Quantifying Losses in Photoelectrode Performance Due to Single Hydrogen Bubbles

Photoelectrochemical (PEC) water splitting is a promising route to solar-driven H2 production, but the efficiency and stability of semiconducting photoelectrodes must be improved for commercialization of this technology. Among the many challenges of developing and optimizing durable and efficient photoelectrode materials are the inherent complexities that arise from the heterogeneous nature of many real-world materials. For example, the surface of a given photoelectrode can possess substantially different crystal phases and orientations, composition, morphology, dopant distribution, defect structures, and more. Different structures and compositions can have vastly different optical, electronic, and catalytic properties, resulting in a photoelectrode whose macro-scale properties and performance is often a complex convolution of the properties and performance of the micro- and/or nano-scale regimes that comprise it. Being able to establish the structure-property relations that exist on these heterogeneous surfaces is therefore of great importance for optimizing photoelectrode performance. Fortunately, there are several complementary experimental techniques that can be used to analyze the spatial variation in performance and properties of photoelectrodes at the micro- or nano-scale. In this work, we have used two scanning probe techniques to investigate spatial variation in the optical, electronic, and catalytic properties of p-Si-based photocathodes (Fig. 1). The first technique is scanning photocurrent microscopy (SPCM), a technique in which a laser beam is focused onto the photoelectrode surface, resulting in a “local” photocurrent that is measured by a potentiostat. Using a nano- or micro-positioning stage, the position of the laser beam is scanned across the sample surface while recording change in local photocurrent. The second technique employed in this work is scanning electrochemical microscopy (SECM), in which an ultramicroelectrode (UME) is used to monitor spatial variation in the concentration of product species evolved from the photoelectrode surface. For our work on p-Si photocathodes, a positive potential is applied to the UME such that the H2 evolved from the photoelectrode surface is oxidized, resulting in an electrical current in the UME (JUME). This current is proportional to the amount of H2 evolved from the photoelectrode surface in the vicinity of the UME and therefore strongly reflects the catalytic activity of the surface positioned beneath it. As a platform for these SPCM and SECM investigations, our work has focused on the study of p-Si-based photocathodes for H2 evolution based upon the metal-insulator-semiconductor (MIS) architecture that have demonstrated great potential for highly efficient and stable solar water splitting.(1,2,3,4) Our studies have involved both planar (Fig. 2c) and 3D-structured MIS photoelectrodes (Fig. 2a). SPCM and SECM have been used to investigate spatial variation in performance relating to 3D structuring (Fig. 2b), intentionally introduced defects (Fig. 2d), and more. Knowledge gained from these micro-scale measurements is shown to be invaluable for i.) engineering more efficient photoelectrodes, and ii.) developing a deeper understanding of fundamental optical, electronic, and catalytic phenomena in MIS photoelectrodes. In addition to presenting results from the application of SPCM and SECM to the study of p-Si MIS photocathodes, this talk will also highlight the challenges and opportunities for applying these techniques more broadly to the field of photoelectrochemistry. Figure 1

Hydrodynamic study of bubble characteristics and bubble rise velocities in batch electrocoagulation with vibration-induced electrode plates using the PIV technique

One-dimensional (1D) arrayed photoelectrodes usually present superior performance in photoelectrochemical (PEC) water splitting. This superiority is known to be attributed to directional transport of photogenerated charge carriers. Herein, we show that in addition to this intrinsic charge transport property, a 1D arrayed structure introduces nanoscale capillary wetting, which is also believed to contribute to the improved PEC performance. Our theoretical model predicts that this morphology-dependent capillarity leads to the formation of a liquid film between the photoelectrode surface and the adhered bubble (the generated H2/O2 bubble), thus largely reducing the blockage of active sites at the bubble base. This prediction has been experimentally demonstrated by taking arrayed TiO2 nanorods as a model photoelectrode, with the observation of the PEC activity within the bubble base. This work extends our knowledge toward hydrodynamic functionality involved in morphology-controlled photoelectrodes for enhanced PEC performance.

A model for concentration of proteins from dilute solutions in a continuous foam fractionation column is proposed. This model accounts for (1) kinetics of adsorption of proteins in the liquid pool as well as in the foam, (2) liquid drainage from thin films due to Plateau border suction and disjoining pressure, (3) gravity drainage of liquid from Plateau borders, and (4) bubble coalescence in the foam. Protein enrichment and recovery were found to increase as the liquid pool height increased, eventually attaining constant values (corresponding to adsorption equilibrium), thus demonstrating the strong dependence of protein separation on adsorption kinetics. Higher enrichments and lower recoveries were obtained for smaller gas velocities, larger bubble sizes, and higher feed flow rates. Enrichments as well as recoveries were higher at lower feed concentrations. Coalescence was found to lead to higher enrichments and lower recoveries, this dependence being stronger for larger inlet bubble sizes.

The production of solar fuels by using concentrated irradiation allows for smaller and cheaper devices, and ultimately lower fuel production costs, with the added benefit of increased performances if operating conditions are chosen judiciously [1]. Photoelectrochemical (PEC) water splitting has been studied mostly at room temperature and under 1 sun illumination, hence there are still unrealized possibilities for solar fuel production by using high photon fluxes. However, the potentially advantageous synergy between higher temperatures and irradiation, also renders the system complex and difficult to study. The effect of temperature alone on photoelectrode performances has been reported for several semiconducting materials, e.g. Fe2O3 [2] and BiVO4 [3]; while the effect of high photon fluxes has been investigated separately up to 30 suns [4]. The combined effects of temperature and high irradiations on the performance of PEC devices has been rarely investigated experimentally, this is mostly due to the complexity of decoupling the effects due to increased surface temperature, photon fluxes and degradation rates.With the aim to address this challenge, we developed a PEC cell for operation at high irradiations, 30 – 360 kW m-2, along with a multiphysics model to predict the increased surface temperature and the inevitable effects related to ohmic drop, bubble evolution and current density distribution. The PEC cell was equipped with thermocouples to measure the temperature of the photoelectrode and electrolyte at different positions inside the device. Additionally, the effects of temperature alone were investigated under 1 sun by heating the electrolyte between 20 and 55 °C. Two photoelectrodes were investigated: BiVO4 and Sn-doped Fe2O3, deposited via spray pyrolysis on FTO or Ti foil.Our findings suggest that the losses due to bubbles and ohmic drop could account for up to 90% and 50% of the losses in photocurrent densities for FTO and Ti substrates, respectively. And although, increased temperatures improve the charge carrier transport, electrolyte conductivity and oxygen evolution kinetics and thermodynamics, these effects could be offset by the aforementioned losses. For hematite operating at high temperatures (55 °C) under 1 sun illumination, it was found that the surface charge transfer efficiency decreases at low electrode potentials; in contrast, impedance spectroscopy under high irradiation indicates that the charge transfer efficiencies actually increases, even when the surface of the photoelectrodes were at higher temperatures. This suggests that although e-h recombination kinetics increases with temperature, the increased charge transport kinetics and higher exciton concentration could ultimately favor the charge transfer at the photoelectrode | electrolyte interface, see Fig 1a. In the case of BiVO4, the rate of photocurrent decay does not follow a linear relationship with increased irradiation, and its performance can be predicted by a transient model accounting for dissolution and changing thicknesses. Our results at higher irradiations reveal that the highest normalized current density for BiVO4 was achieved at ca. 120 kW m-2 as shown in Fig. 1b.Hence, the electrolyte flow and the irradiation concentration should be carefully chosen in order to keep the photoelectrode surface at appropriate temperatures, while also preventing the buildup of bubbles and current density distribution. This work shows the promise of using the synergy of temperature and high photon fluxes for PEC water splitting, while also warning about engineering and material challenges that should be addressed for a successful application of this approach for solar fuel production.

In 2023, global hydrogen production reached 97 Mt (million tons), but low-emission hydrogen production technologies accounted for less than 1% of this total, resulting in an estimated 920 Mt of CO2 emissions. Green hydrogen or renewable hydrogen is produced by coupling water electrolysers to renewable energy systems. Among the water electrolyser technologies, the most advanced and widely deployed is the alkaline water electrolyser (AWE), which constitutes 60% of the global electrolyser capacity [1].To make hydrogen production via water electrolysis competitive with hydrocarbon-based hydrogen production methods, innovative strategies need to be developed, such as identifying more cost-effective and efficient electrodes and electrocatalysts like nickel (Ni)-based materials to minimize power losses through AWE sub-components and system modifications. AWEs operate at relatively low temperatures (compared to solid oxide electrolyser cells), leading to significant cell voltage and thus power losses caused by the formation of molecular hydrogen and oxygen gas bubbles (H₂ and O₂). These bubbles cover large portions of the electrode surface, acting like a “passivation layer” and thus reducing its electrocatalytic activity [2].Additionally, the gas release creates a bubble curtain near the surface, in turn lowering the electrolyte conductivity. Therefore, finding ways to remove these bubbles more efficiently is crucial for improving AWE efficiency. Two strategies can be applied, depending on the use of an external power source.In this study, we explore the combination of both methods—using a porous electrode (passive) and power ultrasound (active)—to enhance the removal of gas bubbles at the hydrogen electrode. The use of a porous electrode offers two key advantages: (i) it increases the electrochemically active surface area (ECSA), and (ii) it enhances the superaerophobicity of the surface [3]. These electrodes are fabricated through a two-step process: a first planar Ni layer is electrodeposited, followed by the creation of a porous Ni layer using the Dynamic Hydrogen Bubble Template method [4]. The process starts with the electrodeposition of a 1x1 cm² planar Ni layer on a copper (Cu) substrate for circa 20 minutes in a Ni Watts bath containing 1.33 M NiSO₄·6H₂O, 0.21 M NiCl₂·6H₂O, and 0.65 M H₃BO₃ at 60°C. Next, the porous layer is formed in an undisclosed electrolyte for 2 minutes at a current density of 1 A·cm⁻². Additionally, power ultrasound (20 and 580 kHz in the acoustic power range of 0 to 67 W) is known to significantly improve gas bubble removal from the electrode surface, aid in solution/electrolyte degassing, disrupt the Nernst diffusion layer thickness (d), enhance the mass transport of electroactive species through the double layer, and promote both the activation and cleaning of the electrode surface [5]. This study builds on previous studies involving ultrasonication of Ni electrode surfaces, as a pretreatment activation process [6].In our conditions, it was observed that the cathodic HER overpotential (η) at -10 mA.cm-2 (j) and Tafel slope (b) both decrease significantly from the planar to the porous electrode with a further reduction occurring upon the addition of ultrasound, depending on the acoustic power emitted by the sonotrode. However, erosion was observed during ultrasonication when the electrode was placed at pressure nodes, where mass transfer and performance are maximal. This study thus paves the way for further exploration of optimization strategies for sonoAWE.[1] U. Remme, Global Hydrogen Review 2024, (2024).[2] K. Zouhri, S. Lee, Evaluation and optimization of the alkaline water electrolysis ohmic polarization: Exergy study, Int. J. Hydrog. Energy 41 (2016) 7253–7263. https://doi.org/10.1016/j.ijhydene.2016.03.119.[3] R. Andaveh, Gh. Barati Darband, M. Maleki, A. Sabour Rouhaghdam, Superaerophobic/superhydrophilic surfaces as advanced electrocatalysts for the hydrogen evolution reaction: a comprehensive review, J. Mater. Chem. A 10 (2022) 5147–5173. https://doi.org/10.1039/D1TA10519A.[4] M. Hao, V. Charbonneau, N.N. Fomena, J. Gaudet, D.R. Bruce, S. Garbarino, D.A. Harrington, D. Guay, Hydrogen Bubble Templating of Fractal Ni Catalysts for Water Oxidation in Alkaline Media, ACS Appl. Energy Mater. 2 (2019) 5734–5743. https://doi.org/10.1021/acsaem.9b00860.[5] J. Hihn, F. Touyeras, M. Doche, C. Costa, B.G. Pollet, Sonoelectrodeposition: The Use of Ultrasound in Metallic Coatings Deposition, in: B.G. Pollet (Ed.), Power Ultrasound Electrochem., 1st ed., Wiley, 2012: pp. 169–214. https://doi.org/10.1002/9781119967392.ch6.[6] F. Foroughi, M. Tintor, A.Y. Faid, S. Sunde, G. Jerkiewicz, C. Coutanceau, B.G. Pollet, In Situ Sonoactivation of Polycrystalline Ni for the Hydrogen Evolution Reaction in Alkaline Media, ACS Appl. Energy Mater. 6 (2023) 4520–4529. https://doi.org/10.1021/acsaem.2c02443.

Study on gas–liquid flow characteristics in stirred tank with dual-impeller based on CFD-PBM coupled model

Oxygen gas generated at the anode of PEM electrolyzers turns the flow in the anodic flow field into two-phase regime. Along with upscaling the electrolyzers and the objective to reach higher current densities, the impact of the two-phase flow regime becomes an issue of major concern for their future design and operating conditions. To observe and quantify gas bubble formation and local gas content in the flow field channels during operation using a high-speed camera, a test rig for a 25 cm2 scaled laboratory electrolyzer cell with a transparent flow field has been set up. Current-voltage curves were recorded, while monitoring at the same time the impedance and the gas evolution in the anode flow field. Gas bubbles were distinguished by a deep learning-based image processing algorithm, revealing enhanced gas area coverage and bubble size with increasing current density. Corresponding calculations from a two-phase flow mixture model show a similar trend for local gas content in the flow field. In the high current density region, a strongly increasing voltage along with changes in the impedance pattern coincide with the transition from the flow regime with individual gas bubbles to an interconnected gas flow regime in the majority of the cell.

Quantitative analysis of the upwelling behavior of methane bubbles in nature and numerical simulations

Formation and detachment of the enclosing water film as a bubble passes through the water-oil interface

Gas and water migration in cement slurries at different curing times

We investigate numerically the dynamics of a drop containing a bubble impacting onto a pool of the same liquid. We show that the bubble can be engulfed into the pool after impact only for a limited range of impact velocities and bubble sizes. Below a critical Weber number, the compound drop bounces from the surface. By contrast, above a second threshold in Weber number, the bubble bursts during impact. Depending on the bubble size, we identify two different mechanisms responsible for this higher impact velocity threshold, with central bursting at lower bubble sizes, or dimple bursting at larger bubble sizes. We then characterize and model the dynamics of the cavity, to finally provide an overview of the mechanisms affecting the bubble stability in the liquid.

Sonochemical and sonoelectrochemical production of hydrogen

Gas bubbles are an important pathway for methane release from peatlands. The mechanisms controlling gas bubble transport and emissions in peat remain uncertain. The effects of hydrostatic pressure and peat structure on the dynamics of gas bubbles in shallow peat were therefore tested in laboratory experiments. A peat monolith was retrieved from a raised bog and maintained in a saturated state. Three distinct layers were identified from noninvasive permittivity measurements supported by soil physical properties (porosity, bulk density). Phase I of the experiment involved monitoring for the accumulation of gas bubbles under steady pressure and temperature conditions. The data showed evidence for gas bubbles being impeded by a shallow semiconfining layer at depths between 10 and 15 cm. Visible gas bubbles observed on the side of the sample box were recorded over time to estimate changes in the vertical distribution of volumetric gas content. Porosity estimates derived using the Complex Refraction Index Model (CRIM) suggest that gas bubbles enlarge the pore space when the exerted pressure is high enough. Phase II involved triggering release of trapped bubbles by repeatedly increasing and decreasing hydrostatic pressure in an oversaturated condition. Comparison of changes in pressure head and methane density in the head space confirmed that the increasing buoyancy force during drops in pressure is more important for triggering ebullition than increasing mobility during increases in pressure. Our findings demonstrate the importance of changes in hydrostatic pressure on bubble size and variations in resistance of the peat fabric in regulating methane releases from peatlands.

The mechanisms of the liquid jet formation from the interaction of the laser-induced and gas bubble pair are investigated and compared with the jet formation from the interaction of the laser-induced anti-phase bubble pair. The strobe photography experimental method and numerical simulations are implemented to obtain the parameter space of the optimum liquid jet, i.e. highest speed and lowest diameter. It is found that due to the enhanced “catapult effect”, which is induced by the protrusion of the first bubble into the second bubble and the flip back of the elongated part of the first bubble, the optimum liquid jet of the second bubble of the laser-induced anti-phase bubble pair compared to that of the laser-induced and gas bubble pair is 54 %, 65 % and 11 % faster in speed, and 4 %, 44 % and 64 % smaller in diameter, for the 500 μm, 50 μm and 5 μm sized bubbles, respectively. The optimum dimensionless distance for the optimum jet of the laser-induced and the gas bubble is around 0.7, when the maximum bubble radius increases from ∼ 5μm to ∼500 μm, which is different from the laser-induced anti-phase bubble pairs. Besides, the optimum jet of the laser-induced bubble appeared when the bubbles are equal sized, while that of the gas bubble is independent of the relative bubble size, i.e. the liquid jet of the gas bubble has higher robustness in real liquid jet assisted applications when the laser-induced bubble size varies. However, the jet of bubble 2 could maintain a high speed (20 m/s - 35 m/s) and a low diameter (∼5 % of the maximum bubble diameter) over a big range of the dimensionless distance (0.6 - 0.9) for both of the 50 μm and 500 μm sized laser-induced equal sized anti-phase bubble pairs.