Platinum overlayers characterization and fabrication via self-terminating electrodeposition

Abstract

Hydrogen is considered a promising alternative fuel for the future. However, despite its perception as a “zero-carbon” energy carrier, fossil fuels remain the primary feedstocks for hydrogen production, involving significant carbon emissions. Although a considerable push exists to generate green hydrogen using electrolysis via renewable energy resources, its contribution remains <0.1% today. Geological hydrogen (or natural hydrogen) may play a game-changer role in realizing a large-scale hydrogen economy. Polymer Electrolyte (or Proton Exchange) Membrane Fuel cells (PEMFCs) extract power from hydrogen and oxygen using electrochemical pathways and are energy-efficient alternatives to traditional combustion engines for automotive and mobile applications. Fuel Cell Vehicles (FCVs) offer the potential for 'zero' tailpipe emissions, significantly propelling the idea of a 'hydrogen economy'. Among the various components of PEMFC, the catalyst layer contributes to over 40% of the overall fuel cell stack cost, especially when scaled for production. As a result, extensive research efforts have been focused on reducing the amount of platinum (Pt) electrocatalyst used over the last few decades, aiming to achieve targets of cost (Platinum Group Metal – PGM loading <100 µg cm-2), durability (<40% loss of activity after 30,000 cycles), and performance (0.44 A mgPt-1 @0.9 V).

Over several decades, the structural configuration of the electrocatalyst layer in PEMFC has evolved to fulfil the crucial requirements for heat, electron, and chemical transport while providing a substantial surface area for efficient conversion. Carbon-based supports are currently favoured due to their high surface area, but there remains scope for innovation concerning electrical conductivity and water regulation. Additionally, the durability of PEMFCs in automotive applications is significantly limited by the corrosion of the carbon support, particularly during start-up and shut-down cycles. Extensive research has been conducted on developing carbon-free nanostructured thin films to act as electrocatalyst layers in PEMFC applications. The efficacy of these thin-film structures relies on establishing adhered platinum layers on conductive nanostructured surfaces. This procedure aims to reduce activation losses linked to load cycling, predominantly triggered by ionomer absorption and the breakdown of catalyst particles. The Nanostructured Thin Film (NSTF) design, pioneered by 3M, presents an alternative approach that enhances specific activity and durability. However, the complexity of fabricating using vacuum-sputtering processes could hinder their mass adoption.

The Self-Terminating Electrodeposition (STED), a double-pulse voltammetric technique, provides an additive and scalable process for electrodepositing atomic-scale platinum overlayers under ambient conditions. Platinum atomic overlayers synthesized through STED have displayed promising performance across various contemporary reactions. Despite these advancements, platinum overlayers' electrochemical potential cycling durability has not been thoroughly explored. To bridge this knowledge gap, we have utilized STED to synthesize platinum overlayers on metallic thin film substrates and conducted an extensive experimental analysis, specifically from the electrochemical durability standpoint.

The ex-situ measurements on platinum-based electrocatalysts for fuel cells last more than a day and require careful data handling during acquisition and analysis. To begin with, we developed prudent practices for obtaining reliable data without introducing any artefacts by considering the effects of choice of appropriate electrolyte, potential limits, baseline selection, inert gas purging, and organic impurities on the ECSA measurement. Following up, we conducted studies on platinum overlayer deposition on rough substrates, which has practical relevance, by STED and characterised their electrochemical durability. In our control experiments using e-beam evaporated gold (Au) substrates, we discovered the non-linear growth of platinum overlayers on rough gold with an increasing number of STED cycles. Our results indicate that to be electrochemically durable, the platinum overlayers must completely cover the surface, achieved after 4 STED cycles (> 2.21 ± 0.74 µg cm-2 platinum loading) on evaporated gold substrates.

Platinum deposition via STED can be extended to encompass “reactive” substrates such as nickel, cobalt, iron and oxides such as TiOx. On these substrates, the platinum adlayer is no longer limited to atomic thickness; “approximate” monolayers of nanoparticulate deposits are formed. The differences in the deposit structures can be attributed to the changes in the rates of platinum chloride reduction and surface termination by underpotentially deposited hydrogen atoms (Hupd) on these substrates. Silver (Ag) is an excellent conductor of electricity. If one can make nm thin-platinum skin electrocatalysts for PEMFCs, several challenges related to dynamic degradation mechanisms can be overcome. To this end, in the third part, we have discussed our results on the electrochemical performance of platinum overlayers deposited on commercially available silver leaf (“Varakh”) substrates using a modified STED protocol. Additionally, we propose a potential strategy for mitigating challenges in employing silver leaf as a standalone substrate by incorporating gold. We compare the performance of platinum overlayers on silver, gold, and various gold-silver alloy leaf substrates and determine the minimum amount of platinum needed to attain electrochemically durable platinum film.

Finally, some initial experimental results with silver nanostructure substrates are discussed, wherein the process parameters are optimised to obtain electrochemically active platinum overlayers. This exploration marks a significant step towards combining the deposition of platinum overlayers through STED with the utilization of inkjet-printed silver nanostructures developed in our group. The ease of printing silver nanostructures using a simple inkjet printer and the ability to coat them with platinum atomic layers via electrode potential cycling can pave the way for a cost-effective, additive, and scalable process for manufacturing durable membrane electrode assemblies for PEMFCs.