# Bubble evolution during hydrogen production under normal gravity and microgravity: A review

Canghao Zhen, Yangbo Xue, Bo Zhang, Yechun Wang, Yichuang Xing, Qiangqiang Zhao, Qiuyang Zhao, Kevin Andrew Tucker, Zhihui Chen, Eric Lichtfouse, et al.

► To cite this version:

Canghao Zhen, Yangbo Xue, Bo Zhang, Yechun Wang, Yichuang Xing, et al. Bubble evolution during hydrogen production under normal gravity and microgravity: A review. International Journal of Hydrogen Energy, 2025, 163, pp.150380. 10.1016/j.ijhydene.2025.150380. hal-05204433

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# Bubble evolution during hydrogen production under normal gravity and microgravity: A review

Canghao Zhen<sup>1 ID</sup>, Yangbo Xue<sup>1</sup>, Bo Zhang, Yechun Wang<sup>*</sup>, Yichuang Xing, Qiangqiang Zhao,<br>Qiuyang Zhao<sup>**</sup>, Kevin Andrew Tucker<sup> ID</sup>, Zhihui Chen, Eric Lichtfouse<sup> ID</sup>, Jinwen Shi<sup>*** ID</sup>

International Research Center for Renewable Energy, State Key Laboratory of Multiphase Flow in Power Engineering, Xi'an Jiaotong University, Xi'an, 710049, China

## ABSTRACT

Hydrogen production by water electrolysis is a promising energy source for space missions, yet the lack of buoyancy in space reduces the efficiency of electrochemical reactions because bubbles of hydrogen accumulate on electrode surfaces. In this review, we systematically compare and discuss the differences in bubble evolution behaviors and key factors under normal gravity and microgravity environments, focusing on nucleation, growth, detachment, coalescence, and adhesion. We discover higher bubble nucleation probability and faster growth direction are provided in hydrophobic electrode, and this effect being further amplified in microgravity. Moreover, Marangoni forces are accentuated in microgravity, further inhibiting bubble detachment. Then, effective strategies under normal gravity, like applying ultrasonic fields, magnetic fields, flow fields, and electrode structure modifications, are proposed to mitigate bubble coverage, offering valuable references for space applications. Lastly, future perspectives are proposed to address technical challenges and advance the development of reliable water electrolysis systems for space missions.

### Keywords:

Water electrolysis

Normal gravity

Microgravity

Bubble evolution

Hydrogen production

## 1. Introduction

The energy demand of human society has increased significantly with the growth of global population and economy, and fossil fuels account for 80% of global energy consumption [1]. A major challenge of the 21st century is sustainably meeting the ever-growing global energy demands. The combustion of fossil fuels will release greenhouse gases, particularly carbon dioxide (CO₂), which severely damages the ecological environment and contributes to adverse climate change [2]. Hydrogen, known for its high energy density and compatibility with renewable energy systems, offers a promising clean energy solution for reducing greenhouse gas emissions [3]. Additionally, hydrogen can be transported over long distances through pipelines at a low cost [4]. It also serves as a backup power source, helping to maintain grid stability and manage peak load demands [5].

Hydrogen can be produced from a wide range of feedstocks, pathways, and technologies [6]. A classic method involves cracking or reforming fossil fuels, such as steam methane reforming, which emits

around 9 kg of CO₂ per kg of H₂ produced [7]. Water electrolysis is one of the cleanest and most efficient methods for producing hydrogen [8]. This process can be powered by renewable energy sources such as solar, hydropower, and wind energy, thus minimizing environmental impact and enhancing sustainability [9]. Nowadays, Green hydrogen production driven by renewable energy is increasingly considered a viable approach to reducing greenhouse gas emissions and environmental pollution, with its production projected to reach 300–800 Mt yr⁻<fnref n="1" /> by 2050 [10].

Water electrolysis is an electrochemical reaction and the evolution is shown in Fig. 1a. In this process, hydroxide ions (OH⁻) move toward to anode driven by the electric field, forming an electric double layer. At the cathode, water molecules undergo a reduction reaction, producing H₂ and OH⁻ ions. Meanwhile, at the anode, the OH⁻ ions undergo an oxidation reaction, losing electrons to produce oxygen (O₂) and water (H₂O) [11].

Mainstream water electrolysis technologies can be classified into four types based on different electrolytes, including alkaline water electrolysis, proton exchange membrane water electrolysis, solid oxide

<footnote>* Corresponding author.</footnote>

<footnote>** Corresponding author.</footnote>

<footnote>*** Corresponding author.</footnote>

<footnote>E-mail addresses: wangycc@mail.xjtu.edu.cn (Y. Wang), qyzhao@mail.xjtu.edu.cn (Q. Zhao), jinwen_shi@mail.xjtu.edu.cn (J. Shi).</footnote>

<footnote n="1">¹ These two authors contribute equally to this work.</footnote>
Graphical Abstract

<figure><img src="image_3.png" /></figure>
Nomenclature

Abbreviations

<table><tr><td>VBM</td><td>valence band maximum</td></tr><tr><td>NHE</td><td>normal hydrogen electrode</td></tr><tr><td>CBM</td><td>conduction band minimum</td></tr><tr><td>NASA</td><td>National Aeronautics and Space Administration</td></tr><tr><td>Bo</td><td>Bond number</td></tr><tr><td>Eo</td><td>Eötvös number</td></tr><tr><td>Ca</td><td>Capillary number</td></tr><tr><td>Ar</td><td>Archimedes number</td></tr><tr><td>Fr</td><td>Froude number</td></tr><tr><td>pH</td><td>potential of hydrogen</td></tr><tr><td>Pt</td><td>Platinum</td></tr><tr><td>PFOS</td><td>perfluorooctane sulfonate</td></tr><tr><td>ECSA</td><td>electrochemical active surface areas</td></tr><tr><td>SDS</td><td>sodium dodecyl sulfate</td></tr></table>

Equation parameters

<table><tr><td>R</td><td>Bubble radius</td></tr><tr><td>K</td><td>Henry's constant</td></tr><tr><td>δ<sub>C</sub></td><td>Degree of supersaturation of dissolved gas</td></tr><tr><td>ρ<sub>L</sub></td><td>Electrolyte density</td></tr><tr><td>C</td><td>Coefficient</td></tr><tr><td>D</td><td>Diffusion coefficient</td></tr><tr><td>C<sub>s</sub></td><td>Bubble growth coefficient</td></tr><tr><td>α<sub>l</sub></td><td>Liquid's thermal diffusivity</td></tr><tr><td>I</td><td>Current</td></tr><tr><td>R<sub>g</sub></td><td>Gas constant</td></tr><tr><td>T</td><td>Temperature</td></tr><tr><td>F</td><td>Faraday constant</td></tr></table>

<table><tr><td>p</td><td>Pressure</td></tr><tr><td>φ</td><td>Correction factor</td></tr><tr><td>λ</td><td>Liquid's thermal conductivity</td></tr><tr><td>ρ</td><td>Density</td></tr><tr><td>c</td><td>specific heat capacity</td></tr><tr><td>J<sub>a</sub></td><td>Jakob number</td></tr><tr><td>σ</td><td>Surface tension</td></tr><tr><td>Δg<sub>ν</sub></td><td>energy density difference</td></tr><tr><td>d(t)</td><td>Bubble diameter at time t after nucleation</td></tr><tr><td>R(t)</td><td>Bubble radius at time t after nucleation</td></tr><tr><td>β</td><td>Growth coefficient</td></tr><tr><td>x</td><td>Time coefficient</td></tr><tr><td>f<sub>G</sub></td><td>Gas-evolution efficiency</td></tr><tr><td>C<sub>d</sub></td><td>Drag coefficient</td></tr><tr><td>R<sub>s</sub></td><td>Contact line radius</td></tr><tr><td>θ</td><td>Contact angle</td></tr><tr><td>ρ<sub>G</sub></td><td>Gas density</td></tr><tr><td>ΔT</td><td>Temperature difference between the solution and the gas-producing surface</td></tr><tr><td>ΔC</td><td>Concentration difference between the bulk electrolyte and the dissolved gas on the gas-producing surface</td></tr><tr><td>F<sub>d</sub></td><td>Hydrodynamic drag force</td></tr><tr><td>F<sub>s</sub></td><td>Surface tension force</td></tr><tr><td>F<sub>M</sub></td><td>Marangoni force</td></tr><tr><td>F<sub>i</sub></td><td>Inertia force</td></tr><tr><td>F<sub>p</sub></td><td>Pressure force</td></tr><tr><td>F<sub>b</sub></td><td>Buoyancy force</td></tr><tr><td>d<sub>b</sub></td><td>Bubble detachment diameter</td></tr><tr><td>R<sub>p</sub></td><td>Contact circle radius</td></tr></table>

electrolysis cell, and anion-exchange membrane water electrolysis [12]. Alkaline water electrolysis and proton exchange membrane water electrolysis [13] have been widely used for industrial hydrogen production, while solid oxide electrolysis [14] and anion-exchange membrane water electrolysis [15] are two emerging water electrolysis technologies.

Another method for producing clean hydrogen is photocatalytic water-splitting technology, which was first discovered by Fujishima and Honda in 1972 [16]. The feasible semiconductor photocatalyst material is required to satisfy the following requirements for water splitting under visible light, (i) have a narrow band gap, (ii) be stable under photoirradiation, and (iii) have suitable conduction and valence band levels for H₂ and O₂ production [17]. The reaction mechanisms of photocatalytic water-splitting technology are shown in Fig. 1b. Step 1, photon absorption generates electron-hole pairs. Step 2, charges separate and migrate to trapping sites on the catalyst surface. Step 3, construction of surface chemical reactions for H₂ and O₂ generation. To achieve stoichiometric H₂ and O₂ evolution, the valence band maximum (VBM) should be above 1.23 V (vs. normal hydrogen electrode, NHE), while the conduction band minimum (CBM) should be below 0.00 V (vs. NHE) [18,19]. Photocatalytic water-splitting technology effectively utilizes solar energy for hydrogen production and holds significant potential for applications in space, where solar energy is abundant.

Pure photocatalytic hydrogen production has low efficiency, which can be enhanced through photothermal synergies [20]. Currently, Zhou et al. [21] have achieved solar-to-hydrogen efficiency of 9.2 % by using an indium gallium nitride photocatalyst at the optimal reaction temperature of 70 °C. However, the water-splitting thermochemical cycle system, which utilizes heat generated by focused sunlight to drive the thermochemical process, has achieved a higher solar-to-hydrogen efficiency of 17.4 % [22]. A typical water-splitting thermochemical cycle is

shown in Fig. 1c. In this process, the oxygen carrier, such as ceria, releases oxygen at higher temperatures and lower oxygen partial pressures. Subsequently, it extracts oxygen atoms from water to produce hydrogen at lower temperatures and higher oxygen partial pressures.

Photovoltaic-driven water electrolysis technology can indirectly produce hydrogen by coupling photovoltaic cells with water electrolysis. In this system, photovoltaic cells convert solar energy into electrical energy, which is then transmitted through an external circuit to an electrolyzer for the process of water splitting [23]. Currently, while water electrolysis efficiency has reached 80 % [24], commercial photovoltaic cell efficiency is typically below 20 % [25], leading to an overall solar-to-hydrogen efficiency of 10 %–20 %.

In electrolysis and photoelectro catalytic water splitting, hydrogen is typically released from the electrode surface in the form of bubbles. Bubbles are commonly observed in processes such as water electrolysis, photocatalytic water splitting, and liquid boiling. Among these, the study of boiling bubbles is the earliest and most extensive. Although boiling bubbles do not involve hydrogen generation, they still provide a substantial theoretical foundation and experimental methods for researching bubbles in water electrolysis and photoelectrochemical water splitting. Bubbles produced during water electrolysis and photocatalytic water splitting have many comparable kinetic features with boiling bubbles. The comparison of bubble dynamics characteristics under the three systems is shown in Table 1. However, since boiling bubbles do not directly release hydrogen, this review focuses on bubble behavior during water electrolysis and photoelectrochemical water splitting under normal gravity and microgravity environments.

In space, solar energy can be converted into storable hydrogen through Photovoltaic-driven water electrolysis technology and photocatalytic water-splitting technology, providing a stable energy supply for space missions. Since the 1970s, technologies for space exploration
have developed rapidly. One notable application is the Environmental Control and Life Support System based on water electrolysis, which has been implemented in space station systems and manned spacecraft [26]. For instance, the National Aeronautics and Space Administration (NASA) installed its solid polymer electrolyte water electrolyzer on the International Space Station in July 2006 [27,28]. Additionally, the regenerative fuel cell system designed for lunar exploration equipment also employs water electrolysis technology for hydrogen production [29].

Due to the absence of atmosphere to absorb or scatter sunlight, the intensity of solar radiation in space is significantly higher than that on Earth. Consequently, photocatalytic systems can capture more solar energy to drive water-splitting reactions, thereby improving the overall energy utilization efficiency. However, the application of photocatalytic hydrogen production in space remains at an early stage of experimental validation. Some researchers have already conducted fundamental studies related to this field, such as exploring material synthesis [30], electrode structure optimization [31], and bubble behavior control in microgravity environments [32,33].

Gravity is one of the key factors influencing bubble behavior. Cooper [34] developed an equation that describes the relationship between bubble detachment diameter and gravity, which pointed out that the detachment diameter of bubbles tends to increase as gravity decreases. The dimensionless numbers used to quantify its effects include the Bond number (Bo, also known as the Eötvös number, Eo) [35,36], Capillary number (Ca) [36], Archimedes number (Ar) [36], and Froude number (Fr) [37–39]. Both a large bubble size and a significant density difference between the liquid and gas lead to increases in the Bo and Ar numbers. In such cases, gravity plays a major role in determining the bubble’s shape, detachment, and rising behavior. In microgravity, the Bo and Ar are very small. As a result, bubbles remain nearly spherical,

and their detachment is significantly slowed.

A more detailed understanding of both bubble evolution behavior in microgravity and its key influencing factors is needed to mitigate the adverse effects of bubbles on water electrolysis systems in space applications. There are several high-quality reviews on the dynamics of bubble evolution, the effect of bubbles on water electrolysis efficiency, the principles and applications of water electrolysis technology [40–44]. However, most of these reviews are limited to normal gravity environments. While a few reviews focus on hydrogen production via electrolysis in microgravity, their focus is more on the application of the electrolyzer in space, with limited discussion on bubble evolution [45]. This review systematically summarizes the similarities and differences in bubble evolution behavior under normal gravity and microgravity environments, and proposes strategies of significant reference value for mitigating the adverse effects of bubble adhesion in space environment.

## 2. Bubble nucleation

Nucleation is the initial phase of bubble evolution [43]. In water electrolysis systems, hydrogen molecules readily accumulate near the electrodes [46]. When these hydrogen molecules locally reach super-saturation levels, a metastable condition will occur, where any local fluctuation could result in bubble nucleation, as shown in Fig. 2a. Bubble nucleation from as few as tens of H₂ molecules on the electrocatalyst surface and then rapidly grow to tens of µm in diameter [43].

The process of bubble nucleation is generally explained by bubble nucleation theory [47]. The variation in Gibbs free energy during homogeneous bubble nucleation is shown in Fig. 2b [48]. The surface free energy, bulk-free energy, homogeneous nucleation free energy, and heterogeneous nucleation free energy are presented as a function of the radius. The Gibbs free energy required for homogeneous nucleation

<figure><img src="image_4.png" /><figcaption>**Fig. 1.** (a) Evolution of water electrolysis system in alkaline electrolyzers. (b) Water splitting over semiconductor photocatalyst. (c) Thermochemical water-splitting cycle system [22]. (d) Is reprinted with permission from Elsevier.</figcaption></figure>
<table><caption>**Table 1**
Comparison of bubble dynamic characteristics in different systems.</caption><thead><tr><th>Characteristics</th><th>Water electrolysis</th><th>Photocatalytic water splitting</th><th>Boiling</th></tr></thead><tbody><tr><td>Interfacial interaction</td><td>Heterogeneous nucleation</td><td>Heterogeneous nucleation</td><td>Heterogeneous nucleation</td></tr><tr><td>Driving force</td><td>Supersaturation of liquid phase</td><td>Supersaturation of liquid phase</td><td>Liquid superheat</td></tr><tr><td>Physical process</td><td>Electro catalytic reactions generate gas molecules that form the gas phase</td><td>Light energy causes photocatalytic reactions that generate gas molecules, forming the gas phase</td><td>Heat causes the liquid phase change, forming the gas phase</td></tr><tr><td>Energy and mass transfer</td><td>Involving the transfer of thermal energy, chemical energy, and mass</td><td>Involving the transfer of thermal energy, chemical energy, light energy, and mass</td><td>Involving the transfer of heat energy and mass</td></tr><tr><td>Bubble effect</td><td>Bubble coverage reduces the mass transfer area, and microconvection affects the mass transfer coefficient</td><td>Bubbles affect light distribution and absorption, their coverage reduces the mass transfer area, while microconvection influences the mass transfer coefficient</td><td>Microlayer evaporation and microconvection affect the heat transfer coefficient</td></tr><tr><td>Bubble growth modes</td><td>Inertia-controlled stage<br>R = (Kδ<sub>c</sub>/6ρ<sub>L</sub>)<sup>0.5</sup> t<br>Diffusion-controlled stage<br>R = C(Dt)<sup>0.5</sup><br>Chemical reaction-controlled stage<br>R = (3IR<sub>g</sub>T/(8πFp))<sup>0.33</sup> t<sup>0.33</sup></td><td>Inertia-controlled stage<br>R = (Kδ<sub>c</sub>/6ρ<sub>L</sub>)<sup>0.5</sup> t<br>Diffusion-controlled stage<br>R = C(Dt)<sup>0.5</sup><br>Chemical reaction-controlled stage<br>R = (3IR<sub>g</sub>T/(8πFp))<sup>0.33</sup> t<sup>0.33</sup></td><td>Inertia-controlled stage<br>R = (2Δp/(3ρ<sub>L</sub)))<sup>0.5</sup> t<br>Heat transfer-controlled stage<br>R = 2C<sub>s</sub>(α<sub>i</sub>t)<sup>0.5</sup><br>Adjacent to the heated surface<br>R = (√(12/π)φ + <sup>1</sup>/<sub>√π</sub>(√(λρc)<sub>w</sub> + (√(λρc)<sub>w</sub> + (λρc)<sub>l</sub>))J<sub>a</sub>√α<sub>i</sub>t)</td></tr><tr><td>Factors affecting bubble detachment</td><td>Phase properties, reaction rate, reaction surface wettability, and polarization conditions</td><td>Phase properties, reaction rate, reaction surface wettability, and light conditions</td><td>Phase properties, heat flux, wall temperature, heat transfer surface wettability</td></tr><tr><td>Bubble size</td><td>10<sup>-9</sup> ~ 10<sup>-4</sup> m</td><td>10<sup>-9</sup> ~ 10<sup>-4</sup> m</td><td>10<sup>-3</sup> ~ 10<sup>-2</sup> m</td></tr><tr><td>Growth time</td><td>10<sup>-3</sup> ~ 10<sup>-2</sup> s</td><td>10<sup>-3</sup> ~ 10<sup>-2</sup> s</td><td>10<sup>-2</sup> ~ 10<sup>-1</sup> s</td></tr></tbody></table>

Where R is the bubble radius, K is the Henry constant, δ<sub>c</sub> is the degree of supersaturation of dissolved oxygen, ρ<sub>L</sub> is the electrolyte density, t represents time, Δp is the excess pressure, C is a coefficient, D is the diffusion coefficient, C<sub>s</sub> is the bubble growth coefficient, α<sub>i</sub> is the thermal diffusivity, I is the current, R<sub>g</sub> is the gas constant, T is the temperature, F is the Faraday constant, p is the pressure, φ is the correction factor, λ is the liquid thermal conductivity, ρ is the density, c is the specific heat capacity. The coefficients (λρc)<sub>w</sub> and (λρc)<sub>l</sub> refer to the wall and the liquid material. J<sub>a</sub> is the Jakob number, representing the ratio of the sensible heat absorbed by the superheated liquid to the latent heat required to vaporize the same volume of liquid.

arises from the surface free energy associated with the formation of the gas-liquid interface and the bulk free energy related to the formation of the gas phase. The free energy of homogeneous nucleation is given by Eq. (1).

$$ \Delta G_{Hom} = 4\pi\sigma R^2 - \left(\frac{4\pi}{3}\right) \Delta g_v R^3 \quad (1) $$

where σ is the surface tension, and Δg<sub>v</sub> represents the energy density difference.

Heterogeneous nucleation commonly occurs at non-uniform interfaces, such as within crevices on the electrode surface [49]. These interfaces create favorable energy conditions by reducing the critical supersaturation required for nucleation. A typical heterogeneous nucleation process is illustrated in Fig. 2a. Heterogeneous nucleation requires a lower free energy barrier than homogeneous nucleation due to non-uniform interfaces. It can be described by introducing the geometric function f(θ), which relies upon the contact angle (θ) at the substrate interface.

$$ \Delta G_{Het} = f(\theta) \Delta G_{Hom} \quad (2) $$

Bubble nucleation is affected by electrode surface wettability [51], and its effect is more significant in microgravity due to the absence of buoyancy. The likelihood of cavity formation on a hydrophobic electrode surface is greater than that on a hydrophilic surface [52], increasing the probability of bubble nucleation. Hydrophilic electrodes feature a larger electrochemical active area and a lower adhesion force, both of which facilitate bubble release and provide space for the formation of new bubble nuclei [53].

The wettability of the electrode surface is influenced by the surfactants added, the electrode surface morphology, and the electrolyte's potential of hydrogen (pH). Adding a suitable surfactant to the electrolyte can effectively adjust the electrode surface wettability, improve concentration overpotential, and reduce initial bubble nucleation current. For example, Platinum (Pt) achieved a current density of 26.77

mA/cm² at an overpotential of 0.04 V after adding perfluorooctane sulfonate (PFOS), which is 2.4 times higher than that of pure Pt [54]. Nanostructured electrode surfaces are favorable for bubble nucleation [55]. Mixed-wettable nanoelectrodes, which combine the advantages of both hydrophilic and hydrophobic surfaces, significantly enhance nucleation rate [56]. In acidic solutions, changes in the charge state of the electrode surface and the surface functional groups weaken the interaction between the electrode surface and water molecules. This results in increased surface tension and a more hydrophobic surface. In contrast, in alkaline solutions, the interaction between the electrode surface and OH⁻ ions is strengthened, which enhances the hydrophilicity of the electrode surface [57]. The variation in surface properties caused by adjusting the electrolyte pH can significantly impact the process of bubble nucleation, thereby altering the nucleation density and the initial growth rate of the bubble nuclei.

The growth process of bubble nuclei is not significantly affected by microgravity environment. Matsushima et al. [50] researched the temporal variations in the growth rate of a single bubble in 0.36 M KOH under a constant cathodic current of 80 µA, as shown in Fig. 2c. He divided the bubble growth process into three stages based on time: 1st stage to 0.1 s, 2nd stage to 1.5 s, followed by 3rd stage in which the bubble diameter exceeds that of the electrode one. During the 1st stage, the initial bubble growth rate follows the linear function of time.

$$ d(t) = \left( \frac{2K\delta_c}{3\rho_L} \right)^{0.5} t \quad (3) $$

The growth stage of the bubble nucleus accounts for only a part of the 1st stage. The transient growth curves for the 1st stage in both normal gravity and microgravity environments have nearly identical slopes, around 1.1. This indicates that the growth characteristics of bubble nuclei in microgravity are largely similar to those under normal gravity.
<figure><img src="image_5.png" /></figure>

<figure><img src="image_6.png" /></figure>

<figure><img src="image_7.png" /><figcaption>**Fig. 2.** (a) Heterogeneous nucleation process of an H₂(g) bubble during the hydrogen evolution reaction. (b) The variation in Gibbs free energy during bubble nucleation [48]. (c) Transient behavior of single bubble diameter of hydrogen gas under normal and microgravity environments [50]. (d) is reprinted with permission from ECS. (e) is reprinted with permission from Elsevier.</figcaption></figure>

### 3. Bubble growth

Bubble growth begins from nucleation and continues until it enters the necking phase, just before detaching from the electrode surface [58]. This process involves three stages, involving inertia-controlled growth, diffusion-controlled growth, and chemical reaction-controlled growth [59]. Inertia-controlled growth occurs immediately after the bubble initiates growth and stops when its interfacial acceleration reaches maximum. During this stage, the bubble is controlled by momentum interaction from the ambient fluid [60]. The inertial-controlled and chemical reaction-controlled growth modes are illustrated in Fig. 3a and b, respectively [61]. Diffusion-controlled growth is caused by the reduction of available gas and is controlled by the diffusion of dissolved hydrogen molecules from surrounding medium into the bubble [62]. At this stage, bubble growth depends on hydrogen molecules transfer from the high concentration gradient near the bubble bottom [50]. When current density reaches a high level, the mode of bubble growth is chemical reaction-controlled. The gas produced during electrolysis diffuses directly into the bubble, rather than first diffusing into the solution [63]. The empirical formulas relating bubble radius and growth time for the three modes are presented in Table 1. The key factor in determining the bubble growth modes lies in the relative size between the effective

reactive surface and the bubble diameter [59]. Therefore, by controlling this relative size, it is possible to achieve a transition in the bubble growth modes.

The empirical equation $R(t) = \beta t^x$ is usually used to describe the process of bubble growth in an electrolytic system under normal gravity. In this equation, $R(t)$ denotes the bubble radius at time $t$ after nucleation, $\beta$ denotes the growth coefficient, and $x$ indicates the time coefficient, which varies based on the experimental conditions during electrolysis [64]. Additionally, the time coefficient is also associated with the ratio of the solid electrode surface size to the bubble diameter. Brandon et al. [59] evaluated the bubble growth process when $R(t) \propto t^x$ following the relationships $x = 1, 0.5, \text{and } 0.33$ by controlling the electrode diameter. The results of their research indicate that the time coefficient $x$ increases when the ratio of the electrode surface size to the bubble diameter decreases.

In microgravity, the evolution of electrolytic bubble diameter is described by the empirical equation $R(t) = \beta t^x$. Using parabolic flight, Sakuma et al. [65] investigated the effect of three electrodes with different wettability (hydrophobic, bare, and hydrophilic) on bubble growth in 0.1 M KOH solutions. The difference in time coefficients for inertia-controlled growth mode, diffusion-controlled growth mode, and chemical reaction-controlled growth mode between normal gravity and
<figure><img src="image_8.png" /><figcaption>Fig. 3. (a) Gas molecules transfer into bubble from gas dissolved in electrolyte [61]. (b) Gas molecules injected into bubble directly from gas reaction. (c) Hydrogen bubble growth on hydrophobic and hydrophilic surfaces [51]. (d) Gas-evolution efficiency ($f_g$) on electrodes with different wettabilities under microgravity [65]. (e) Transient behavior of mass transfer coefficients of dissolved hydrogen gas, where $k_s$ and $k_b$ are mass transfer coefficients corresponding to the two categories of microconvection [50]. a) and b) are reprinted with permission from Wiley. c), d), and e) are reprinted with permission from Elsevier.</figcaption></figure>

microgravity are not significant, with values of $x = 1$, 0.5, and 0.33, respectively. It is only that the time coefficient in diffusion-controlled growth slightly decreases on hydrophobic electrodes and slightly increases on hydrophilic electrodes.

Bubble growth is always accompanied by the transfer of substances from the liquid phase to the bubble. This implies that the bubble growth rate is related to the interfacial mass transfer [66]. The mass transfer process near the electrode involves the adsorption of gas molecules on
the electrode surface, mass transfer between solution and electrode surface, diffusion of gas products, and mass transfer of dissolved gas molecules to the growing bubbles [67]. The transfer coefficient of bubbles is less than $5.1 \times 10^{-5} \text{ m} \cdot \text{s}^{-1}$ in nucleation, while it increases rapidly during the growth process and reaches its maximum of $2.8 \times 10^{-4} \text{ m} \cdot \text{s}^{-1}$ before detachment [68].

Electrode surface wettability has a significant impact on bubble mass transfer in the microgravity environment [65]. Xu et al. [51] simulated the bubble growth behavior on hydrophilic and hydrophobic surfaces, as shown in Fig. 3c. A bubble tends to grow preferentially along hydrophobic surface and supersaturated layer, thereby accelerating the diffusion of dissolved gas molecules to the bubble. In contrast, the bubble on hydrophilic electrode surface tends to maintain a spherical shape. This growth mode is less effective at absorbing dissolved gas because it grows toward areas of lower gas concentration in the electrolyte. The fraction of the total dissolved gas absorbed by bubbles near the electrode is referred to as the gas-evolution efficiency ($f_G$) [69]. Sakuma discovered that $f_G$ near the electrode decreases significantly with the increase of electrode hydrophobicity in microgravity, as shown in Fig. 3d. The increase of $f_G$ indicates that hydrophobic electrodes have an advantage of directly collecting dissolved gas, thereby promoting bubble growth.

Mass transfer between bubbles and the surrounding solution is influenced by both natural convection and microconvection. Natural convection is caused by changes in electrolyte temperature and plays a crucial role in bubble growth under normal gravity [70]. However, natural convection is suppressed in microgravity. Meanwhile, surface tension induced by the temperature gradient guides electrolyte flow [71].

Transient curves of the mass coefficients in both normal and microgravity environments are shown in Fig. 3e, measured in 0.36 M KOH with a current density of $-2.6 \times 10^3 \text{ A} \cdot \text{m}^{-2}$. In microgravity, although temperature or concentration gradients can also cause density differences, the absence of buoyancy prevents natural convection. Therefore, single-phase free convection is not considered under microgravity conditions. When the bubble size is small, single-phase free microconvection plays a decisive role in mass transfer under normal gravity, while the influence of natural convection has not been confirmed. Because the bubble size is very small in the 1st stage, the bubble-induced microconvection is not very significant. As bubble growth enters the 2nd stage, gravitational differences begin to emerge and affect the bubble growth rate. Single-phase free microconvection remains almost constant, and mass transfer is more influenced by bubble-induced microconvection. At the same time, the effect of bubble-induced microconvection on mass transfer is more pronounced in microgravity than in normal gravity. In normal gravity, the two types of microconvections can effectively stir the supersaturated gas layer around the bubble, reducing its growth rate. When bubbles reach the 3rd stage, their diameter starts to exceed the electrode diameter. The bubble-induced microconvection is sharply decreased in both normal gravity and microgravity environments, corresponding to a reduced mass transfer rate and a slower bubble growth rate. At this stage, although the growth rate is slower than that in the 2nd stage, the difference in bubble diameter becomes more pronounced. As shown in Fig. 2c, the growth rates during the 3rd stage are 0.32 under microgravity and 0.26 under normal gravity.

## 4. Bubble detachment

When a bubble reaches its critical diameter, it becomes unstable due to the combined effects of various forces. Detachment will occur when it is not possible anymore for forces acting on the bubble to satisfy the momentum balance of the attachment stage [72]. Detachment of bubbles can release the active electrode area that was previously blocked. This process induces convective flow and enhances mass transfer, which

is crucial for efficient gas evolution [73].

For an isolated bubble, the detachment process on the electrode surface can be viewed as an equilibrium state influenced by multiple dynamic forces. The force promoting bubble detachment is regarded as a positive force, while the force preventing bubble detachment is regarded as a negative force [74]. Common expressions for these forces are presented in Table 2.

Karri [75] evaluated the forces acting on the bubbles growing on a horizontal surface and provided an equilibrium expression, which is given by Eq. (4). Chen et al. [76] found that its predicted results tended to underestimate the actual bubble detachment diameter. To resolve this issue, Chen optimized the model by adding the Marangoni force $F_M$, as shown in Fig. 4. In the above model, $F_b$, $F_i$, and $F_p$ tend to lift the bubble, while $F_s$, $F_d$, and $F_M$ resist its detachment. When a surface tension gradient exists at the gas-liquid interface, the Marangoni force occurs, resulting in convective motion near the interface and generating shear stress at the gas-liquid interface. In the case of bubble formation during water electrolysis for hydrogen production, two factors of the Marangoni force should be considered: thermal Marangoni force and solutal Marangoni force. Although the Marangoni force is much smaller than other forces, its effect on microbubbles cannot be neglected.

The expression for the revised force equilibrium is given in Eq. (5).

$$F_d + F_s = F_i + F_p + F_b \quad (4)$$

$$F_d + F_s + F_M = F_i + F_p + F_b \quad (5)$$

Bubble detachment diameter can be predicted by balancing capillarity with buoyancy, which is shown in Fritz's equation [77]. However, it has been observed that the experimental value is 20 % lower than the predicted value. Lv et al. [78] considered the effect of the contact angle $\theta$ based on Fritz's equation, and the corrected predicted values fitted well with the experimental results.

The relationship between gravitational fields and bubble detachment diameter is given in Table 3 [34]. The bubble detachment diameter in microgravity is significantly larger than that in normal gravity. In the photoelectrocatalytic hydrogen production system, both the Marangoni force and surface tension increase with increasing bubble diameter, and the proportion of the Marangoni force and surface tension among the forces resisting bubble departure remains at 18 % and 82 % [79]. Under normal gravity, buoyancy is a vital factor in driving bubbles away from the electrode surface. However, the balance of forces acting on the bubble changes in microgravity environment. Buoyancy is absent in microgravity, and the Marangoni force plays a greater role in preventing bubble detachment. Bubbles generated during water electrolysis for

<table><caption>**Table 2**
Expression of forces acting on the bubble.</caption><thead><tr><th>Force name</th><th>Expressions</th></tr></thead><tbody><tr><td>Hydrodynamic drag force F<sub>d</sub></td><td>F<sub>d</sub> = C<sub>d</sub>ρ<sub>L</sub>(dR/dt)<sup>2</sup>πR<sub>s</sub></td></tr><tr><td>Surface tension force F<sub>s</sub></td><td>F<sub>s</sub> = 2πR<sub>s</sub>σ sin θ</td></tr><tr><td>Marangoni force F<sub>M</sub></td><td>F<sub>M</sub> = |dσ/dT|ΔTπR + |dσ/dC|ΔCπR</td></tr><tr><td>Inertia force F<sub>i</sub></td><td>F<sub>i</sub> = (4πR<sup>3</sup>/3) ρ<sub>L</sub> d<sup>2</sup>R/dt<sup>2</sup></td></tr><tr><td>Pressure force F<sub>p</sub></td><td>F<sub>p</sub> = (2σπR<sub>s</sub><sup>2</sup>/R)</td></tr><tr><td>Buoyancy force F<sub>b</sub></td><td>F<sub>b</sub> = (πd<sub>b</sub><sup>3</sup>/6)(ρ<sub>L</sub> - ρ<sub>G</sub>)g</td></tr></tbody></table>

In the equations, $C_d$ denotes the drag coefficient, $\rho_L$ denotes the electrolyte density, $R_s$ expresses the contact line radius, $\theta$ denotes the contact angle, $\rho_G$ expresses the gas density. $\Delta T$ denotes the temperature difference between the solution and the gas-producing surface. $\Delta C$ expresses the concentration difference between the bulk electrolyte and the dissolved gas on the gas-producing surface.
<figure><img src="image_9.png" /><figcaption>Fig. 4. Forces acting on a bubble [76]. Reprinted under Elsevier permission.</figcaption></figure>

<table><caption>**Table 3**
Expression of the detachment diameter prediction equation of the bubble.</caption><thead><tr><th>Prediction equation for diameter</th><th>Expressions</th><th></th></tr></thead><tbody><tr><td>Fritz's equation [77]</td><td>d<sub>b</sub> = 2(3 σR<sub>p</sub> / (2 ρ<sub>l</sub>g))<sup>1/3</sup></td><td>(6)</td></tr><tr><td>Lv's equation [78]</td><td>d<sub>b</sub> = 2(3 σR<sub>p</sub> sin θ / (2 ρ<sub>l</sub>g))<sup>1/3</sup></td><td>(7)</td></tr><tr><td>Cooper's equation [34]</td><td>d<sub>b</sub> (g/b<sup>4</sup>)<sup>1/3</sup> = 12</td><td>(8)</td></tr></tbody></table>

In the equations, $d_b$ is the bubble detachment diameter, $R_p$ is the radius of the contact circle between the bubble and the electrode surface, which can be obtained by the shadow imaging method, and $b = Ja\sqrt{\alpha_l}$.

hydrogen production are influenced by two types of Marangoni forces: thermal Marangoni force and solutal Marangoni force. The temperature gradient generated by local ohmic heating near the platinum surface can induce Marangoni flow. Massing et al. [80] applied a -7 V voltage to a platinum wire electrode (100 µm in diameter) and produced a bubble with a maximum diameter of 1 mm. When the bubble grew to 0.56 mm in diameter, the calculated Marangoni force was 0.79 µN, directed toward the electrode, which is only one order of magnitude smaller than the buoyancy, 7.2 µN. Ion concentration gradients in the electrolyte can alter surface tension. Park et al. [81] found that increasing the concentration of H₂SO₄ raises the surface tension acting on the bubble ($\sigma_C = \partial\sigma/\partial C$, $\sigma_C > 0$), while increasing the concentrations of HCl, HNO₃, and HClO₄ decreases the surface tension ($\sigma_C < 0$). During water electrolysis, the solutal Marangoni force acting on the bubbles points toward the electrode in H₂SO₄ solutions, while it points away from the electrode in HCl, HNO₃, and HClO₄ solutions. The average detachment diameter of individual hydrogen bubbles follows the order: $d_{H₂SO₄} \gg d_{HCl} > d_{HNO₃} > d_{HClO₄}$. The difference in buoyancy calculated from the bubble detachment diameters in the four electrolytes is approximately 0.3 µN, which is consistent with the magnitude of the Marangoni force reported in related studies, around 1 µN [80,82,83]. Therefore, although the Marangoni force is much smaller than the buoyancy, its effect on bubbles should not be overlooked.

Besides buoyancy and Marangoni forces, bubble detachment is also influenced by electric force. The electric force Fe acting on a hydrogen bubble is generated by the interaction between the cathode electric field and the charges adsorbed on the bubble surface, and its direction points

toward the electrode. In his experiments on water electrolysis in acidic solutions, Bashkatov et al. [84] discovered that the electrode surface was covered by a carpet of microbubbles below a hydrogen bubble. The bubble above the carpet experiences an electric force that varies with its distance from the electrode. This force acts like a spring, pulling the bubble back within a certain range and causing it to oscillate continuously above the carpet. The oscillation frequency of the bubble is closely related to the applied voltage and the bubble growth time. As the bubble grows larger, its buoyancy increases, weakening the counteracting effect of the electric force on the buoyancy, which results in a gradual decrease in the amplitude of the bubble's oscillation. As the bubble volume increases further, the electric force can no longer counteract the buoyancy, ultimately causing the bubble to detach. Buoyancy is absent in microgravity, and the excess electric force is directed toward the electrode. Therefore, the effect of electric force on bubbles during water electrolysis in microgravity cannot be ignored.

## 5. Bubble coalescence

In multi-bubble systems, bubble coalescence is a common phenomenon, which is helpful to bubble detachment. When bubbles coalesce, the resulting bubble's volume increases significantly, which in turn enhances its buoyancy. Additionally, excess surface energy released during the coalescence process is partially converted into kinetic energy. These two factors work together to speed up the new bubble detachment [85].

Common double-bubble coalescence modes are mainly classified into three types: coalescence-adhesion mode, coalescence-bounce mode, and coalescence-detachment mode [86,87]. When the excess surface energy released during bubble coalescence is small, it is entirely absorbed by the deformation of the merged bubble, and the bubble remains attached to the electrode surface throughout the process, which is known as the coalescence-adhesion mode. When more surface energy is released, part of it is converted into deformation energy of the coalesced bubble, while the remaining energy provides the kinetic energy needed for the bubble to rise. However, the downward Marangoni flow caused by concentration and temperature gradients on the electrode surface may pull the recently detached bubble back to the surface, resulting in the coalescence-bounce mode. When the released surface energy exceeds a certain threshold, the bubble gains sufficient kinetic energy to overcome the Marangoni flow resistance and, aided by buoyancy, permanently detaches from the electrode surface, which leads to the coalescence-detachment mode.

Chen et al. [88] and Zhao et al. [85] found that the bubble diameter before coalescence and the bubble contact angle with electrode surface are both factors related to bubble coalescence-induced detachment. If the bubble diameter is not large enough before coalescence, energy generated during coalescence may be insufficient for detachment, resulting in continued adherence on the electrode surface. Coalescence-induced detachment regime is bounded by the Fritz model and the critical contact angle $\theta_c$ of about 30°, below which detachment occurs, as shown in Fig. 5a [89]. The gray region represents detachment driven by buoyancy, the blue region represents detachment driven by coalescence, and the red region represents no detachment. The bubble diameter for coalescence-induced detachment is smaller than the theoretical detachment diameter [90].

Bubble coalescence-induced detachment behavior is affected by buoyancy, which is absent in microgravity. Bubble detachment from the electrode surface after coalescence does not follow uniformly accelerated upward motion. As shown in Fig. 5b, Bashkatov et al. [91] researched the coalescence-induced bubble detachment process on a microelectrode in 0.02 mol L⁻¹ H₂SO₄ at -5 V under parabolic flight conditions. The shaded area represents the Pt electrode, and the blue region is the glass bottom. The target bubble is circled in red and was first observed at 0.02 s. Its further growth is primarily governed by continuous coalescence with newly nucleated bubbles on a time scale O
<figure><img src="image_10.png" /><figcaption>**Fig. 5.** (a) Functional relationship between effective bubble diameter and contact angle [89]. (b) Bubble Coalescence Behavior in Microgravity [91]. (c) Evolution of bubble level set profiles and velocity fields in microgravity and normal gravity [92]. (a) is reprinted with permission from ACS. (b) is reprinted with permission from RSC. (c) is reprinted with permission from Springer Nature.</figcaption></figure>

(µs). When two bubbles of a similar size coalesce, the symmetrical deformation wave causes the resultant bubble to jump [90]. In Fig. 5b, the two bubbles differ significantly in size. The symmetry of the much smaller deformation wave breaks and induces only a slight disturbance in the larger bubble. Each coalescence event exerts a lateral force on the large bubble, pushing it slightly to the right. With progressing time during the parabola, the residual buoyancy, forces the marked bubble to move to the right side away from the electrode until it finally departs.

Reducing gravity extends the stagnation time of bubbles within the equipment [93]. Pu et al. [92] used numerical simulations to reveal and compare the bubble coalescence process under microgravity and normal gravity environments, as shown in Fig. 5c. The level set profiles and velocity fields are shown at 0, 10, 40, 80 ms, and 0.4 s for microgravity (10⁻² g), and at 0, 10, 40, and 80 ms for normal gravity. In both gravity environments, bubbles show comparable velocity fields and level set distributions, with vortices forming on either side of each bubble. However, vortex strength is significantly greater under normal gravity. At a gravitational level of 10⁻² g, the coalescence time for two bubbles is 0.16 s, compared to 0.06 s under normal gravity. Consequently, the coalescence time for bubbles in microgravity is significantly longer than

that in normal gravity, and their deformation is also much smaller.

## 6. Bubble adhesion

In microgravity, the absence of buoyancy makes bubbles more likely to adhere to the electrode surface due to surface tension, compared to normal gravity [45]. This phenomenon can be clearly observed in Fig. 6a, b, and 6c.

Fig. 6a illustrates the bubble evolution phenomena near the electrode surface under both normal gravity and microgravity environments in 25 wt% KOH and 0.1 N H₂SO₄ solutions [94]. In microgravity, the number of bubbles adhering to the electrode surface increases significantly. Bubbles in the KOH solution are smaller, more numerous, and more tightly packed than those in the H₂SO₄ solution.

The residence time of the H₂ bubble for both normal gravity and microgravity is shown in Fig. 6b. Residence time of H₂ bubbles in alkaline solutions under microgravity remains consistent with that in normal gravity environments. This phenomenon may be attributed to the H₂ bubbles jumping movement from the cathode surface or the microconvection induced by H₂ bubble growth. The residence time of H₂
<figure><img src="image_11.png" /></figure>

<figure><img src="image_12.png" /></figure>

<figure><img src="image_13.png" /></figure>

<figure><img src="image_14.png" /></figure>

<figure><img src="image_15.png" /></figure>

<figure><img src="image_16.png" /></figure>

<figure><img src="image_17.png" /></figure>

<figure><img src="image_18.png" /><figcaption>**Fig. 6.** (a) Bubble evolution phenomena near the electrode surface in both normal gravity and microgravity environments [94]. (b) Adhesion time of hydrogen bubbles at −0.8V [94]. (c) Thickness of the hydrogen bubble froth layer on the cathode surface after 8 s of electrolysis under various electrode potentials [94]. (d) Time variation of ohmic resistance in 2 wt% KOH solution at 140 mA cm⁻² [96]. (e) Bubble evolution on the inverted horizontal electrode surface in both normal gravity and microgravity environments [96]. (f) Time variation of the ohmic resistance during oxygen gas evolution on hydrophobic, bare platinum, and hydrophilic electrodes under microgravity, with a current density of 200 mA cm⁻² [65]. (g) Current-voltage curve of electrolytic water after 8 s under microgravity and normal gravity [97]. (h) Time variation of the anodic potential during oxygen gas evolution, with a current density of 200 mA cm⁻² [65]. a), b), c), f), g), and h) are reprinted with permission from Elsevier. d) and e) are reprinted with permission from IOP Publishing.</figcaption></figure>

bubbles in acidic solutions can reach up to 6.0–6.5 s, which is significantly longer than that under normal gravity. This phenomenon may be related to the increased hydrophobicity of the electrode surface induced by the acidic environment. In contrast, the residence time of O₂ bubbles under microgravity is significantly longer than that under normal gravity in both KOH and H₂SO₄ solutions.

The thickness of H₂ bubble froth layer formed on the cathode surfaces under microgravity is significantly thicker than that under normal gravity, and its thickness increases as the electrode potential decreases, as shown in Fig. 6c. The thickness of H₂ bubble froth layer in the two alkaline solutions is less than that in the acidic solution under normal gravity, and it slightly decreases with lower electrode potential.

The thickness of the bubble froth layer is correlated with the

electrode surface ohmic resistance, potential variation, and current density. The bubble froth layer impedes contact between the electrolyte and the electrode, increasing ohmic losses [95]. As shown in Fig. 6d, the ohmic resistance near the inverted horizontal electrode in microgravity is significantly higher than that in normal gravity [96]. The images of bubble evolution on the inverted horizontal electrode surface are shown in Fig. 6e. In normal gravity, H₂ bubbles generated on the inverted horizontal electrode adhere to the surface and coalesce due to buoyancy. This adhesion reduces the effective cathode area, resulting in increased ohmic resistance. In microgravity, H₂ bubbles produced on the inverted horizontal electrode remain suspended near the surface due to the lack of buoyancy, forming a denser bubble froth layer that obstructs contact between the electrolyte and electrode. An increase in ohmic resistance
leads to a higher electrode potential in a constant current system and a lower current density in a constant voltage system. By comparing the ohmic resistance, electrode potential, current density, and the factors influencing them under normal gravity and microgravity environments, we can indirectly infer the thickness of the bubble froth layer and the important factors regulating it in both environments.

The ohmic resistance near the electrode is influenced by its wettability. The variation in ohmic resistance over electrolysis time for electrodes with different wettability is shown in Fig. 6f. The initial ohmic resistance of electrodes with different wettability is consistent in microgravity environments and gradually increases with the duration of electrolysis [65]. However, due to the variation in bubble adhesion on electrodes with different wettability, the ohmic resistance of hydrophobic electrodes is greater than that of bare Pt electrodes, which is in turn greater than that of hydrophilic electrodes.

In microgravity, the bubble froth layer thickness on the electrode surface can be influenced by the electrode wettability, which is reflected in potential changes [65]. The initial potentials of electrodes with different wettabilities are nearly the same in different gravity environments. However, during the electrolysis process, these potentials change based on the electrode wettability, as shown in Fig. 6h. Due to the influence of bubble coverage, the electrode surface potential is generally higher in microgravity environments. In normal gravity, only the hydrophobic electrode exhibits a higher potential due to bubble adhesion.

Current density during water electrolysis differs between normal gravity and microgravity environments [97]. In normal gravity, the evolution of multiple bubbles generates macroscopic rising flows, which suppress bubble coverage and enhance mass transfer, leading to a higher current density than that in microgravity. Additionally, as shown in Fig. 6g, the difference in current density under normal gravity and microgravity environments becomes greater with increasing electrolyte alkalinity. This is due to the lower conductivity of the 2 wt% KOH solution compared to the 25 wt% KOH solution. Moreover, the decrease in electrolyte pH leads to a reduction in current density, indicating that electrolyte pH affects the compactness of the bubble froth layer on the electrode surface. Despite the thick layer of gas bubbles covering the electrode surface, the current density never drops to zero, and the electrolysis process is not interrupted.

In summary, bubble adhesion behavior in microgravity has a significant impact on water electrolysis systems. Fig. 6f, g, and 6h show that under the same conditions of other variables, the electrode surface in a microgravity environment exhibits higher ohmic resistance, higher electrode potential, and lower current density compared to a normal gravity environment. This indicates that the bubble froth layer formed on the electrode surface is denser under microgravity. Moreover, the bubble froth layer in the electrolyzer is closely related to the electrode wettability and electrolyte pH. Increasing electrode wettability and electrolyte pH helps to reduce bubble accumulation, both of which are beneficial for improving electrolysis efficiency.

## 7. Regulation of bubble detachment

Bubbles adhering to the electrode surface can significantly reduce the effective reaction area of the electrode and also cause fluctuations in the reaction potential during electrolysis, hindering the electrolysis reaction [98]. In microgravity, the degree of bubble adhesion to the electrode surface is further intensified, and the bubble detachment diameter is even ten to twenty times larger than that under normal gravity [99]. Therefore, it is necessary to develop effective methods to eliminate or reduce the adverse effects of bubbles covering the electrode surface and promote bubbles' detachment from the electrode surface.

The regulation of the bubble detachment process can be divided into active and passive regulation depending on whether additional energy is introduced. Active regulation includes the application of ultrasonic or magnetic fields. Although the active regulation strategies are effective, the additional equipment increases the operating and maintenance costs

of the water electrolysis system. Passive regulation includes changing the electrode structure and adding surfactants. Researchers have mainly focused on regulation modes under normal gravity environments. However, because the microconvection of action and effects are relatively similar, these findings still provide valuable reference for regulating bubble detachment under microgravity environments.

The active regulation approach requires the use of an external energy source. For example, an ultrasonic field can be applied to the electrolysis process to reduce the reaction's activation energy, thus effectively enhancing the electrochemical reaction and improving gas-evolution efficiency [100]. In addition, as shown in Fig. 7a and b, the ultrasound-induced acoustic flow can cause bubble oscillation, which helps accelerate the bubble detachment from the electrode surface and forms a smaller and denser bubble plume [101]. As shown in Fig. 7c and d, if an external magnetic field is added to the electrolysis process, magnetic convection can be triggered in the electrolyte [102], which requires utilizing the inherent magnetic properties of water and water-based electrolytes [103]. Magnetic convection causes a local non-uniform force on the electrolyte, regulating the bubble detachment process [104]. Furthermore, the rotation of the electrolytic cell can promote bubble detachment. Experimental research by Weng et al. [105] has demonstrated that even a small rotational speed is sufficient to remove bubbles from the electrode surface and increase the current density at the electrode.

The passive regulation method does not require introducing energy sources external, such as using unique electrode structures to influence the bubble detachment process [106]. As can be seen from Fig. 7e and f, the nanoelectrode can reduce the contact area between the electrode and the bubble compared to the flat electrode [107,108]. Using shadow nanosphere lithography to create catalytic "hot spots" on the surface of nanoelectrodes makes it possible to effectively overcome bubble coalescence and mass transfer limitations [109], without requiring additional surface chemical functionalizations that might impact catalytic performance [110]. In addition, adding surfactants to the solution can regulate bubble detachment. As illustrated in Fig. 7g, the surfactants will accumulate at the gas-liquid interface and reduce the surface tension, thus promoting bubble detachment [111,112].

A reasonable porous structure can effectively enhance the electrochemical reaction process [113]. These porous structures not only provide large internal reactive surface, but also accelerate the transport of mass and electrons during the chemical reaction process [114,115]. Paul et al. compared the bubble removal efficiencies of three different Ni electrode structures (Ni planar, Ni pillar, and Ni recesses electrodes), and found that the Ni recesses have the lowest bubble adhesion force, which allows bubbles to release more easily [116]. Xiao et al. [117] found that, compared to foam nickel and nickel fiber felt, aligned porous electrodes possess larger electrochemical active surface areas (ECSA), stronger bubble removal capabilities, and superior hydrogen evolution reaction performance.

Currently, the mainstream mechanisms for bubble transport within the porous layer can be categorized into two types: capillary-driven transport and buoyancy-driven transport [118]. Capillary-driven bubble transport typically occurs at low Eotvos numbers [119,120], where the capillary number is usually less than $10^{-6}$. However, at high current densities, bubble transport is driven by buoyancy, as bubble clusters grow large enough to overcome the capillary pressure [121]. Although porous layer structures are highly attractive for achieving high efficiency in space applications, their practical implementation faces significant challenges. In microgravity environments, buoyancy is weakened or even absent, causing bubbles to become trapped within the pores of the porous layer, which hinders effective bubble removal [45]. Therefore, new design solutions are urgently needed to ensure the stable and efficient operation of porous electrolyzers under microgravity environments.
<figure><img src="image_19.png" /><figcaption>**Fig. 7.** (a) Efficiency of H₂ production with and without the application of an ultrasonic field, and (b) enhancement of H₂ generation under a constant current density in 0.1, 0.5, and 1.0 M NaOH [101]. (c) Application of a parallel magnetic field and (d) its influence on bubble size distribution [102]. (e) Bubble adhesion behavior on flat film; (f) on nanoelectrode [106]. (g) Change in release frequency with bubble variation upon addition of sodium dodecyl sulfate (SDS) [111]. a), b), c), and d) are reprinted with permission from Elsevier. e) and f) are reprinted with permission from John Wiley and Sons. g) is reprinted with permission from ACS Publication.</figcaption></figure>
## 8. Conclusion and outlooks

Understanding bubble evolution behavior in microgravity is crucial for optimizing the design of electrolyzers in aerospace equipment. Therefore, this review compares the evolution of bubbles during electrolysis in both normal and microgravity environments and discusses the related influencing factors.

In microgravity, bubble evolution behavior exhibits significant differences compared to that under normal gravity. For instance, bubbles have a longer growth time, larger separation diameters, smaller coalescence deformations, and tend to form froth layers on the electrode surface that hinder electrochemical reactions. Hydrophobic electrode surfaces increase the probability of bubble nucleation, and bubbles tend to grow along the hydrophobic surface, which accelerates their growth rate. These effects become more pronounced in microgravity. The absence of buoyancy enhances the effect of Marangoni forces, further suppressing bubble detachment and increasing the detachment diameter. While coalescing bubbles under both normal and microgravity show similar level set profiles and velocity fields, its process in microgravity is slower and involves less deformation. The presence of bubble froth layers leads to increased ohmic resistance and diminished current density. Applying ultrasound fields, magnetic fields, and flow fields, as well as modifying electrode surface structure and adding surfactants, have significant references for facilitating bubble detachment in microgravity environments.

Water electrolysis technology, as an efficient and sustainable source of hydrogen, has shown significant potential for space missions. However, the unique challenges posed by microgravity, high radiation, and extreme temperatures in space impact the stability and efficiency of water electrolysis systems. Therefore, the design and optimization of water electrolysis systems tailored to space environments have become a significant area of current research.

The overall stability and reliability of water electrolysis systems in space missions must be ensured. Therefore, it is essential to develop and utilize materials with excellent resistance to radiation and tolerance to extreme temperatures for manufacturing key components, such as electrolysis cells and electronic components. Additionally, optimizing the structure of the electrolysis cell is crucial to meet the stringent volume and weight requirements of spacecraft water electrolysis systems.

In the microgravity environment, bubbles that are generated during water electrolysis frequently adhere to electrode surfaces. Developing electrode materials with excellent catalytic performance and resistance to bubble adhesion, as well as designing water electrolysis systems capable of detaching hydrogen bubbles autonomously and collecting pure hydrogen, could significantly improve electrolysis efficiency.

Available energy in space is limited, making it necessary to design an energy recovery system to capture excess heat and unused electrical energy generated during the water electrolysis process. These resources could be used to preheat the electrolyte or drive other auxiliary equipment, thus enhancing the overall energy efficiency of the water electrolysis system.

### CRediT authorship contribution statement

**Canghao Zhen:** Writing – review & editing, Writing – original draft, Visualization, Investigation, Conceptualization. **Yangbo Xue:** Writing – review & editing, Writing – original draft, Visualization, Investigation, Conceptualization. **Bo Zhang:** Writing – review & editing, Writing – original draft. **Yechun Wang:** Writing – review & editing, Supervision, Project administration, Investigation, Funding acquisition, Conceptualization. **Yichuang Xing:** Visualization, Data curation. **Qiangqiang Zhao:** Data curation. **Qiuyang Zhao:** Writing – review & editing, Supervision. **Kevin Andrew Tucker:** Data curation. **Zhihui Chen:** Data curation. **Eric Lichtfouse:** Writing – review & editing. **Jinwen Shi:** Writing – review & editing, Supervision, Project administration, Investigation, Funding acquisition, Conceptualization.

### Declaration of interest statement

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

### Acknowledgment

The authors sincerely appreciate the support for this work from the National Key Research and Development Program of China (No. 2022YFB3803600), the Key Research and Development Program of Shaanxi Province (No. 2024GX-YBM-459), and the Fundamental Research Funds for the Central Universities.

## Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2025.150380.

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