsorbed CO and lowering onset and peak potentials. However, challenges persist, such as a limited potential window for CO oxidation and decreased CO adsorption affinity at higher potentials. Addressing these issues requires understanding factors affecting Mo‐based electrocatalysts (Mo‐ECs) activity, including PtMo alloy composition, Mo's chemical state, cell temperature, and the role of carbon support. This article provides a comprehensive review of Mo‐ECs role in CO electrooxidation in direct alcohol fuel cells (DAFCs) over the past two decades.

Assessing the Intrinsic Activity of Pt‐Group Electrocatalysts for Carbon Monoxide Oxidation: Best Practices and Benchmarking Parameters

“…The electrochemical CO Oxid on the surface of Pt‐group catalysts in or out of alcohol fuel cells is undeniably a complex process, so it is essential to decipher its pathway and mechanisms to improve the catalytic performance of such fuel cells and reduce the CO‐poisonous [22] . There are various proposed processes for the CO Oxid process on Pt‐group catalysts, but mainly four mechanisms are the most acceptable, including the ligand effect, electronic effect, bi‐functional effect, and hydrogen spillover effect, [6] which all start with CO adsorption, followed by diffusion, oxidation at Pt‐group active sites, and finally, CO 2 ‐desorption (Figure 2a). In the ligand effect, the metal interacts with CO molecules, decreasing the binding energy between CO and the Pt surface and thus easing the CO Oxid .…”

“…In the electronic effect, the metal alters the electronic structure by modulating the d‐band center of Pt. Meanwhile, in the bifunctional effect, the delocalized d‐orbital electrons on the metal surface induce the electron transfer to the Pt‐group [6,23] . In the hydrogen spillover, a hydrogen atom is dissociated from the Pt‐group surface and transferred to the metal sites, creating an interaction of CO−Mo bond, whereas the metal−H interaction decreases the bond between CO and metal, resulting in facilitating the CO oxidation on the Pt surface.…”

“…In the hydrogen spillover, a hydrogen atom is dissociated from the Pt‐group surface and transferred to the metal sites, creating an interaction of CO−Mo bond, whereas the metal−H interaction decreases the bond between CO and metal, resulting in facilitating the CO oxidation on the Pt surface. Various studies reported that Pt‐based electrocatalysts with a uniform dispersion of Pt tend to show a stronger spill effect [6,24] . These four processes are driven mainly by two factors: the first is driven by the oxophilic effect in the case of mixing Pt with other transition metals that induce the generation of OH species on the oxophilic sites of transition metals via the activation and dissociation of water in electrolytes (H 2 O+*⇌OH ads +H + +e − ) then OH is transferred to the Pt‐group to oxidize CO adsorbed on Pt (Pt−CO) [6] .…”

“…Pt‐group elements are the main anodes or cathodes for such fuel cells, but their high‐cost, earth rarity, and poisoning by CO are crucial barriers in the commercialization process [5] . Mainly, Pt‐group catalysts are easily poisoned and loose catalytic active sites by adsorption of small traces of adsorbed CO (i. e., 10–100 ppm), so the CO should be promptly removed from the cells via the electrochemical oxidation to allow the long‐term operation [6] . Thus, optimization and understanding the fundamental factors controlling the electrocatalytic CO Oxid on Pt‐based electrocatalysts is a crucial step toward large‐scale utilization of PEMFCs as green and efficient energy resources [7] .…”

“…Thus, enormous efforts were devoted to tolerating the CO‐tolerance on Pt‐group electrocatalysts for the oxidation of organic molecules (i. e., ethanol, methanol, and formic acid), which led to over ~104000 published articles according to; however, their CO Oxid activity was highlighted in ~230 articles (Figure 1a). Thus, this research arena was comprehensively reviewed in various excellent reviews on Pt‐group catalysts for electrocatalytic CO Oxid and others, but not on assessment Factors [6,16] . Thereby, it is urgent to better understand the inherent CO Oxid performance of Pt‐group electrocatalysts, besides precise assessments of parameters for analyzing and optimizing the catalytic performance [17]…”