A thermoelectric conversion element includes a thermoelectric conversion material portion having a compound semiconductor composed of first base material element A and second base material element B and represented by A.sub.x-cB.sub.y with value of x being smaller by c with respect to a compound A.sub.xB.sub.y according to a stoichiometric ratio, a first electrode disposed in contact with the thermoelectric conversion material portion, and a second electrode disposed in contact with the thermoelectric conversion material portion and apart from the first electrode. An A-B phase diagram includes a first region corresponding to low temperature phase, second region corresponding to high temperature phase, and third region corresponding to coexisting phase, sandwiched between the low temperature phase and the high temperature phase, in which the low and high temperature phases coexist. A temperature at a boundary between the first region and the third region changes monotonically with a change in c.

A thermoelectric conversion element includes a thermoelectric conversion material portion having a compound semiconductor composed of first base material element A and second base material element B and represented by A.sub.x-cB.sub.y with value of x being smaller by c with respect to a compound A.sub.xB.sub.y according to a stoichiometric ratio, a first electrode disposed in contact with the thermoelectric conversion material portion, and a second electrode disposed in contact with the thermoelectric conversion material portion and apart from the first electrode. An A-B phase diagram includes a first region corresponding to low temperature phase, second region corresponding to high temperature phase, and third region corresponding to coexisting phase, sandwiched between the low temperature phase and the high temperature phase, in which the low and high temperature phases coexist. A temperature at a boundary between the first region and the third region changes monotonically with a change in c.

An exemplary method for producing a mixed metal chalcogenide under atmospheric pressure may include forming a reaction mixture by mixing a first metal chalcogenide and a second metal chalcogenide. An exemplary method may further include pouring a first layer of NaCl within a reactor, where an exemplary reactor may include a container and a cap. Pouring an exemplary first layer of NaCl within an exemplary reactor may include pouring an exemplary first layer of NaCl on an exemplary base end of an exemplary container of the exemplary reactor. An exemplary method may further include pouring an exemplary reaction mixture into an exemplary container on top of an exemplary first layer of NaCl, pouring a second layer of NaCl into an exemplary container on top of an exemplary reaction mixture, sealing an exemplary container by closing an exemplary cap and pouring molten NaCl on top of the exemplary cap, and heating an exemplary reactor at a predetermined temperature for a predetermined time.

An exemplary method for producing a mixed metal chalcogenide under atmospheric pressure may include forming a reaction mixture by mixing a first metal chalcogenide and a second metal chalcogenide. An exemplary method may further include pouring a first layer of NaCl within a reactor, where an exemplary reactor may include a container and a cap. Pouring an exemplary first layer of NaCl within an exemplary reactor may include pouring an exemplary first layer of NaCl on an exemplary base end of an exemplary container of the exemplary reactor. An exemplary method may further include pouring an exemplary reaction mixture into an exemplary container on top of an exemplary first layer of NaCl, pouring a second layer of NaCl into an exemplary container on top of an exemplary reaction mixture, sealing an exemplary container by closing an exemplary cap and pouring molten NaCl on top of the exemplary cap, and heating an exemplary reactor at a predetermined temperature for a predetermined time.

A thermoelectric conversion element includes a thermoelectric conversion material portion having a compound semiconductor composed of first base material element A and second base material element B and represented by A.sub.x-cB.sub.y with value of x being smaller by c with respect to a compound A.sub.xB.sub.y according to a stoichiometric ratio, a first electrode disposed in contact with the thermoelectric conversion material portion, and a second electrode disposed in contact with the thermoelectric conversion material portion and apart from the first electrode. An A-B phase diagram includes a first region corresponding to low temperature phase, second region corresponding to high temperature phase, and third region corresponding to coexisting phase, sandwiched between the low temperature phase and the high temperature phase, in which the low and high temperature phases coexist. A temperature at a boundary between the first region and the third region changes monotonically with a change in c.

A thermoelectric conversion element includes a thermoelectric conversion material portion having a compound semiconductor composed of first base material element A and second base material element B and represented by A.sub.x-cB.sub.y with value of x being smaller by c with respect to a compound A.sub.xB.sub.y according to a stoichiometric ratio, a first electrode disposed in contact with the thermoelectric conversion material portion, and a second electrode disposed in contact with the thermoelectric conversion material portion and apart from the first electrode. An A-B phase diagram includes a first region corresponding to low temperature phase, second region corresponding to high temperature phase, and third region corresponding to coexisting phase, sandwiched between the low temperature phase and the high temperature phase, in which the low and high temperature phases coexist. A temperature at a boundary between the first region and the third region changes monotonically with a change in c.

Copper sulfide of the formula Cu.sub.xS.sub.y, wherein x and y are integer or non-integer values, wherein (i) the copper sulfide has a sulfur 2p XPS spectrum with peaks at 162.3 eV (±1 ev), 163.8 eV (±1 ev) and 68.5 eV (±1 ev), characterised in that the peak at 168.5 eV has a lower value of counts per second (CPS) than both the peak at 162.3 eV and the peak at 163.8 eV; and (ii) the copper sulfide has a copper 2p XPS spectrum with peaks at 932.0 eV (±2 ev) and 933.6 eV (±3 eV) and characterised in that the XPS spectrum does not comprise identifiable satellite peaks at 939.8 eV and 943.1 eV (±3 eV).

Copper sulfide of the formula Cu.sub.xS.sub.y, wherein x and y are integer or non-integer values, wherein (i) the copper sulfide has a sulfur 2p XPS spectrum with peaks at 162.3 eV (±1 ev), 163.8 eV (±1 ev) and 68.5 eV (±1 ev), characterised in that the peak at 168.5 eV has a lower value of counts per second (CPS) than both the peak at 162.3 eV and the peak at 163.8 eV; and (ii) the copper sulfide has a copper 2p XPS spectrum with peaks at 932.0 eV (±2 ev) and 933.6 eV (±3 eV) and characterised in that the XPS spectrum does not comprise identifiable satellite peaks at 939.8 eV and 943.1 eV (±3 eV).

A method of making copper sulfide electrode material comprising steps of: 1) stirring and dissolving copper(ii) nitrate hydrate (Cu(NO.sub.3)2.3H.sub.2O) and Thiourea (CH.sub.4N.sub.2S) in a mixed solution consisting of ethylene glycol and deionized water; 2) adding hexadecyl trimethyl ammonium bromide (C.sub.19H.sub.42N.Br) to mixture A; 3) placing the mixture B into a roaster, raising a temperature of the roaster to 100° C. to 180° C. for 10 hours to 18 hours; 4) washing the crude CuS by using a mixed fluid of ethanol absolute (C.sub.2H.sub.6O) and deionized water to be cooled in a room temperature, placing the crude CuS in the roaster and raising a temperature of the roaster to 50° C. to 80° C.; 5) producing cathode electrode of asymmetric capacitive deionization module by using the copper sulfide electrode material; 6) producing anode electrode of asymmetric capacitive deionization module by using activated carbon electrode material.

A method of making copper sulfide electrode material comprising steps of: 1) stirring and dissolving copper(ii) nitrate hydrate (Cu(NO.sub.3)2.3H.sub.2O) and Thiourea (CH.sub.4N.sub.2S) in a mixed solution consisting of ethylene glycol and deionized water; 2) adding hexadecyl trimethyl ammonium bromide (C.sub.19H.sub.42N.Br) to mixture A; 3) placing the mixture B into a roaster, raising a temperature of the roaster to 100° C. to 180° C. for 10 hours to 18 hours; 4) washing the crude CuS by using a mixed fluid of ethanol absolute (C.sub.2H.sub.6O) and deionized water to be cooled in a room temperature, placing the crude CuS in the roaster and raising a temperature of the roaster to 50° C. to 80° C.; 5) producing cathode electrode of asymmetric capacitive deionization module by using the copper sulfide electrode material; 6) producing anode electrode of asymmetric capacitive deionization module by using activated carbon electrode material.