Highly Efficient Synthesis of Hydrogen Storage Material of Formate

Review pubs.acs.org/IECR

Highly Efficient Synthesis of Hydrogen Storage Material of Formate from Bicarbonate and Water with General Zn Powder Jingwen Song,†,# Yang Yang,‡,# Guodong Yao,‡ Heng Zhong,*,§ Runtian He,‡ Binbin Jin,‡ Zhenzi Jing,† and Fangming Jin*,‡ †

School of Materials Science and Engineering, Tongji University, 1239 Siping Road, Shanghai 200092, China School of Environmental Science and Engineering, State Key Lab of Metal Matrix Composites, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, China § Research Institute for Chemical Process Technology, National Institute of Advanced Industrial Science and Technology, Sendai, 983-8551, Japan ‡

ABSTRACT: Formate, as an excellent hydrogen-storage material, has recently become increasingly important because formate has low toxicity, is easy to store and transport, and contains relatively high energy density. In this paper, we give an overview of the recent strategies in the conversion of bicarbonate into formate by water splitting with a general metallic Zn powder, which mainly includes: (1) hydrogen production from water with Zn; (2) formate syntheses from bicarbonate with water-derived hydrogen; (3) catalytic role of ZnO and Zn/ZnO interface and reducing role of active intermediate of Zn hydride species (Zn−H); (4) density function theory calculation for formate synthesis from bicarbonate and water with Zn; (5) solar-tochemical energy conversion efficiency estimation. The novel options of this proposed strategy are mainly the hydrogen being in situ formed from water with metal Zn as a reductant, and the plausible autocatalytic reaction mechanism. When coupled with reducing ZnO into Zn by solar energy, a highly efficient solar-fuel process can be expected.

1. INTRODUCTION The sustainable development of humankind has been threatened by the energy crisis due to the shortage of fossil fuels. Utilization of renewable energies, such as solar, wind, and biomass energies is regarded as one of the most promising solutions.1−3 However, due to the instability and unequal distribution of the renewable energies, methods for their conversion, storage, and transportation are indispensable.4−8 Hydrogen produced from the dissociation of water using renewable energies is one of the most prevalent materials for storing renewable energy due to its environmental benign impact.9−11 However, the application of hydrogen is quite limited since it exists in the gaseous state at ambient conditions, which leads to its energy consuming storage and transportation problems. Numerous hydrogen storage materials have been examined to face the challenge, including metal hydrides, metal organic frameworks and carbon nanostructures.12−15 Although these materials can provide high hydrogen storage ability, they are still not entirely satisfactory due to their high activation energy.15 It has been reported that formic acid, which contains 4.4 wt % of hydrogen, can be readily decomposed into hydrogen through a variety of homogeneous and heterogeneous catalysts,16−20 which implies the validity of using formic acid and related formate salts as the hydrogen carrier.21−24 The idealization of formate/formic acid using as hydrogen storage © 2017 American Chemical Society

materials comes from its advantages of safety handling, low toxicity, high energy density and convenient transportation with the existing infrastructures for gasoline and oil.25,26 Furthermore, the only byproduct of the formate or formic acid dehydrogenation is bicarbonate or CO2, which can be recovered and hydrogenated back to formate27,28 or formic acid29−37 again to realize a HCO3−−HCOO− or CO2− HCOOH cycle.24,38,39 More importantly, in the case of formate salts decomposition, H2 is the sole gas product and CO2 is “trapped” in bicarbonate salts, which can avoid the separation of H2 from CO2. On the other hand, the fast accumulation of CO2 in the atmosphere has become a serious global problem that threatens human beings and the environment.40−43 If a satisfactory method for the formate/formic acid production from bicarbonate/CO2 can be proposed, the concept of hydrogen storing in formate/formic acid would be quite attractive as it can mitigate both problems of the global warming and the energy crisis. Although recent research has developed vast approaches for rapid decomposition of formic acid into hydrogen and CO2 with both homogeneous and heterogeneous catalysts,44 the Received: Revised: Accepted: Published: 6349

January 14, 2017 May 3, 2017 May 10, 2017 May 10, 2017 DOI: 10.1021/acs.iecr.7b00190 Ind. Eng. Chem. Res. 2017, 56, 6349−6357

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Industrial & Engineering Chemistry Research

kept in the salt bath. Gaseous, liquid, and solid samples after the reactions were collected and analyzed by GC-TCD, HPLC, GC−MS, and XRD. More details of the experimental method can be found in our previous report.49 As shown in Table 1, hydrogen was easily formed under hydrothermal conditions, and interestingly, the addition of

methods for highly efficient conversion of CO2 back to formic acid to close the CO2−HCOOH cycle still remain a great challenge. Most of the reported research related to the CO2 hydrogenation to formic acid requires a gaseous hydrogen source and/or elaborately prepared noble metal catalysts, which limit the practical application.45 In this work, we outline our recent advances in the highly efficient formate production from bicarbonate with water-derived hydrogen, in which Zn was used to produce hydrogen from water for in situ bicarbonate hydrogenation. Thus, the requirements of gaseous hydrogen source and noble metal catalyst are avoided. The overall reaction can be expressed as in eq 1, in which the standard Gibbs free energy (ΔGo) and the standard enthalpy of reaction (ΔHo) were calculated with available thermodynamic data:

Table 1. Effect of NaHCO3 on the Production of Hydrogena

(1)

H2/mL

0 1 2 0

52 74 78 70

NaHCO3 enhanced the production of hydrogen from water. The promotion effect of NaHCO3 for the hydrogen production might be caused by the pH difference in the solution. As shown in Table 1, 70 mL of hydrogen was produced when the pH of the solution was adjusted to 8.6, which is similar to that obtained with the NaHCO3. Zn(OH)2 might have formed during the oxidation of Zn into ZnO, which can act as a passivating layer to stop the Zn from further oxidation. However, as an amphoteric oxide, the Zn(OH)2 can further transform to Zn(OH)42− in alkaline solution, removing the passivating layer and enabling the remainder of the Zn to react with water to produce hydrogen. It can be suggested that, from these results, hydrogen can be effectively produced from the reaction of Zn with water, and NaHCO3 is a strong promoter for the hydrogen production as it can provide an alkaline condition for the promotion of Zn oxidation.48

Both the ΔG and ΔH values are negative, which means this reaction does not require extra energy input and can be potentially self-supported. Although Zn is oxidized after the reaction, it can be reduced to its zerovalent state readily by solar energy,46,47 indicating the reaction can be regarded as an indirect using of solar energy. Therefore, an integrated technology for highly efficient formate production could be developed by coupling the bicarbonate reduction into formate with the thermochemical reduction of metal oxides into metals driven by the solar energy (Figure 1), which could link to the development of a solar-fuel concept.48 o

NaHCO3/mmol

1 2 3 4b

a Reaction conditions: 4 mmol Zn, 300 °C, 2 h. Table adapted with permission from ref 48. Copyright 2014 Nature Publishing Group. b pH of the solution was adjusted to 8.6 with NaOH.

Zn + HCO3− = HCOO− + ZnO ΔGo(298K) = −67.25kJ/mol; ΔHo(298K) = −66.18kJ/mol

entry

o

3. FORMATE SYNTHESIS FROM NaHCO3 HPLC and GC−MS analyses of the liquid products after the reaction of NaHCO3 with Zn and high temperature water (HTW) revealed that sodium formate was the only product in the liquid phase. The selectivity for formate was approximately 100%, determined by comparing the total organic carbon concentration (TOC) to the formate concentration in the solution.48 Analyses for gas samples with GC−TCD showed that a small amount of CO was observed. These results indicate that the formate can be easily and selectively produced when the NaHCO3 reacted with Zn in HTW.48 Figure 2a shows the effect of reaction time and pH on the formate yield, which is defined as the percentage of the produced formate to the initial amount of NaHCO3 in a carbon basis. The production of formate from NaHCO3 proceeded efficiently, and a high formate yield of 40−60% could be obtained after only 5 min at 300 °C. After 10 min, the increasing rate of the formate yield slowed down significantly, which suggests two different mechanisms of formate production probably exist. XRD analysis of the solid residue after the reaction showed that the Zn was oxidized to ZnO rapidly within 10 min (Figure 3). It has been reported that the initial pH of the solution is an important factor in the production of formate, as pH can affect the oxidation of Zn as well as the decomposition of the produced formate.50−52 Results showed that in a lower, acidic pH of 4.0, the yield of formate was only 27.2% (Figure 2a). An increase in the pH from 4 to 8.6, which is the natural pH value of the aqueous

Figure 1. Proposed integrated technology for formate production.

2. HYDROGEN PRODUCTION FROM WATER WITH METALLIC Zn To examine whether the bicarbonate can be reduced into formate in water-derived hydrogen, the investigation of hydrogen production from water with Zn is required. All experiments were conducted using a stainless steel (SUS-316) tubular reactor with an inner volume of 5.7 mL. The typical reaction procedure is as follows: a desired amount of NaHCO3, Zn powder, and deionized water were loaded into the reactor, and then, the reactor was sealed and immersed into a salt bath which had been preheated to the desired temperature. The temperature in the reactor can reach to a typical reaction temperature of 300 °C within about 30 s. At desired reaction time, the reactor was removed from the salt bath and immediately immersed in a cold-water bath. The reaction time was defined as the period during which the reactor was 6350

DOI: 10.1021/acs.iecr.7b00190 Ind. Eng. Chem. Res. 2017, 56, 6349−6357

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Figure 2. (a) Effect of reaction time and pH on the yield of formate (NaHCO3, 1 mmol; Zn, 6 mmol; 300 °C, 2 h for the effect of pH); (b) effect of temperature on the yield of formate (NaHCO3, 1 mmol; Zn, 6 mmol); (c) effect of the ratio of Zn to NaHCO3 on the yield of formate (300 °C, 2 h).

Figure 3. (a) XRD patterns and (b) conversion of Zn into ZnO after the reactions at 300 °C with 10 mmol Zn and 1 mmol NaHCO3 at different reaction times. Figure adapted with permission from ref 48. Copyright 2014 Nature Publishing Group.

show that the formate can be efficiently and selectively produced from NaHCO3 with Zn under hydrothermal conditions, and the reaction parameters have a strong effect on the yield of formate.

NaHCO3 solution, led to an increase in the yield of formate from 28% to 78%. These results indicate that a mildly alkaline pH is favorable for the production of formate. In Figure 2b, the production of formate was promoted by elevating reaction temperature obviously, and the effect of the reaction temperature was more significant than that of the reaction time. A temperature of 300 °C is preferred for obtaining a high yield of formate. Figure 2c shows the effect of the ratio of Zn to NaHCO3 on the yield of formate. When the amount of NaHCO3 was set as 1 mmol, an increase in the ratio of Zn to NaHCO3 from 2 to 12 resulted in a significant increase in yield of formate from 21% to 80%. However, when the amount of Zn was fixed as 10 mmol, varying the ratio of Zn to NaHCO3 did not obviously affect the yield of formate. These results indicate that the amount of Zn is a more significant factor than the amount of NaHCO3 on the formate yield. The above results

4. FORMATE SYNTHESIS WITH GASEOUS CO2 Although many researchers have reported that the formate can be synthesized with the use of NaHCO3,49,53−55 there are only a few reports in the literature on the direct conversion of gaseous CO2 into formic acid. The conversion of gaseous CO2 into formic acid at different initial pH showed that almost no formic acid was formed when using gaseous CO2 without NaOH (Table 2), which is probably caused by the low dissolution of CO2 in water. When alkali (NaOH) was added to the solution, the formate formation was clearly observed. However, the formate yield was still lower than that obtained 6351


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When a commercial reagent of ZnO was added, the yield of formate increased to 16%, suggesting that the ZnO can slightly promote the formation of formate. With the dry ZnO collected from the reaction of Zn with NaHCO3, the yield of formate was close to that obtained with the commercial reagent of ZnO. Interestingly, the collected wet ZnO exhibited better performance in promoting the yield of formate, suggesting the in situ formed ZnO has a higher catalytic activity.48 However, compared to the experiments with Zn reducing NaHCO3, the formate yield with wet ZnO was much lower. As already discussed in Figure 2a, the formate production rapidly increased in the first 10 min and then slowed down thereafter. Since Zn was completely oxidized to ZnO after 10 min (Figure 3), the in situ formed ZnO probably catalyzed the slow formation of formate at the reaction time over 10 min. However, the other reaction mechanism should dominate the rapid formate generation in the first 10 min. 5.2. Catalytic Role of Zn/ZnO Structures. It has been reported that a number of oxide-supported metal catalysts, such as Cu/ZnO, Ni/γ-Al2O3, and Pd/b-Ga2O3, are active in the selective hydrogenation of CO2.58,59 Recent research has demonstrated that Zn/ZnO in a core−shell configuration had a great potential in fabricating devices with improved features,60−62 which is attributed to the point defects in the interfacial regions of Zn/ZnO. Therefore, the coexistence of Zn and ZnO at the beginning of the hydrogenation reaction (within 10 min) might act as a catalyst in the reduction of NaHCO3 to formate. 5.2.1. TEM/HRTEM Analysis of Zn/ZnO Structures. TEM analysis showed that the morphology of the solid residue after 1 min was rodlike, about 180 nm long and 60 nm wide. The dspacing of two particles was 0.1684 and 0.2603 nm in the HRTEM analysis of selected area, corresponding to the (102) spacing of wurtzite−Zn and (002) spacing of wurtzite−ZnO, respectively (Figure 4). These results revealed the coexistence of Zn and ZnO in the starting stage of the reaction, which is in agreement with the XRD analysis in Figure 3. Further analysis from the HRTEM patterns showed disorderly lattices on the Zn/ZnO interface, which suggests possible defects or dislocations formed, particularly on the interfaces of Zn/ZnO.63 5.2.2. XPS Analysis of Zn/ZnO Structures. Figure 5 shows the XPS spectra for Zn 2p and O 1s of samples after the reactions as well as bulk ZnO and Zn. The two Zn 2p peaks at 1021.3 ± 0.1 eV and 1045.6 ± 0.1 eV and O 1s peak at 530.5 ± 0.1 eV for samples after 10 min are almost the same as those of bulk ZnO. However, shifts in the peak positions toward high energy are clearly observed in samples after the reactions for short durations (less than 10 min) for both Zn 2p and O 1s. The deconvolution results of the O 1s peak using a Gaussian distribution reveals three peaks marked with Oa, Ob and Oc correspond to oxygen ions on the wurtzite structure of ZnO, oxygen-deficient regions within the ZnO matrix, and chemisorbed or dissociated oxygen, respectively.63 And with the reaction time decreasing, the relative intensity of Oa decreased, whereas the relative intensity of Ob increased (Table 4). These results indicated that oxygen vacancies increased at short reaction times; specifically, the presence of Zn could create more oxygen vacancies in ZnO, which is most likely caused by the interaction between Zn and ZnO. Hence, the shifts in the peak positions for Zn 2p and O 1s are most likely caused by the interaction of Zn/ZnO due to the increased concentration of oxygen vacancies.63

Table 2. Yield of Formate/Formic Acid with Gaseous CO2 at Various Reaction Conditionsa reaction condition entry

initial pH

NaOH

dissolution time/h

1 2 3 4 5 6 7 8 9

6.7 8.8 11.4 13.5 14 14 14 14 14

without with with with with with with with with

0 0 0 0 0 1 2 24 48

pH after dissolution

yield /%

13 10 9.2 9

0.02 0.03 0.74 5.9 16.6 28.6 35.4 51.2 60

a

Table adapted with permission from ref 48. Copyright 2014 Nature Publishing Group. Reaction conditions: CO2, 2 mmol; Zn, 6 mmol; 300 °C, 2 h.

with the NaHCO3.48 Further, by keeping CO2 in the alkaline solution (pH = 14) at room temperature for several hours, the formate yield increased significantly with the dissolution/ keeping time. In the meantime, the pH of the solution decreased incessantly with time, and the highest formate yield was obtained when the pH value of the solution changed to 9, which is very close to the pH value of the NaHCO3 solution. These results indicate that the formation of formate should take place from the hydrogenation of HCO3−, rather than CO2.48

5. EXPERIMENTAL STUDY OF THE REACTION MECHANISM OF FORMATE SYNTHESIS FROM NaHCO3 WITH Zn IN HTW 5.1. Catalytic Activity of the in Situ Formed ZnO. Usually, catalysts are needed in the hydrogenation of CO2; however, no extra catalyst was added in the above researches. One possible reason is that the in situ generated ZnO in the oxidation of Zn in HTW plays an autocatalytic role in the hydrogenation reaction as ZnO is a traditionally good hydrogenation catalyst.56,57 Results of the effect of different ZnO samples on the formate yield are summarized in Table 3. When gaseous hydrogen was used as the reducing agent in the absence of ZnO, the formate yield decreased tremendously compared with that obtained with the Zn as the reductant. Table 3. Formate Yield from NaHCO3 in the Presence and Absence of Different ZnOa entry reductant

reductant/NaHCO3 (mol/mol)

1 2 3 4

Zn H2 H2 H2

6/1 6/1 6/1 6/1

5

H2

6/1

additives without without reagent ZnOb collected dry ZnOc collected wet ZnOd

yield/% 57.5 13.0 16.0 15.5 23.0

a

Table adapted with permission from ref 48. Copyright 2014 Nature Publishing Group. Reaction conditions: NaHCO3, 1 mmol; 300 °C, 2 h; the molar amount of ZnO is the same as the reductant for all experiments. bReagent ZnO: in powder with 200-mesh size. c Collected dry ZnO: The solid residue collected after the reaction of using Zn and was treated by washing with deionized water for several times, filtrating, and drying in air. dCollected wet ZnO: The solid residue collected after the reaction of using Zn. 6352


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Figure 4. TEM (a) and HRTEM (b, c) images of Zn/ZnO interface generated after the hydrogenation reaction (Zn, 10 mmol; NaHCO3, 1 mmol, 300 °C; 1 min). Figure adapted with permission from ref 63. Copyright 2017 Elsevier.

Figure 5. Zn 2p (a) and O 1s (b) spectra of solid samples obtained at different reaction times and bulk ZnO (Zn, 10 mmol; NaHCO3, 1 mmol, 300 °C).63

complex in H−Zn···O−H respectively appeared in the solid residue (Figure 6). However, when NaHCO3 was added, the absorption peak of Zn−H disappeared, with two new absorption peaks corresponding to the asymmetrical and symmetrical stretching vibrations of the absorbed HCOO− respectively being observed. These results indicate that when Zn reacted with water, an active intermediate structure of H− Zn···O−H was probably formed due to the water splitting reaction on Zn, and the absence of the Zn−H complex absorption peak was probably caused by the hydrogen in Zn−H reacting with NaHCO3 for the formation of formate.48 Zn hydride species (Zn−H) can be formed by Zn reacting with H2O at 700 K,64 and can act as a possible active hydrogen source for the hydrogenation reactions, such as the Cu/ZnO catalyzed hydrogenation of CO and CO2 to methanol,65 and the reduction of CO2 to formic acid.66,67 Thus, the hydrogen in the Zn−H complex, which was formed in the first 10 min, could act as an active hydrogen contributing to the rapid generation of formate from HCO3− due to the weak Zn−H bond.48 5.4. Proposed Mechanism of Formate Synthesis from NaHCO3 with Zn in HTW. On the basis of the above results, a possible reaction mechanism of the Zn autocatalytic HCO3− reduction into formate in HTW is proposed in Figure 7. Initially, H−Zn···O−H complex 1 is generated from the water splitting on Zn (route I). Then the anionic proton of complex 1 with nucleophilic tendency attacks the bicarbonate ion 2, yields the compound formate 4, with the generation of Zn complex 5, which loses water to form ZnO 6 (an SN2-like mechanism).48 On the other hand, the mechanism of hydrogen absorption onto ZnO or Zn/ZnO (route II and III) to reduce NaHCO3 may also occur. A similar structure of H−Zn···O−H (complex 1) is probably formed because of the chemisorption of hydrogen on ZnO (route II).48 For route III, hydrogen is adsorbed on the surfaces of Zn/ZnO interface. Since many oxygen vacancies are formed there, the hydrogen is easily

Table 4. Relative Intensity of Three Oxygen Types of Bulk ZnO and Solid Samples after Reaction for Different Timesa percentage of binding oxygen to total oxygen (%)

EB (eV) reaction time (min)

Oa

Ob

Oc

Oa

Ob

Oc

0.5 1 5 10 30 120 bulk ZnO

530.0 530.2 530.1 529.8 529.7 529.8 529.9

531.3 531.7 531.4 530.9 530.7 530.8 531.1

532.6 532.9 532.6 532.2 532.0 532.1 532.3

24.5 31.1 30.5 30.2 36.5 45.0 37.9

41.5 41.3 39.1 32.4 35.2 32.9 33.5

34.0 27.6 30.4 37.4 28.2 22.2 28.6

a

Table adapted from ref 63. Copyright 2017 Elsevier. Oa: Oxygen ions on the wurtzite structure of ZnO. Ob: Oxygen ions that are in oxygendeficient regions within the ZnO matrix. Oc: Chemisorbed or dissociated oxygen or to OH species on the surface of the ZnO.

5.2.3. Experimental Confirmation of the Role of Zn/ZnO Interfaces. With the assumption that the solid sample after a short reaction time should exhibit higher catalytic activity, the activeness of Zn/ZnO interface was demonstrated by a twostep experiment, in which Zn reacted with NaHCO3 in the firststep for different reaction times, and hydrogen was introduced to the reactor in the second-step to reduce NaHCO3. As expected, the yield of formate after 5 min in the first-step reaction was 38.4%, which was higher than that after 2 h in the first-step reaction (23.0%). These results evidenced the catalytic activity of Zn/ZnO structure.63 5.3. Reducing Role of Active Intermediate of Zn Hydride Species (Zn−H). The infrared (IR) absorption peaks of solid samples after the reactions at different reaction times in the presence and absence of NaHCO3 showed that when Zn solely reacted with water, two absorption peaks attributed to the stretching vibration of the O−H complexes and Zn−H 6353


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6. QUANTUM CHEMICAL CALCULATIONS OF THE REACTION MECHANISM Density functional theory (DFT) calculations on the intermediate (Zn−H) formation and formate production verified the proposed route I in Figure 7. A model system containing a Zn5 cluster with two H2O molecules was adopted for all DFT investigations. H2O fragments distribution on the Zn5 model contained the step (linear), terrace (surface), and kink (vertex) types. The Zn5 clusters were first optimized with all the possible fragments of H+, OH−, and H2O that are generated from two H2O molecules, which comprise a total 859 conformations. When the free energy was adopted as the screening criteria, relative to the reference consisting of the optimized Zn5 cluster and the two nonfragmented H2O molecules, calculations showed the existence of the energetically favorite Zn−H intermediate at 300 °C as a product of the Zn5 + 2H2O reactions.68 Formate formation through the Zn−H intermediate was determined by the calculation of the geometry of the transition state (TS) and its formation energy (Figure 8).

Figure 6. FT-IR absorption spectra of (a) wet solid sample after reactions of Zn with water in the absence of NaHCO3 (Zn 10 mmol, 300 °C, 10 min), (b) bulk ZnO, and (c−f) solid samples after reactions of Zn with water in the presence of NaHCO3 (NaHCO3 1 mmol, Zn 10 mmol, 300 °C). Figure adapted with permission from ref 48. Copyright 2014 Nature Publishing Group.

Figure 8. TS geometry (left) and activation energy (right) of Zn5−H + HCO3− → Zn5−OH + HCOO−. Figure reproduced with permission from ref 68. Copyright 2014 PCCP Owner Societies.

activated on this interface, and then, nucleophilic attack of the H− on bicarbonate ion occurs, and finally formate is obtained, along with H2O.63 As route II and III are diffusion-limited reactions, it is possible route I contributed mainly to formate production. Since Zn was oxidized to ZnO totally after 10 min, route II possibly dominated formate production after 10 min.

Using the most stable form of Zn−H, the activation energy of the TS from the initial state (Zn−H and HCO3−) is 24.1 kcal/ mol, which can be readily achieved at 300 °C. The Zn−H bond distance is approximately 1.94 Å, and the charge of H in the Zn−H species is −0.221, which is assigned to be the hydride rather than the proton. These results reflect the production of HCOO− from HCO3−, with an important implication that this is an SN2-like reaction, as shown in Figure 7.68

Figure 7. Proposed mechanism of reduction of HCO3− into formate in HTW with Zn (M represents the metal cation in the bicarbonate salts. Figure reproduced with permission from ref 48. Copyright 2014 Nature Publishing Group. 6354


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values of Zn and formate, the energy conversion efficiency of eq 1 can be estimated as

Figure 9 shows the results of intrinsic reaction coordinate (IRC) calculations and the TS shapes of the highest occupied

η1% =

0.7 × (− 254.34) × 100 = 16.9% 3 × ( −350.46)

(5)

Generally, an energy conversion efficiency of 29−36% for ZnO−Zn−H2 by solar energy can be obtained;46 then, an energy efficiency of 30% was adopted for η2. Thus, the overall solar-to-formate energy conversion efficiency can be obtained at around 5%,48 which is competitive with other related technologies.69−71 Figure 9. IRC calculation (a) and HOMO and LUMO orbital shapes (b) of the TS. Figure reproduced with permission from ref 68. Copyright 2014 PCCP Owner Societies.

8. CONCLUSIONS In this review, our recent advances in a new strategy of the synthesis of formate from bicarbonate by water dissociation with metallic Zn are reviewed. As nascent works, results herein show that the formate can be efficiently and rapidly synthesized from NaHCO3 or CO2 by water splitting with metallic Zn under hydrothermal conditions, and a high yield (70−80%) of formate from HCO3− with nearly 100% selectivity can be achieved. Based on the experimental results and DFT studies, the in situ formed intermediate of H−Zn···O−H during the water dissociation with Zn played a key role in converting the HCO3− into formate. By coupling the proposed method with the solar-driven reduction of ZnO to Zn, a Zn−ZnO cycle for the continuously reduction of bicarbonate into formate with water-derived hydrogen can be achieved, which means a highly efficient solar-fuel process can be expected.

molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO). The plain IRC curve of the TS led to a smooth transformation of the reactant Zn−H + HCO3− to the product formate. The occupied HOMO of the TS shows a bonding interaction between the carbon (C) atom of HCO3− and the hydrogen (H) atom of the Zn−H intermediate, whereas the unoccupied LUMO shows an antibonding state in the Zn−H bond as well as between the carbon of HCO3− and OH−. The HOMO and LUMO of the TS clearly demonstrated that formate production from HCO3− followed an SN2-like mechanism.68

7. Zn-ZnO CYCLE AND ASSESSMENT OF THE ENERGY CONVERSION EFFICIENCY The aforementioned research showed a highly efficient way to simultaneously produce hydrogen from water and its in situ storage in formate from the reaction of Zn with NaHCO3 in HTW. Although Zn oxidized into ZnO after the reaction, the generated ZnO can be readily reduced to Zn by concentrated solar energy, which is a promising technology for industrial utilization in the near future.46 By combining the solar reduction of ZnO to Zn with the presented method of Zn reduction of HCO3− to formate, a highly efficient method for converting solar energy to chemical fuels could be achieved (Figure 1). The energy conversion efficiency from solar to fuel (formate) was evaluated according to eq 1 and 2, in which the solar reduction of ZnO to Zn is written as ZnO + solar energy → Zn + 0.5O2 (2)

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*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Fangming Jin: 0000-0001-9028-8818 Author Contributions #

J.S. and Y.Y. contributed equally.

Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This paper was identified by Session Chair Yun Hang Hu (Michigan Technological University, USA) as the Best Presentation from the session, “USA−China Symposium on Energy” from the 2016 ACS Fall Meeting in Philadelphia, PA. The authors gratefully acknowledge financial support from the Natural Science Foundation of China (Grant No. 21077078, 21277091), Key Basic Research Projects of Science and Technology Commission of Shanghai (No. 14JC1403100), and China Postdoctoral Science Foundation (No. 2013M541520).

Thus, the solar-to-formate energy conversion efficiency is ηsolar − formate = η1 × η2 (3) Where η1 and η2 are the energy conversion efficiencies of eqs 1 and 2, respectively. Since eq 1 represents the chemical energy of Zn transformed into the chemical energy of formate, η1 can be expressed as η1 =

MHCOO− × ΔHHCOO− M Zn × ΔHZn

AUTHOR INFORMATION

Corresponding Authors

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(4)

REFERENCES

(1) Dincer, I. Renewable energy and sustainable development: a crucial review. Renewable Sustainable Energy Rev. 2000, 4, 157. (2) Gross, R.; Leach, M.; Bauen, A. Progress in renewable energy. Environ. Int. 2003, 29, 105. (3) Iqbal, M.; Azam, M.; Naeem, M.; Khwaja, A. S.; Anpalagan, A. Optimization classification, algorithms and tools for renewable energy: A review. Renewable Sustainable Energy Rev. 2014, 39, 640. (4) Hou, Y.; Vidu, R.; Stroeve, P. Solar Energy Storage Methods. Ind. Eng. Chem. Res. 2011, 50, 8954.

where MHCOO− and MZn are the production of formate and consumption of Zn in mole, and ΔHZn and ΔMHCOO− are the higher heating values of Zn and formate, respectively. As discussed in section 3, over 70% conversion efficiency from bicarbonate to formate can be reached at the reaction condition of Zn/NaHCO3 = 3:1 which means 3 mol of Zn could produce 0.7 mol of formate. Hence, by substituting the higher heating 6355


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Highly Efficient Synthesis of Hydrogen Storage Material of Formate

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