Functional materials with high-efficiency energy storage and conversion for batteries and fuel cells
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aInstitute of New Energy Material Chemistry, Nankai University, 94 Weijin Road, Tianjin 300071, People's Republic of China
bEngineering Research Center of Energy Storage and Conversion (Ministry of Education), Chemistry College, Nankai University, 94 Weijin Road, Tianjin 300071, People's Republic of China
Received 24 January 2009;
accepted 19 April 2009.
Available online 24 April 2009.
Abstract
Electrochemical energy storage and conversion with high efficiency and cleanliness is unquestionably one challenge for the sustainable development of the society of human beings. The functional materials can be applied in the systems of electrochemical energy storage and conversion such as in the fields of batteries and fuel cells. For the aspect of energy storage, high efficiency is closely connected with lightweight and high energy density materials, such as hydrogen, lithium, and magnesium. While for energy conversion, two major problems exist namely the diffusion/migration of ions and the transportation of electrons. The properties of the corresponding materials directly affect the solution of these challenges. Thus, in this review we concentrate on the crystal structures and the properties of functional materials applied in electrochemical energy storage and conversion systems with selected primary and secondary batteries and hydrogen fuel cells. In particular, the design, synthesis, structure and property of the materials, containing (1) cathode, anode and electrolyte for non-aqueous Li/Li+; (2) various Mg, MgxMo6S8 (0 < x < 2) Chevrel phase cathode and electrolyte solutions for primary and secondary Mg batteries; (3) proton exchange membranes, electrode catalysts, hydrogen production and storage for aqueous H2(H)/H+ fuel cells. The advantages and disadvantages involved in the batteries and fuel cells using functional materials are also discussed.
Keywords: Functional materials; Batteries; Fuel cells; Energy storage and conversion; High efficiency
Fig. 1. Schematic diagram showing energy densities (ρ) and how they correlate with the elements in the periodic table. In general, more electronegativity (χ) and less atomic weight (M) will lead to higher energy density for an element in the energy storage and conversion systems. Thus, we have qualitatively estimated the relative energy densities by the equation of ρ = χ/M for all the elements within the periodic table. Of all materials, the highest capacity is 26,590 mA h/g from H to H+. The data of atomic weight and electronegativity for each element are taken from Ref. [3].
Fig. 2. Theoretical and practical specific energy of Zn-Mn, Li-Mn, Ni-MH, Li-ion and Zn-air batteries plus H2–O2 fuel cell with their nominal voltages. The inset table lists the theoretical capacity of the typical electrode materials involved. Data are taken from Ref. [1].
Fig. 4. Crystal structures of LiCoO2 and LiFePO4. (a) LiCoO2: a = b = 2.9557(4) Å, c = 14.532(3) Å; α = β = 90° and γ = 120°; space group, R−3m; data are taken from Ref. [10]. (b) LiFePO4: a = 10.3290(3) Å, b = 6.0065(2) Å, and c = 4.6908(2) Å; space group, Pnma; data are taken from Ref. [13].
Fig. 5. Crystal structures of α-, β-, and γ-MnO2. (a) α-MnO2: a = b = 9.815(1) Å and c = 2.847(1) Å; space group, I4/m; data are taken from Ref. [21]. (b) β-MnO2: a = b = 4.4041(1) Å and c = 2.8765(1) Å; space group, P42/mnm; data are taken from Ref. [22]. (c) γ-MnO2: a = 9.3229(11) Å, b = 4.4533(7) Å, and c = 2.8482(3) Å; space group, Pnam; data are taken from Ref. [23].
Fig. 6. Crystal structures of Chevrel phases. (a) MgMo6S8: a = b = 9.4592714(6) Å and c = 10.5588446(5) Å; α = β = 90°, γ = 120°; space group, R−3. (b) Mg2Mo6S8: a = b = 9.7630596(10) Å and c = 10.3689375(10) Å; α = β = 90°, γ = 120°; space group: R−3. Data are taken from Ref. [59].
Fig. 9. Two primary ways which currently utilize a solar source to produce H2: (a) driving water electrolysis with solar cells and (b) direct photocatalytic splitting of water.
Fig. 10. Crystal structure, electronic structure and theoretical hydrogen content of 18-electron structural Mg2NiH4, Mg2CoH5 and Mg2FeH6. Data are taken from Refs. [101], [102], [103] and [104].
Corresponding author at: Institute of New Energy Material Chemistry, Nankai University, 94 Weijin Road, Tianjin 300071, People's Republic of China. Tel.: +86 22 23506808; fax: +86 22 23506808.
Corresponding author at: Institute of New Energy Material Chemistry, Nankai University, 94 Weijin Road, Tianjin 300071, People's Republic of China. Tel.: +86 22 23506808; fax: +86 22 23506808. 
