Electrochemical characterisation of copper thin-film formation on polycrystalline platinum.

Electrochemically formed thin films are vital for a broad range of applications in virtually every field of modern science and technology. Understanding the film formation process could provide a means to aid the characterisation and control of film properties. Herein, we present a fundamental approach that combines two well-established analytical techniques (namely, electrochemical impedance spectroscopy and electrogravimetry) with a theoretical approach to provide physico-chemical information on the electrode/electrolyte interface during film formation. This approach allows the monitoring of local and overall surface kinetic parameters with time to enable an evaluation of the different modes of film formation. This monitoring is independent of surface area and surface concentrations of electroactive species and so may allow current computational methods to calculate these parameters and provide a deeper physical understanding of the electrodeposition of new bulk phases. The ability of this method to characterise 3D phase growth in situ in more detail than that obtained by conventional approaches is demonstrated through the study of a model system, namely, Cu bulk-phase deposition on a Pt electrode covered with a Cu atomic layer (Cu(ad)/Pt).

Towards a detailed in situ characterization of non-stationary electrocatalytic systems.

A complementary combination of cyclic voltammetry, impedance spectroscopy and quartz crystal microbalance techniques was used to: (i) control the assembly of a model electrocatalytic system consisting of monolayer and sub-monolayer amounts of Ag and Pb on a Au electrode, (ii) evaluate the system performance for the reduction of NO(3)(-) and (iii) study the disassembly of the electrocatalytic system to explore any changes which occurred during the assembly and/or catalytic stages. Physical models of the electrochemical interface (described in terms of equivalent electric circuits) at all stages are found to be considerably different but consistent with each other. Deposition of the Ag atomic layer on Au is accompanied by spontaneous surface alloying and specific adsorption of anions. In the following, deposition of the Pb atomic layer triggers further alloying in the Ag(ad)/Au layer while perchlorate-ions leave the surface. Approximately 1/3 of the Pb atomic layer on Ag(ad)/Au was found to demonstrate the best activity towards nitrate reduction. The developed experimental approach shows promise for the in situ characterization and control of all the non-stationary stages which are usually of particular importance in electrocatalytic research.

Tuning the activity of Pt(111) for oxygen electroreduction by subsurface alloying.

To enable the development of low temperature fuel cells, significant improvements are required to the efficiency of the Pt electrocatalysts at the cathode, where oxygen reduction takes place. Herein, we study the effect of subsurface solute metals on the reactivity of Pt, using a Cu/Pt(111) near-surface alloy. Our investigations incorporate electrochemical measurements, ultrahigh vacuum experiments, and density functional theory. Changes to the OH binding energy, ΔE(OH), were monitored in situ and adjusted continuously through the subsurface Cu coverage. The incorporation of submonolayer quantities of Cu into Pt(111) resulted in an 8-fold improvement in oxygen reduction activity. The most optimal catalyst for oxygen reduction has an ΔE(OH) ≈ 0.1 eV weaker than that of pure Pt, validating earlier theoretical predictions.

The Pt(111)/electrolyte interface under oxygen reduction reaction conditions: an electrochemical impedance spectroscopy study.

The Pt(111)/electrolyte interface has been characterized during the oxygen reduction reaction (ORR) in 0.1 M HClO(4) using electrochemical impedance spectroscopy. The surface was studied within the potential region where adsorption of OH* and O* species occur without significant place exchange between the adsorbate and Pt surface atoms (0.45-1.15 V vs RHE). An equivalent electric circuit is proposed to model the Pt(111)/electrolyte interface under ORR conditions within the selected potential window. This equivalent circuit reflects three processes with different time constants, which occur simultaneously during the ORR at Pt(111). Density functional theory (DFT) calculations were used to correlate and interpret the results of the measurements. The calculations indicate that the coadsorption of ClO(4)* and Cl* with OH* is unlikely. Our analysis suggests that the two-dimensional (2D) structures formed in O(2)-free solution are also formed under ORR conditions.

Screening of electrocatalytic materials for hydrogen evolution.

A general scheme for high-throughput screening of electrocatalysts is presented. By systematically exploiting a collection of theoretical and experimental materials databases, supplemented with quantum mechanical calculations, it locates systems that meet a set of pre-imposed selection criteria. As an example, the scheme is used to identify a binary "substrate-overlayer" electrocatalytic system for the hydrogen evolution reaction. The best catalysts found in this screening are based on Cu and W. The hydrogen evolution activity of W-Cu catalysts is evaluated by means of cyclic voltammetry. It turns out to be considerably more active than any of its constituents, pure W and Cu.

Analysis of large experimental datasets in electrochemical impedance spectroscopy.

An approach for the analysis of large experimental datasets in electrochemical impedance spectroscopy (EIS) has been developed. The approach uses the idea of successive Bayesian estimation and splits the multidimensional EIS datasets into parts with reduced dimensionality. Afterwards, estimation of the parameters of the EIS-models is performed successively, from one part to another, using complex nonlinear least squares (CNLS) method. The results obtained on the previous step are used as a priori values (in the Bayesian form) for the analysis of the next part. To provide high stability of the sequential CNLS minimisation procedure, a new hybrid algorithm has been developed. This algorithm fits the datasets of reduced dimensionality to the selected EIS models, provides high stability of the fitting and allows semi-automatic data analysis on a reasonable timescale. The hybrid algorithm consists of two stages in which different zero-order optimisation strategies are used, reducing both the computational time and the probability to overlook the global optimum. The performance of the developed approach has been evaluated using (i) simulated large EIS dataset which represents a possible output of a scanning electrochemical impedance microscopy experiments, and (ii) experimental dataset, where EIS spectra were acquired as a function of the electrode potential and time. The developed data analysis strategy showed promise and can be further extended to other electroanalytical EIS applications which require multidimensional data analysis.

Evaluation of the catalytic performance of gas-evolving electrodes using local electrochemical noise measurements.

Characterization of gas evolution reactions at the electrode/electrolyte boundary is often difficult due to the dynamic behavior of interfacial processes. Electrochemical noise measurements determined by scanning electrochemical microscopy were used to characterize Cl(2) evolution at gas-evolving electrodes (GEEs). Analysis of the electrochemical noise is a powerful method to evaluate the efficiency of the catalyst layer at a GEE. The high sensitivity of the developed measurement system enabled accurate monitoring of the current fluctuations caused by gas-bubble detachment from the electrode surface. Fourier transform analysis of the obtained current responses allows extraction of the characteristic frequency, which is the main parameter of the macrokinetics of GEEs. The characteristic frequency was used as part of a methodology to evaluate the catalyst performance and, in particular, to estimate the fraction of the catalyst layer that is active during the gas evolution reaction.