1 00:00:05,660 --> 00:00:11,370 This week I shall discuss different ways on how to use solar energy in a less conventional way. 2 00:00:12,200 --> 00:00:17,470 In past weeks, we have discussed how to convert light into electricity. 3 00:00:17,470 --> 00:00:23,960 Also, in the previous blocks this week, we talked about conversion of solar energy into heat. 4 00:00:25,320 --> 00:00:33,500 The last option is to convert solar energy into chemical energy, which allows us to directly create fuels. 5 00:00:33,600 --> 00:00:35,800 But why is that so important? 6 00:00:35,800 --> 00:00:41,940 Why would we want to create fuels instead of electricity or heat? 7 00:00:41,940 --> 00:00:46,260 Since the sun does not always shine, solar energy is not constant. 8 00:00:46,260 --> 00:00:52,289 There are two types of variations, the daily variations, meaning the difference between 9 00:00:52,289 --> 00:00:58,570 day and night, and the seasonal variations, because the sun's irradiation in the winter 10 00:00:58,570 --> 00:01:02,120 is not the same as in the summer. 11 00:01:02,120 --> 00:01:08,410 But even if there is no sun, we expect our energy need to be always covered. 12 00:01:08,410 --> 00:01:14,580 That is why we need some kind of storage, to be able to cover the electricity demand 13 00:01:14,580 --> 00:01:16,990 even when there is no sun. 14 00:01:16,990 --> 00:01:23,160 Actually, the biggest problem in the current energy scenario is not the production of renewable 15 00:01:23,160 --> 00:01:26,849 energy, but its storage. 16 00:01:26,849 --> 00:01:32,360 We know how to efficiently harvest the energy around us, but we don't have a reliable way 17 00:01:32,360 --> 00:01:35,390 to store it. 18 00:01:35,390 --> 00:01:39,970 There are several forms of storing energy that we can use. 19 00:01:39,970 --> 00:01:43,000 The graph here is called a Ragone plot. 20 00:01:43,000 --> 00:01:50,000 Ragone plots show the amount of energy stored in a certain storage technology per kilogram 21 00:01:50,050 --> 00:01:57,050 of material with respect to the power provided by the technology per unit of mass. 22 00:01:57,789 --> 00:02:02,489 The most used form of stored energy are fossil fuels. 23 00:02:02,489 --> 00:02:09,900 As the graph shows, fossil fuels have high energy density properties, are stable and reliable. 24 00:02:10,530 --> 00:02:17,530 But fossil fuels are depleting and their use is detrimental to our environment. 25 00:02:18,470 --> 00:02:25,470 Hydrogen has an even better gravimetric energy density compared to fossil fuels, but it is 26 00:02:25,720 --> 00:02:31,830 a very light gas, so the volumetric energy density, i.e. the energy per unit of volume, 27 00:02:31,830 --> 00:02:33,690 is much lower. 28 00:02:33,690 --> 00:02:38,220 That is one of the reasons it has not been widely used until now. 29 00:02:38,220 --> 00:02:44,730 However, if we compare batteries and hydrogen for the energy storage of a solar cell, batteries 30 00:02:44,730 --> 00:02:51,680 have a lower energy density and assume a much higher initial investment than hydrogen. 31 00:02:51,680 --> 00:02:58,330 Also, hydrogen can effectively be produced from solar energy by utilizing electrochemistry. 32 00:02:58,330 --> 00:03:05,330 This is why the concept of solar fuels is interesting and being discussed here. 33 00:03:05,790 --> 00:03:11,420 Nature has been converting the energy from the sun into chemical energy for a long time 34 00:03:11,420 --> 00:03:18,420 by converting carbon dioxide and water into oxygen and sugars using sunlight. 35 00:03:18,470 --> 00:03:20,900 That is what we call photosynthesis. 36 00:03:20,900 --> 00:03:27,900 Now, we want to imitate this process with inorganic semiconductor materials. 37 00:03:28,900 --> 00:03:35,900 These materials are able to split a water molecule into oxygen and hydrogen using the 38 00:03:35,900 --> 00:03:38,709 energy of sunlight. 39 00:03:38,709 --> 00:03:45,709 That is why sometimes these technologies are referred to as artificial photosynthesis. 40 00:03:45,959 --> 00:03:49,950 Let's now focus on the chemical cycle of hydrogen. 41 00:03:49,950 --> 00:03:54,020 Water is split into hydrogen and oxygen by electrolysis. 42 00:03:54,020 --> 00:04:01,020 Then, the hydrogen is combined with carbon dioxide in a water gas shift reaction to obtain 43 00:04:01,180 --> 00:04:03,540 carbon monoxide and hydrogen. 44 00:04:03,540 --> 00:04:10,540 This gas mixture, known as synthesis gas, can be then refined in a Fischer-Tropsch reaction 45 00:04:10,910 --> 00:04:13,270 to finally obtain methane. 46 00:04:13,270 --> 00:04:18,930 This conversion from hydrogen to methane is done because methane is easier to store and 47 00:04:18,930 --> 00:04:22,630 has less hazardous problems than hydrogen. 48 00:04:22,630 --> 00:04:28,530 When the energy needs to be released, the methane can be burnt in a combustion reaction, 49 00:04:28,530 --> 00:04:34,520 which will also need oxygen and will give water and carbon dioxide as by-products. 50 00:04:34,520 --> 00:04:40,000 This water is reused for the electrolysis and the carbon dioxide for the water gas shift 51 00:04:40,000 --> 00:04:43,960 reaction, thus closing the cycle. 52 00:04:44,150 --> 00:04:49,750 Each reaction and each process of the cycle will have a certain efficiency, and the overall 53 00:04:49,750 --> 00:04:54,870 efficiency is the combination of all those efficiencies. 54 00:04:54,870 --> 00:05:01,870 Now that we know why we want to produce hydrogen, let's see how we can do it with solar energy. 55 00:05:02,060 --> 00:05:07,760 One of the best ways to produce hydrogen using solar energy is using a photoelectrode. 56 00:05:07,760 --> 00:05:14,360 A photoelectrode is named as such because it uses light to produce an electrochemical 57 00:05:14,360 --> 00:05:19,699 reaction, in which water is split into oxygen and hydrogen. 58 00:05:19,699 --> 00:05:25,940 In this process, the photons reach the surface of the photoelectrode, which is made of a 59 00:05:25,940 --> 00:05:27,800 photoactive semiconductor. 60 00:05:27,800 --> 00:05:33,460 As in any other semiconductor, the photons with the same or higher energy than the semiconductor 61 00:05:33,460 --> 00:05:37,389 band gap energy create an electron-hole pair. 62 00:05:37,389 --> 00:05:42,310 The electrons and holes will be separated by an electric field, and both will be used 63 00:05:42,310 --> 00:05:47,639 in the two half reactions involved in the overall water splitting process. 64 00:05:47,639 --> 00:05:54,400 To create that electric field, this is coupled with a voltage source, for example a solar cell. 65 00:05:54,430 --> 00:06:00,180 The solar cell will receive the transmitted light from the photoanode, creating another 66 00:06:00,180 --> 00:06:05,440 electron-hole pair and electric field that will bring the electrons to the photoanode 67 00:06:05,440 --> 00:06:11,509 and the holes to the photocathode with enough potential to drive the redox reaction in the 68 00:06:11,509 --> 00:06:17,139 electrolyte, splitting the water molecule into oxygen and hydrogen. 69 00:06:17,139 --> 00:06:22,990 The reaction of water splitting is a reduction-oxidation reaction, or how they are commonly known, 70 00:06:22,990 --> 00:06:24,850 a redox reaction. 71 00:06:24,850 --> 00:06:31,320 In redox reactions, the reaction happens due to the exchange of electrons between elements 72 00:06:31,320 --> 00:06:33,229 or molecules. 73 00:06:33,229 --> 00:06:40,160 They can be divided in two half reactions: the oxidation or loss of electrons and the 74 00:06:40,160 --> 00:06:43,810 reduction or gain of electrons. 75 00:06:43,810 --> 00:06:49,539 The oxidation happens in the anode and the reduction happens in the cathode. 76 00:06:49,539 --> 00:06:53,669 In this case the reduction reaction is the formation of hydrogen with protons in the 77 00:06:53,669 --> 00:06:59,870 solution and electrons coming from the electrode, and the oxidation is the splitting of water 78 00:06:59,870 --> 00:07:06,870 giving oxygen molecules, protons to the solution and electrons to the anode. 79 00:07:06,880 --> 00:07:13,300 Each half reaction has a potential associated, and the sum of the potential of each half 80 00:07:13,300 --> 00:07:19,319 reaction gives us the potential for the whole redox reaction. 81 00:07:19,319 --> 00:07:26,240 But potentials are always defined with respect of a reference, a zero that we define. 82 00:07:26,240 --> 00:07:32,160 For redox reactions, the zero is defined as the hydrogen half reaction. 83 00:07:32,160 --> 00:07:39,600 So in the case of water splitting, the oxygen production reaction has a potential of 1.23V 84 00:07:39,680 --> 00:07:45,700 with respect to the hydrogen reaction, and therefore, the overall potential needed for 85 00:07:45,700 --> 00:07:51,900 the reaction to happen will be 1.23V. 86 00:07:51,910 --> 00:07:57,410 Depending on the electronic properties of the semiconductor material it can either be 87 00:07:57,420 --> 00:08:04,700 treated as a donor or n-type semiconductor material, or as an acceptor or p-type semiconductor. 88 00:08:04,760 --> 00:08:12,640 The band bending of the semiconductor can then go in two ways, depending on the semiconductor type. 89 00:08:12,699 --> 00:08:19,999 If it is p-type, the material will attract more holes to the interface, and that will 90 00:08:20,000 --> 00:08:26,720 enhance the reduction half reaction, producing hydrogen in the photoelectrode or photocathode. 91 00:08:27,300 --> 00:08:31,200 If the semiconductor is n-type, the opposite happens. 92 00:08:31,209 --> 00:08:36,969 Electrons are moved to the interface by the internal electric field, and those electrons 93 00:08:36,969 --> 00:08:41,409 are involved in the oxidation half reaction, producing oxygen. 94 00:08:42,560 --> 00:08:48,040 N-type semiconductor photoelectrodes are also called photoanodes. 95 00:08:48,709 --> 00:08:56,569 As mentioned, in the photoelectrolysis, a photoactive semiconductor is used as an electrode. 96 00:08:57,100 --> 00:09:00,540 This material has to fulfill several requirements. 97 00:09:01,020 --> 00:09:06,020 First, it has to absorb the light that arrives to the surface. 98 00:09:06,820 --> 00:09:12,520 Another important feature is an efficient charge carrier transport inside the material 99 00:09:12,680 --> 00:09:15,320 and separation into the two electrodes. 100 00:09:15,580 --> 00:09:22,580 It has been estimated that material with an energy band gap close to 2.1 eV has the potential 101 00:09:22,680 --> 00:09:28,760 to split water, taking into account required overpotentials to drive the reaction. 102 00:09:29,460 --> 00:09:35,240 In addition, the energy levels of the material have to be adequate to couple with the energy 103 00:09:35,300 --> 00:09:36,840 needed for the reaction. 104 00:09:37,580 --> 00:09:44,240 This is what is called a favorable band edge position, meaning that the energy levels of 105 00:09:44,320 --> 00:09:49,900 the reactions have to be located somewhere in the energy band gap of the semiconductor. 106 00:09:50,720 --> 00:09:56,600 To further enhance the reaction, a catalyst may be added to the semiconductor surface. 107 00:09:57,320 --> 00:10:03,900 Finally, on the practical side, it is important that the material is photochemically stable 108 00:10:04,000 --> 00:10:05,500 and relatively cheap. 109 00:10:05,960 --> 00:10:11,680 From these criteria, the main technical challenges to be addressed are the light absorption, 110 00:10:11,780 --> 00:10:16,640 the separation of charges and the catalysis of the reaction. 111 00:10:17,500 --> 00:10:23,380 The absorption and catalysis problems can be tackled by carefully choosing the semiconductor 112 00:10:23,420 --> 00:10:26,640 material and its corresponding catalyst. 113 00:10:27,920 --> 00:10:32,260 There are several materials that can be considered for solar water splitting. 114 00:10:32,800 --> 00:10:38,220 Some of the most popular materials studied for this application are titanium dioxide (TiO2), 115 00:10:38,720 --> 00:10:45,600 tungsten oxide (WO3), bismuth vanadate (BiVO4), iron oxide (Fe2O3) or silicon carbide (SiC). 116 00:10:46,440 --> 00:10:52,880 These last three materials are the ones with a more promising future judging by their potential 117 00:10:52,880 --> 00:10:56,040 solar-to-hydrogen efficiency, as shown in the graph. 118 00:10:56,660 --> 00:11:03,100 When looking at the optimum band gap, iron oxide and silicon carbide are the best choice for 119 00:11:03,100 --> 00:11:09,980 the optimum absorption of light, since they have band gap energies closest to the optimal 2.1 eV. 120 00:11:10,560 --> 00:11:17,000 But if considering the other factors like band position or stability, materials like 121 00:11:17,020 --> 00:11:19,980 bismuth vanadate may also be a viable option. 122 00:11:20,880 --> 00:11:27,060 The rest of the materials are not considered, such as titanium dioxide or tungsten oxide, 123 00:11:27,280 --> 00:11:33,060 because they have lower maximum theoretical efficiencies due to their large band gap energies. 124 00:11:33,700 --> 00:11:38,480 There are two main factors on which the efficiency of the overall water splitting device depends: 125 00:11:38,960 --> 00:11:44,600 The catalytic efficiency and the separation efficiency of the photoelectrode. 126 00:11:45,460 --> 00:11:52,080 The catalytic efficiency can be improved by placing a catalyst in the surface of the semiconductor. 127 00:11:52,780 --> 00:11:59,040 For example, for a bismuth vanadate photoanode the inclusion of a cobalt phosphate catalyst 128 00:11:59,060 --> 00:12:04,880 on the surface will ease the oxidation reaction and the water splitting efficiency will be higher 129 00:12:05,640 --> 00:12:12,400 The separation efficiency can be improved by introducing an electric field inside the material. 130 00:12:12,799 --> 00:12:20,819 One way to do this is to introduce gradient doping, from no doping at the surface to 1% 131 00:12:20,900 --> 00:12:23,240 doping close to the back of the electrode. 132 00:12:24,180 --> 00:12:30,620 That way we create a depletion region in between the semiconductor and the electrolyte 133 00:12:30,940 --> 00:12:35,980 that will more easily move the electrons from the electrolyte into the semiconductor. 134 00:12:36,840 --> 00:12:42,980 The combination of both effects can highly improve the efficiency of the overall device. 135 00:12:44,660 --> 00:12:49,700 For the photoelectrochemical water splitting process, at least the potential difference 136 00:12:49,760 --> 00:12:53,720 of 1.23 V must be given to directly split water. 137 00:12:54,180 --> 00:13:00,300 Then an overpotential has to be applied to compensate for the extra losses in the electrodes 138 00:13:00,400 --> 00:13:03,300 and the activation energy needed for the reaction. 139 00:13:03,920 --> 00:13:11,240 This value will depend on the materials and electrolyte used, but it is usually around 0.8 V. 140 00:13:12,360 --> 00:13:19,820 Both potentials added will result in the total potential difference needed to drive the redox reaction. 141 00:13:20,640 --> 00:13:26,580 This voltage will be partly covered by the potential difference created within the photoelectrode 142 00:13:26,680 --> 00:13:28,200 when light shines on it. 143 00:13:28,760 --> 00:13:35,500 But that only compensates for around 0.6 V of the needed voltage, depending on the material 144 00:13:35,500 --> 00:13:37,700 used, which is not enough. 145 00:13:38,820 --> 00:13:43,820 That is why these photoelectrodes are often combined with solar cells that give the extra 146 00:13:43,820 --> 00:13:45,980 potential needed for the reaction to happen. 147 00:13:46,820 --> 00:13:52,460 The combination of a photoelectrode and a solar cell forms a photoelectrochemical device. 148 00:13:53,180 --> 00:13:58,800 And if you are asking yourself how such a device will work, I will explain it right now. 149 00:14:00,260 --> 00:14:05,980 The photoelectrode, let's say a photoanode, is connected in series to a solar cell. 150 00:14:06,440 --> 00:14:11,820 So the photoanode of the example will be connected to the positive part of the solar cell 151 00:14:12,120 --> 00:14:17,340 and then the negative part of the solar cell will be connected through an external circuit 152 00:14:17,600 --> 00:14:22,280 to another electrode, which may or may not be photoactive. 153 00:14:23,060 --> 00:14:26,220 The circuit will be closed by the electrolyte. 154 00:14:26,540 --> 00:14:31,480 Since both devices are in series, the same current should go through both parts 155 00:14:31,760 --> 00:14:34,540 the same way we explained in tandem solar cells. 156 00:14:36,260 --> 00:14:39,200 In this device, the light is also utilized more. 157 00:14:40,040 --> 00:14:45,060 When the light strikes the photoelectrode surface, the light corresponding to the band gap 158 00:14:45,100 --> 00:14:47,760 of the photoelectrode will be absorbed there. 159 00:14:48,720 --> 00:14:54,780 The rest of the light that has not been absorbed or reflected, called the transmitted spectrum, 160 00:14:54,940 --> 00:15:00,220 goes to the solar cell where it can be absorbed to produce the extra potential difference 161 00:15:00,300 --> 00:15:03,580 to separate charges and drive the reaction. 162 00:15:04,420 --> 00:15:09,020 The solar cell must be especially designed to function with the transmitted spectrum, 163 00:15:09,260 --> 00:15:16,500 since the light will have a different spectrum and intensity than the standard solar spectrum AM1.5. 164 00:15:18,440 --> 00:15:24,780 The photoelectrode, as any other semiconductor device, has its own characteristic J-V curve. 165 00:15:25,380 --> 00:15:30,740 When a solar cell and a photoelectrode are combined, the conditions at which they will work 166 00:15:30,740 --> 00:15:35,260 can be calculated by studying their J-V curve characteristics. 167 00:15:36,380 --> 00:15:41,640 I already discussed the J-V curve of a solar cell in previous lectures 168 00:15:41,640 --> 00:15:43,840 which looks like the red curve here. 169 00:15:44,720 --> 00:15:50,800 Now, if we include the J-V curve of the photoelectrode, a photoanode in this case, 170 00:15:50,900 --> 00:15:55,940 represented by the black line, we can get the current and voltage at which the device will work, 171 00:15:56,240 --> 00:15:57,700 the operational point. 172 00:15:58,640 --> 00:16:04,000 Since both elements are connected in series, the current of both the solar cell 173 00:16:04,000 --> 00:16:06,740 and the photoelectrode must be the same. 174 00:16:07,440 --> 00:16:13,040 So the operational point will be where both J-V curves cross. 175 00:16:13,960 --> 00:16:19,720 The solar-to-hydrogen efficiency of the device is related to the amount of hydrogen 176 00:16:19,720 --> 00:16:25,820 that is produced, which is directly calculated by the current density measured at the operating point. 177 00:16:26,620 --> 00:16:32,180 The idea is that current is basically charges moving per unit of time, and we assume that 178 00:16:32,320 --> 00:16:37,680 all charges produced are involved in the reaction of hydrogen production. 179 00:16:38,200 --> 00:16:43,140 Mathematically, the overall solar-to-hydrogen conversion efficiency, is described as the 180 00:16:43,320 --> 00:16:51,380 operational current density J_photo multiplied by 1.23 V, the redox potential, 181 00:16:51,520 --> 00:16:57,640 and divided by the irradiance arriving to the surface of the photoelectrode, P_0. 182 00:16:58,580 --> 00:17:04,660 The higher the operational current, the more hydrogen is produced and the higher the efficiency 183 00:17:04,680 --> 00:17:06,140 of the device. 184 00:17:06,740 --> 00:17:12,400 That is why researchers are focusing in improving the current density that this device can give. 185 00:17:13,120 --> 00:17:17,280 We talked about gradient doping and the use of water oxidation catalysts. 186 00:17:17,800 --> 00:17:22,640 There are other improvements that can be done to increase the performance of such a device, 187 00:17:22,980 --> 00:17:28,660 such as the design of the materials and thickness of a silicon thin-film tandem cell 188 00:17:28,880 --> 00:17:32,000 or the texturing of the photoanode for better light trapping. 189 00:17:32,520 --> 00:17:39,040 Applying all those concepts, we here at the Delft University of Technology have developed 190 00:17:39,040 --> 00:17:44,700 a device made with a bismuth vanadate photoanode combined with a double junction amorphous 191 00:17:44,700 --> 00:17:47,740 silicon solar cell and a platinum cathode. 192 00:17:48,100 --> 00:17:55,820 This device has achieved a solar-to-hydrogen efficiency of 4.9%, which is actually 193 00:17:55,880 --> 00:18:02,000 the highest efficiency reported for such devices based on a metal oxide photoanode. 194 00:18:03,300 --> 00:18:10,180 This week we have discussed the third generation technologies, solar thermal and solar fuel approaches. 195 00:18:10,760 --> 00:18:15,160 Next week, we will start building and designing a PV system. 196 00:18:15,840 --> 00:18:20,260 I will introduce you to all important components in the PV system. 197 00:18:20,820 --> 00:18:22,500 See you next week!