Michael R. Nellist

Junction behavior of n-Si photoanodes protected by thin Ni elucidated from dual working electrode photoelectrochemistry

Si is a desirable photoanode material for use in photoelectrochemical water-splitting devices. However, Si self-passivates during the oxygen evolution half reaction and requires a protection layer to maintain high photoanodic efficiency. Thin evaporated metallic Ni layers have been reported to protect Si while also enhancing the kinetics for oxygen evolution. Maximizing performance of these and related protected/catalyzed semiconductors requires a fundamental understanding of the semiconductor|catalyst|solution interface. We use dual-working-electrode (DWE) photoelectrochemistry measurements to directly measure the interfaces electronic properties in situ during operation. By controlling the Ni thickness (3, 5, and 20 nm), we confirm that favorable shifts in photocurrent onset are correlated with thinner protection layers. Photoelectrochemical DWE measurements are used to test various prevailing hypotheses for the origin of this behavior. We find evidence that increased photovoltage is due to the development of a spatially inhomogeneous buried junction wherein high barrier regions arise via adventitious SiO2 growth. Thinner protection layers more readily promote this behavior by facilitating solution permeation to the n-Si|Ni interface. Repeated electrochemical cycling of thicker catalyst layers can achieve similar behavior and improve the photocurrent onset by as much as 300 mV. The results are discussed in the context of the general design principles for metal–insulator–semiconductor protected photoanodes.

Atomic force microscopy with nanoelectrode tips for high resolution electrochemical, nanoadhesion and nanoelectrical imaging

Multimodal nano-imaging in electrochemical environments is important across many areas of science and technology. Here, scanning electrochemical microscopy (SECM) using an atomic force microscope (AFM) platform with a nanoelectrode probe is reported. In combination with PeakForce tapping AFM mode, the simultaneous characterization of surface topography, quantitative nanomechanics, nanoelectronic properties, and electrochemical activity is demonstrated. The nanoelectrode probe is coated with dielectric materials and has an exposed conical Pt tip apex of ∼200 nm in height and of ∼25 nm in end-tip radius. These characteristic dimensions permit sub-100 nm spatial resolution for electrochemical imaging. With this nanoelectrode probe we have extended AFM-based nanoelectrical measurements to liquid environments. Experimental data and numerical simulations are used to understand the response of the nanoelectrode probe. With PeakForce SECM, we successfully characterized a surface defect on a highly-oriented pyrolytic graphite electrode showing correlated topographical, electrochemical and nanomechanical information at the highest AFM-SECM resolution. The SECM nanoelectrode also enabled the measurement of heterogeneous electrical conductivity of electrode surfaces in liquid. These studies extend the basic understanding of heterogeneity on graphite/graphene surfaces for electrochemical applications.

Morphology Dynamics of Single-Layered Ni(OH)2/NiOOH Nanosheets and Subsequent Fe Incorporation Studied by in Situ Electrochemical Atomic Force Microscopy

Nickel (oxy)hydroxide-based (NiOxHy) materials are widely used for energy storage and conversion devices. Understanding dynamic processes at the solid-liquid interface of nickel (oxy)hydroxide is important to improve reaction kinetics and efficiencies. In this study, in situ electrochemical atomic force microscopy (EC-AFM) was used to directly investigate dynamic changes of single-layered Ni(OH)2 nanosheets during electrochemistry measurements. Reconstruction of Ni(OH)2 nanosheets, along with insertion of ions from the electrolyte, results in an increase of the volume by 56% and redox capacity by 300%. We also directly observe Fe cations adsorb and integrate heterogeneously into or onto the nanosheets as a function of applied potential, further increasing apparent volume. Our findings are important for the fundamental understanding of NiOxHy-based supercapacitors and oxygen-evolution catalysts, illustrating the dynamic nature of Ni-based nanostructures under electrochemical conditions.

Direct in Situ Measurement of Charge Transfer Processes During Photoelectrochemical Water Oxidation on Catalyzed Hematite

Electrocatalysts improve the efficiency of light-absorbing semiconductor photoanodes driving the oxygen evolution reaction, but the precise function(s) of the electrocatalysts remains unclear. We directly measure, for the first time, the interface carrier transport properties of a prototypical visible-light-absorbing semiconductor, α-Fe2O3, in contact with one of the fastest known water oxidation catalysts, Ni0.8Fe0.2Ox, by directly measuring/controlling the current and/or voltage at the Ni0.8Fe0.2Ox catalyst layer using a second working electrode. The measurements demonstrate that the majority of photogenerated holes in α-Fe2O3 directly transfer to the catalyst film over a wide range of conditions and that the Ni0.8Fe0.2Ox is oxidized by photoholes to an operating potential sufficient to drive water oxidation at rates that match the photocurrent generated by the α-Fe2O3. The Ni0.8Fe0.2Ox therefore acts as both a hole-collecting contact and a catalyst for the photoelectrochemical water oxidation process. Separate measurements show that the illuminated junction photovoltage across the α-Fe2O3|Ni0.8Fe0.2Ox interface is significantly decreased by the oxidation of Ni2+ to Ni3+ and the associated increase in the Ni0.8Fe0.2Ox electrical conductivity. In sum, the results illustrate the underlying operative charge-transfer and photovoltage generation mechanisms of catalyzed photoelectrodes, thus guiding their continued improvement.

PeakForce Scanning Electrochemical Microscopy with Nanoelectrode Probes

This article describes new batch-fabricated, robust, and easyto-use scanning electrochemical microscopy (SECM) nanoelectrode probes with a characteristic dimension of about 50 nm. The resulting microscopy method, PeakForce SECMTM, provides electrochemical images with sub-100-nm resolution, as well as simultaneously acquired topographical, electrical, and mechanical maps on the nanometerscale. Using Bruker’s existing high-bandwidth electronics, PeakForce SECM also provides the capability for high-quality nanoelectrical imaging in liquid. This article describes several applications of this new PeakForce SECM capability that enable various types of multidisciplinary research.

Transient photocurrents on catalyst-modified n-Si photoelectrodes: insight from dual-working electrode photoelectrochemistry

Semiconductor photoelectrodes coated with electrocatalysts are an important component of water-splitting cells that convert and store solar energy. Surface states on light-absorbing semiconductors can function as recombination centers and lower the performance of water-splitting systems. To characterize the presence and impact of surface states on catalyst-coated semiconductors, transient photoelectrochemical behavior is often studied. These experiments typically assume that the filling/emptying of surface states at the semiconductor interface causes transients to occur whenever the incident illumination intensity is perturbed. Analyzing transients may then reveal the density of surface states and their effect on carrier recombination. However, the transient technique does not directly measure the origin of the transient behavior, and the utility of the experiment requires assuming an underlying process. Here, we use a dual-working-electrode technique applied to Ni-protected n-Si photoanodes coated with Ni(Fe) (oxy)hydroxide catalyst to examine transient behavior of catalyst-coated photoelectrodes. We find that the most pronounced transients are due to catalyst redox activity. By directly measuring the catalyst redox state, we confirm that transients are related to either catalyst oxidation to Ni(Fe) oxyhydroxide or reduction to Ni(Fe) hydroxide. We also find that the redox-active catalyst moderates how quickly the depletion region and Helmholtz electrostatic potentials relax after each illumination perturbation. The results indicate that a redox-active catalyst can serve as a “parallel capacitor” which influences both the decay time and shape of transients. This data shows that photocurrent transients on catalyzed photoanodes are influenced by the catalysts redox-activity and are not solely based on surface state loading/emptying.

Operando X‐Ray Absorption Spectroscopy Shows Iron Oxidation Is Concurrent with Oxygen Evolution in Cobalt–Iron (Oxy)hydroxide Electrocatalysts

Iron cations are essential for the high activity of nickel and cobalt-based (oxy)hydroxides for the oxygen evolution reaction, but the role of iron in the catalytic mechanism remains under active investigation. Operando X-ray absorption spectroscopy and density functional theory calculations are used to demonstrate partial Fe oxidation and a shortening of the Fe-O bond length during oxygen evolution on Co(Fe)Ox Hy . Cobalt oxidation during oxygen evolution is only observed in the absence of iron. These results demonstrate a different mechanism for water oxidation in the presence and absence of iron and support the hypothesis that oxidized iron species are involved in water-oxidation catalysis on Co(Fe)Ox Hy .

Structural Evolution of Metal (Oxy)hydroxide Nanosheets during the Oxygen Evolution Reaction

Metal (oxy)hydroxides (MO xH y, M = Fe, Co, Ni, and mixtures thereof) are important materials in electrochemistry. In particular, MO xH y are the fastest known catalysts for the oxygen evolution reaction (OER) in alkaline media. While key descriptors such as overpotentials and activity have been thoroughly characterized, the nanostructure and its dynamics under electrochemical conditions are not yet fully understood. Here, we report on the structural evolution of Ni1-δCoδO xH y nanosheets with varying ratios of Ni to Co, in operando using atomic force microscopy during electrochemical cycling. We found that the addition of Co to NiO xH y nanosheets results in a higher porosity of the as-synthesized nanosheets, apparently reducing mechanical stress associated with redox cycling and hence enhancing stability under electrochemical conditions. As opposed to nanosheets composed of pure NiO xH y, which dramatically reorganize under electrochemical conditions to form nanoparticle assemblies, restructuring is not found for Ni1-δCoδO xH y with a high Co content. Ni0.8Fe0.2O xH y nanosheets show high roughness as-synthesized which increases during electrochemical cycling while the integrity of the nanosheet shape is maintained. These findings enhance the fundamental understanding of MO xH y materials and provide insight into how nanostructure and composition affect structural dynamics at the nanoscale.