Sunilkumar Khandavalli

• Doctor of Philosophy
• Staff Scientist at National Renewable Energy Laboratory

While anion exchange membrane water electrolyzers (AEMWEs) have achieved significant performance advances in recent decades, overpotentials remain high relative to their proton exchange membrane water electrolyzer (PEMWE) counterparts, requiring AEMWE-specific catalyst layer design strategies to further advance this technology. In this work, the ro...

Iridium oxide (IrO2) is recognized as a state-of-art catalyst for anodes of low-temperature polymer-electrolyte membrane water electrolyzers (PEMWE), one of the promising clean energy technologies to produce hydrogen, a critical energy carrier for decarbonization. However, typical IrO2 ink formulations are challenging to process in liquid-film coat...

The transition of the United States energy infrastructure towards hydrogen energy involves the optimization of proton exchange membrane water electrolyzers (PEMWEs) for reliable hydrogen generation. PEMWE device typically consists of titanium porous transport layer (PTL) and an iridium-based anode catalyst layer. Several directions are being pursue...

Perflourosulfonic acid (PFSA) ionomers are used in many applications including polymer-electrolyte membrane fuel cells and electrolysers owing to their many attractive properties such as excellent ion-conductivity, chemical resistance, mechanical properties, and thermal stability. In polymer-electrolyte membrane fuel cells and electrolysers, they a...

With the recent trends in hydrogen technologies, scale-up fabrication of membrane electrode assemblies (MEAs) for the electrochemical systems such as fuel cells and electrolyzers is gaining significant attention. Companies and researchers are focusing on diverse large-scale electrode fabrication processes, such as roll-to-roll and screen-printing....

With the move towards commercialization of proton exchange membrane water electrolyzers (PEMWE’s), the integration of the porous transport layer (PTL) and the anode catalyst layer (CL) into the porous transport electrode (PTE) is of much interest. Specifically, it is important to understand how different CL fabrication routes and ink formulations a...

We investigate the effect of alcohol fraction (isopropanol, IPA) in a binary water‐alcohol solvent mixture on the shear and extensional rheological properties, as well as the role of viscoelasticity on fiber formation of poly(acrylic acid) (PAA) in electrospinning. Comparison of the scaling of both specific viscosities η sp and extensional relaxati...

Leveraging the interactions between ionomer and catalyst can increase the performance of proton exchange membrane fuel cells. The impacts of the equivalent weight (EW) of perfluorosulfonic acid–based ionomers on the platinum group metal‐free electrode structure and fuel cell performance have not been fully explored. Four membrane electrode assembli...

To reduce hydrogen production costs for low temperature water electrolysers to meet the Hydrogen Shot goal of $1/kg manufacturing methods need to be translated from batch to continuous processes. For production of IrO 2 anode layers, roll-to-roll (R2R) methods are well suited due to their potential for high throughput and uniformity. There are a va...

Low-temperature polymer electrolyte membrane water electrolyzers (PEMWE) are an attractive clean energy technology to produce hydrogen (H 2 ), which is an energy carrier for several applications such as transportation and grid-scale energy storage and distribution (as supported by the US Department of Energy’s H2@Scale initiative). The catalyst lay...

Polymer electrolyte membrane water electrolyzers (PEMWEs) are promising hydrogen production systems for fuel cell vehicles and industrial uses with virtually no greenhouse gas emissions when using renewable energy sources [1]. Unsupported iridium oxide (IrO 2 ) is a typical oxygen evolution reaction (OER) catalyst utilized in the anode of the PEMWE...

his work investigates how local ionomer/platinum (Pt) interactions and ionomer distribution in electrospun Pt/Vulcan nanofiber electrodes impact ionomer coverage, proton accessibility, and oxygen reduction reaction (ORR) performance in proton-exchange membrane fuel cells. Insights from various in situ electrochemical diagnostics were utilized in co...

The microstructure of platinum-group-metal-free (PGM-free) catalyst layers is known significantly impact mass and proton transport and is critical for optimization of PGM-free fuel cells. ¹ It is known that catalyst ink formulation can influence interactions between catalyst and ionomer which in turn influence the catalyst layer microstructure. In...

We present an investigation of the microstructure and rheological behavior of catalyst inks consisting of Fe-N-C platinum-group-metal-free catalysts and perfluorosulfonic acid (PFSA) ionomer in a dispersion medium (DM) of water and 1-propanol (nPA). The effects of ionomer to catalyst (I/C) ratio and weight percentage of water (H2O%) in the DM on in...

As a result of the expansion of material handling, backup power, and now, automotive markets for low-temperature fuel cells, the need to understand and overcome challenges to high-volume electrode production has never been greater. OEMs and system integrators globally are addressing these challenges specific to their systems and materials. However,...

With lower site density and turnover frequency, polymer electrolyte platinum group metal (PGM)-free catalysts based electrodes are often greater than 50 μm thick in order to increase performance across the fuel cell operating range. Consequently, PGM-free electrodes have an additional bulk electrode transport resistance beyond the local or aggregat...

In situ electrochemical diagnostics designed to probe ionomer interactions with platinum and carbon were applied to relate ionomer coverage and conformation, gleaned from anion adsorption data, with O2 transport resistance for low loaded (0.05 mgPt cm⁻²) platinum supported Vulcan carbon (Pt/Vu) based electrodes in a polymer electrolyte fuel cell. C...

We present an investigation of the structure and rheological behavior of catalyst inks for low-temperature polymer electrolyte membrane water electrolyzers (PEMWE). The ink consists of iridium oxide (IrO2) catalyst particles and Nafion ionomer dispersed in a mixture of 1-propanol and water. The effects of ionomer concentration and catalyst concentr...

Many electrochemical devices such as low temperature fuels utilize ionomers to improve efficiency and overall performance. In polymer electrolyte fuel cells (PEFCs), perflurosulfonic acid ionomers improve ionic (protonic) conductivity and Pt utilization within the catalyst layer. However, the strong interaction between sulfonate (-SO 3 ⁻ ) function...

Low-temperature polymer electrolyte membrane water electrolyzers (PEMWE) are an attractive clean energy technology to produce hydrogen (H 2 ) as an energy carrier for several applications such as transportation and grid-scale energy storage and distribution (as supported by the US Department of Energy’s H2@Scale initiative). A critical component of...

Ink formulation and electrode design optimization is often an iterative process, in which conclusions from in situ performance/ diagnostics as well as ex situ characterization (e.g. microscopy and tomography) are relayed to a model. The model then identifies fundamental limiting phenomena which are to be improved in the next iterative cycle. This p...

The rational design of electrode structures and interfaces has emerged as a potential route for increasing the efficiency of electrochemical devices. As a fabrication technique, electrospinning allows for the creation of electrode structures possessing purported improvements in proton conductivity, gas transport and durability[1], while economic as...

Proper integration of materials to enable high performing low temperature fuel cell operation has never been more crucial to realizing the isolated (e.g. half-cell or ex-situ) improvements of advanced electrocatalysts, supports and ionomers. Variables such as ink formulation (i.e. solvents and solvent ratios), processing method, ionomer chemistry,...

We present a rheological investigation of fuel cell catalyst inks. The effects of ink parameters, which include carbon black-support structure, Pt presence on carbon support (Pt-carbon), and ionomer (Nafion) concentration, on the ink microstructure of catalyst inks were studied using rheometry in combination with ultrasmall-angle X-ray scattering (...

In this letter, we report a roll-to-roll fabrication method to develop a hierarchical nanopatterned superhydrophobic surface. The hierarchical pattern includes a primary micropattern with an overlayed secondary nanopattern. The primary pattern of 15–30 μm length scales was fabricated through UV nanoimprint lithography. The secondary nanopattern of...

In the presented study, we have investigated the effect of a complex flow field consisting of a combination of both shear and extensional deformation on the liquid transfer from an idealized gravure cell. The study was conducted for two classes of non-Newtonian fluid; a shear and extensional thickening nanoparticle dispersion and a extensional thic...

In this study, we present a comparative investigation of linear and branched wormlike micelles using two nonlinear rheological tools: orthogonal superposition rheology and large amplitude oscillatory shear (LAOS) rheology. The surfactants were a series of mixtures of octyl trimethyl ammonium bromide (C8TAB) and sodium oleate (NaOA). A transition fr...

Slot-die coating is an economical roll-to-roll processing technique with potential to revolutionize the fabrication of nano-patterned thin films at high throughput. In this study, the impact of shear-thickening of the coating fluid on the stability of slot-die coating was investigated. For the coating fluid, a model system fumed silica nanoparticle...

We present a large amplitude oscillatory shear rheology (LAOS) investigation of three different shear-thickening particle dispersions - fumed silica in polyethylene oxide (FLOC), fumed silica in polypropylene glycol (HydroC), and cornstarch in water (JAM). These systems shear-thicken by three different mechanisms - shear-induced formation of partic...

Gravure printing is an economical roll-to-roll processing technique with potential to revolutionize the fabrication of nano-patterned thin films at high throughput. In the present study, we investigated the impact of shear-thickening on the liquid transfer from an idealized gravure cell by a combination of experiments and numerical computations. We...

In this paper, the shear and extensional rheology of fumed silica nanoparticles dispersed in an aqueous polyethylene oxide (PEO) solution is investigated. The role of particle concentration, polymer concentration, and polymer molecular weight on both the shear and the elongational behavior of the dispersions was examined. The fumed silica dispersio...

In this fluid dynamics video, we show the spontaneous random motion of thin filaments of a shear-thickening colloidal dispersions floating on the surface of water. The fluid is a dispersion of fumed silica nanoparticles in a low molecular weight polypropylene glycol (PPG) solvent. No external field or force is applied. The observed motion is driven...