Yu Shi,Liang Zhang,Jun Li,Qian Fu,Xun Zhu,Qiang Liao,Yongsheng Zhang

Key Laboratory of Low-grade Energy Utilization Technologies and Systems,Ministry of Education,Chongqing 400030,China

Institute of Engineering Thermophysics,School of Energy and Power Engineering,Chongqing University,Chongqing 400030,China

Keywords:Electrochemistry Thermally regenerative ammonia-based battery Recovery Renewable energy

ABSTRACT This study investigated the important factors that affect the operating parameters of thermally regenerative ammonia-based batteries (TRABs),including the metal electrode type,membrane type,electrode surface area,electrode distance,electrolyte concentration,and ammonia concentration.The experimental results showed that the maximum power density of TRABs with a Cu electrode was 40.0 W﹒m-2,which was considerably higher than that with Ni(0.34 W﹒m-2)and Co(0.14 W﹒m-2)electrodes.TRABs with an anion exchange membrane had a 28.6% higher maximum power density than those with a cation exchange membrane.An increased electrode surface resulted in an increased maximum power but a decreased maximum power density.Within a certain range,TRAB performance was enhanced with decreased electrode distance and increased electrolyte concentration.An increased ammonia concentration resulted in enhanced ammonia transfer and improved the TRAB performance.

1.Introduction

Low-grade heat (<130 °C) obtained from geothermal and solar energy and from industrial processes can be converted to usable energy and can considered as a new type of energy to address the energy problems faced in industrial production [1,2].At present,low-grade heat recovery and utilization technologies mainly include several thermoelectric systems based on semiconductor materials (STEs),organic Rankine cycle systems (ORCs),and membrane-based thermo-osmotic systems (MTOs) [3–5].However,the relatively complex systems and expensive materials limit their developments[5].It is clear that a valuable thermo-electricity technology should have the advantages of a low production cost,high power density,and high efficiency [6].Therefore,this is the main focus of research on low-grade thermal energy utilization technology.

Recently,an efficient thermo-electricity technology,known as the thermally regenerative ammonia battery (TRAB),has been developed;it has demonstrated the ability to produce a high power density [7].The TRAB system mainly consists of a cell reactor for power generation and a heat exchanger for electrolyte regeneration using thermal energy (Fig.1).The anode metal electrode reacts with ammonia to form metal-ammonia complex and produces electrons.The metal ion in the cathode electrolyte receives electrons and is reduced on the cathode.After discharging,the metal-ammonia complex in the anode electrolyte,heated by lowgrade thermal energy in the heat exchanger,can decompose into ammonia and be separated from the solution and reused in the next electricity generation cycle [7].In the next circle,the anode and cathode are swapped and ammonia is added into anode to continue discharging.

Fig.1.Schematic of thermally regenerative ammonia-based battery system.

Previous studies reported several influencing factors on performance and conducted several proposals to improve the power generation of TRABs.Zhang et al.found that increasing ammonia concentration was beneficial to the transmission in the anode reaction,resulting in high performance of TRABs [7].It was also reported that temperature not only affected the electrochemical reaction rate,but also induced the ammonia crossover in TRABs[8].A poly (phenylene oxide) based anion exchange membrane was used in TRABs to effectively improve the performance [9].With respect to the structure of TRABs,a compact reactor with a flow condition was used to enhance the mass transfer [10] and a zero gap cell was designed to decrease internal resistance and improve performance [11].To solve the problem of low reversibility over several cycles,ethylenediamine was used instead of ammonia [12] and silver was used as an electrode instead of copper [13].To obtain greater power output,a bimetallic thermally regenerative ammonia-based battery (B-TRAB) was developed,realizing high-voltage discharge and low-voltage charge [14,15]and on the base of B-TRAB,a kind of TRAB reactor with decoupled electrolytes was proposed to enhance energy and high temperature performance [16].In addition,from the view of increasing the reaction area,researchers investigated a TRAB with 3-D copper foam electrodes to realize high power generation and long operation time [17].Considering the improvement of mass transfer and uneven anodic current distribution on the 3-D electrode,a flow-through design reactor [18] and Cu/C composite electrode with gradient pore diameter [19] were put forward to increase power generation of TRAB with 3-D electrode,respectively.These existing studies made efforts to improve the performance and practicability of TRABs.However,the existing research on the new technology is still very limited so it is necessary to further investigate many aspects,especially the basic operating parameters,which directly influence the battery performance and are significant for practical application.

In this study,for a better understanding of TRAB operation,a rectangular TRAB was constructed to investigate the effects of the basic and important operating parameters,such as the metal electrode type,membrane type,electrode area,electrode distance,concentration of electrolytes,and anodic ammonia,on the battery performance.

2.Materials and Methods

2.1.TRAB construction and operation

As shown in Fig.1,a single TRAB cell used in this experiment consists of anode and cathode chambers,which were separated by an ion exchange membrane (60 mm × 60 mm).The size of the two chambers are both 60 mm × 60 mm × 80 mm.Metal sheets(60 mm×60 mm and 1 mm in thickness)are used as electrodes.In order to monitor the electrode potentials,two Ag/AgCl reference electrodes were inserted in the anode and cathode chambers.The supporting electrolyte is a mixed solution of 0.1 mol﹒L-1Cu(NO3)2and 5 mol﹒L-1NH4NO4.Ammonia was added to the anode supporting electrolyte to form a reaction system.All experiments were performed in a temperature-controlled room at 30°C.

In this study,the effects of operating parameters,including the metal electrode type,membrane type,electrode area,electrode distance,concentrations of NH4NO3and ammonia,on the performance of TRABs were evaluated.Copper,nickel,and cobalt sheets(20 mm × 8 mm and 1 mm in thickness) were used as electrodes using a 5 mol﹒L-1solution of NH4NO3mixed with 0.1 mol﹒L-1of Cu(NO3)2,0.1 mol﹒L-1Co(NO3)2,and 0.1 mol﹒L-1Ni (NO3)2as electrolytes,respectively.To study the effect of membrane types on the TRAB,a cation exchange membrane (CEM,thickness (0.45 ± 0.025) mm,electrical resistance <30 Ω﹒cm2,permselectivity 94%)and an anion exchange membrane (AEM,thickness (0.45 ± 0.025)mm,electrical resistance <40 Ω﹒cm2,permselectivity 90%) were used to separate the cathode and anode electrolytes.Copper sheets(with a projected surface area of 15 cm2and 1 mm in thickness)were used as electrodes and the mixed solution of 0.1 mol﹒L-1Cu(NO3)2and 5 mol﹒L-1NH4NO3was selected as the electrolyte to study the effect of the electrode area and distance on performance.Different copper electrode areas(1,2,6,8,10,20,and 25 cm2)and electrode distances(1.5,4,6,8,10,12,and 14 cm)were used as the experimental variables in a single TRAB cell.Experiments were performed to explore the influence of different concentrations of NH4NO3and ammonia on the performance.The size of the copper electrode was 20 mm×8 mm,and the distance between the electrodes was 4 cm in these experiments.

2.2.Measurement and calculations

The voltage (U) across the external resistant and electrode potentials of the TRAB were measured and recorded by an Agilent 34,970 data acquisition tool.The external resistant was changed through a resistant box ZX95.The measurement of potential began with the open circuit,and then the external resistance gradually decreased from 100 to 0.1 Ω at 2 min intervals.The current (I)was calculated by Ohm’s law and the power was calculated using the formula P=U×I.Both the current density and the power density were normalized to a single electrode projected surface area of a TRAB cell.In this article,power and power density were calculated to examine performance of TRABs with different operating parameters.

3.Results and Discussion

3.1.Effect of the electrode metal types

According to the reaction principle of TRABs,electrical power is generated by an anodic metal oxidation reaction and a cathodic metal ion reduction.Therefore,the performance of TRABs depends on the different redox potential differences caused by the types of reaction metals.Based on three common ammonia complexes,the effects of three different metal electrodes (cobalt,nickel and copper sheets,with the size of 20 mm × 8 mm) on the performance of TRABs were studied.As shown in Fig.2,a TRAB with a nickel metal electrode had the highest open circuit voltage (0.63 V),followed by copper(0.45 V)and cobalt(0.15 V).However,the power density of a TRAB with a copper electrode could reach 40 W﹒m-2at 203 A﹒m-2,which was much higher than nickel (0.34 W﹒m-2) or cobalt(0.15 W﹒m-2).Compared with the maximum power density of other previous studies,the maximum power density in this work (40 W﹒m-2) was within the range of TRAB with copper electrode(9.3–136 W﹒m-2)[7,8,17,20].This result was mainly because copper reacted more easily with ammonia and caused lower reaction resistance.The copper also had the largest redox potential difference in the electrode reaction (0.38 V) compared with nickel(0.23 V) and cobalt (0.15 V).In addition,the copper electrode had the highest electrical conductivity,leading to the lowest ohmic resistant battery compared to TRABs using the other two electrodes.The above research showed that copper metal was the most suitable electrode for TRABs out of the three metals(Cu,Ni and Co)and copper was used as electrodes in follow experiments.

Although the Ni and Co could hardly be used as electrode of TRAB directly,it should be noted that they were expected to be used as the skeleton of composite electrode to improve electrode stability because they were difficult to react with ammonia.In addition,other metals were used as electrodes of TRAB in other studies such as Zn and Ag.Among these metal electrodes,TRAB with Ag electrode could obtain higher anode coulombic efficiency than copper electrode,hence better electrode reversibility,which could balance the mass of anode and cathode in the discharging process [13].And the studies of bimetallic TRAB with Zn-Cu electrodes suggested much higher output power generation could be obtained than that of TRAB with copper electrodes because of the large redox potential difference in the electrode reaction [14,15].Besides,the use of the porous composite electrodes could improve its surface area and stability,which was benefit for the long-time discharge operation of TRAB [19,20].

3.2.Effect of the membrane types

Membranes are widely used to separate the anode and cathode in bioelectrochemical systems,and the ions flow through the membrane.Different membranes would lead to differences in iron transfer and may influence the battery performance.In this section,the effect of membrane types (AEM and CEM) on the performance of TRABs is investigated.TRABs with copper sheets electrodes (60 mm × 60 mm × 1 mm) were operated with the ammonia concentration of 1 mol﹒L-1.As shown in Fig.3,the open-circuit voltage of TRABs using AEMs and CEMs were essentially equal (0.44 V),whereas the values of maximum power were 16.3 W﹒m-2and 14.0 W﹒m-2,obtained at current densities of 75.4 A﹒m-2and 62.3 A﹒m-2,respectively.This result indicated that the performance of TRABs using AEMs is 16.4% better than that of TRABs using CEMs.This was mainly due to the difference between the ions crossing the membrane.Nitrate ions could be transported across the membranes for ion migration between the cathode and anode in TRABs using AEMs.However,in TRABs using CEMs,copper ions were mainly transported across the membrane.The transmission of copper ions from the cathode to the anode occurred in TRABs using CEM and the copper ions reacted with ammonia in anode.Meanwhile,the concentration of ammonia in the anode and that of copper ions in the cathode were decreased.This resulted in a degradation of both cathode and anode performance[7].These results showed that the crossover of ions other than anions had adverse effects on TRABs.Considering the powergeneration performance and stability of TRABs,the AEM was more suitable.Moreover,in future research,a new anion exchange membrane with low internal resistance and better selectivity of ammonia to reduce effect of the ammonia crossover on the power generation.In addition,in consideration of the ohmic resistance caused by membrane,the application of membrane-less reactor for TRAB could be a feasible method to reduce the internal resistance of a TRAB.It should be noted that the ammonia-crossover from cathode chamber to anode chamber in a membrane-less reactor may be a potential problem resulted in the deterioration of performance and stability,which suggests the attention should be paid in the design of membrane-less reactor.

Fig.2.Performance of TRAB using different metal electrode types ((a) Co and Ni,(b) Cu) with 5 M solution of NH4NO3 mixed with 0.1 mol﹒L-1 of Cu(NO3)2,0.1 mol﹒L-1 Co(NO3)2,and 0.1 mol﹒L-1 Ni(NO3)2 as electrolytes corresponding to electrode type and 1 mol﹒L-1 NH4OH in anode.

Fig.3.Performance of TRAB using AEM and CEM with 5 mol﹒L-1 solution of NH4-NO3 mixed with 0.1 mol﹒L-1 Cu(NO3)2 as electrolyte and 1 mol﹒L-1 NH4OH in anode.

3.3.Effect of electrode surface areas

In a single cell of a TRAB,both the anode and cathode reactions occur on the surface of the electrodes,and thus,the area of the electrodes affect the rate of the electrochemical reaction [21].Therefore,copper sheet electrodes with different projected areas(1,2,6,8,10,20,and 25 cm2) were selected to investigate the influence of electrode area on the maximum power and power density.As shown in Fig.4,when the projected surface area was 1 cm2,the maximum power and maximum power density of the TRAB were 3.6 mW and 36.4 W﹒m-2,respectively.With an increase in the electrode area,the maximum power of the TRAB increased,but growth tendency would be obviously slow.This indicates that when the area of the electrode exceeded 25 cm2,it was no longer the limiting factor for the maximum output power of the TRAB.However,the maximum power density of the TRAB showed the opposite trend with the maximum power.This was mainly because the increase in the maximum power of the TRAB was considerably less than the increase in the electrode area.This result showed that with the increase in the electrode area,the maximum power of the TRAB increased gradually,whereas the maximum power density decreased continuously.This was because the power did not increase proportionally with the increasing electrode area [22].It was reported that the increased surface area increased the distance of electrons from the point of generation to the external circuit,which led to power loss,especially in the system with small external resistance[23].Therefore,the 3-D porous electrode can provide larger electrode area and more active sites in the same projected surface area and the volume of reactor,which could be an effective method to improve power generation[17–20].In addition,considering the limitation of power density,the output power can be further improved by using single cells employing electrodes with small projected area to construct stacks,instead of increasing the projected surface area of a single TRAB cell.

3.4.Effect of the electrode distance

The distance between the anode and cathode electrodes could influence the ohmic resistance of a single TRAB cell.In this section,TRABs with different electrode distances (1.5,4,6,8,10,12,and 14 cm) are investigated.As shown in Fig.5,it can be seen that the maximum power density of TRAB cells was 25.5 W﹒m-2when the distance between the electrodes was 1.5 cm.As expected,with the increasing of electrode distance,the maximum power density would gradually decrease.When the electrode distance increased to 4 cm and 6 cm,the maximum power density of the TRAB dramatically decreased to 18.1 W﹒m-2and 9.4 W﹒m-2,respectively.When the electrode distance was further increased,it was found that maximum power density declined slowly and finally maintained at nearly 6 W﹒m-2.This was mainly due to the increase of the distance between the electrodes,which led to the increase of the ion transfer distance between the anode and cathode,and the increase of the ohmic resistance of ion transfer,resulting in the decline of the performance of the TRAB[24].The above results showed that the performance of TRABs would increase with the decrease of electrode distance.Therefore,in future practical application,a compact reactor with short electrode distance is more suitable for the promotion of power output.

3.5.Effect of the concentration of NH4NO3

In addition to the electrode distance,the concentration of NH4-NO3used as the supporting electrolyte is also one of the important factors affecting internal resistance of a TRAB cell [7,17].In this experiment,TRAB cells with different NH4NO3concentration electrolytes(1,3,5,and 8 mol﹒L-1)were used,and the result is shown in Fig.6.When the concentration of NH4NO3was 1 mol﹒L-1,the maximum power density was extremely low (9.7 W﹒m-2).When the concentration increased to 3 mol﹒L-1,the maximum power density of the TRAB (15.0 W﹒m-2) increased by 54.6%,which was mainly due to the increase of the NH4NO3concentration.This led to a significant increase in the conductivity of the electrolyte,thus improving the power density.When the concentration of NH4NO3was increased to 5 mol﹒L-1,the power density was further increased to 18 W﹒m-2.However,when further increasing the concentration of NH4NO3to 8 mol﹒L-1,the maximum power density did not increase dramatically,which indicated that when the concentration of the electrolyte exceeded 5 mol﹒L-1,the concentration of NH4NO3was no longer the limiting factor of TRAB performance.The above results show that TRAB performance increases with the increase of NH4NO3concentration within a certain range.

Fig.4.Effect of different surface area on the maximum power(a)and maximum power density(b)of a TRAB with 5 mol﹒L-1 solution of NH4NO3 mixed with 0.1 mol﹒L-1 Cu(NO3)2 as electrolyte and 1 mol﹒L-1 NH4OH in anode.

Fig.5.Effect of electrode distance on the maximum power density of a TRAB with 5 mol﹒L-1 solution of NH4NO3 mixed with 0.1 mol﹒L-1 Cu(NO3)2 as electrolyte and 1 mol﹒L-1 NH4OH in anode.

Fig.6.Effect of NH4NO3 concentration on power generation of TRAB with 0.1 mol﹒L-1 Cu(NO3)2 as electrolyte and 1 mol﹒L-1 NH4OH in anode.

3.6.Effect of the concentration of NH3

Fig.7.Effect of NH3 concentration in anode on power generation with 5 mol﹒L-1 solution of NH4NO3 mixed with 0.1 mol﹒L-1 Cu(NO3)2 as electrolyte.

As the anodic reactant,the concentration of ammonia directly affected the anode and TRAB performances.It was obvious that low ammonia concentration would cause limited ammonia transfer and affect the anode reaction intensity,while high ammonia concentration would aggravate ammonia crossover and cause mixed potential in the cathode,leading to the deterioration of performance.To explore the suitable ammonia concentration in TRABs,operation conditions with four ammonia concentrations were tested,as shown in Fig.7.As expected,a low maximum power (14.5 mW) was observed in the TRAB with a low ammonia concentration of 0.3 mol﹒L-1.With the increase of ammonia concentration from 0.3 to 2 mol﹒L-1,the maximum power relatively increased from 14.5 mW to 22.1 mW,which indicated that the increasing of ammonia concentration within a certain range could enhance the power generation performance of TRABs.It is worth mentioning that a 42.1% increase in the maximum power was observed after increasing the ammonia concentration from 0.3 to 1 mol﹒L-1,while a further increase from 1 to 2 mol﹒L-1resulted in only a 7.3% improvement in the maximum power generation.It was obvious that the performance improvement was not as pronounced when the concentration of ammonia was greater than 1 mol﹒L-1.This trend was due to a limitation of the anodic reactions.In the low ammonia concentration range,the increasing of ammonia could result in the intensification of anodic reaction and concurrently solve the limitation of ammonia transfer in low concentration.After increasing the ammonia concentration over 1 mol﹒L-1,the reduced influence of increased ammonia concentration on the anode reaction led to the slowly increasing trend of performance.The maximum power might be significantly reduced due to the decreased cathode performance with the further increase of ammonia concentration [7,17].Thus,considering the limitation of ammonia mass transfer in the anode,high power generation of TRABs could be obtained at an appropriately increased range of ammonia concentration.In addition,ammonia regenerated from thermally reactor is in gaseous form and can be added in regenerated anode in the form of gas.Therefore,the TRAB structure should be optimized in the future research.For example,a compact structure with ammonia gas chamber could be designed to make the ammonia gas easier access to the regenerated anode chamber and the ammonia concentration in the anode chamber should be adjusted by the structure of the gas diffusion layer.

4.Conclusions

In this study,the effects of operating parameters on the performance of TRABs were studied experimentally,including metal electrodes types,membrane types,electrodes areas,electrode distance and electrolyte concentrations.The results showed that the maximum power density (40 W﹒m-2) of a TRAB with copper electrodes was much higher than that with nickel and cobalt electrodes.The measurement of power density showed that AEM was more suitable for TRABs compared with CEM.With the increase of the electrode area within a certain range,the maximum power of the battery increases,while the maximum power density decreases.The maximum power density increased with the decreased electrode distance due to the decreased internal resistance.In addition,the maximum power density of TRABs increased with the increase of electrolyte concentration and ammonia centration,within certain ranges.

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.

Acknowledgements

This work was supported by the National Natural Science Foundation of China(No.51976018),the National Natural Science Foundation for Young Scientists of China (No.51606022),Natural Science Foundation of Chongqing,China(No.cstc2017jcyjAX0203),Scientific Research Foundation for Returned Overseas Chinese Scholars of Chongqing,China (No.cx2017020),the Fundamental Research Funds for the Central Universities (No.106112016CDJXY145504) and Research Funds of Key Laboratory of Low-grade Energy Utilization Technologies and Systems (No.LLEUTS-2018005).