Major research efforts have been made to explore the degradation mechanisms and to improve the stability of electrocatalysts. Pt nanoparticles (NP) supported on carbon (Pt|C) have been a frequent subject of electrocatalyst stability studies due to the common use of this catalyst|support combination in commercial fuel cells and electrolyzers.69–73 Degradation of Pt|C electrocatalysts can depend strongly on the operating conditions (temperature, potential cycling, current density, electrolyte pH) and the nature of the catalysts.69,71,74 Several degradation mechanisms of supported Pt NPs have been proposed and are illustrated in Figure 1.7:75–77
i. Dissolution of Pt to form dissolved species in the electrolyte
ii. Growth of Pt NPs via modified Ostwald ripening (re-deposition of dissolved Pt onto other particles) and aggregation (particle migration and coalescence)
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iv. Poisoning of Pt by impurity species in the electrolyte that deposits onto the surface of the catalyst and decreases its catalytic activity or enhances dissolution (section 1.6.2)
All of these degradation mechanisms decrease ECSA and increase ηs.
Figure 1.7. Schematic of four common electrocatalyst degradation mechanisms: i) dissolution, ii) re- deposition and/or aggregation (particle migration and coalescence), iii) detachment, and iv) poisoning by an impurity species.
Electrocatalyst dissolution is a common concern in electrocatalytic applications. Although Pt is thermodynamically stable in a large pH and potential window,78 Pt is susceptible to oxidation and dissolution within the operating window of PEMFC and DAFC cathodes (> 0.7 V vs. RHE and pH < 2 at 25 °C).79–81 Additionally, due to the harsh oxidation environment at the anode of an electrolyzer, OER catalysts are especially susceptible to dissolution.72,82 For example, Ru-based catalysts are one of the most active OER catalysts,83 but they are not stable in acidic environments.82 Furthermore, it has been demonstrated that Pt dissolution is dependent on the particle size and oxide coverage in which the dissolution rate increases with decreasing particle size.84,85 Particles smaller than 2 nm in diameter are expected to dissolve at accelerated rates, often orders of magnitude faster than larger (>10 nm) particles.86 Besides decreasing ECSA, particle dissolution negatively impacts membrane performance in a membrane electrode assembly (MEA) by increasing ionic resistance as ions deposit in the membrane.87
Migration & Coalescence Pt2+ (aq) Pt0 2 e- Dissolution & Re-deposition 2 e- Detachment support < 5 nm diam. catalytic NP Poisoning impurity/ intermediate n e-
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Particle growth via Ostwald ripening and aggregation also reduces the ECSA.79,80,88,89 Modified Ostwald ripening is a subsequent process of Pt dissolution,88,89 in which dissolved Pt is redeposited on larger platinum particles driven by difference in surface free energy and local adsorbed atom concentration on the support surface, causing larger particles to grow at the expense of smaller particles. Aggregation involves the Brownian motion of NPs. This migration eventually leads to coalescence when particles come in close proximity to each other causing particle growth.88 Physical detachment of electrocatalyst particles from the support generally results from a weak or weakening interaction between the Pt particles and support. This is especially challenging for Pt nanoparticle electrocatalysts on oxide-covered MIS photoelectrodes, which is the focus of Chapter 3. Particle detachment and aggregation are typically observed in parallel because the rates of both processes increase as the interaction between the electrocatalyst and support becomes weaker.
Under oxidizing conditions, these degradation mechanisms are exacerbated due to the electrochemical corrosion of the carbon supports, which directly initiates particle detachment and aggregation.90–94 The corrosion of the support and the concomitant structural and chemical changes to the catalyst|support interface inevitably initiates a series of secondary degradation processes. For example, electrochemical corrosion of the support separates and electrically isolates catalysts particles from the support (increasing ηΩ) and facilitates particle aggregation.
The strong influence of support oxidation and Pt dissolution on the overall degradation behavior emphasizes the importance of enhancing the oxidation resistance of both the support/substrate and Pt particles.95 As a result, a vast majority of strategies to mitigate these degradation mechanisms have focused on modifying either the support material96–106 or the active electrocatalyst.72,107–110 A more detailed description of these approaches are discussed in Chapter
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4 section 4.1. Researchers who have focused on increasing the stability of the support investigate interactions between the support and Pt NPs and methods to improve the adhesion of electrocatalysts to the support.111–114 However, none of these solutions can completely eliminate catalyst poisoning because the active sites are still directly exposed to the electrolyte and any impurities.
Despite the promising solutions to minimize dissolution and aggregation, electrocatalyst performance can still degrade from a poison or impurity species. Precious metal HER catalysts, such as Pt, are particularly prone to adverse effects by poisons.115–118 Contaminants from feed water or corrosion of stack components such as S2-, Cu2+, Ni2+, or Fe3+ or impurities from air including SOx, NOx, and H2S, can strongly bind or deposit to the Pt surface and drastically decrease the electrocatalyst activity.115–118 In PEM electrolyzers, impurities can also impact the stability of IrO2 OER electrocatalysts.72 To avoid impurities and concomitant shortened MEA lifetime, a water purification unit is implemented upstream of the electrolyzer and system components are constructed with more expensive noncorrosive materials, which can add to the balance of system costs.