A high-performance H2O2-based fuel cell for air-free applications

Depletion of resources and energy crisis in tandem with the increase in global population are undoubtedly some of the threatening challenges in our modern society [1,2]. To significantly cut down the massive energy dependence on the limited and inefficient fossil fuels as well as their corresponding greenhouse gas emissions, the development and use of clean and renewable power sources are now of increasing importance and thus remain as one of the ultimate targets for researchers and scientists across the globe [[3], [4], [5], [6]]. Over the years, fuel cell technology has emerged as an efficient and environmentally friendly energy conversion option as it converts the chemical energy available in fuel directly to electrical energy [7,8]. Beginning with the use of hydrogen, a chemical energy carrier, hydrogen fuel cells have received a broad array of applications, including the transport sector. However, the high-pressure requirement, safety and flammability issues, storage as well as transport difficulties of hydrogen seriously limit the continuous application and large-scale deployment of hydrogen fuel cells [[8], [9], [10]]. This therefore necessitated the needs to introduce and employ alternative fuels for the operation of fuel cell systems.

Due to the modularity and flexibility of utilization of fuel cells, a number of liquid fuels, including alcohols such as methanol and ethanol, have been employed to operate fuel cells. These liquid fuel cells have also been demonstrated to offer several benefits, including high energy density, high safety, and easy storage, in comparison to hydrogen fuel cells [11,12]. Nevertheless, the high catalyst cost, fuel crossover, short durability, limited energy efficiency, and low power density associated with these direct liquid fuel cells later raised major concerns on the needs to utilize more efficient liquid fuels towards improving the cell performance. In response to this, the idea of using an electrically rechargeable liquid fuel (e-fuel) was recently put forward [13] and is currently receiving an increasing research attention [14,15]. Interestingly, both organic and inorganic materials were stated as prospective and suitable candidates to produce e-fuels. Subsequently, an e-fuel solution which contains vanadium ions dissolved in sulfuric acid was used to fuel the anode of a fuel cell and thereafter showcased its promising capabilities for running a fuel cell system [[16], [17], [18]]. Addressing the fuel issues prevalent in hydrogen and alcoholic fuel cells, the use of this e-fuel offers several advantages including the elimination of catalyst material at the anode, thereby reducing the fabrication cost of the cell. This particular e-fuel has therefore been demonstrated to possess excellent potential and fascinating properties to remain dominant over a long term in fuel cell applications.

Generally, an oxidant is required at the cathode of fuel cells. Gaseous oxygen or ambient air has been mostly employed for this purpose. However, their utility and performance are largely limited in air/oxygen-free environments and similar special conditions, such as space propulsion, underwater power systems, and applications requiring compact systems [[19], [20], [21], [22]] As an alternative and a realistic oxidant, hydrogen peroxide has been demonstrated to be workable in fuel cells under the above-mentioned air-independent systems and for high power applications. In addition, the use of hydrogen peroxide provides several advantages in comparison with gaseous oxygen [[23], [24], [25]]: (1) significantly increases the theoretical voltage of the fuel cell, (2) offers low activation loss of reduction reaction due to two-electron transfer, (3) avoids water flooding issue and simplifies heat removal as a result of its intrinsic liquid phase, (4) offers higher current density due to its higher mass density compared to gas, and thus ultimately improves the performance of the fuel cell. Following these intriguing advantages, hydrogen peroxide has been employed as oxidant in the operation of fuel cells.

Interestingly, the reduction reaction of hydrogen peroxide can occur in both acidic and alkaline media [23]. However, to achieve high fuel cell performance, the reduction reaction is mostly preferred in acidic media as a result of its inherently higher potential (1.78 V) compared to alkaline media (0.87 V) [23,26]. In addition, hydrogen peroxide is less stable in alkaline solution leading to its high decomposition rate [27]. On the other hand, the addition of sulfuric acid not only stabilizes the hydrogen peroxide solution but also enhances its electrochemical reduction [23]. While various fuels, including hydrazine [20] and ethylene glycol [28], have been paired with hydrogen peroxide for fuel cell operations, sodium borohydride is the mostly used fuel in H₂O₂-based fuel cells with numerous studies, including the development of anode [[29], [30], [31], [32]] and cathode electrocatalysts [[33], [34], [35]], and bipolar interface membrane electrode assembly [36,37] with the aim to improve the performance of direct borohydride hydrogen peroxide fuel cells. However, the toxic nature and safety concerns regarding the hydrogen evolution of the borohydride solution and the undesired disparity between the pH environment of the alkaline anolyte and acidic catholyte [38,39] have largely constrained the commercialization of direct borohydride hydrogen peroxide fuel cells.

Herein, the operation as well as the performance of an inherently compact H₂O₂-based e-fuel cell, as it combines an all-aqueous reactant and the superior advantages of both e-fuel (at the anode) and H₂O₂ (at the cathode), is experimentally examined. The hydrogen peroxide aqueous solution used at the cathode is, however, directly acidified using sulfuric acid. The operation principle of this H₂O₂-based cell system, schematically presented in Fig. 1, is simply described below:

At the anode, the e-fuel is oxidized as follows [40]:$V^{2+}\to V^{3+}+e^{-}{E}_a=-0.26Vvs.SHE$

At the cathode, the reduction reaction of the hydrogen peroxide is as follows [41]:$H_2{O}_2+2H^{+}+2e^{-}\to 2H_{2}O{E}_c=1.78Vvs.SHE$

The overall reaction of the e-fuel/hydrogen peroxide cell is:$2V^{2+}+H_2{O}_2+2H^{+}\to 2V^{3+}+2H_{2}O{E}_0=2.04V$

The experimental investigation of this novel e-fuel/hydrogen peroxide fuel cell produced a peak power density of 1456.0 mW cm⁻² at 60 $°C$, which is 70% higher than the use of oxygen (857.0 mW cm⁻²) [16], and also indicates a maximum current density exceeding 3000 mA cm⁻². Such impressive performance not only outshines the e-fuel cells receiving oxygen as oxidant, but also exceeds many of the regular direct liquid fuel cells whose oxidant is H₂O₂. This therefore reveals the capability of this e-fuel/hydrogen peroxide fuel cell as a promising power generation system in airtight environments and high-power applications. We further investigated the performance of the cell at various operating conditions including varying hydrogen peroxide concentration, sulfuric acid concentration, vanadium ion concentration, operating cell temperature, Nafion membrane thicknesses, and its constant-current discharge characteristics. The details of the experiment and the various results obtained from the investigations are discussed in subsequent sections.

Membrane electrode assembly

For the membrane electrode assembly, a catalyst-free graphite-felt electrode having a size of 4.0 cm² and treated by heating for 5 h in the air while subjected to a temperature of 500 $°C$ was used as the anode. A pretreated Nafion membrane, adopting the preparation method as reported in detail elsewhere [42], of size 9.0 cm² was used for the experiment. A Pt/C coated carbon paper of 0.50 mg cm⁻² loading produced by simply following the procedure used in one of our previous studies [43] was used

General cell performance

The general performance of this H₂O₂-based fuel cell is presented in this section. The anolyte delivered into the cell during its operation is composed of 1.5 M V²⁺ in 4.0 M H₂SO₄ while the catholyte is a combination of 4.0 M H₂O₂ in 1.0 M H₂SO₄. Both anolyte and catholyte are pumped and circulated through the cell at a flow rate of 50.0 mL min⁻¹. The operating temperature of the cell is set at 60 $°C$. Under such an operating condition, the cell achieved a peak power density of 1456.0 mW cm⁻²

Summary

This study presents the operation and performance of a high-performance H₂O₂-based fuel cell for application in air-tight environments. As an alternative oxidant, in place of gaseous oxygen or ambient air, hydrogen peroxide is used at the cathode side of an e-fuel cell. The results obtained from this experimental study show that the application of hydrogen peroxide as oxidant substantially bolsters the performance of the cell to achieve a peak power density of 1456.0 mW cm⁻² at 60 $°C$ and a

CRediT authorship contribution statement

Oladapo Christopher Esan: Investigation, Methodology, Formal analysis, Writing - original draft, Writing - review & editing. Xingyi Shi: Methodology, Formal analysis, Writing – review & editing. Zhefei Pan: Formal analysis. Yun Liu: Resources. Xiaoyu Huo: Resources. Liang An: Conceptualization, Funding acquisition, Supervision, Writing – review & editing. T.S. Zhao: Conceptualization.

Declaration of competing interest

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.

Acknowledgement