Simulation of electrochemical processes during oxygen evolution on Pb-MnO2 composite electrodes

The geometric properties of Pb-MnO2 composite electrodes are studied, and a general formula ispresented for the length of the triple phase boundary (TPB) on two dimensional (2D) compositeelectrodes using sphere packing and cutting simulations. The difference in the geometrical properties of2D (or compact) and 3D (or porous) electrodes is discussed. It is found that the length of the TPB is theonly reasonable property of a 2D electrode that follows a 1/r particle radius relationship. Subsequently,sphere packing cuts are used to derive a statistical electrode surface that is the basis for the earlierproposed simulations of different electrochemical mechanisms. It is shown that two of the proposedmechanisms (conductivity and a two-step-two-material kinetic mechanism) can explain the currentincrease at Pb-MnO2 anodes compared to standard lead anodes.The results show that although MnO2 has low conductivity, when combined with Pb as the metal matrix,the behaviour of the composite is not purely ohmic but is also affected by activation overpotentials,increasing the current density close to the TPB. Current density is inversely proportional to the radius ofthe catalyst particles, matching with earlier experimental results. Contrary to earlier SECM experiments,mass transport of sulphuric acid is not likely to have any influence, as confirmed with simulations.A hypothetical two-step-two-material mechanism with intermediate H2O2 that reacts on both the Pbmatrix and MnO2 catalyst is studied. It was found that assuming quasi-reversible generation of H2O2followed by its chemical decomposition on MnO2, results are obtained that agree with the experiments.If the quasi-reversible formation of H2O2 occurs near the peroxide decomposition catalyst, currentincreases, leading to an active TPB and to the current density that scales with 1/r. It is furtheremphasised that both the Pb matrix and MnO2 catalyst are necessary and their optimum ratio dependson the used current density. Yet, additional experimental evidence is needed to support the postulatedmechanism.Peer reviewe

Composite electrodes for oxygen evolution in metal electrowinning

Oxygen evolution is the most common anode reaction in the electrowinning (EW) of metals from acidic sulfate based electrolytes and is a reaction that requires high activation overpotentials. Since the the oxygen evolution reaction contributes roughly 500-800 mV to the cell voltage which is roughly 15-25% of the total cell voltage, there has been increasing interest to replace the traditionally used lead anode by alternative anodes employing better electrocatalysts that show lower oxygen evolution overpotentials.In this work an alternative anode concept is explored, the composite anode consisting of ex situ prepared MnO2 and lead metal as the composite matrix material, where a special focus has been to investigate the role of the triple phase boundary and microscopic processes that would explain why this material combination has been showing relatively low oxygen evolution overpotential (up to 250 mV lower than the traditional lead anode).  After initial characterisation of different types of MnO2 as electrocatalysts for the oxygen evolution reaction (OER), it was noticed that the oxygen evolution mechanism was mass transfer dependent and that the current density measured at contant electrode potential was inversely proportionally dependent on the particle size of the MnO2 catalyst material (1/r), indicating edge effects on a microscopic level. This lead to the development of a stochastic model describing the total triple phase boundary length (Pb, MnO2 and electrolyte) proportional to 1/r. It was followed by a characterisation of the microscopic process using scanning electrochemical microscopy (SECM) and conductive atomic force microscopy (CAFM) showing that the triple phase boundary was characterised by special electrical properties and that hydrogen peroxide was generated as an intermediate. Different microscopic processes were simulated and it was shown that the conductivity of MnO2 and a newly postulated 2-step 2-material mechanism could serve as an explanation of the observed experimental results.  Since the involvement of H2O2 as an intermediate in the OER was not well supported, it was attempted to measure H2O2 reactions on lead electrode, which was not successful. As a consequence a methodology involving potential step transients on a rotating disc electrode was developed and tested for one electron transfer reactions. It was furthermore shown how this method could be used to measure rate constants relating to H2O2 reactions on Pb and MnO2

Electrochemical determination of hydrogen entry to HSLA steel during pickling

Pickling with hydrochloric acid is a standard method to clean steel surfaces before hot-dip galvanizing. When normal low strength steels are pickled, hydrogen formed in pickling reactions does not have any significant harmful effect on the mechanical properties of steel. However, in pickling of steels with higher strength, the penetration of hydrogen into the steel may cause severe damages. The effect of pickling of high-strength low-alloy (HSLA) steels was investigated using a cell construction based on the Devanathan-Stachurski method with modified anodic surface treatment and hydrogen production using acid. The penetration and the permeability of hydrogen were measured using an electrochemical cell with hydrochloric acid on the one side of the steel sample and a solution of NaOH on the other side. No protective coating, for example, palladium on the anodic side of the sample, is needed. The penetration rate of hydrogen into the steel and exit rate from the steel were lower for higher strength steel.Peer reviewe