In an era where technological innovation races ahead at an unprecedented pace, the world’s appetite for energy grows ever more insatiable. Against this backdrop, the call for clean, sustainable energy sources has never rung louder; solutions that can fuel progress without leaving behind the scars of environmental degradation and global warming. Among the promising candidates stands the aluminium-air battery, celebrated for its remarkable theoretical specific discharge capacity of approximately 2980 mAh/gm and an impressive energy density nearing 8100 Wh/kg [1]. Yet, like many great ideas, it grapples with its own set of challenges. The conventional aluminium-air battery, reliant on aqueous electrolytes, faces formidable obstacles: relentless parasitic corrosion of the aluminium anode, the formation of a stubborn passive aluminium hydroxide layer, unwelcome hydrogen evolution, and a discouragingly high rate of self-discharge [2],[1].

Moreover, a significant amount of aqueous solvent is consumed during the discharge process of an aluminium-air battery [3]. Another well-established aqueous primary battery — the Daniell Cell, also known as the Zinc-Copper battery — employs a metallic copper (Cu) cathode, a copper sulfate (CuSO₄) catholyte, and a metallic zinc (Zn) anode, typically in the presence of an aqueous sulfuric acid (H₂SO₄) or aqueous zinc sulfate (ZnSO₄) anolyte. However, the Daniell Cell is inherently limited by the relatively low theoretical specific discharge capacity of zinc, which is approximately 820 mAh/gm [4], especially when compared to aluminium's much higher theoretical specific discharge capacity of about 2980 mAh/gm [1].

The Leclanché cell, a type of primary battery, was invented and patented by the French scientist Georges Leclanché in 1866 [5]. Commonly known as the Zinc-Carbon (Zn-C) battery, this cell uses an electrolyte of ammonium chloride (NH₄Cl), a carbon (C) cathode, manganese dioxide (MnO₂) as a depolarizer, and a zinc (Zn) anode. The battery’s electrochemical process involves the oxidation of zinc (Zn) at the anode, forming Zn²⁺ ions, while at the cathode, manganese dioxide (Mn⁴⁺) is reduced to manganese oxyhydroxide (MnO(OH), Mn³⁺). Carbon serves as the current collector in this system. The overall cell reaction can be represented as:

Zn(s) + 2 MnO₂(s) + 2 NH₄Cl(aq.) → ZnCl₂(aq.) + 2 MnO(OH)(s) + 2 NH₃(aq.).

Similar to the Daniell cell, the Leclanché cell is constrained by the relatively low theoretical specific discharge capacity of zinc, which is approximately 820 mAh/gm [4].

One of the primary limitations of the Leclanché cell is its inability to sustain a continuous current over an extended period. Prolonged usage initiates internal chemical reactions that increase the cell's internal resistance, leading to a decline in output voltage. However, when the battery is left idle, these reactions partially reverse, allowing the cell to recover some of its performance for intermittent use. This characteristic makes the Leclanché cell well-suited for applications that require short bursts of power followed by rest periods, but ill-suited for continuous, long-duration operation [6].

A variety of primary battery chemistries—such as alkaline batteries, lithium-based primary cells, silver oxide cells, mercury batteries, and zinc–air systems are commercially available. However, these technologies are fundamentally constrained by their comparatively low theoretical specific discharge capacities, which limit their energy density and long-term applicability in high-demand applications.

Considering the factors outlined above, and given aluminum’s abundance, low cost, and environmentally favorable profile, battery systems incorporating aluminum anodes, while addressing the inherent limitations of traditional aluminum–air technologies, represent a compelling solution for large-scale energy storage. These systems are particularly well-suited for deployment in electrical applications, offering a practical approach to reducing dependence on fossil fuels and supporting the transition to more sustainable and resilient energy infrastructure.

This work has led to the development of a primary aqueous battery comprising a cathode made from either high-purity copper (Cu) or a copper-based alloy, and an anode fabricated from either high-purity aluminium (Al) or an aluminium alloy. These electrodes are separated by a membrane composed of ceramic, wood, or any other inert, ion-permeable material that facilitates selective ionic transport between the two compartments. The cathodic compartment is filled with an aqueous solution of copper (II) sulfate (CuSO₄), serving as the catholyte, with a minimum Cu²⁺ ion concentration of 1.0 M (mol/L).

The anodic compartment contains an aqueous solution of aluminium sulfate (Al₂(SO₄)₃), acting as the anolyte, with a minimum Al³⁺ ion concentration of 1.0 M (mol/L). To mitigate the undesired migration of Cu²⁺ ions from the cathode to the anode compartment, both electrolytes are supplemented with an aqueous potassium nitrate (KNO₃) solution at a concentration of at least 1.0 M (mol/L), functioning as a supporting electrolyte (Fig. 1).

The net cell reaction is represented by the following redox equation:

The cathode half-cell reaction for the battery can be given as (Fig. 2),

3 Cu²⁺ (aq.) + 6 e = 3 Cu (s) with standard electrode reduction potential E⁰ value (E⁰_cathode) of + 0.337 volts against SHE (standard hydrogen electrode) at 298 K.

Similarly, the anode half-cell reaction for the battery can be given as (Fig. 2),

2 Al (s) = 2 Al³⁺(aq.) + 6 e with standard electrode reduction potential E⁰ value (E⁰_anode) of -1.662 volts against SHE (standard hydrogen electrode) at 298 K. The overall battery electromotive force (EMF) at standard conditions (E⁰_cell) and at 298 K operating temperature will be as given below.

E⁰_cell = E⁰_cathode - E⁰_anode = [0.337 – (-1.662)] volts = 1.999 volts ~ 2.0 volts at 298 K.

The equilibrium constant K_eq at 298 K for the battery redox reaction can be estimated as,

ln K_eq = E⁰_cell / (RT/6F) =[2.0/(8.3144*298/6*96500)] = 467.37. R being the gas constant; T being the operating temperature of the battery, and F is the Faraday constant; Therefore, K_eq = e^467.37 = 9.4670 x 10²⁰², which is enormously large. Similarly, the Gibbs’ free energy change at standard conditions (ΔG⁰) and at 298 K for the redox reaction will be as given below.

(ΔG⁰) = -6FE⁰_cell = -(6*96500*2.0) = -1158 kJ, which indicates that the reaction is spontaneous.

The equation for the determination of electromotive force (EMF) of the battery under non-standard conditions (E_cell) can be derived from the formulation of the thermodynamic potential of the battery as outlined below.

The net cell reaction of the battery can also be written as (Fig. 2),

At equilibrium,

3 µ^eS(Cu2+) + 2 µ^eAl(Al) + 6 µ^eAl(el) = 3 µ^eCu(Cu) + 2 µ^eS(Al3+) + 6 µ^eCu(el) (1)

In (Eq. 1), µ^eS(Cu2+) is the electrochemical potential of Cu²⁺ ions in the aqueous solution phase; µ^eAl(Al) is the electrochemical potential of metallic aluminium (Al) in the aluminium phase; µ^eAl(el) is the electrochemical potential of electrons in the aluminium phase; µ^eCu(Cu) is the electrochemical potential of metallic copper (Cu) in the copper phase; µ^eS(Al3+) is the electrochemical potential of Al³⁺ ions in the aqueous solution phase and µ^eCu(el) is the electrochemical potential of electrons in the copper phase.

From (Eq. 1), after rearranging, one can write the following equation.

6 (µ^eAl(el) - µ^eCu(el)) = 3 µ^eCu(Cu) + 2 µ^eS(Al3+) − 3 µ^eS(Cu2+) − 2 µ^eAl(Al) (2)

However,

In (Eq. 3), F is the Faraday constant (~ 96500 C/mol of electrons), ϕ^Al is the electric potential in metallic aluminium (Al) phase, ϕ^Cu is the electric potential in metallic copper (Cu) phase, and E_cell is the electromotive force (EMF) of the battery under non-standard conditions.

Expanding (Eq. 2), the following expression can be given.

-6FE_cell = 3 µ^0Cu(Cu) − 2 µ^0Al(Al) + 2 µ^0S(Al3+) − 3 µ^0S(Cu2+) + 2RTln (a_Al3+) – 3RTln (a_Cu2+) (4)

In (Eq. 4), µ^0Cu(Cu) is the standard chemical potential of Cu in the copper phase; µ^0Al(Al) is the standard chemical potential of aluminium (Al) in the aluminium phase; µ^0S(Al3+) is the standard chemical potential of aluminium ions (Al³⁺) in the aqueous solution phase; µ^0S(Cu2+) is the standard chemical potential of copper ions (Cu²⁺) in the aqueous solution phase; R is gas constant; T is the operating temperature of the battery; a_Al3+ is the activity of Al³⁺ ions in the aqueous solution phase; a_Cu2+ is the activity of Cu²⁺ ions in the aqueous solution phase.

In (Eq. 4), (3 µ^0Cu(Cu) − 2 µ^0Al(Al) + 2 µ^0S(Al3+) − 3 µ^0S(Cu2+)) is equal to ∆G⁰ or the standard Gibbs free energy change of the net cell reaction.

Thus,

-6FE_cell = ∆G⁰ + 2RTln (a_Al3+) – 3RTln (a_Cu2+) = ∆G⁰ + RTln (a²_Al3+) –RTln (a³_Cu2+) = ∆G⁰ + RT ln [ (a²_Al3+)/(a³_Cu2+)] (4a)

Since ∆G⁰ = -6FE⁰_cell, the electromotive force (EMF) under non-standard conditions (E_cell) can be expressed as,

-6FE_cell = -6FE⁰_cell + RT ln [ (a²_Al3+)/(a³_Cu2+)]

Or dividing both sides by -6F, one can arrive at the following expression.

E_cell = E⁰_cell – (RT/6F) ln [(a²_Al3+)/(a³_Cu2+)] (5)

It may be stated here that (Eq. 5) is the Nernst equation for the battery.

It is usually difficult to use activities in evaluations of cell potentials, as activity coefficients of ionic species are almost always unknown. A convenient way for avoiding them is the concept of the usage of formal potential instead of standard electrode potential.

The Nernst equation for the battery (Eq. 5) can be expanded as,

E_cell = E⁰_cell – (RT/6F) ln [(γ_Al3+²C²_Al3+)/(γ_Cu2+³C³_Cu2+)]

Or

E_cell = E⁰_cell – (RT/6F) ln[(γ_Al3+²)/(γ_Cu2+³)]- (RT/6F) ln [(C²_Al3+)/(C³_Cu2+)]

Or

E_cell = E^0f_cell - (RT/6F) ln [(C²_Al3+)/(C³_Cu2+)] = E^0f_cell + (RT/6F) ln [(C³_Cu2+)/(C²_Al3+)] (6)

In (Eq. 6), E^0f_cell = E⁰_cell – (RT/6F) ln[(γ_Al3+²)/(γ_Cu2+³)]. E^0f_cell is the formal potential of the battery. γ_Al3+ and γ_Cu2+ are the activity coefficients of Al³⁺ and Cu²⁺ ions respectively. C_Al3+ and C_Cu2+ are molar concentrations of Al³⁺ and Cu²⁺ ions, respectively.

It may be construed from (Eq. 6) that the formal potential of the battery (E^0f_cell) will be the measured cell potential of the battery when [(C³_Cu2+)/(C²_Al3+)] is unity. Alternatively, one may find out the formal potential of the battery (E^0f_cell) from a plot of the measured cell potential (E_cell) against ln [(C³_Cu2+)/(C²_Al3+)], where the intercept of the plot is the formal potential (E^0f_cell). Although individual activity coefficients of ionic species γ_Al3+ and γ_Cu2+ cannot be easily determined, the parameter [(γ_Al3+²)/(γ_Cu2+³)] can be readily determined from the relation: E^0f_cell = E⁰_cell – (RT/6F) ln[(γ_Al3+²)/(γ_Cu2+³)] when one has evaluated the formal potential (E^0f_cell).

The following equation can estimate the power output, P_B, from a battery.

P_B = I*V_CCV (7)

In (Eq. 7), I is the current delivered by the battery, and V_CCV is the closed-circuit voltage of the battery. It is defined as the voltage across the negative and positive terminals of the battery when it is delivering current I. When the battery is not delivering any current (i.e., I = 0), the voltage across the negative and positive terminals of the battery is different, and it is known as the open-circuit voltage V_OCV of the battery. The relationship between open-circuit voltage V_OCV and the closed-circuit voltage V_CCV can be expressed by the following equation (if surface and concentration overpotentials are negligibly small for any operating current I).

V_CCV = V_OCV -IR_Bat (8)

In (Eq. 8), R_Bat is the internal resistance of the battery. (Eq. 8) implies that V_CCV will generally be less than V_OCV, and when R_Bat is zero, then they are equal. However, it should be noted that practically R_Bat cannot be equal to zero in any realistic battery design, and one can only minimise R_Bat to offer a good battery design. When the battery is producing current I in the presence of an external load of resistance R_ext then the following equation can be written using the Ohm’s law.

V_CCV = IR_ext (9)

Thus, referring to (Eq. 8) and (Eq. 9), one can develop the following relationship amongst current I, open circuit voltage V_OCV, internal resistance of the battery R_Bat and the external load resistance R_ext as,

I = V_OCV/(R_Bat + R_ext) (10)

V_OCV is also the electromotive force (EMF) of the battery, E_cell. The drop in voltage or (V_OCV - V_CCV) is an indication of battery internal resistance R_Bat if the contribution of the surface overpotentials at the cathode and anode, as well as concentration overpotentials at the cathode and anode, are very small so that they can be neglected at a given current I.

Moreover, the achievable magnitude of V_OCV is dependent on the electrochemical potential difference of electrons between the anode and cathode of a battery (Eq. 3).

The magnitude of electrochemical potential difference is dependent on the activities of participating ions in the electrolyte (e.g., aAl³⁺, aCu²⁺), standard Gibbs free energy change (∆G⁰) for battery reaction, and battery operating temperature (T) (Eq. 3, Eq. 4a).

It is important to mention here that for the invented aqueous copper-aluminium primary battery, the magnitude of V_OCV lies between 0.61 to 0.7 volts. Following the discussion given above, one can state that for maximising power output from a battery P_B, it is necessary to design a battery which has the least internal resistance R_Bat and a large open circuit voltage V_OCV (Eq. 7, Eq. 8) for a desired output current I. Internal resistance R_Bat is composed of electrolyte resistance, electrode-electrolyte interfacial resistance, and separator resistance. Minimization of the electrolyte resistance can be achieved by decreasing the effective length between the anode and cathode and enhancing the geometrical area between these two electrodes. As ionic conductivities of electrolytes are known to be directly proportional to the operating temperature of the battery (T), the internal resistance of a battery (R_Bat) reduces with increasing operating temperature. One can lower the electrode-electrolyte interfacial resistance by the use of porous electrodes, where the ratio of the geometrical area to the interfacial area shall be minimum at the cathode and anode. Summarizing, one should design a battery with the least effective inter-electrode distance as maximum as possible, high effective geometrical areas of cathode and anode, and maintaining the minimum possible ratio of the geometrical area to the interfacial area. While maximizing the open circuit voltage V_OCV of a battery, one should endeavour to ensure activities of participating ions in the electrolytes to such values that lead to the maximum electrochemical potential difference of electrons between the anode and the cathode of the battery. If one plots the closed-circuit voltage V_CCV of a battery against the discharge time t during which the battery delivers a varying current I under a constant external load resistance R_ext, the plot is termed a discharge curve for the battery.

On the other hand, a plot of the closed-circuit voltage V_CCV against discharge time t under a constant discharge current I produced by the battery by altering the external load resistance R_ext, is also known as the discharge curve for the battery.

Specific discharge capacity C of a battery in mAh/gm can be estimated as,

C = (I*t_dis)/δm (11)

In (Eq. 11) I, is the constant current delivered by the battery (in mA), t_dis is time of discharge (in hours) of the battery, and δm is the weight loss (in gm) of the anode during the discharge process of time t_dis. If the discharge current I is time (t) dependent, then one should evaluate the specific discharge capacity C of a battery as,

C = (11a)

In (Eq. 11a), I(t), is the time-dependent discharge current obtained from the battery in (in mA), t is time (in hours), t_dis is time of discharge (in hours), and δm is the weight loss (in gm) of the anode during the discharge process of time t_dis .

Anodic efficiency η of a battery is defined as the ratio of the specific discharge capacity C to the theoretical specific discharge capacity of the anode material Co and is hence evaluated by dividing the specific discharge capacity C obtained by usage of (Eq. 11) or (Eq. 11a) by the theoretical specific discharge capacity of the anode material Co. In case of the newly developed aqueous copper-aluminium primary battery, Co is 2980 mAh/gm [1]. Therefore, in case of the constant discharge current I, η can be calculated as,

η = (I*t_dis)/(δm*Co) * 100% (12)

and if the discharge current I is time (t) dependent, then η should be evaluated as,

η = (12a)

The energy density P(t) at any time t per active material for the anode (in Wh/kg) of a battery with constant discharge current I can be evaluated as,

P(t) = [(I*t_dis)/δm]*V_ccv (t) (12b)

In (Eq. 12b), V_ccv (t) is the closed-circuit voltage of the battery (in volts), which is always time (t) dependent. Similarly, in case of the discharge current I being time (t) dependent, the energy density P(t) at any time t per active material of the anode (in Wh/kg) of a battery should be estimated as,

P(t) = = (12c)

In (Eq. 12c) Ep(t) is power delivered by the battery in volt-mA, which is also always time (t) dependent.

The current density I_D (in mA/gm) of a battery is estimated by dividing the constant delivered current I (in mA) by the weight loss δm (in gm) of the anode during discharge as,

I_D= (I/δm) (12d)

If the discharge current I is time (t) dependent, then one should calculate the average current density I_DA (in mA/gm) of a battery with the weight loss δm (in gm) of the anode during discharge as,

I_DA = (12e)

Figure 3: Front view and Top view of an experimental aqueous Copper-Aluminium (Cu || Al) primary battery [7].

In Fig. 3, front and top views of an experimental aqueous Copper-Aluminium (Cu || Al) primary battery are shown. With reference to Fig. 3, the battery had been constructed using a rectangular bar of aluminium (Al) or its alloy anode, which had an effective area of more than 15872 mm² and a cylindrical copper

(Cu) or its alloy cathode case that had possessed an effective area higher than 21566 mm².

The battery was provided with a ceramic cylindrical separator that was closed at the bottom end and had an opening at the top end. The ceramic cylindrical separator had an outer diameter of 49 mm and an inner diameter of 41 mm. The separator had a thickness of 4 mm. The rectangular bar of aluminium (Al) or its alloy anode had been placed inside the ceramic cylindrical separator and had been fixed at the centre of the ceramic cylindrical separator using some insulating spacers. Before installation, the rectangular bar aluminium (Al) or its alloy anode had been treated with approximately 0.4 (M) potassium hydroxide (KOH) aqueous solution for 24 hours in a separate polypropylene vessel. The rectangular bar aluminium (Al) or its alloy anode had been washed thoroughly, then with distilled water, dried by a hot air stream, abraded with emery paper, and then fixed inside the ceramic cylindrical separator. The cylindrical copper (Cu) or its alloy cathode case had also been abraded with emery paper from inside, and then was washed thoroughly with distilled water and then dried in a hot air stream. The external electrode terminals (cathode and anode) of the

fully constructed battery (Fig. 3) had then been attached with connecting wires. An external load resistor having a resistance of 8.6 ohms had been used in the external circuit for discharge testing of the constructed battery (Fig. 3). The open circuit voltage V_OCV and the closed-circuit voltage V_CCV of the battery were measured by usage of a high impedance voltmeter before and during the discharge tests. Discharge current readings had been recorded by using an ammeter that had been connected in series with the resistive load of 8.6 ohms.

The cylindrical copper (Cu) or its alloy cathode case (Fig. 3) that is the copper or its alloy electrode compartment in the battery, had been filled with aqueous copper sulfate (CuSO₄) solution, which had a concentration of Cu²⁺ ions of 1.0 mole/lit or more as catholyte up to a minimum 62 mm or more liquid level from the bottom. The catholyte had also been supplemented with aqueous potassium nitrate (KNO₃) solution of at least 1mole/lit concentration or more as a supporting electrolyte to prevent cupric ion (Cu²⁺) crossover to the aluminium or its alloy electrode compartment. The aluminium or its alloy electrode compartment that consisted of a cylindrical ceramic separator and the rectangular bar aluminium (Al) or its alloy anode in the battery (Fig. 3) had been filled with an aqueous aluminium sulfate solution having concentration of Al³⁺ ions of at least 1.0 mole/lit or more as the anolyte up to at least 62 mm or more liquid level from the bottom. This anolyte had also been supplemented with aqueous potassium nitrate (KNO₃) solution of at least 1mole/lit concentration or more as a supporting electrolyte to prevent cupric ion (Cu²⁺) crossover to the aluminium or its alloy electrode compartment.

It is found that if the catholyte and anolyte (Fig. 3) had not been supplemented with the supporting electrolyte (i.e., aqueous KNO₃ solution), cupric ion (Cu²⁺) crossover does take place to the aluminium or its alloy electrode compartment, which causes electroplating of copper (Cu) on the aluminium or its alloy electrode. This phenomenon is undesirable as it may cause a lessening of the specific discharge capacity of the battery.

It is also observed that as soon as the catholyte and anolyte (Fig. 3) are added in the copper (Cu) or its alloy electrode compartment and aluminium or its alloy electrode compartment, respectively, the battery becomes activated immediately, indicating a come-up time (i.e., activation time) as low as less than 15 seconds.

In this work, it is considered that the come-up time (or activation time) is the time required for the output voltage or the open-circuit voltage V_OCV of the battery to reach 2/3rd of its maximum output voltage or open-circuit voltage V_OCV from the time of addition (i.e., zero time) of the catholyte and anolyte to the copper (Cu) or its alloy electrode compartment and aluminium or its alloy electrode compartment, respectively, at an environment temperature of 23 + − 3 ^oC. This battery indicated a maximum output voltage or open-circuit voltage V_OCV plateau greater than 0.6 volt at an environment temperature of 23 + − 3 ^oC (Fig. 6).

Figure 4 and Fig. 5 show the discharge curves of an experimental aqueous Copper-Aluminium (Cu || Al) primary battery. These curves reveal that the battery could deliver very high specific discharge capacity exceeding 2500 mAh per gram of the anode material. These curves also prove that the battery possesses a reasonably high energy density that exceeds 600 Wh per kg of the anode material. The battery also has very good anodic efficiency, which is as high as 86% or more. Moreover, the battery possesses the capability of producing a large electric current output exceeding 5 mA per gram of the anode material, approximately.

It is found that while discharging an experimental aqueous Copper-Aluminium (Cu || Al) primary battery, evolution of hydrogen gas (H₂) does not take place at the anode. Furthermore, the discharge process does not cause consumption of the aqueous solvent, nor does it lead to parasitic corrosion of the aluminium or its alloy anode. It is noticed that the aqueous solution of the aluminium sulfate (Al₂(SO₄)₃) anolyte is acidic in nature having pH value around 3.5 to 4. The acidic characteristic of the anolyte indicates that each Al³⁺ metal ion in the aqueous solution of the anolyte must be hydrated with six water (H₂O) molecules forming an octahedral structure and the strong metal (Al³⁺)-oxygen (O) bond causes weakening of the oxygen (O)- hydrogen (H) bonds leading to hydrolysis and release of protons (H⁺) making the aqueous solution acidic [8]. The relevant reaction is,

[(H₂O)₆Al]³⁺ ◊[(H₂O)₅Al-(OH)]²⁺ + H⁺

The probable reason for non-evolution of hydrogen gas (H₂) at the anode of an experimental aqueous Copper-Aluminium (Cu || Al) primary battery comes from the fact that in aqueous acidic solution, metallic aluminium (Al) can exist only in zero (0) and + III oxidation states as Al/Al³⁺ couple with the reduction potential of the Al/Al³⁺ couple being − 1.66 volts whereas in aqueous basic solution, the aluminium (Al) metal will exist as Al/Al(OH)₃ couple (in zero (0) and + III oxidation states only) with the reduction potential of the Al/ Al(OH)₃ couple being − 2.31volts [8]. The alkaline solution of the Al/Al(OH)₃ couple will eventually release hydrogen (H₂) gas due to following reaction.

Al + 3H₂O ◊[Al(OH)₃] + 3/2H₂

Therefore, as the aluminium sulfate (Al₂(SO₄)₃) anolyte is acidic in nature, Al/Al³⁺ couple with the reduction potential of -1.66 volts will be relevant without any evolution of hydrogen (H₂) gas.

Figure 6 shows the output voltage or the open-circuit voltage V_OCV profile of an experimental aqueous Copper-Aluminium (Cu || Al) primary battery before the start of discharge through an external resistive load of 8.6 ohms. Figure 6 indicates that the maximum output voltage or open-circuit voltage V_OCV of the battery is higher than 0.6 volt at an environmental temperature of 23 +- 3 ^oC. Figure 6 also establishes that the battery possesses a stable electrochemical behaviour, with an output voltage or open-circuit voltage V_OCV plateau at around 0.61 volts. Figure 7 is a photograph indicating the open-circuit voltage of an experimental aqueous Copper-Aluminium (Cu || Al) primary battery as 0.7 volts before discharge. Figure 8 is a photograph of the experimental aqueous Copper-Aluminium (Cu || Al) primary battery powering a red LED light. Figure 9 is a photograph showing the condition (corrosion morphology) of the aluminium or its alloy anode surfaces of an aqueous Copper-Aluminium (Cu || Al) primary battery, which is subjected to continuous discharge for 48 hours, powering a red LED light (Fig. 8). Figures 10 and 11 are photographs showing the treated (with 0.4 (M) aqueous KOH solution for 24 hours) but undischarged anode surface and the untreated and undischarged anode surface, respectively. As is evident from Fig. 9, the morphology indicates intense corrosion throughout the anode surface on account of continuous discharge of the aqueous Copper-Aluminium (Cu || Al) primary battery. The corrosion morphology of the aluminium or its alloy anode surface in Fig. 10 indicates that the surface is seriously attacked by OH⁻ ions from the aqueous solution of 0.4 (M) KOH, which was used during treatment. The reason for this corrosion is due to the reaction of the oxide film present on the anode surface with the aqueous KOH solution, as per the following reaction.

Al₂O₃ + 2KOH = 2KAlO₂ + H₂O

Moreover, during the treatment, in 0.4(M) aqueous KOH solution, aluminium gets oxidised, solvent water gets reduced, producing hydrogen (H₂) gas and an aluminium hydroxide layer on the anode surface on account of the following reactions.

Al + 2H₂O +(OH)⁻ = Al(OH)₃ + H₂ + e⁻

Al(OH)₃ + H₂O + e⁻ = [Al(OH)₄]⁻ + ½ H₂

It is to be noted that the aluminium hydroxide layer on the anode surface is to be removed by abrading with emery paper and then washing thoroughly with distilled water. Figure 11 does not show any corrosion on the untreated and undischarged anode surface.

In conclusion, the first example of a functional aqueous primary battery composed of a copper or copper alloy cathode, an aluminium or aluminium alloy anode, an aqueous copper sulfate catholyte, and an aqueous aluminium sulfate anolyte [Al/Al³⁺,SO₄^2- // Cu^2+,SO₄^2-/Cu] is reported. The battery is capable of delivering high specific discharge capacity exceeding 2500 mAh per gm of anode material with a reasonably stable electrochemical behavior having an output voltage plateau greater than 0.6 volt at an environment temperature of 23 + − 3 ^oC. The energy density of the battery was determined to be exceeding 600 Wh per kg of anode material, and the calculated anodic efficiency of the battery was greater than 86% with the capability of producing an electric current density higher than 5 mA per gram of anode material, approximately. It is noticed that the battery does not lead to hydrogen gas (H₂) evolution at the anode, nor does it cause consumption of aqueous solvent during discharge, nor does it lead to parasitic corrosion of the anode. Finally, it is worth mentioning that if the catholyte and anolyte of the battery are not supplemented with the supporting electrolyte (i.e., aqueous KNO₃ solution), cupric ion (Cu²⁺) crossover does occur to the aluminium or its alloy electrode compartment, causing electroplating of copper (Cu) on the aluminium or its alloy electrode.

a_Al3+ activity of Al³⁺ ions in the aqueous solution phase

a_Cu2+ activity of Cu²⁺ ions in the aqueous solution phase

C_Al3+ molar concentration of Al³⁺ ions

C_Cu2+ molar concentration of Cu²⁺ ions

C specific discharge capacity of anode of a battery

Co theoretical specific discharge capacity of anode of a battery

E⁰ standard electrode reduction potential

E⁰_cathode standard electrode reduction potential of cathode

E⁰_anode standard electrode reduction potential of anode

E⁰_cell cell (battery) electromotive force (EMF) at standard conditions

E_cell electromotive force (EMF) of the battery under non-standard conditions

E^0f_cell formal potential of a battery

Ep power delivered by a battery (in volt-mA)

F faraday’s constant

ΔG⁰ Gibbs free energy change at standard conditions

I current delivered by a battery

I_D current density of a battery (in mA/gm)

I_DA average current density of a battery (in mA/gm)

K_eq equilibrium constant

P_B power output from a battery

P energy density

R gas constant

R_Bat internal resistance of a battery

R_ext external load of resistance

T operating temperature of a battery

t time

t_dis time of discharge of a battery

V_CCV closed-circuit voltage of a battery

V_OCV open-circuit voltage of a battery

µ^eS(Cu2+) electrochemical potential of Cu²⁺ ions in the aqueous solution phase

µ^eAl(Al) electrochemical potential of metallic aluminium (Al) in the aluminium phase

µ^eAl(el) electrochemical potential of electrons in the aluminium phase

µ^eCu(Cu) electrochemical potential of metallic copper (Cu) in the copper phase

µ^eS(Al3+) electrochemical potential of Al^3⁺ ions in the aqueous solution phase

µ^eCu(el) electrochemical potential of electrons in the copper phase

ϕ^Al electric potential in metallic aluminium (Al) phase

ϕ^Cu electric potential in metallic copper (Cu) phase

µ^0Cu(Cu) standard chemical potential of Cu in the copper phase

µ^0Al(Al) standard chemical potential of aluminium (Al) in the aluminium phase

µ^0S(Al3+) standard chemical potential of aluminium ions (Al³⁺) in the aqueous solution phase

µ^0S(Cu2+) standard chemical potential of copper ions (Cu²⁺) in the aqueous solution phase

γ_Al3+ activity coefficient of Al³⁺ ions

γ_Cu2+ activity coefficient of Cu²⁺ ions

δm weight loss of the anode during discharge

η anodic efficiency of a battery