Advantages of Solar Hydrogen Compared to Direct Carbon Dioxide Reduction for Solar Fuel Production
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billion tons of CO2 emitted each year. The manufacture of cement is also a very large scale industrial process that annually produces about 800 million tons of CO2. Therefore, to have any influence on global CO2 levels, one would have to develop and pay for implementing an energy-intensive industrial-scale process at some significant fraction of this huge scale to have any impact at all on atmospheric CO2 levels. This huge industrial-scale process would also have to produce fuels that are low enough in cost to be affordable even in rich countries or with reasonable subsidies. For comparison, worldwide, the largest industrially produced chemical is sulfuric acid at about 230 million tons per year in a downhill reaction. The second argument concerns a simple energy return on energy investment (EROEI) argument. At first thought, photoelectrochemical CO2 reduction looks attractive because one will have a ready supply of CO2 at the stack of a coal burning power plant, fermentation-based process, or cement plant. The raw materials needed (CO2 and sunlight) are free, and one might even get a subsidy to utilize or prevent the release of some or all of the CO2. However, this analysis only takes into account a small part of the entire coal burning system, the CO2 coming from the stack. To perform a proper analysis, as in thermodynamics, one must put the box around the whole coal mining and burning system not just the stack. Figure 1 shows a simplified graphic of the inputs and outputs of
ver the last 4 decades, there have been several surges of interest in research targeted toward the renewable production of fuels from solar energy. The first push was in the late 1970s and early 1980s following the oil embargo that created fuel shortages, raised fuel prices, and encouraged western governments to look to conservation and domestic sources of renewable fuels. Then, as now, the intermittency of wind and solar was also pointing toward the need to store renewably produced energy. One such approach is to store wind and solar energy as chemical fuels. The end of the oil embargo, and return of relatively cheap gasoline prices, resulted in the loss of funding for much of the research into the development of renewable fuel production for the next 15 years. Fossil fuels, currently more than 75% of total energy use, are a finite resource, and the cost of burning these fuels is more than just the price for their production when the eventual costs of climate change are considered. These facts have produced a new surge in funding for storing the energy as fuels from intermittent renewable energy sources because burning of carbon-containing fossil fuels produces most greenhouse gas emissions. Both in the past and the present the photoelectrochemical reduction of water to hydrogen and reducing carbon dioxide to produce carbon-containing fuels, such as methane or methanol, have been the main goals of solar fuels research. In this opinion piece, I will present arguments that if we are to spend considerable research and development dollars to actually deploy a renewable fuel-based economy the main effort should be on producing hydrogen from water rather than the direct reduction of CO2. However, I will state up front that this does not mean that no research funding should be spent on the basic science of CO2 reduction because this is an interesting and fundamental challenge due to the inertness of this molecule and the complex multielectron/multiproton chemistry involved. I will present several arguments of why hydrogen is the preferred product for at least the first step in the storage of renewable energy, and I will focus this Viewpoint on solar energy and photoelectrochemical processes although similar arguments might be made for solar thermal fuel-producing reactions. If the goal of CO2 reduction is to contribute to the mitigation of climate change by producing carbon-free or carbon-neutral fuels, the first argument against reducing CO2 can be summed up in a single word, scale. Coal burning, the largest fossil fuel contributor to greenhouse gas production, is the largest-scale industrial process on the planet. Although world coal consumption is now declining, now around 7 billion tons per year are still mined and burned, resulting in emission of more than twice as many billion tons of CO2. In addition, CO2 produced by other fossil fuels brings this total to around 32 © 2016 American Chemical Society
Figure 1. Simplified flowchart of the coal mining and utilization system.
the entire coal production and utilization system. It is apparent that there is a release of CO2 from burning fossil fuels in machines and explosives for mining the coal and then in shipping it to the power plants at locations far from the mine (e.g., Wyoming to the Midwestern U.S. or from Australia to China) and pulverizing the coal before combustion. The coal is then burned to produce heat that is converted to electricity at an efficiency of about 33% in a modern coal-fired plant (older Received: August 23, 2016 Accepted: October 17, 2016 Published: October 31, 2016 1057
DOI: 10.1021/acsenergylett.6b00377 ACS Energy Lett. 2016, 1, 1057−1059
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actually been reduced because the produced fuel is burned, producing CO2 that is returned to the atmosphere, resulting in only recycling the carbon once before it contributes to the greenhouse effect. The argument that more valuable products can be made from CO2 is countered by the fact that a “useful” fuel, if defined as one that can be used on a large scale, cannot be valuable because it has to be inexpensive and valuable products will still be made more cheaply from precursors that do not need a large energy input such as CO2. Large-scale production of valuable polymer products will still be made in the future, as they are today, from fossil resources in mostly downhill reactions with small CO2 emissions and hopefully more recycling. Leaving these fossil fuel resources for chemical product production for future generations is not often cited as another compelling reason to move away from burning them as quickly as we can produce them. The question then arises: Is there a better way for using sunlight to produce any liquid fuels needed in the future other than by direct photodriven reduction of CO2? A better way may well be through solar hydrogen production from water. Intensive research over the last decades has yielded some improvements in our ability to directly photoelectrolyze water, but we are still not very close to a practical deployable system. The reduced cost of photovoltaic electricity, and ongoing research to reduce the cost and increase the efficiency of electrolyzers, is also serious competition to the direct photoelectrolysis approach because there are some major advantages for producing hydrogen on demand where it is needed and at pressure. The lack of major progress in implementing a direct photoelectrolysis of water is partly due to the fact that cheaply implementing the efficient four-electron, four-proton chemistry to produce hydrogen and oxygen from water is a difficult enough problem that is currently unsolved. It is less difficult than CO2 reduction not only due to the fewer electrons and protons but also due to the substrate, water, being at very high concentration (55 M). The solubility of CO2 in water, even as carbonate, is quite limited. There are also mass transport limitations for doing electrochemical reactions at current densities needed from an efficient solar converter (>10 mA/ cm2), especially considering the required investment in expensive and energy-consuming collection systems to concentrate the CO2 from the small atmospheric concentration of around 400 ppm that given the mass transfer limitations would require 100−1000 times the collector area of the solar photon collectors to match their current density. Planting more and not cutting down self-replicating solar powered trees is the still the best option for collecting and storing dilute CO2. The direct photoelectrolysis of water still needs intensive research to solve basic scientific problems such as the lack of inexpensive scalable semiconducting materials with proper band gaps and band positions that are stable in electrolyte solutions for many years while under illumination. There are also many engineering problems to solve when the materials are identified. However, if one can cost-effectively produce hydrogen from water, again also possibly with photovoltaics connected to electrolyzers, one can mitigate many of the CO2 emissions associated with transportation by the use of hydrogen fuel cells in light-duty transportation. If liquid fuels are required, as they most likely will be for aircraft and ships, and some sort of incentive or tax credit is available to use CO2 as a feedstock, then the hydrogen could be used in the well-known water−gas
plants can be as low as 20%). If one plans to use CO2 from coal burning, there will be an additional expense associated with cleaning up the gas stream that is not unlike what is needed for simply sequestering the carbon, a technology that has its own challenges but may be viable in some locations. Along with CO2, burning coal also produces sulfur dioxide, nitrogen oxides, mercury, and other heavy and radioactive metal emissions. Huge volumes of coal ash, which also contains toxic metals, must be disposed of by shipping it to off site (and out of sight) locations using more fossil fuel (although some of the ash can be used in building materials and soil conditioning, but this is controversial due to the aforementioned contaminants). These wastes have resulted in widespread water pollution and some catastrophic incidents because their disposal is not federally regulated. Focusing on CO2 however, given the efficiency, there are more than three and perhaps as many as four or more energy equivalents of CO2 emitted to the atmosphere from burning the coal to produce a 1 equiv amount of electrical energy considering the previously mentioned CO2producing fossil fuel-based energy inputs that complete the coal mining to electricity cycle. Sunlight is a diffuse energy source, and currently its utilization requires investments to cover large areas with either photovoltaic cells or solar concentrators. Given the above reasoning, it would take more than 4 times the area, and thus capital investment in hardware, installation costs, and land, to employ solar collectors to reverse the emissions of a coal-fired plant than to replace its electrical energy production up front with solar electricity and/or other renewable electricity. Due to the intermittent nature of solar energy, some part of this replacement may have to come from wind or nuclear, especially given that nuclear is a base load power generator as is coal. Also, this investment would have the usual ∼25% duty cycle of solar collectors, and therefore, CO2 would have to be stored at night in order to utilize it the during daylight hours or the majority of it would still be released to the atmosphere. Industries are reluctant to invest in any high capital cost system that has a low duty cycle. Recently, the balance of systems’ costs of photovoltaic systems has exceeded the cost of the solar panels in photovoltaic installations. The balance of systems’ costs for a system that has to purify and concentrate and perhaps store the CO2 and include gas handing, perhaps under pressure, with liquid flow and then separation of products would be considerably larger for a CO2 reduction system than that for a photovoltaic system or even a hydrogen-producing system. The cost and stability of an as yet unknown very selective low overpotential catalyst needed for the complex multistep reduction reactions also needs to be considered. The bottom line is that investment in such a fuel-producing system would likely be more than 5 times the investment in producing the renewable energy to replace the electrical output of the coalfired plant. In addition to removing the CO2 emissions, the renewable energy would eliminate the production of toxic metal emissions, reduce the number of deaths from coal mining, which averages thousands per year worldwide (think of the outrage if a single worker died of radiation poisoning in a nuclear plant), and remove the need to mitigate the other environmental effects of coal mining such as ash disposal and mine reclamation. Additionally, even if one were to invest in and implement a process that selectively reduces CO2 from a coal-fired plant to a useful liquid fuel, carbon dioxide in the atmosphere has not 1058
DOI: 10.1021/acsenergylett.6b00377 ACS Energy Lett. 2016, 1, 1057−1059
ACS Energy Letters
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shift reaction to first produce CO at high temperatures inherent in flue gas
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AUTHOR INFORMATION
Notes
Views expressed in this viewpoint are those of the author and not necessarily the views of the ACS. The author declares no competing financial interest.
CO2 + H 2 → CO + H 2O
This reaction is already used on a large scale with nearly optimized catalysts and reactor designs. Additional renewable hydrogen can be added to produce the well-named synthesis gas or Syngas, a mixture of CO and H2, from which methanol can be produced directly with known catalysts. Higher hydrocarbon fuels such as gasoline and diesel can be made on a large scale either from methanol using known catalysts or using the Fischer−Tropsch reaction, as was done in Germany during World War II and in South Africa in more recent times.
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ACKNOWLEDGMENTS I wish to thank many colleagues in the photoelectrochemical community for their helpful comments and critiques.
(2n + 1)H 2 + nCO → CnH 2n + 2 + nH 2O
Indeed, even gasification of coal or biomass and adding solar hydrogen to make a liquid fuel would be more cost-effective and be less CO2-intensive than coal burning because there will be some renewable hydrogen in the fuel. Photoelectrochemical or photovoltaic/electrolysis-generated hydrogen from water would also be useful for nitrogen fixation via the Haber Bosch process. N2 + 3H 2 → 2NH3
Currently, most of the hydrogen used from ammonia production is derived from steam reforming methane followed by the water−gas shift, another large industrial process that releases CO2: CH4 + H 2O → CO + 3H 2 CO + H 2O → CO2 + H 2
This process already consumes 3−5% of the world’s natural gas and 1−2% of the world’s energy supply; therefore, using renewable hydrogen produced from water would have a large global impact in an already established infrastructure. In summary, I argued that although fundamental research on the direct electrochemical reduction of CO2 is an interesting and challenging academic exercise, it will not lead to large-scale implementation as a source of fuels. Except in limited off-grid situations, the value of electrons is determined by the cost of grid electricity that will be the competition for any on- or offgrid use of electrons for water or CO2 reduction. If any significant fraction of these grid electrons comes from the Carnot efficiency-limited burning of fossil fuels, as they will for the foreseeable future, it makes no sense to then use them to reduce that produced carbon dioxide, collected with significant energy input at the source or from the atmosphere, just to burn it, again with Carnot efficiency limits, and release it back to the atmosphere. Instead, transportation, which will still constitute a large fraction of world energy use, should be transitioned to much more efficient hydrogen fuel cells and batteries with no CO2 footprint as the grid continues to have a much higher fraction of carbon-free renewable or nuclear-produced electricity. Transportation for planes and ships, which still require liquid fuels, would have to be provided by near carbon neutral biofuels or other carbon sources that can also be upgraded by addition of renewable hydrogen to reduce their carbon footprint using well-known gas-phase chemistry.
Bruce Parkinson
Department of Chemistry and School of Energy Resources, University of Wyoming, 1000 East University, Laramie, Wyoming 82071, United States 1059
DOI: 10.1021/acsenergylett.6b00377 ACS Energy Lett. 2016, 1, 1057−1059
