Self-Terminated Electrodeposition Research Articles

Self-terminated electrodeposition of Pt group metal: principles, synthetic strategies, and applications

Hydrogen, characterized by its carbon-neutral attributes and high energy density, is gaining momentum as a promising energy source. Platinum group metal (PGM) catalysts have emerged as pivotal components in water electrolysis and fuel cell technologies. However, their constrained availability and high cost impede the advancement of energy conversion systems. To address these challenges, various strategies have been explored within the realm of PGM catalysts. Particularly noteworthy are catalysts that exhibit an overlayer structure, offering exceptional catalyst utilization efficiency, bimetallic synergies, and strain-induced enhancements. Self-terminated electrodeposition (SED) stands out as a technique that enables precise atomic layer electrodeposition within an aqueous electrolyte environment. It allows meticulous control of metal loading quantities and surface coverage while operating at low temperatures and without the need for vacuum conditions. Catalysts with tailored properties achieved through SED exhibit distinct electrochemical reactivity compared to bulk catalysts, showcasing exceptional electrocatalytic activity, particularly in terms of mass and specific activity. This comprehensive review provides insights into the SED phenomenon, elucidates methodologies for fabricating PGM electrocatalysts using SED, and highlights their applications in water electrolysis and fuel cells.

Electrochemical cycling durability of platinum overlayers formed by self-terminating electrodeposition

Platinum thin films comprising nominally 1 to 20 atomic overlayers were formed on a gold-coated substrate using the self-terminating electrodeposition (STED) method. The platinum overlayer loading and morphology were characterized using X-ray Photoelectron Spectroscopy (XPS), Inductively Coupled Plasma – Optical Emission Spectroscopy (ICP-OES), Field Emission Scanning Electron Microscopy (FESEM), Cyclic Voltammetry (CV) and Chronoamperometry (CA). The durability of these platinum overlayer samples was estimated by subjecting the samples to ex-situ electrochemical potential cycling. The extent of platinum overlayer growth is lower than expected till four STED cycles and then increases non-linearly, leading to increased roughness. The shifts of platinum redox peaks in CVs and Pt4f signals in XPS spectra indicate surface-alloying of gold and platinum. After the durability test, the morphological evolution of platinum overlayers conforms with the extent of platinum dissolution estimated from CVs. Our results reveal that more than four STED cycles are needed to form platinum overlayers that meet the US Department of Energy (DOE) criteria for durability.

Voltage-Controlled ON-OFF-Switching of Magnetoresistance in FeOx/Fe/Au Aerogel Networks.

Voltage control of magnetoresistance (MR) in nanoscale three-dimensional (3D) geometries is interesting from a fundamental point of view and a promising route toward novel sensors and energy-efficient computing schemes. Magneto-ionic mechanisms are favorable for low-voltage control of magnetism and room-temperature operation, but magneto-ionic control of MR has been studied only for planar geometries so far. We synthesize a 3D nanomaterial with magneto-ionic functionality by electrodepositing an iron hydroxide/iron coating on a porous nanoscale gold network (aerogel). To enable maximum magneto-ionic ON-OFF-switching, the thickness of the coating is adjusted to a few nanometers by a self-terminating electrodeposition process. In situ magnetotransport measurements during electrolytic gating of these nanostructures reveal large reversible changes in MR, including ON-OFF-switching of MR, with a small applied voltage difference (1.72 V). This effect is related to the electrochemical switching between a ferromagnetic iron shell/gold core nanostructure (negative MR at the reduction voltage) and an iron oxide shell/gold core nanostructure (negligible MR at the oxidation voltage).

A Pt cathode with high mass activity for proton exchange membrane water electrolysis

To reduce the fabrication cost of proton exchange membrane water electrolyzers (PEMWEs), low-loading platinum-based cathodes were fabricated through constant potential (−1.1 VSCE) electrodeposition of Co, galvanic displacement with Pt in K2PtCl4 + NaCl solution, and subsequent chemical (immersion in 0.5 M H2SO4 for 5 min) or electrochemical dealloying (2000 potential cycles at −0.2 to −0.5 VSCE in 0.5 M H2SO4) processes. These simple electrochemical steps produced a bimodal structure of layered Pt/Co/carbon paper (CP) and particulate Pt/CP catalysts on a porous transport layer of carbon fiber. Heat treatment of Co deposits (500 °C for 2 h, H2/Ar atmosphere) prior to Pt displacement was found to strongly affect the stability of Co and the electronic structures of subsequently placed Pt, resulting in high activity (an overpotential of 18.4 mV at −10 mA cm−2) and durability (∼6000 potential cycles) of the electrochemically dealloyed catalyst in hydrogen evolution reaction. A single cell with this catalyst at low cathode Pt loading of 18.4 μg cm−2 further demonstrated excellent performance of 2.39 A cm−2 @ 1.9 V. The cathode Pt mass activity is superior to previous works on other low Pt electrode fabrication methods using pulse electrodeposition or self-terminated electrodeposition, providing a new lower limit of precious metal usage for PEMWE.

Pt Island Decorated Au Anode for Formic Acid Oxidation Reaction

For direct formic acid fuel cell (DFAFC), Pt-based catalyst is widely used as an anode. The Pt catalyst shows high activity and stability for formic acid oxidation reaction (FAOR) in acidic condition. For FAOR, it is generally accepted that Pt follows dual pathway including direct and indirect pathway. During the indirect pathway the Pt surface is vulnerable to CO poisoning, which cause the activity degradation. To enhance the CO tolerance, various Pt-based bi/ternary catalysts have been investigated. In recent, Pt–Au alloy catalysts have reported higher current density for direct FAOR pathway compared with pure Pt catalyst. The enhanced FAOR activity is originated from lower CO adsorption energy. However, the alloy structure has low catalyst utilization, which is not applicable to practical device.In this study, we report Pt islands decorated porous Au electrode for anode of the DFAFC. The porous Au transport layer was fabricated on carbon fiber by galvanic displacement and subsequent electrochemical etching process. Using Au transport layer as a substrate, Pt self-terminated electrodeposition (SED) was performed. By adjusting the SED condition, the Pt loading and coverage on Au surface cab be easily controlled. The FAOR activity test of Pt–Au electrodes was performed by cyclic voltammetry (CV). It was revealed that the current density for direct FAOR is linearly increased as the Pt coverage on Au surface is lower. By increasing the CV cycles, the current density for direct FAOR is gradually increased owing to enhanced ensemble effect. Furthermore, the cycled electrode was applicated into practical DFAFC single cell. The single cell demonstrates more than two-order higher Pt mass activity compared with previously reported single cell using Pt–Au anode.

Stabilization of nanoscale iron films by self-terminated electrodeposition in sulfate electrolyte

Iron and iron oxide nanostructures are of broad interest for numerous applications, such as in the fields of magnetic data storage, spintronics, biosensing and catalysis. In all of these cases, defined deposition on nanometer scale is essential for functionality. For conventional electrodeposition of transition metals, precise thickness control and layer stability at the nanoscale are difficult due to dissolution tendencies in acidic electrolytes after the voltage is switched off. In contrast to previous studies that focused on self-termination of Ni and Ni-based alloys, we investigate the thickness control of nanoscale iron oxide/iron layers using self-terminated electrodeposition from sulfate electrolytes. Electrochemical quartz crystal microbalance measurements show that self-terminated thickness can be controlled by both deposition potential and iron ion concentration. Comparison of experimental results with model calculations based on diffusion theory reveal two different growth modes for self-termination. At low iron ion concentration, self-termination of iron proceeds via the formation of an ultrathin iron hydroxide layer. At larger iron ion concentration, precipitation of bulk Fe(OH)2 dominates the film growth and self-termination is shifted to more negative potentials. All self-terminated layers exhibit enhanced stability in the electrolyte after the voltage is switched off compared to those deposited in the conventional deposition regime. With in situ Rutherford backscattering spectrometry measurements, we can follow the self-terminating deposition and the stability after voltage switch-off for longer times online. Surface analytical and morphological analyses show that the self-terminated layers exhibit a higher iron oxide/iron ratio and are smoother than layers obtained by conventional electrodeposition.

Durability of Platinum Overlayers formed By Self-Terminating Electrodeposition

In Proton Exchange Membrane Fuel Cell (PEMFC), the membrane electrode assembly (MEA), especially the electrode, is considered 'the heart' and is designed to accommodate the need for efficient transport of electrons, reactants, and heat as well as constraints imposed by the platinum cost. Advances in PEMFC electrode design over the years, which range from using Pt black films with a loading of 10 gpt/cm2 in the 1970s to present-day Pt/ PGM (Platinum Group Metal) nanoparticle coated carbon-black particles (Pt/C) that use about 0.3 mgpt/cm2, have led to significant cost-reduction and performance enhancement. However, durability problems associated with corrosion of carbon support and subsequent loss of active surface area during start-up or shut down cycles have led to renewed interest in carbon-free nanostructured electrodes that employ a thin coating of Platinum or PGM-based catalytic layer on mesostructured conductive support ((Debe 2012), (Liu et al. 2012)).In this context, we carried out systematic ex-situ investigations on the durability of platinum thin films comprising of atomic layers formed by Self-Terminating platinum electrodeposition (Liu et al. 2012). We characterized the sequential growth of platinum film using XPS and AFM measurements; and analyzed the electrocatalytic performance as a function of the number of self-terminating pulses. We found that eight pulses of electrodeposition, corresponding to ~10 nm thick platinum film, can meet stringent ex-situ durability targets set, by the Department of Energy (DOE), USA, for a potential electrocatalyst in Fuel Cell Vehicles (FCVs) application. Our results also show that platinum layers formed using four pulses (< 4 µg/cm2 loading) and even one pulse (< 1 µg/cm2 loading) are electrochemically active. For platinum samples formed using eight or more pulses, we observed ~ 20% loss in Electrochemically Active Surface Area (ECSA) during the first 3000 cycles and 10-15% ECSA loss in the remaining 27000 cycles, indicating stabilization of platinum activity over time. We will present the results of our studies on the minimum number of platinum pulses needed to form a complete overlayer of electrochemically active platinum and discuss the platinum loss during the ex-situ durability tests using ICP-OES and Cyclic Voltammetry measurements. These studies pave the way for an additive, roll-to-roll, and cost-effective manufacturing process of ultra-low platinum loaded MEAs for PEMFCs by building on our earlier results on printing silver nanostructures using a simple inkjet printer by Print-Expose-Develop technique (Parmar and Santhanam 2014) in conjunction with the ability to controllably form atomic layers of platinum on metallic substrates by Self-terminating electrodeposition. Figure 1

Durability of Platinum Overlayers formed By Self-Terminating Electrodeposition

The self-terminating electrodeposition (STED) method was used to synthesize platinum (Pt) atomic overlayers comprising 20 layers of Pt on gold substrates. The deposition of Pt on evaporated gold (Au) films was studied through linear sweep voltammetry (LSV) and potentiostatic deposition. The thickness of the synthesized Pt thin films after 20 cycles of STED was estimated from Energy Dispersive Spectroscopy (EDS) and X-ray Photoelectron Spectroscopy (XPS) measurements. The Pt loading was measured using Inductively coupled plasma-optical emission spectrometry (ICP-OES) and corroborated with chronoamperometry (CA). We further demonstrate the ability of these thin films to withstand the stringent targets of <40% loss in ECSA after 30,000 cycles of ex-situ durability analysis for Fuel Cell Vehicle (FCV) application. The ability to synthesize durable Pt films in the atomic layer regime by electrodeposition is a promising step towards a cost-effective route for PEMFC manufacturing.

Prudent Practices in ex situ Durability Analysis Using Cyclic Voltammetry for Platinum-based Electrocatalysts.

Platinum (Pt)-based electrocatalysts are at the vanguard of research initiatives to meet activity and durability targets for promoting large-scale adoption of fuel cell vehicles. Ex situ characterization of electrocatalyst activity and durability using cyclic voltammetry (CV) has a steep learning curve. Thus, many researchers who do not receive formal training in electrochemistry are left unsure how to proceed. Herein, we identify and compile prudent practices for reliable assessment of ECSA values with examples from our research on nanoscale catalytic films formed by the self-terminating electrodeposition of Pt. Starting with a conceptual framework to understand typical features in the CV of reversible redox couples, we present prudent practices in acquiring CV data aimed at nonelectrochemists. We then highlight specific features related to ECSA computation from Pt CV. Finally, we suggest safeguards that help avoid missteps and achieve repeatable results while conducting ex situ durability tests that extend over days.

An Ultra-Low Loading Iridium Electrode for Proton Exchange Membrane Water Electrolyzer

With interest in global warming, hydrogen produced by water electrolysis is a promising candidate for alternative for fossil fuel. For proton exchange membrane water electrolyzer (PEMWE), Ir-based oxide used as an anode which showed high performance for oxygen evolution reaction (OER). However, its high price and scarcity prevent from commercialization. In order to increase the mass activity, various methods have been conducted. In recent, many studies on the electronic structure modified catalyst have been reported and their high OER activity is originated from decreased energy barrier of intermediate for OER. Notwithstanding, it is difficult to facilitate practical application as an anode for PEMWE due to its poor structure for mass transfer at high current range. In general, powder-type of IrOx was used with ionomer solution. On the the hand, self-terminated electrodeposition (SED) facilitates controlling the Ir loading amounts in sub-microgram scale via repeated deposition pulses on Au substrate.In this study, Ir-based electrodes were directly deposited on dendritic Au substrate by SED and subsequent electrochemically (EC) and thermochemically (TC) oxidation process. Metallic Ir nano particles were electrodeposited on Au/carbon paper (Au/CP). Depending on the number of the Ir SED pulses, Ir loading mass and Ir coverage easily controlled. Then the optimized metallic Ir/Au/CP oxidized EC and TC, respectively. The electrochemical surface area (ECSA) and geometric OER activity were measured. The specific OER activities were obtained by normalized the geometric OER activity by ECSA. The oxidation state and changes of phase were investigated by transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). Through XPS fitting analysis, the Ir valency and electronic structure were investigated. By adjusting the number of the Ir SED pulses and further oxidation process, Ir valency can be changed. Finally, relationship between specific OER activity and Ir valency was investigated. Conclusively, it was confirmed that the specific OER activity was strongly affected by Ir3+/(Ir3++Ir4+).

Hydrogen evolving electrode with low Pt loading fabricated by repeated pulse electrodeposition

Reducing the Pt loading amount in an electrode is essential for the commercialization of water electrolyzers. We report a simple method for the fabrication of a low Pt loading electrode, using an electrochemical method named self-terminated electrodeposition, at room temperature and under ambient pressure. Controlling the deposition conditions enables the quenching deposition of Pt on a C-coated gas diffusion layer by H passivation at a highly negative potential. Repeating deposition pulses facilitate the facile control of the Pt surface composition and electrochemical surface area, which significantly affects their catalytic performance for the hydrogen evolution reaction. The results presented show that the aforementioned Pt electrode can be a promising cathode for use in membrane-based water electrolyzers.

Electrochemical Atomic Layer Deposition: Self-Terminated Electrodeposition Reactions

Recently, an inexpensive “wet form” of ALD based on self-terminated electrodeposition reactions was uncovered that enables controlled formation of ultrathin films of Pt, Ir and iron group metals and alloys thereof. Common to all these systems is the role of reaction intermediates, namely adsorbed H or OH-, in the quenching of metal deposition reactions. Further details on the mechanisms of self-terminated deposition reactions will be discussed. Likewise, the utility and relevance of the process to the synthesis of nanoparticles and thin films, and the study of bimetallic electrocatalysts will be detailed.

Self-terminated electrodeposition of platinum on titanium nitride for methanol oxidation reaction in acidic electrolyte

The development of high-performance electrocatalysts for methanol oxidation is an urgent task to enhance the efficiency of direct methanol fuel cells. We report a simple and controllable method to fabricate Pt-decorated TiN electrocatalysts using self-terminated electrodeposition at room temperature and ambient pressure. Under optimized deposition parameters such as electrolyte pH, TiN substrate pretreatment, and pulsed deposition potential, quenching of the Pt electrodeposition facilitates obtaining an extremely low Pt mass loading (0.93 μg/cm2) on the TiN substrate. Repeated deposition potential pulses enable a gradual increase in Pt loading, with a precise control of the loaded Pt mass. Maximum intrinsic and mass activities for the methanol oxidation reaction are achieved for the catalyst with a Pt loading mass of 55.0 μg/cm2, prepared by 20 deposition pulses. The maximum intrinsic activity achieved with the Pt-decorated TiN electrocatalyst is five times higher than that obtained with bulk Pt. The present results thus provide a facile method for the fabrication of cost-effective electrocatalysts.

Self-terminating electrodeposition of Pt on WC electrocatalysts

Self-terminated electrochemical deposition is used to grow Pt nanoparticles on tungsten monocarbide (WC) from a pH 4 electrolyte of 3 mmol/L K2PtCl4-0.5 mol/L NaCl. An unconventional potentiodynamic deposition program is used where nucleation is promoted at large overpotentials followed by growth termination at still larger overpotentials to yield a high coverage of Pt nanoparticles. Following three deposition cycles between −0.8 VSCE and −0.45 VSCE, the surface is covered by a monolayer equivalent charge of Pt in the form of ≈3 × 1011 particles/cm2 that are ≈6.7 ± 1.1 nm in diameter. The number and size of nanoparticles increase monotonically for five deposition cycles. Area-normalized kinetics for hydrogen evolution (HER) and oxidation (HOR) on Pt-WC were determined in 0.5 mol/L H2SO4. For the lowest surface coverage of Pt nanoparticles on WC, ≈ 0.01, an exchange current density of ≈ 100 mA/cm2 is achieved, comparable to the highest reported values for Pt nanoparticles and ultramicroelectrodes. The area normalized apparent exchange current density decreases with increasing Pt coverage as the relative contribution of point versus planar diffusion decreases. Self-terminated electrodeposition of Pt provides an attractive approach to achieving ultra-low loadings of well-dispersed Pt nanoparticles on a non-precious metal support like WC.

Electrochemical Atomic Layer Deposition: Self-Terminated Electrodeposition Reactions

Recently, an inexpensive “wet form” of ALD based on self-terminated electrodeposition reactions was uncovered that enables controlled formation of ultrathin films of Pt, Ir and iron group metals and alloys thereof. Common to all these systems is the role of reaction intermediates, namely adsorbed H or OH-, in the quenching of metal deposition reactions. Further details on the mechanisms of self-terminated deposition reactions including its extension to substrates beyond Au and role in alloy co-deposition will be discussed. Likewise, the utility and relevance of the process to the synthesis of nanoparticles and thin films, and the study of bimetallic electrocatalysts will be detailed.

In Situ stress Measurements during Strained-Layer Electrodeposition

Enhanced catalytic activity has been demonstrated in elastically strained thin films, pseudomorphically grown on substrates with an appropriate lattice mismatch. The enhancement in catalytic activity is typically attributed to both electronic interactions and the epitaxial mismatch strain in the overlayer. The magnitude of the strain is often estimated from lattice-constant measurements using x-ray diffraction. An additional probe gaining popularity in electrochemistry involves the measurement of surface and growth stress using cantilever curvature. Cantilever curvature offers a nice complement to the direct strain measurementand can provide an in situ measurement while the strained layer is being formed. This talk will focus on the biaxial growth stress that develops during the self-terminated electrodeposition of ultrathin Pt films onto a variety of cantilever substrates that introduce different levels of misfit strain into the Pt. The cantilever was a borosilicate glass strip measuring 60 mm × 3 mm × 0.108 mm and was metallized on one side by electron beam evaporation, resulting in substrates with strong (111) crystallographic orientation. The electrolyte was 0.5 mol/L NaCl containing 3 mmol/L K2PtCl4. Deposition consisted of stepping the potential from the open circuit potential (~ -0.1V vs SSE) down to -1.2V for 10s and then back to -0.1V for another 30s, following the procedure established by Liu et al.1Figure 1 shows the stress response for Au, Pt and Ir substrates. Pt grown on Au exhibits considerable tensile stress, which is consistent with the +3.9% lattice mismatch between Au and Pt. In contrast, Pt deposition onto a Pt cantilever deposits homoepitaxially as an unstrained continuation of the substrate. The slight increase in stress with continued deposition is likely microstructural. Pt grown on Ir, with its -2.1% lattice misfit, initially exhibits compressive stress, before leveling off and turning tensile, similar to the growth of Pt on Pt. Although it is tempting to focus on lattice misfit as the primary source of stress, one must consider other contributions, such as step line tension associated with the growth of small islands, possible intermixing, and surface roughness. References Y. Liu, D. Gokcen, U. Bertocci and T. P. Moffat, Science 338, 1327 (2012). Figure 1

Co-Deposition of Catalytic Pt-Rh Thin Films in the Self-Terminated Regime

The drive to adopt alternative energy technologies for a more sustainable energy future has been inhibited by insufficient activity, poor durability and high cost of fuel cell catalysts. To overcome these drawbacks there has been significant interest in the exploration of core-shell nanoparticles, related overlayer thin films and alloy architectures. These bimetallic systems offer significant tunability based on ligand effects associated with nearest neighbor bonding 1, strain effects associated with lattice mismatch between the constituent metals , and geometric effects dictated by the local atomic distribution of elements. In order to realize such effects significant effort is underway to develop inexpensive, easy fabrication methods for forming these alloy and over-layer architectures 2-7. Recently, self-terminated electrodeposition reactions have been discovered and adopted for the fabrication of ultra-thin metal films with monolayer level control 8-11. For the electrodeposition of Pt and/or Ir on Au and Ni12 the deposition of growing metal at high overpotentials is quenched at the onset of hydronium reduction by hydrogen adsorption. In this work, the electrodeposition of Pt-Rh alloys at potentials coincident with hydronium reduction will be discussed. In contrast to Pt, the electrodeposition of Rh is not terminated by hydrogen adsorption. The introduction of minimal quantities of [RhCl6]3- into the NaCl-[PtCl4]2- electrolyte disrupt the quenching of the Pt partial deposition reaction. The impact of Rh concentration on deposition rate, film morphology and composition was examined with voltammetry, SEM, XPS, and ISS. Finally, the tunability of as deposited films of varied thickness and composition toward the ethanol oxidation reaction will be discussed. (1) Bligaard, T.; Norskov, J. K. Electrochimica Acta 2007, 52, 5512. (2) Nutariya, J.; Fayette, M.; Dimitrov, N.; Vasiljevic, N. Electrochimica Acta 2013. (3) Viyannalage, L. T.; Vasilic, R.; Dimitrov, N. Journal of Physical Chemistry C 2007, 111, 4036. (4) Brankovic, S. R.; Dimitrov, N.; Sieradzki, K. Electrochemical and Solid- State Letters 1999, 2, 443. (5) Brankovic, S. R.; Wang, J. X.; Adzic, R. R. Surface Science 2001, 474, L173. (6) Ambrozik, S.; Dimitrov, N. Electrochimica Acta 2015, 169, 248. (7) Adzic, R. R.; Zhang, J.; Sasaki, K.; Vukmirovic, M. B.; Shao, M.; Wang, J. X.; Nilekar, A. U.; Mavrikakis, M.; Valerio, J. A.; Uribe, F. Top Catal 2007, 46, 249. (8) Ahn, S. H.; Tan, H.; Haensch, M.; Liu, Y.; Bendersky, L. A.; Moffat, T. P. Energy Environ. Sci. 2015, 8, 3557. (9) Liu, Y.; Gokcen, D.; Bertocci, U.; Moffat, T. P. Science 2012, 338, 1327. (10) Ritzert, N. L.; Moffat, T. P. J. Phys. Chem. C 2016, 120, 27478. (11) Wang, R.; Bertocci, U.; Tan, H.; Bendersky, L. A.; Moffat, T. P. J. Phys. Chem. C 2016, 120, 16228 (12) Liu, Y.; Hangarter, C. M.; Garcia, D.; Moffat, T. P. Surface Science 2015, 631, 141.

(Invited) Self-Terminated Electrodeposition Reactions

Recently, an inexpensive “wet form” of ALD based on self-terminated electrodeposition reactions was uncovered that enables controlled formation of ultrathin films of Pt, Ir and iron group metals and alloys thereof. Common to all these systems is the role of reaction intermediates, namely adsorbed H or OH-, in the quenching of metal deposition reactions. Further details on the mechanisms of self-terminated deposition reactions will be discussed. Likewise, the utility and relevance of the process to the synthesis of nanoparticles and thin films, and the study of bimetallic electrocatalysts will be detailed.

Atomic Layer Deposition Using Self-Terminated Electrodeposition Reactions

Recently, an inexpensive “wet form” of ALD based on self-terminated electrodeposition reactions was uncovered that enables controlled formation of ultrathin films of Pt, Ir and iron group metals and alloys thereof. Common to all these systems is the role of reaction intermediates, namely adsorbed H or OH-, in the quenching of metal deposition reactions. Further details on the mechanisms of self-terminated deposition reactions will be discussed. Likewise, the utility and relevance of the process to the synthesis of nanoparticles and thin films, and the study of bimetallic electrocatalysts, will be outlined.

Electrochemical Atomic Layer Deposition: Self-Terminated Electrodeposition Reactions

Development of a sustainable hydrogen economy based on water electrolysis and fuel cells is constrained by the performance, longevity and high cost of Pt group electrocatalysts. Important strategies for improving catalytic reactivity include alloying, forming bimetallic or alloyed surfaces, and engineering substrate–catalyst interactions. At the same time, significant effort is underway to minimize the loading of Pt group metals by examining new synthesis methods and electrode geometries. Of specific interest is atomic layer deposition (ALD) that can be used for growing ultrathin films to maximize utilization of rare, expensive metals and facilitate the exploration of bimetallic effects in electrocatalysis. Recently, an inexpensive “wet form” of ALD based on self-terminated electrodeposition reactions was uncovered that enables controlled formation of ultrathin films of Pt, Ir and iron group metals and alloys thereof. The mechanism of self-terminated deposition reactions will be discussed and the utility of these films as bimetallic electrocatalysts will be demonstrated. For example, monolayer equivalent Pt and Ir films were found to match or exceed the best known specific HER/HOR activity for bulk polycrystalline Pt or Ir electrodes and thereby offer the promise of increasing the scalability of these important precious metals in energy conversion devices. The effectiveness of such reaction on a variety of different substrates will also be examined.