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Photocatalytic Hydrogen Production in the Context of Sustainable Energy

Alberto Puga

Universitat Rovira i Virgili (URV), Department of Chemical Engineering, Av. Països Catalans 26, 43007 Tarragona, Spain

1.1 The Transition to Sustainable Energy

1.1.1 Trends in Primary Energy Production

The production of energy has not ceased to increase during the last two centuries [1]. This fact is rooted in the Industrial Revolution since the machinery representing its very heart was (and still is) powered by the exploitation of massive amounts of fossil carbon resources [2]. One key consequence of the industrial economy has been the improvement of living conditions, which, in turn, has resulted in sustained population growth worldwide. A greater population then needs more energy, leading to ever-increasing energy production. Therefore, it is not surprising that the general trend is still clearly upward for the last few decades (Figure 1.1).

Despite transient declines due to contingencies such as the credit crunch of 2007-2008 or the coronavirus pandemic in 2020, this steady and relentless increment in energy demand experienced in recent times is expected to continue for several more decades, in parallel to global demography. This is because world population growth might not reach a plateau until the end of this century, even if natural increase rates are slowing down. Although energy use is extraordinarily unequal across the planet, the overall current trend is also one of increasing energy demand in relative terms, that is, each generation consumes more energy per person than the previous one. In terms of sustainability, and considering that an excess of energy use above a certain threshold does not lead to improved living standards, an optimal and reasonable average annual per capita energy consumption at 2.8 tons of oil equivalent (toe) has been advocated [4]. Along these lines, global energy production would have to almost double up to almost 3?×?105 TWh in 2050. It is apparent that coping with demand from our near-future energy-hungry societies will represent a gigantic challenge. Therefore, energy security will also require judicious consumption and smart and efficient production, distribution, and use systems [5].

Figure 1.1 Global primary energy supply by source since 1965. Traditional biomass represents raw or gently processed solid fuels such as wood, peat, or charcoal. Modern biofuels include biomass-derived liquid fuels such as biodiesel or bioethanol. Source: OurWorldInData.org/energy, based on data from Refs. [1, 3]; Creative Commons 2022 Our World In Data.

Regarding the shares of different sources in our energy mix, it is striking to note that little has changed in the last half century besides the surge of nuclear power and the rather modest development of renewables. In fact, fossil fuels represented almost 80% of all primary energy supply in 2019 (Figure 1.1). This situation is evolving along a decarbonization pathway owing to the formidable improvement of solar and wind energy technologies, which are becoming cost-competitive at a rapid pace, motivated by the depletion of fossil fuels and the dreadful negative effects of CO2 emissions on the global climate (see following sections). However, it is naive to assume that the enormous amounts of fossil fuel in use today (see Figure 1.1) will be easily replaced with renewables.

1.1.2 Fossil Reserves

The depletion of reserves of carbon-based fossil fuels, chiefly natural gas, crude oil, and coal, is proceeding at a rapid pace due to our obstinate dependence on them. Proponents of persisting on their mass extraction and exploitation argue that technological advances are constantly enabling new discoveries. Therefore, the so-called proven reserves - i.e. those accessible with current technologies - have been increasing, being hitherto able to meet demand in a satisfactory fashion. It should be noted, though, that most of the newly discovered oil and gas fields are nonconventional and/or extremely difficult to reach, such as extra heavy crude, tar sands, shale oil and gas, or deep offshore fields.

The data in Table 1.1 establish a comparison of reserves and consumption data. Proven reserves in present times are significant, but somewhat lower than total reserves by the end of last century. These numbers must be taken with care due to the uncertainty in their estimation methods, but it is important to note that most of the existing oil and gas (coal is an exception) can be accessed using current means. What is even more relevant is assessing how much of the fossil fuel reserves have been already used and how much is still left. Only during the last 55?years, a significant proportion of all initially available crude oil and natural gas has been used by humanity (e.g. 155 and 60?Gt(C) equivalent, respectively, as compared to 208 and 96?Gt(C) in predicted reserves as of 2020, see Table 1.1). It should be emphasized that proven reserves are expected to be progressively more challenging for future extraction, and hence, less efficient and more expensive. Another matter of concern is their concentration in certain areas of the planet, creating dramatic geopolitical tensions and conflicts for the control of production [7, 8].

Table 1.1 Comparison of total reserves, proven reserves, and consumption of fossil fuels.

a) According to geological inventories performed in the late twentieth century [6].

b) According to the BP Statistical Review of World Energy [3].

Data in Table 1.1 clearly reveal that almost half of our underground battery of stored fossil oil and gas energy is already gone, and for obvious reasons, it will not be replenished to any practically meaningful extent on a human timescale. An unavoidable question follows: how long can we still rely on fossil fuels? One straightforward way to calculate this is the reserves-to-production ratio based on current data, which points to only another half century until total depletion of oil and gas (54 and 49?years, respectively, Table 1.1), and somewhat longer for coal. Future events may of course alter such projections, but based on all the above data, it would not be surprising if production of fossil fuels will stop being able to cope with world energy demand at some not-so-distant point, probably within the next couple of generations.

1.1.3 Carbon Dioxide Emissions and Global Warming

The finite nature of fossil fuels should be a strong enough argument in itself to seriously and strategically plan our economies ahead of their inaccessibility or depletion. In addition to that, their mass consumption is noticeably affecting global carbon cycles, especially with regard to emissions, and consequent atmospheric accumulation, of carbon dioxide [6, 9]. As illustrated in Figure 1.2, anthropogenic emissions pump more CO2 into the atmosphere than nature is able to fixate into biomass or to store into oceans (9?Gt(C) emitted, vs. 3?and 2?Gt(C) used by plants and algae to grow or absorbed by oceans, respectively, Figure 1.2).

Figure 1.2 Global carbon inventories and flows on Earth, showing natural carbon cycle events and the influence of human activities. Source: P, Müller [9]/with permission of Royal Society of Chemistry.

Among the effects of CO2 accumulation in the atmosphere, global warming is the most threatening and worrisome. The correlation of average temperatures with atmospheric CO2 levels has been established even in a pre-industrial time frame with little influence from human activities, confirming its greenhouse effect [10]. Moreover, the extraordinary buildup of carbon dioxide in the Earth's atmosphere from burning fossil fuels in recent times is now completely out of doubt [6, 11]. The extent to which this will affect global climate is a matter of intense debate, but many different models predict a dangerous rise in temperatures, probably leading to other uncharted consequences such as extreme weather events.

1.1.4 Strategic Low-carbon Goals and Energy Sustainability

Both eventual shortages of fossil fuels and global warming due to the greenhouse effect of anthropogenically emitted CO2 will sooner or later force humankind to a determined decarbonization. The sustainability of future energy schemes will thus largely depend on a successful transition from the current overexploitation of fossil fuels to the efficient and judicious use of renewable energy sources. Developing low-carbon, circular-carbon, or carbon-free energy sources are in principle valid options to strive on this...