Scanning Electrochemical Microscopy: Using the Potentiometric Mode of SECM To Study the Mixed Potential Arising from Two Independent Redox Processes

This study demonstrates how the potentiometric mode of the scanning electrochemical microscope (SECM) can be used to sensitively probe and alter the mixed potential due to two independent redox processes provided that the transport of one of the species involved is controlled by diffusion. This is illustrated with the discharge of hydrogen from nanostructured Pd hydride films deposited on the SECM tip. In deareated buffered solutions the open circuit potential of the PdH in equilibrium between its β and α phases (OCP_β→α) does not depend on the tip–substrate distance while in aerated conditions it is found to be controlled by hindered diffusion of oxygen. Chronopotentiometric and amperometric measurements at several tip–substrate distances reveal how the flux of oxygen toward the Pd hydride film determines its potential. Linear sweep voltammetry shows that the polarization resistance increases when the tip approaches an inert substrate. The SECM methodology also demonstrates how dissolved oxygen affects the rate of hydrogen extraction from the Pd lattice. Over a wide potential window, the highly reactive nanostructure promotes the reduction of oxygen which rapidly discharges hydrogen from the PdH. The flux of oxygen toward the tip can be adjusted via hindered diffusion. Approaching the substrate decreases the flux of oxygen, lengthens the hydrogen discharge, and shifts OCP_β→α negatively. The results are consistent with a mixed potential due to the rate of oxygen reduction balancing that of the hydride oxidation. The methodology is generic and applicable to other mixed potential processes in corrosion or catalysis

Sub-surface seawater temperature in the Menai Strait (Wales, UK) between October 2011 and October 2012.

<p>Temperature was measured at 3</p

Shell morphometrics and shell composition.

<p>Mean values ±SE for shell dry weight, shell length, shell width, shell height and shell thickness (umbo and distal edge), and molar fraction of calcite:aragonite in <i>M. edulis</i> following a six month exposure period to four pH-temperature treatments: ambient pH and ambient temperature (ambient); ambient pH and elevated temperature (warming); reduced pH and ambient temperature (acidified); or reduced pH and elevated temperature (warming+acidified). N = 18 per treatment for the morphometric determinations, and N = 4 for shell composition, with the exception of the acidified treatment when only 2 values were recorded and both of these are given. ML: maximum load, E: extension, MF: molar fraction, U: umbo, DE: distal edge.</p

Seawater carbonate chemistry values for the four treatments.

<p>Values represent the mean ±SD of bimonthly measures taken over the six months exposure period (n = 12 for each treatment). Mussels were either exposed to: ambient pH and ambient temperature (ambient); ambient pH and elevated temperature (warming); reduced pH and ambient temperature (acidified); or reduced pH and elevated temperature (warming+acidified). TRT represents treatment, T = temperature, Sal = Salinity, pH_T = pH (total scale), TA = total alkalinity, DIC = dissolved inorganic carbon, pCO₂ = CO₂ partial pressure, HCO₃⁻ = bicarbonate, CO₃²⁻ = carbonate, Ω_arag = aragonite saturation state, and Ωcalc = calcite saturation state. Measured values are: temperature, salinity, TA, and DIC. Calculated values are pH, pCO₂, HCO₃, CO₃²⁻, Ω_arag and Ω_calc.</p

Effects of warming and/or acidification on shell strength.

<p>A) Maximum load endured until fracture and B) extension or distance a shell will bend/flex before failure of shells from <i>M. edulis</i> held for six months under: ambient temperature and ambient pH (ambient); ambient temperature and reduced pH (acidified); elevated temperature and ambient pH (warming); and elevated temperature and reduced pH (warming+acidified). Values given as means <b>±</b>SE (n = 18 per treatment). Acidified treatments shown in dark grey. Significant differences indicated by different lowercase letters (p<0.05).</p

Effects of warming and/or acidification on shell surface area.

<p>Shell surface area was determined in at least three subsamples consisting of a number of shell fragments (of 0.1–0.3 g total mass) per shell from <i>M. edulis</i> held for six months at either: ambient temperature and ambient pH (ambient); ambient temperature and reduced pH (acidified); elevated temperature and ambient pH (warming); or elevated temperature and reduced pH (warming+acidified). Values given as means <b>±</b>SE (n = 3). Acidified treatments shown in dark grey. Significant differences indicated by different lowercase letters (p<0.05).</p