1 Introduction

Renewable energy, particularly solar, often experiences imbalances between production and consumption, leading to surplus generation during summer when electricity demand is lower and deficits in winter when consumption spikes. To address this, alternative energy sources that can quickly adjust to these variations are essential. Hydrogen production emerges as a key solution for storing excess renewable energy. The process harnesses solar power for electrolysis, a method that cleaves water into hydrogen and oxygen, utilizing the excess solar capacity. This approach not only stores energy efficiently but does so through a method that is environmentally benign. Off-grid solar power often relies on diesel generators, which emit harmful pollutants such as nitrogen oxide and carbon dioxide and are also affected by the fluctuating prices of fuel, impacting operating costs. While battery storage is an alternative for storing solar energy, the capacity required for seasonal storage makes it an impractical solution. In contrast, electrolysis, powered by renewable energy, effectively handles both short-term and long-term variations in renewable energy supply. Electrolysis, combined with renewable energy, forms a more effective hybrid system than steam reforming, which cannot utilize solar or wind energy and results in carbon dioxide emissions. Utilizing electricity from renewables makes hydrogen production through electrolysis particularly advantageous. Among electrolyzers, alkaline and proton exchange membrane (PEM) types stand out. PEM electrolyzers are noted for their superior efficiency, longevity of about 10 years, and compatibility with renewable energy resources [1], capable of producing high-pressure hydrogen suitable for refueling stations [2]. They operate at efficiencies over 80%, significantly higher than other electrolyzers, allowing for optimal conversion of solar energy into hydrogen. PEM electrolyzers can work under pressures up to several hundred bars, removing the need for extra compression equipment and thus simplifying the system and cutting costs. The hydrogen purity from PEM electrolyzers reaches 99.9%, compared to 99.5% from alkaline ones. Their fast response to fluctuations in solar power or electrical input makes them flexible for systems dependent on intermittent renewables. Due to these advantages, including effective solar energy utilization and meeting high-pressure hydrogen demands, this study prioritizes PEM electrolyzers.

PV-hydrogen systems typically employ either direct or indirect coupling through power electronics converters for electrical power conditioning. Direct coupled systems link the PV source and the electrolyzer using a mechanical or electrical switch [3,4,5,6,7,8,9,10], appreciated for their simplicity, reliability, and efficiency. However, they face limitations at the system level, particularly in the PV array’s configuration. Optimal energy extraction from PV becomes challenging when their I-V characteristics closely match those of the electrolyzer, influenced by the number of cells in series within the electrolyzers. While direct coupling can increase system efficiency under various weather conditions, it might not always ensure maximum power transfer [4, 5] and restricts the system’s size adaptability since the electrolyzer’s operational voltage must match the PV’s maximum power point (MPP) voltage. Powered by solar PV energy, the electrolyzer operates at peak capacity for about 5 to 6 h daily, limited by the period during which PV panels produce maximum power due to optimal sunlight exposure. Consequently, the electrolyzer does not function at full capacity outside these peak hours. This discrepancy between the PV’s output and the electrolyzer’s requirements can lead to inefficiencies in converting power to hydrogen, necessitating a system design that maintains a balance between supply and demand while accommodating fluctuations.

The combination of batteries with PV and PEM electrolysis presents a novel approach by stabilizing energy output and minimizing reliance on grid electricity, particularly beneficial in remote or off-grid areas where grid access is limited or inconsistent. This combination allows for the deployment of PV-PEM systems in locations detached from the grid, where connecting to utility services is either not possible or cost-prohibitive. It paves the way for local hydrogen production in areas rich in solar energy, promoting energy independence and supporting rural development initiatives. Additionally, the system can act as a backup power source, maintaining electrolyzer operation and continuous hydrogen production, regardless of solar variability. In settings connected to the grid, battery storage plays a critical role in managing peak demand charges by storing electricity during off-peak periods and releasing it when demand and prices spike. This strategy lowers electricity costs and improves the economic viability of PV-PEM systems, further encouraging decentralized hydrogen production in regions with abundant renewable resources. This approach enhances the alignment between the intermittent nature of solar energy and the continuous operational demands of the electrolyzer, optimizing energy efficiency and reducing the necessity for excessively large electrolyzer units. This not only brings down costs but also improves the economic aspect of the system. Such enhancements are vital for ensuring the system’s reliability and resilience, critical for applications that depend on a continuous hydrogen supply, like industrial operations or backup power systems.

Incorporating an energy storage device within a PV array or power plant, managed by solar charge controllers, effectively stabilizes the energy supplied to the electrolyzer, achieving a consistent or specific load profile. Utilizing maximum power point trackers (MPPTs) [11,12,13,14,15] addresses the issue of power variability, facilitating precise control and optimization of solar power, crucial for maximizing energy extraction across different environmental conditions. This is especially important as the optimal point for energy extraction, the MPP, shifts with changes in sunlight and temperature, necessitating ongoing adjustment by the converter.

However, for standalone PV-electrolyzer systems, constant MPP tracking may not be obligatory. Situations such as maintenance periods or when hydrogen storage reaches capacity might call for reduced PV power to accurately manage hydrogen production rates. In these instances, adjusting the electrolyzer’s current for direct control over hydrogen output proves more difficult than focusing solely on MPP tracking [16, 17]. This approach gains particular significance in applications like spacecraft or remote systems, where adding additional electrical loads is undesirable due to the extra power required. Hence, PV converters in such scenarios are equipped to handle both MPPT and current modulation [18,19,20,21]. This dual capability underscores the need for further research into PV-PEM system integration, a field that has yet to be fully explored, particularly concerning standalone power converters for PV-electrolyzer setups.

In this study, a DC-DC buck converter is utilized to bridge the gap between the electrolyzer and battery storage, regulated by a charge controller that adjusts the voltage and current to meet the demand of the load. This setup effectively adapts the DC output from the solar panels to the specific voltage levels required for hydrogen production, overcoming the limitations seen in direct PV to PEM electrolyzer connections. In direct coupling, increased solar irradiance leads to higher power output but also to energy waste, as the surplus energy generated cannot be stored or utilized beyond the electrolyzer’s immediate demand, thus decreasing overall energy efficiency. The approach discussed here mitigates this inefficiency by storing excess PV energy in batteries, enhancing energy utilization, and slightly reducing the electrolyzer’s need for constant high capacity. The stored power can then be drawn upon during periods of low solar output, ensuring uninterrupted operation and offering a versatile design approach that circumvents inefficiencies related to grid transmission losses, particularly relevant in isolated locations with scant grid access. This method not only secures a steadier and more reliable energy supply for hydrogen production but also underscores the viability of hybrid renewable energy setups, especially those incorporating solar PV, water electrolysis, and battery storage, towards sustainable hydrogen generation. Despite the availability of numerous simulations and theoretical studies on battery-supported hydrogen generation, there remains a significant gap in research concerning the design of system components. The focus of this paper is to explore the optimization of solar energy use through battery assistance, investigating the water electrolysis process and evaluating the performance of a laboratory-scale PEM electrolyzer powered by rooftop solar PV panels for the continuous production of green hydrogen.

2 Water electrolysis principle

Water electrolysis is a recognized method for generating oxygen and hydrogen gas. In the process of PEM electrolysis, water is introduced at the anode, where it undergoes electrochemical decomposition into oxygen, protons, and electrons, driven by an electric current in the presence of electrolytes. The protons then move across a proton-conducting membrane to the cathode, while electrons travel through the external circuit, supplying the necessary cell voltage for the reaction. At the cathode, protons and electrons reunite to form hydrogen gas. The reaction yields hydrogen and oxygen in a volumetric ratio of two to one, respectively. The design of the electrolyte membrane ensures the separation of the gases, preventing the formation of a potentially explosive mixture within the electrolysis device. The production rate of these gases is directly proportional to the current flowing through the cell. The illustrated scheme, referred to as Fig. 1, outlines the basic principle behind water electrolysis, including the two half-cell reactions involved in the process.

Fig. 1
figure 1

Schematic illustration of proton exchange membrane (PEM) electrolyzer, featuring the core components such as the anode, cathode, and PEM, along with the direction of water input, oxygen and hydrogen gas output, and the flow of proton (H+) ions across the membrane, powered by a DC source

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At anode:

$$\begin{array}{cc}{H}_{2}O\to {2H}^{+}+{2e}^{-}+\frac{1}{2}{O}_{2}& {E}_{Cell}=-1.23V\end{array}$$
(1)

At cathode:

$$\begin{array}{cc}{2H}^{+}+{2e}^{-}\to {H}_{2}& {E}_{cell}= 0.00V\end{array}$$
(2)

Therefore, the overall reaction of this decomposition can be written as

$$\begin{array}{cc}{H}_{2}O\to {H}_{2}+\frac{1}{2}{O}_{2}& {E}_{rev}= 1.23V\end{array}$$
(3)

This minimum necessary cell voltage for electrolysis start-up can be calculated using Gibbs free energy (ΔG). At standard conditions, ΔG for water splitting into hydrogen and oxygen is determined by Eq. (4).

$$\Delta G = nF{E}_{rev}$$
(4)

where n represents the number of electrons involved, F is the Faraday’s constant (96,500 coulombs), and Erev is the reversible voltage, calculated as

$${E}_{rev}=\frac{{\Delta G}^{^\circ }}{nF}=1.23V$$
(5)

However, due to entropy generation during water splitting, it is more accurate to use enthalpy (ΔH) instead of ΔG for potential calculation. At standard conditions, ΔH is 285.84 kJ mol−1, ΔS (entropy change) is 0.1631 kJ/mol K, and ΔG is 237.22 kJ mol−1. Thus, the thermoneutral voltage (VTN), which is the minimum voltage required for electrolysis, is calculated as following Eq. (6).

$${V}_{TN}=\frac{\Delta H}{nF}=\frac{\Delta G}{nF}+\frac{T\Delta S}{nF}=1.48V$$
(6)

where T is the temperature. This voltage, 1.484 V, represents the energy provided entirely by electricity at low output rates, which approximates practical cell performance with a 20% efficiency loss compared to the ideal situation.

In practical conditions, the operating voltage of commercial electrolyzers exceeds theoretical estimates by more than 20% due to power losses in the electrolyte and electrodes, which stem from irreversible processes during gas generation by current flow in an electrolyzer cell. These losses include nonlinear irreversibility at each electrode and ohmic losses from the internal resistance of the cell’s materials and the electrolyte. The primary mechanisms of loss are as follows:

  1. (1)

    Electrokinetic limitations: At the electrode–electrolyte boundary, irreversible electron transfer processes, governed by the Butler-Volmer equation, result in activation overpotentials at both anode and cathode. Catalysts at these sites expedite reaction rates. The oxygen evolution reaction at the anode, necessitating a four-electron transfer, is inherently more complex than hydrogen evolution at the cathode; hence, the anode experiences greater overpotentials.

  2. (2)

    Concentration overpotentials: These are due to the slow diffusion rates of reactants, causing a scarcity of reactants at the electrode surface.

  3. (3)

    Ohmic losses: Result from the resistance to ionic flow in the electrolyte and electrical resistance of electrodes, current collectors, bipolar plates, and the resistances at interfaces between cell components, all of which accrue additional losses as current flows through the cell. These factors—the overpotentials at the anode (ηAn) and cathode (ηCath), along with the reversible cell voltage (Vrev) and the voltage drop from the cell’s area-specific ohmic resistance (RA)—collectively determine the total cell voltage (Vcell).

    $${V}_{cell}={V}_{rev} +{[\upeta }_{An }+{\upeta }_{Cath}]+i.{R}_{A}$$
    (7)

To compensate all losses, additional electrical energy is required, in line with the first law of thermodynamics, which ensures energy conservation. The conversion efficiency, indicating the conversion of electrical energy into chemical energy, is calculated using the higher heating value (HHV) of hydrogen. Given the electrolysis process involves water in the liquid phase, efficiency is determined by

$$\eta =\frac{{V}_{TN}}{{V}_{Cell}}$$
(8)

where \({V}_{{\text{TN}}}\) represents the thermoneutral voltage and \({V}_{{\text{Cell}}}\) is the actual operational voltage during electrolysis.

3 Experimental methods

3.1 Steps involved to develop a single-cell PEM electrolyzer

3.1.1 Membrane pretreatment

Nafion-117, a perfluoro sulfonic acid polymer from DuPont USA, is widely chosen as a proton-conducting membrane in water electrolysis due to its superior chemical and mechanical stability alongside high proton conductivity [22]. Prior to use, the membrane is subjected to a comprehensive purification process: initially, it is boiled in 3% hydrogen peroxide to remove organic impurities, followed by rinsing with distilled water. It is then boiled in 1 M sulfuric acid to remove any metallic impurities and to transition the membrane entirely to its protonated (H+) state [23]. The final cleaning step involves boiling it in distilled water to ensure it is thoroughly cleaned.

3.1.2 Fabrication of membrane and electrode assembly (MEA)

Catalyst-coated membrane (CCM) fabrication-catalyst ink formulation

Various techniques exist for applying anodic and cathodic catalyst layers to the proton exchange membrane [24, 25], with spray coating being the method of choice for smaller surface membrane electrode assemblies (MEAs) [26,27,28]. For larger MEAs (typically hundreds of square centimeters or more), blade coating or screen printing is favored due to their efficiency, consistency, and the homogeneity of the coatings. Based on extensive research on ink formulation [29], we selected an ink composition with a 3 mg/cm2 loading of a commercial electrocatalyst mix of Pt/Ru/IrO2 (Duralyst®) for the anode and a similar loading of PtB (Duralyst®) for the cathode to facilitate hydrogen evolution. The Pt-Ru-IrO2 alloy is recognized for its stability and efficacy as an anodic electrocatalyst over platinum [30]. Platinum remains the preferred choice for the cathode due to its superior water electrolysis performance. These catalysts, combined with a Nafion® ionomer (5 wt.% in water and ethanol, D520 DuPont), were weighed precisely and dispersed ultrasonically in a mixture of deionized water and isopropanol at a 1:1 volume ratio. The composition of the electrocatalytic layers included 10 wt.% ionomer for the anode and 15 wt.% for the cathode, with loadings determined by Eqs. 9 and 10.

The catalyst weight for the anode and cathode is given by

$$\mathrm{Total \ catalyst \ loading}=\left(\frac{mg}{{cm}^{2}}\right)\times Pt \%\times Area ({cm}^{2})/1000$$
(9)

The weight percentage of ionomer in the ink formulation is calculated by

$$Ionomer \ wt \% =(catalyst \ wt\left(g\right)\times wt \ \% \ of \ ionomer)/100$$
(10)

After mixing, the ink undergoes high-shear mixing. The anodic catalyst ink is then applied to one side of a pre-prepared Nafion membrane measuring 7.35 cm × 7.35 cm using a spray gun connected to an air compressor, and left to dry. The cathodic ink is applied in a similar manner to the opposite side. Following these procedures, the PEM electrolyzer is assembled with the MEA, boasting an active area of 54 cm2.

3.1.3 PEMWE single-cell assembly and resting

The assembly of a PEM water electrolyzer, illustrated in Fig. 2a, involves careful selection of gaskets and their thickness to eliminate any potential leaks of gas and water during operation. The MEA is enclosed by two Teflon gaskets, each 0.22 mm in thickness and positioned between precision-machined titanium grade monopolar plates. To ensure optimal assembly, the thickness of the catalyst-coated membrane (CCM) is measured at nine uniformly spaced points using a digital micrometer, with the average of these measurements guiding the choice of gasket thickness, as shown in Fig. 2b.

Fig. 2
figure 2

a Conceptual representation to calculate the gasket thickness for PEM-based single-cell water electrolyzer; b top view for the construction sequence to build the PEM water electrolyzer single cell: (1) water, gas inlet and outlet ports, (2) screws for tightening the cell, (3, 9) cathode and anode end plates, (4, 8) cathode and anode gaskets, (5, 7) cathode and anode current collectors, and (6) membrane and electrode assembly

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In PEM water electrolyzers (PEMWEs), managing water flow is critical for efficiency. Water is channeled through the electrolyzer’s flow field to ensure even distribution over the anode’s active surface. However, with water being consumed in the electrolysis process and replaced by oxygen at a rate of half a mole for every mole of water, this consumption introduces various challenges. These include a reduction in flow velocity as water is used up, leading to inconsistent delivery of reactants and possible blockages by oxygen bubbles at the anode, which can result in uneven current distribution across the cell.

Choosing the right flow-field pattern is crucial for the performance of electrolyzers. An interdigitated pattern on the anode side often leads to suboptimal performance due to inconsistent water distribution [31], resulting in increased electrolysis voltage. Conversely, a serpentine pattern outperforms the parallel flow-field pattern [32], with both serpentine [33,34,35] and interdigitated patterns [36,37,38] in PEMWEs demonstrating superior efficiency over parallel configurations.

In our study, we opted for a cascade flow-field layout on the Ti plates for both the anode and cathode, ensuring a uniform water flow across the anode, leading to a more balanced current density and enhancing overall electrolyzer efficiency. Figure 3 shows the flow-field configuration employed, measuring 7.35 cm × 7.35 cm and covering an area of 54 cm2, with 1 mm wide and deep channels, separated by 0.5-mm slots between baffles. The cell and its flow pattern were set up vertically for the experiments, aiding in gas expulsion through buoyancy due to the vertical flow-field orientation. A porous, platinized titanium fleece from Applied Porous Technology Co., Sweden, served as the gas diffusion layer in both compartments. This 1-mm thick fleece, created from sintered powder, played a part in the assembly between two aluminum clamping plates, secured with 12 pairs of nuts and bolts at 8 Nm torque, constituting the PEMWE cell’s framework.

Fig. 3
figure 3

Schematic diagram of a 3-D view of PEM water electrolyzer components; inset images depict a schematic representation of flow-field pattern and membrane electrode assembly

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Adjustments in cell assembly, including gasket thickness and screw torque, are vital when facing open- or short-circuit issues during testing, often attributed to uneven internal cell pressure. A torque wrench ensures screws are tightened to a precise torque, with gasket dimensions and quantity tailored to maintain uniform pressure alongside the gas diffusion layer (GDL). The electrolyzer’s design incorporates heating, temperature control, and ports for introducing reactants and removing products. To achieve peak and stable performance, an activation process is necessary, involving the circulation of deionized water through the cell for 20 to 30 min to properly humidify the electrolyte membrane.

3.2 PV battery-assisted PEM electrolyzer system setup

Figure 4 represents a renewable energy-driven PEM electrolysis system setup, including a solar PV panel, electrolysis equipment, an MPPT-based solar charge controller, a valve-regulated lead-acid (VRLA) battery, and a DC-DC buck converter. Lead-acid batteries are the choice for substantial off-grid applications due to their affordability, quick responsiveness, and long service life of 10–15 years. Unlike lithium-ion batteries, they can deliver power up to the inverter’s low voltage cutoff, even when deeply discharged, though this may reduce their overall lifespan slightly. The system uses a solar panel with amorphous silicon cells, capable of generating up to 50 W, with an open-circuit voltage of 23.1 V and a short-circuit current of 2.7 A under standard conditions (1000 W/m2 solar irradiance at 25 °C). Positioned atop the laboratory, this panel achieved a peak output of 48 W in November, correlating with an average solar irradiance of 462 W/m2 (Fig. 4).

Fig. 4
figure 4

Schematic representation of the proposed energy management strategy for PV-electrolysis system for hydrogen production

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An MPPT solar charge controller links to the 50-W solar panel, optimizing the variable voltage for the efficient charging of an 18-mAh capacity battery. To accommodate the experimental setup—a single-cell PEMWE needing just 1.48 V—a DC-DC buck converter (CNC, Model No RD6024 RD6018, China) lowers the battery’s 12 V to an appropriate level for the electrolyzer, thus managing hydrogen generation rates. The electrolyzer operates by connecting the anode and cathode to the respective poles of the voltage regulator. Deionized water, with conductivity below 8 µS/cm, is pumped through the anode at 20 mL/min. Oxygen and residual water from the anode are redirected to the reservoir, with oxygen being extracted. The cathode produces hydrogen gas, which is passed through an empty trap and silica gel to eliminate water and moisture, respectively, before the high-purity hydrogen is collected and quantified.

4 Results and discussion

4.1 Electrochemical impedance spectroscopy (EIS) test

The study commenced with analyzing the operational characteristics of a PEM water electrolyzer (PEMWE) through electrochemical impedance spectroscopy (EIS) at varying temperatures to decipher the dynamics of its membrane and electrode assembly. EIS findings are presented in two formats: Nyquist plots, displaying frequency response with a semicircular trajectory indicating system stability, and Bode plots. A typical Nyquist plot, as shown in Fig. 5a, features an initial high-frequency intercept on the real axis denoting ohmic resistance, with the span between high- and low-frequency intercepts on this axis representing polarization resistance. The semicircle’s diameter corresponds to the latter. Reaction kinetics are primarily reflected in the charge transfer resistance [39], while at lower frequencies, mass transport resistance manifests as a secondary curve or straight line. The overall cell resistance integrates these aspects with the additional resistances identified in polarization studies [40].

Fig. 5
figure 5

a Typical Nyquist plot observed for an electrolyzer; b equivalent circuit model for a single PEM electrolyzer. Rm, membrane resistance; Ra, anode resistance; Rc, cathode resistance; CPEa and CPEc, constant phase elements of anode and cathode, respectively

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The corresponding equivalent circuit model depicted in Fig. 5b elucidates the electrochemical cell’s internal processes, incorporating elements such as inductors (L), resistors (R), and constant phase elements (CPE). Here, resistors play a pivotal role by marking the ion and electron conduction routes, shedding light on interfacial charge transport mechanisms, including electron movement in conductors and ion migration in electrolytes, along with the resistances faced at electrode surfaces during charge transfer. This model also highlights where power losses occur, notably at anodes, cathodes, membranes, and end plates and through contact and interconnector resistances. Capacitors and inductors represent the polarization regions relevant to adsorption/desorption phenomena and the electrochemical double layer at the electrode interfaces, respectively. Inductance generally relates to the measurement setup’s cables and connectors. The constant phase element, indicative of electrode surface irregularities, reflects the nuances of charge transfer and mass transfer dynamics across anodic and cathodic reactions.

Figure 6 displays the initial impedance spectra for our self-constructed single-cell electrolyzer, captured across a frequency range of 10 kHz with an oscillating voltage of 10 mV. Table 1 outlines the ohmic resistances of the single-cell electrolyzer at various temperatures, highlighting a notable decrease in ohmic resistance as temperature increases. This trend is largely attributed to the membrane’s enhanced water absorption at elevated temperatures, which facilitates the formation of ionic channels for more efficient ion transport compared to a dry membrane. As the temperature rises, the mobility of H+ ions within the membrane improves, significantly reducing ohmic resistance and accelerating the reaction rate.

Fig. 6
figure 6

Impedance spectra recorded at 10 kHz for 54-cm2 single-cell PEM water electrolyzer

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Table 1 Various resistance values obtained from the Nyquist plot for 54-cm2 PEMWE single cell
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The principal component of overall resistance was found to be the charge transfer resistance, with recorded values of 1.005 Ω, 0.842 Ω, and 0.752 Ω at different temperatures. The sharp decrease in charge transfer resistance with temperature increase is primarily due to the enhanced ion mobility in a more loosely packed state, allowing ions to move more swiftly between the membrane and catalyst layer.

4.2 Evaluation of PV battery-PEM electrolyzer

The performance of a 54-cm2 PEM electrolyzer, powered by a solar panel, was evaluated for its current–voltage efficiency in terms of hydrogen production. The chosen solar panel was installed on the roof of the laboratory, oriented south, and positioned at an angle exceeding 90° to the horizontal. This setup was optimized for the tilt angle during the month of November to maximize solar irradiation, yielding an average power of 48 W, averaging solar irradiance of 462 W/m2, as depicted in Fig. 7. The system’s energy management includes a solar charge controller equipped with MPPT technology, designed to charge the connected VRLA battery with the generated direct current. The VRLA battery was always kept in float charging mode at 14.4 V during the operation to compensate for self-discharge and maintain the battery’s state of charge. The power from solar irradiation charged a 12 V, 18 Ah rated VRLA battery with a 90% charge–discharge efficiency. The controller has features to prevent overcharging and control the transition from bulk to float charging when the battery is fully charged. When the battery is low, the solar charge controller will let the solar panel charge the battery as fast as it can. When the battery is nearly full, the controller will limit the current, but it never creates more current.

Fig. 7
figure 7

Graphical representation of solar irradiance and peak average power output from the solar panel in Doha, Qatar, for the month of November

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Key parameters included battery size (Ah), state of charge, depth of discharge indicating a null state of charge, C-rate (the rate at which the battery can charge/discharge relative to its capacity), and the efficiency accounting for energy losses during charging/discharging. For example, if the PV and PEM electrolyzers are directly connected, the chosen PV panel could deliver 2.2 to 2.5 amps at maximum irradiance, and the water electrolyzer must operate at this current, reducing the overall efficiency of the system. However, if the peak power of the PV system is stored by charging the integrated VRLA battery, the entire nominal capacity of the PEM electrolyzer can be used for continuous hydrogen production. The integrated batteries stabilized hydrogen production irregularities and extended the operating rate of an off-grid PV-powered electrolyzer. The energy stored in the battery is utilized by the connected DC-DC step-down converter, an important current regulator that enables the battery storage system to supply the electrolyzer with the correct voltage.

4.2.1 Single-cell polarization studies

The PEM electrolyzer system, featuring a single cell, was operated at ambient temperature by circulating deionized water through the anode side with the help of a peristaltic pump. The power required for electrolysis correlates directly with the applied cell voltage; hence, the system’s performance was assessed across various voltages. This assessment involved measuring current density and hydrogen production rate, with results presented in Fig. 8. Hydrogen production commenced at 1.49 V, showcasing the efficacy of Pt/Ru/IrO2 as an oxygen evolution reaction (OER) catalyst. This indicates that the anode electrocatalyst effectively minimized activation overpotential and that the membrane and gas diffusion layers in the cell configuration maintained a zero-gap resistance without contributing to resistivity.

Fig. 8
figure 8

a Polarization graph and power consumption of PEMWE for hydrogen production at room temperature; b the rate of hydrogen production with respect to electrical energy consumption

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With increasing cell voltage, there was a concurrent rise in both current and hydrogen production, reaching a maximum at a current density of 0.55 A/cm2 with a 2.28 V, generating 12.6 L/h of hydrogen and 11.76 L/h at a current density of 0.51 A/cm2 at 2.23 V. This resulted in energy consumptions of around 5.42 kWh/Nm3 and 5.3 kWh/Nm3, respectively. Figure 8b depicts the relationship between hydrogen production rate and electrical energy consumption at different current densities. Theoretical energy demands for water electrolysis stand at 39.4 kWh/kg or 3.54 kWh/Nm3 of hydrogen, with typical commercial water electrolyzers operating at about 50–67 kW h/kg or 5–6 kWh/Nm3 [41]. The energy consumption of our lab-scale cell is consistent with these commercial values. The key benefit of the system developed in this study is its integration with a renewable energy source, enhancing its attractiveness and cost-effectiveness.

4.2.2 Effect of operating temperature

The study explored the influence of temperature on electrolyzer performance by conducting electrolysis at different temperatures in a cell equipped with a Nafion-117 membrane and an active electrode area of 54 cm2. Figure 9a and b show the variations in current density and hydrogen production rate at different cell voltages across these temperatures. The results indicated an increase in both the current density and hydrogen gas production rate with increasing temperature. Notably, the operating voltages at 60 °C were significantly lower than the ambient temperature for identical current densities, as shown in Fig. 9a. For instance, at a current density of 0.51 A/cm2, the cell voltage at room temperature was 2.23 V, which dropped to 2.03 V at 60 °C for the same current density. Correspondingly, the electrical energy required for water electrolysis at 60 °C and a current density of 0.51 A/cm2 was 4.833 kWh/Nm3. This efficiency suggests that operating the cell at even higher temperatures could further reduce energy requirements, potentially competing with commercial PEM electrolyzers. The acquired results support the interpretations drawn from the recorded impedance data. The improved performance of the cell at higher temperatures is attributed to several factors. Firstly, both the kinetic energy of the reaction and the catalytic activity increase with temperature, leading to a reduction in activation overpotential. This is because the Gibbs energy difference between the products and reactants decreases, favoring spontaneous reactions. As a result, the activation resistance of the cell decreases at elevated temperatures. In addition, the surface tension of the aqueous solution decreases with increasing temperature [42]. This reduced surface tension facilitates greater diffusion of the solution through the anode catalyst layer, thereby lowering the diffusion resistance of the reactants. Consequently, these facts collectively contribute to a significant reduction in reaction resistance, encompassing both activation and diffusion resistances, at higher temperatures.

Fig. 9
figure 9

a Effect of operating temperature on 54-cm2 PEM water electrolyzer at various current densities; b effect of temperature with respect to the rate of hydrogen production at various cell voltages

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4.2.3 Durability studies of PV battery-assisted PEMWE

Evaluating the 54-cm2 PEM electrolyzer’s efficiency, powered by solar energy, involved examining its operation at a 10 A current, corresponding to a current density of 0.185 A/cm2 and a cell voltage of 1.72 V. To produce 4.2 L/h or 100 L over 24 h, the system requires 0.4 kWh of electricity. Given a PV system’s capacity factor of 0.2, the PV power needed is approximately 2 kWh. At maximum efficiency, the electrolyzer directly utilizes electricity from the PV system, consuming 0.4 kWh over 6 h to produce 25 L of hydrogen. The excess power, approximately 1.6 kWh from 2 kWh generated, is stored in a 9.6-kWh battery with an 80% storage efficiency. During peak energy generation, this battery system supplies the necessary 0.4 kWh for around 18 h, facilitating the production of the remaining 75 L of hydrogen. This setup represents an optimal integration of the battery system with the PEM electrolyzer and the PV system for green hydrogen production.

Our initial comprehensive study focused on intermediary devices to stabilize PV power variations and converters to match the electrolyzer’s voltage requirements, proposing an integration approach. In this small-scale experiment, we utilized a 50-W solar panel that required 7 h to fully charge a 12 V, 18 Ah battery. During peak solar intensity, the panel produced a maximum of 2.2 to 2.5 A over 4 h (Fig. 7). Direct integration of the electrolyzer with PV power would necessitate operation at these lower currents, affecting efficiency. Therefore, energy from the PV panel is stored in the battery until its float charge is reached and then drawn by the electrolyzer at a 0.55 C discharge rate to produce 4.2 L/h. Opting for a higher-output solar module could reduce charging times. This arrangement ensures steady hydrogen production even during periods of low solar activity by utilizing stored energy, with the system designed to recharge during times of peak solar output, thereby maintaining consistent hydrogen output.

Although the electrolyzer was only tested through a limited number of production cycles in this study, its capability for sustained operation was demonstrated, supporting the potential for long-term, sustainable hydrogen generation using solar hybrid systems. Voltage stability was monitored over 144 h of continuous operation at a constant current density, showing consistent performance with minor fluctuations that stabilized with time, as illustrated in Fig. 10a. The operational scheme for this solar-assisted PEM electrolyzer system is outlined in Fig. 10b. In contrast to electric vehicles, which demand rapid battery response to changes in driving conditions and quick charging, the charging and discharging cycles for hydrogen production are dictated by solar availability and the electrolyzer’s output needs, leading to a more gradual C-rate [43, 44]. These findings on the C-rate are significant for future research into battery use in hydrogen generation from PV systems.

Fig. 10
figure 10

a Green hydrogen production using 54-cm2 PEMWE single cell. b Conceptual diagram for a few cycles of a battery-assisted PV-PEM single-cell water electrolyzer for hydrogen generation (OCP, open-circuit potential of VRLA; COV, cutoff potential of VRLA)

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4.2.4 The PV-PEM system efficiency analysis

The performance of the electrolyzer is significantly impacted by key parameters such as voltage and current density, which in turn affect the efficiency and cost of hydrogen production. A lower cell voltage results in decreased power consumption, thus reducing the electricity costs for the PEM electrolyzer. According to Fig. 11a, the energy consumption of the system at ambient temperature registered at 4.09 kWh/Nm3 or 45.8 kWh/kg, equating to an 86.5% efficiency. At a modest current density and a temperature of 60 °C, efficiency rose to 91.2%, with energy use at 3.88 kWh/Nm3 or 43.46 kWh/kg. Conversely, a higher voltage at an increased current density of 0.51 A/cm2 led to a reduced efficiency of 73%, which remains in line with typical commercial electrolyzers. Reducing electrical energy consumption can significantly lower the operational costs of electrolyzer. Within the total cost composition of a PEM electrolyzer, the stack represents 45%, and the balance of the plant accounts for the remaining 55%. Since the power supply is integral to the plant’s balance, the study aimed at reducing operational expenses to decrease hydrogen production costs, a critical factor for economic viability.

Fig. 11
figure 11

a A 54-cm2 PEM-based single-cell water electrolyzer efficiency operated at low and high current densities at different temperatures; b PV-H2 system efficiency with respect to the electrical energy consumption of hydrogen

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The efficiency of a solar-to-hydrogen system, known as solar hydrogen production, involves multiple conversion stages: solar energy capture, electrical power generation, and hydrogen production through electrolysis. The overall system efficiency results from the combined efficiencies of these distinct stages, as calculated using an equation shown in Fig. 11b. In our setup, with a PV power efficiency (ηPV) at 10%, power conversion efficiency (ηC) at 90%, and an electrolyzer operating at 86.5% efficiency (ηWE) at room temperature, the overall system-to-hydrogen efficiency (ηS) was determined to be 7.78%. At 60 °C, the efficiency (ηS) improves to 8.2% when running at low current densities but decreases to 6.6% at higher current densities. This performance is a significant step forward in our sustainable hydrogen production approach. It is important to note that the most substantial efficiency losses are in the conversion from solar to electrical power, indicating that improvements in PV technology could further boost the system’s overall efficiency. The formula for system-to-hydrogen efficiency (\({\eta }_{{\text{S}}}\)) is as follows:

$${\eta }_{S}=\left({\eta }_{PV }\times {\eta }_{C }\times {\eta }_{WE}\right)\times 100$$
(11)

When compared to other PV-electrolyzer systems for hydrogen production, as detailed in Table 2, our system, which is designed to provide a stable DC voltage and regulated rate of hydrogen production, demonstrates superior performance.

Table 2 Comparison of different PV-PEM electrolyzer coupled systems for hydrogen production
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4.2.5 Techno-economic cost analysis

Assessing hydrogen’s potential as a commercial energy and industrial feedstock hinges on its production cost. The International Energy Agency (IEA) has observed significant variations in fossil fuel, electricity, and carbon prices across different regions. Despite these disparities, there is a strong likelihood that the levelized cost of hydrogen (LCOH) from renewable electricity electrolysis technologies will undercut that of fossil fuel-based technologies [48]. Projections for 2050 indicate that hydrogen produced through water electrolysis, powered by photovoltaic, wind, or nuclear sources, will likely be more cost-effective than that derived from gasification, according to several studies [49,50,51]. The cost of energy significantly influences the expense of producing electrolytic hydrogen, accounting for 80% of the costs across various electrolyzer capacity factors. Currently, the electricity cost for electrolysis-based hydrogen production, ranging from $0.05 to $0.06/kWh, hinders its competitiveness against large-scale steam methane reforming (SMR) methods, which can produce hydrogen at around $2/kg [52] (Table 3). Achieving cost parity with hydrogen from natural gas also requires reducing electrolyzer capital costs, especially when relying on solar power, which tends to have low capacity factors for electrolyzers. The falling cost of solar energy, reaching as low as $0.02/kWh in optimal settings, is a crucial factor. With such reductions, hydrogen costs can approach or fall below $2/kg, creating favorable conditions for income generation and potential hydrogen export. Additionally, water usage is an important cost consideration in water-stressed regions such as the GCC. Qatar, for instance, offers desalinated water at a lower cost compared to neighboring countries, approximately $0.51 per cubic meter. However, the water usage of PEM electrolyzers is relatively minimal, less than 3% of the total plant balance [53]. Given that only about 9.0 L of water are needed to produce 1 kg of hydrogen, its impact on the overall plant balance is negligible.

Table 3 Total electricity cost/operating cost per kilogram of hydrogen produced based on electricity rate and electrolyzer system efficiency
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5 Conclusion

PEM electrolysis, paired with renewable energy sources like solar, emerges as a promising method for hydrogen production. The energy management system presented in this study ensures a consistent voltage and current supply for controlled hydrogen production, despite the variability in the PV panel’s output due to changing irradiance levels. This study demonstrated the advantages of indirect coupling over direct PV-electrolyzer coupling, incorporating an energy storage system and power management tools. Energy storage-assisted electrolysis not only enhances the operation rate and hydrogen output of the electrolyzer but also contributes to lowering the overall costs of the device. This approach effectively harnesses excess energy through battery storage, making it available for use as needed. This reduces energy loss during transmission and minimizes the rate of unused solar energy, thereby enhancing overall system efficiency.

At room temperature, the PEM electrolyzer demonstrated an efficiency of 86.5%, and at 60 °C, it achieved 91.2% efficiency, using a 54-cm2 single cell operating at a low current density of 0.185 Acm2. The integration of solar power with the PEM electrolyzer through energy storage achieved solar-to-hydrogen system efficiency ranging from 7.78 to 8.2%, complementing the electrolyzer efficiencies of 86.5% and 91.2%. A key advantage of this technology is that the battery-assisted solar PEM-based water electrolyzer boosts the electrolyzer’s operation rate by extending the hours of discharge duration, thereby reducing the LCOH and covering device costs. Looking at regional demand, particularly in the GCC, it is anticipated that the need for hydrogen might reach 25–50 million MT by 2050, driven by existing industries like methanol and ammonia production, as well as emerging sectors such as transportation and energy. By 2050, green hydrogen is predicted to become the most cost-effective hydrogen source, with complete reliance on it anticipated to meet local demand, supported by appropriate legislation and regulations. This study, focusing on producing green hydrogen with low electrical energy consumption and high system efficiency, opens the door for future developments in PV battery-H2 integration technology. These developments are expected to further enhance the system’s efficiency and effectiveness.