Acidic or alkaline? Towards a new perspective on the efficiency of water electrolysis

Water electrolysis is a promising technology for enabling the storage of surplus electricity produced by intermittent renewable power sources in the form of hydrogen. At the core of this technology is the electrolyte, and whether this is acidic or alkaline affects the reaction mechanisms, gas purities and is of significant importance for the stability and activity of the electrocatalysts. This article presents a simple but precise physical model to describe the voltage-current characteristic, heat balance, gas crossover and cell efficiency of water electrolyzers. State-of-the-art water electrolysis cells with acidic and alkaline electrolyte are experimentally characterized in order to parameterize the model. A rigorous comparison shows that alkaline water electrolyzers with Ni-based catalysts but thinner separators than those typically used is expected be more efficient than acidic water electrolysis with Ir and Pt based catalysts. This performance difference was attributed mainly to a similar conductivity but approximately 38-fold higher diffusivities of hydrogen and oxygen in the acidic polymer electrolyte membrane (Nafion) than those in the alkaline separator (Zirfon filled with a 30 wt KOH solution). With reference to the detailed analysis of the cell characteristics, perspectives for the improvement of the efficiency of water electrolyzers are discussed. © The Author(s) 2016. Published by ECS. All rights reserved

Domain dynamics in the multiferroic phase of MnWO4

By using broadband linear and nonlinear dielectric spectroscopy we studied the magnetoelectric dynamics in the chiral antiferromagnet MnWO4. In the multiferroic phase the dielectric response is dominated by the dynamics of domains and domain walls which is strongly dependent on the stimulating electric field. The mean switching time reaches values in the minute range in the middle of the multiferroic temperature regime at T approximate to 10 K but unexpectedly decays again on approaching the lower, first-order phase boundary at T-N1 approximate to 7.6 K. The switchability of the ferroelectric domains denotes a pinning-induced threshold and can be described considering a growth-limited scenario with an effective growth dimension of d approximate to 1.8. The rise of the effective dynamical coercive field on cooling below the T-N2 is much stronger compared to the usual ferroelectrics and can be described by a power law E-c proportional to nu(1/2). The latter questions the feasibility of fast-switching devices based on this type of material

The Electrochemical Dissolution of Noble Metals in Alkaline Media

In this study, the electrochemical transient dissolution of polycrystalline silver, gold, iridium, palladium, platinum, rhodium, and ruthenium is examined in 0.05 M NaOH alkaline electrolyte as a function of electrode potential. An inductively coupled plasma mass spectrometer connected to an electrochemical flow cell is used for online detection of the metals dissolution rates. Broad potential windows starting from the hydrogen and going to the oxygen evolution reaction (OER) potentials are used to study the dissolution. The measured dissolution data, such as onsets of dissolution are analyzed and compared with available thermodynamic data. For most metals, at potentials, at which thermodynamics predict metal/solute or metal/oxide transitions, an initiation of the dissolution process is observed. It is suggested that dissolution during metal/oxide transitions is a purely kinetic effect that reflects the solubility of unstable transient oxides. Such oxides can also be formed during the oxygen evolution reaction. The latter fact is used to explain metals dissolution in the region of OER

Using Instability of a Non-stoichiometric Mixed Oxide Oxygen Evolution Catalyst As a Tool to Improve Its Electrocatalytic Performance

Owing to their superior electrocatalytic performance, non-stoichiometric mixed oxides are often considered as promising electrocatalysts for the acidic oxygen evolution reaction (OER). Their activity and stability can be superior to those of the state-of-the-art IrO2 catalysts, although the exact nature of this phenomenon is not yet understood. In the current work, a Ir0.7Sn0.3O2-x thin-film electrode is taken as a representative example for a thorough evaluation of OER activity of the non-stoichiometric oxides. Complementary activity and stability analysis of Ir0.7Sn0.3O2-x electrodes is achieved using a setup based on an electrochemical scanning flow cell and ICP-MS. The obtained ICP-MS data presents an unambiguous proof of the preferential dissolution of the less noble Sn from the mixed oxide during OER. While less than a monolayer of Ir is dissolved after a prolonged electrolysis of 1400 min during which its dissolution rate drops to near zero, the amount of Sn lost is ten monolayers. The latter finding is confirmed by XPS analysis, which besides showing Ir surface enrichment also indicates a gradual transformation of Ir0 to IrIII species. This transition is beneficial for electrode activity, as the overpotential for OER at j = 5 mA cm−2 was decreasing up to 300 mV. The increase in electrode activity is attributed to several mechanisms including generation of IrIII active sites and overall surface area increase. A generalized description of OER catalysis by Ir-based materials is given, including data from the current work as well as from other Ir-based mixed oxides, such as Ir-Ru-O and Ir-Ni-O

Electrochemical stability of hexagonal tungsten carbide in the potential window of fuel cells and water electrolyzers investigated in a half-cell configuration

Tungsten carbide has attracted much interest as possible support for oxygen reduction and hydrogen oxidation in fuel cells and as catalyst itself for the hydrogen evolution reaction in water electrolyzers in the last years. Herein, we investigate the dissolution behavior of hexagonal tungsten carbide in acidic media with cyclovoltammetric and galvanostatic procedures under steady-state and dynamic conditions. The tungsten dissolution rate in the electrolyte was monitored in-situ and time resolved via coupling of the scanning flow cell with an inductively coupled plasma mass spectrometer (SFC-ICP-MS), allowing a direct correlation of potential and amount of dissolved species. The stability and passivation behavior of tungsten carbide was compared to pristine tungsten metal and its highest oxide WO3 in fuel cell/electrolyzer relevant potential ranges. It was found that partial passivation in the oxygen reduction region takes place, accompanied by steady dissolution of tungsten slightly above these potentials. In the HER/HOR region, no significant dissolution was observed. The dissolution rate of WC at high potentials was found to be in many cases almost one order of magnitude lower than for the pristine metal, yet two orders of magnitude higher than for its corresponding highest oxide

Gas permeation through Nafion Part 2: Resistor Network Model

In the first part of this study, the hydrogen and oxygen permeabilities of Nafion were measured. The aim of the second part of this study presented here is to physically characterize the influence of the aqueous phase, the solid phase, and the intermediate phase in Nafion on the macroscopic hydrogen and oxygen permeabilities. Hereto, a resistor network model morphologically representative for Nafion based on structural investigations reported in the literature is presented in which the different phases are described by individual permeabilities. As a result of the simulations, an enlarged permeability of the solid phase in comparison to that of dry Nafion had to be assumed in order to reproduce the measured influence of temperature and relative humidity on the permeability. This increase of the permeability of the solid phase toward greater water uptake was explained by the effect of water as a plasticizer and the resulting softening of the polymeric matrix. On the basis of the identified mechanisms, approaches to reduce the gas permeability of polymer electrolyte membranes are identified and discussed

Domain dynamics in the multiferroic phase of MnWO

By using broadband linear and nonlinear dielectric spectroscopy we studied the magnetoelectric dynamics in the chiral antiferromagnet MnWO4. In the multiferroic phase the dielectric response is dominated by the dynamics of domains and domain walls which is strongly dependent on the stimulating electric field. The mean switching time reaches values in the minute range in the middle of the multiferroic temperature regime at T=10 K but unexpectedly decays again on approaching the lower, first-order phase boundary at T_N1=7.6K. The switchability of the ferroelectric domains denotes a pinning-induced threshold and can be described considering a growth-limited scenario with an effective growth dimension of d=1.8. The rise of the effective dynamical coercive field on cooling below the TN2 is much stronger compared to the usual ferroelectrics and can be described by a power law E_c ~{\nu}^1/2. The latter questions the feasibility of fast-switching devices based on this type of material.Comment: 9 pages, 8 figure