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Electrochemical atomic layer deposition (E-ALD) is a method for the formation of nanofilms of materials, one atomic layer at a time. It uses the galvanic exchange of a less noble metal, deposited using underpotential deposition (UPD), to produce an atomic layer of a more noble element by reduction of its ions. This process is referred to as surface limited redox replacement and can be repeated in a cycle to grow thicker deposits. It was previously performed on nanoparticles and planar substrates. In the present report, E-ALD is applied for coating a submicron-sized powder substrate, making use of a new flow cell design. E-ALD is used to coat a Pd powder substrate with different thicknesses of Rh by exchanging it for Cu UPD. Cyclic voltammetry and X-ray photoelectron spectroscopy indicate an increasing Rh coverage with increasing numbers of deposition cycles performed, in a manner consistent with the atomic layer deposition (ALD) mechanism. Cyclic voltammetry also indicated increased kinetics of H sorption and desorption in and out of the Pd powder with Rh present, relative to unmodified Pd.

Nanofilms of Pd were grown using an electrochemical form of atomic layer deposition (E-ALD) on 100 nm evaporated Au films on glass. Multiple cycles of surface-limited redox replacement (SLRR) were used to grow deposits. Each SLRR involved the underpotential deposition (UPD) of a Cu atomic layer, followed by open circuit replacement via redox exchange with tetrachloropalladate, forming a Pd atomic layer: one E-ALD deposition cycle. That cycle was repeated in order to grow deposits of a desired thickness. 5 cycles of Pd deposition were performed on the Au on glass substrates, resulting in the formation of 2.5 monolayers of Pd. Those Pd films were then modified with varying coverages of Pt, also formed using SLRR. The amount of Pt was controlled by changing the potential for Cu UPD, and by increasing the number of Pt deposition cycles. Hydrogen absorption was studied using coulometry and cyclic voltammetry in 0.1 M H2SO4 as a function of Pt coverage. The presence of even a small fraction of a Pt monolayer dramatically increased the rate of hydrogen desorption. However, this did not reduce the films’ hydrogen storage capacity. The increase in desorption rate in the presence of Pt was over an order of magnitude.

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Atomic layer deposition (ALD) is a group of methods for the formation of nanofilms of materials, an atomic layer at a time using surface limited reactions.  The majority of those methods are based on use of the vacuum environment.  However there are methods such as sequential ionic layer adsorption reaction (SILAR) which uses the condensed phase, that is, aqueous solutions of precursor ions.  In addition, this group has been working on an electrochemical form of ALD for 25 years.   That work is referred to as electrochemical atomic layer deposition (E-ALD) or electrochemical atomic layer epitaxy (EC-ALE), and is based on using underpotential deposition (UPD) for surface limited reactions. The problem with using an electrochemical methodology for the formation of nanofilms is that many compatible with the conductive substrates normally used for E-ALD.  This has stimulated investigations into the growth of materials using an electroless form of ALD (EL-ALD).  The processes being explored are based on use of an adsorbed layer of a precursor, where the solution is then exchanged for a reducing agent, or vis versa.   This cycle is then repeated to desired thickness. One example is the use of an adsorbed (or absorbed) layer of hydrogen as the reducing agent (usually on Pd).  This is then used for surface limited redox replacement (SLRR) of the desired element.  Another example is the use of Sn2+ adsorption followed by Pd2+, which oxidizes the Sn2+ to Sn4+ and forms Pd on the surface. Studies are underway to build this into a cycle for EL-ALD.