12 Search Results

The commercial viability of anion exchange membrane (AEM) electrolysis requires optimization of various stack components, with specific catalyst-ionomer combinations often yielding higher current densities, lowered Tafel slopes, and improved mass activity. In this joint theoretical-experimental study, theoretical calculations detail the impact of Versogen's piperidinium functional group on the complex, kinetically limiting oxygen evolution reaction, finding that the functional group can act as a promoter of specific steps (O*/O2* formation; H2O/O2 desorption with reaction enthalpies ranging between 0.2-0.6 eV at higher coverages of OxHy intermediates) on NiO and NiFeOx catalysts. In particular, Fe sites on the NiFeOx catalyst facilitate concerted mechanismsmore » of O*/O2* formation and H2O desorption with a low enthalpy of 0.5 eV; O2 desorption alone requires only 0.3 eV. In contrast, Versogen-IrO2 results in stronger Ir-O bonds, where the enthalpies for bond breaking (Ir-OH2 and Ir-O2) are considerably higher (1.4 eV and 1.6 eV, respectively). Rotating disk electrode studies utilized commercially available NiO and IrO2 and synthesized 7.5 wt % Fe in NiFeOx catalysts in combination with Versogen, a common AEM ionomer, and Nafion, an alternative binder. Electrochemical testing validated the impact of these mechanistic changes on ionomer-catalyst combinations, finding that Versogen particularly activates NiO and NiFeOx compared to IrO2. Following a 13.5 h hold at 1.8 V, mass activities and Tafel slopes improved to 34 +- 13 A g-1 and 79 +- 2 mV dec-1 (NiO) and 82 +- 4.9 A g-1 and 72 +- 2 mV dec-1 (NiFeOx). In contrast, Versogen-IrO2 only reached 17 +- 2.9 A g-1 and 81 +- 3 mV dec-1. Optimization of the ionomer-catalyst can yield significant increases in performance from initial activity and after an electrochemical conditioning procedure: this enhancement to the mass activity resulted in a 200.9 +- 106.1% improvement for Versogen-NiFeOx and 1284.2 +- 260.5% for Versogen-NiO. In contrast, Nafion-NiFeOx and -NiO offered moderate improvements of 39.1 +- 30.5% and 120.9 +- 59.1%, respectively.« less

To lower the cost of hydrogen produced by anion exchange membrane water electrolysis (AEMWE), it is critical to reduce the use of platinum group metal (PGM) catalysts within the device. While iridium has been successfully replaced with PGM-free catalysts at the anode, platinum-based (Pt) cathode catalysts are still required to meet the activity and durability targets. This study investigates the impact of commercial Pt/C catalyst loading, ionomer type and content, and electrode fabrication method on the cathode catalyst layer properties and AEMWE performance with the aim of determining the feasibility of reduced Pt loadings. While increased Pt loading is foundmore » to improve beginning-of-life performance, the effects are minimal above 0.6 mg/cm². Ink characterization shows that ionomer type and content affect the ink stability, particle size, and percent of unbound ionomer, which further impact the homogeneity of the sprayed catalyst layers. The 5% PiperION cathode exhibited the highest performance, which may be attributed to a balance between the small particle size and the low proportion of unbound ionomer, minimizing kinetic and transport losses. Theoretical calculations show that the ionomers interact differently with the Pt surface, causing different surface charges and water adsorption strength and activating different mechanisms for hydrogen evolution. Pt-PiperION lowered the enthalpy of water-splitting by 0.1 eV compared to Pt alone and allowed for equal site access between adsorbed H* and OH* (both adsorbed at circa −2.2 eV). Although catalyst-coated membrane (CCM) fabrication techniques are desirable for scale-up, no performance enhancement is observed compared with the catalyst-coated substrate approach. Durability, as measured by degradation rates, Pt loss, and catalyst layer restructuring, was found to improve with increased Pt loadings, higher ionomer content, and CCM architectures. These findings provide important insight into the significant role of the cathode in AEMWE and strategies for maintaining the performance with low Pt loading or PGM-free catalysts.« less

For anion exchange membrane (AEM) electrolysis to be cost- and performance-competitive to proton exchange membrane (PEM) electrolysis, evaluating and improving the stability of the ionomer at the ionomer–catalyst interface will be key to this emerging technology. Theoretical calculations of molecular fragments of the ionomers detailed the complex degradation mechanisms accessible to four different classes of ionomers (Nafion, Sustainion, Versogen, and quaternary ammonium types─ETFE, Gen 2, and Georgia Tech) on model catalysts of platinum group metal IrO₂ and earth-abundant NiO. These mechanisms may occur during the making of the ionomer-catalyst ink or in the alkaline environment of AEM electrolysis or aremore » energetically accessible at the electrochemical potentials of electrolysis. We identified diverse degradations such as (H)SO₄ production, water formation, oxidation to an alcohol, and deprotonation, leading to ionomer instability and competing with the oxygen evolution reaction (OER). Theory predicted that the weakly bound, intact cations of Sustainion’s methyl imidazolium on NiO and Versogen’s piperidinium on NiO combinations to be particularly stable and active for OER; these findings were validated by half-cell rotating disk electrode tests, where following break-in, their performance increased by 7–8 times. IrO₂ may be stable and maintain OER activity, but site access remains limited due to the strong binding and reactivity of the ionomer at the high potentials of electrolysis (at 1.4 V, Nafion’s SO₃ splits into SO₂ + O; at 0.6 V, double deprotonation of Versogen can occur; at 1.5 V, ring oxidation of Sustainion to an alcohol initiates).« less

Density functional theory calculations in conjunction with statistical mechanical arguments are performed on the rutile IrO₂ (110) facet in order to characterize multiple reaction pathways on the surface at the highest active limit (the stoichiometric surface with all metal sites available) and at the lowest active limit (the oxygen-terminated surface). Alternative pathways to the oxygen evolution reaction (OER) are found, with multiple pathways determined at each step of the four proton-coupled electron transfer reaction. Of particular interest is the detailed characterization of a co-adsorption pathway utilizing neighboring, adsorbed O, OH species in order to evolve oxygen; activation energies of thismore » pathway are <0.5 eV and therefore easily surmountable at the high operating potentials of OER. We also determined that surface Ir atoms can potentially participate in deprotonating an OOH* intermediate; the activation energy to this is 0.67 eV on the oxygen-terminated surface. These theoretical findings explain in part the high activity present in iridium oxide catalysts and also provide insight into the mechanistic pathways available on metal oxide catalysts, which may require the concerted interaction of nearest neighbor co-adsorbates to produce chemicals of interest.« less

This paper combines density functional theory calculations and electrochemical testing to study activity differences among iridium (Ir) surfaces in the oxygen evolution reaction. Ir metal/hydroxide is significantly more active than Ir oxide, which may be due to oxide skins at the surface weakening O-binding relative to pure metal or oxide surfaces. Here we report a disparity in activity between Ir and Ir oxide in half-cells not observed in single-cells. Extended operation at elevated temperature and potential were found to result in oxide growth, limiting how surface differences affect electrolyzer performance. Comparisons of half- and single-cell testing were used to assessmore » how well rotating disk electrode testing predicts membrane electrode assembly performance and durability. Although oxygen evolution activities in half-cells can translate to single-cells, standard rotating disk electrode test procedures can exaggerate the activity benefit of a metal/hydroxide surface relative to membrane electrode assembly performance under typical operating conditions; it also appears that a half-cell test cannot reasonably accelerate activity loss from continual operation. While a variety of novel catalyst approaches, including alloying, faceting, morphology, and supports can improve oxygen evolution kinetics, these results suggest that Ir surfaces at different oxide states may struggle to improve performance at the device level.« less

We report the ability to tune the catalytic activities for the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) by applying mechanical stress on a highly n-type doped rutile TiO₂ films. We demonstrate through operando electrochemical experiments that the low HER activity of TiO₂ can reversibly approach those of the state-of-the-art non-precious metal catalysts when the TiO2 is under tensile strain. At 3% tensile strain, the HER overpotential required to generate a current density of 1 mA/cm² shifts anodically by 260 mV to give an onset potential of 125 mV, representing a drastic reduction in the kinetic overpotential. Amore » similar albeit smaller cathodic shift in the OER overpotential is observed when tensile strain is applied to TiO₂. Results suggest that significant improvements in HER and OER activities with tensile strain are due to an increase in concentration of surface active sites and a decrease in kinetic and thermodynamics barriers along the reaction pathway(s). Our results highlight that strain applied to TiO₂ by precisely controlled and incrementally increasing (i.e. dynamic) tensile stress is an effective tool for dynamically tuning the electrocatalytic properties of HER and OER electrocatalysts relative to their activities under static conditions.« less