6. Property, Plant and Equipment

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Property, plant and equipment as of June 30, 2024 and December 31, 2023 consisted of the following (in thousands):

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Construction in progress is primarily comprised of construction of three hydrogen production plants. Completed assets are transferred to their respective asset classes, and depreciation begins when an asset is ready for its intended use. Interest on outstanding debt is capitalized during periods of capital asset construction and amortized over the useful lives of the related assets. During the three months ended June 30, 2024 and 2023, the Company capitalized $3.1 million and $2.0 million of interest, respectively. During the six months ended June 30, 2024 and 2023, the Company capitalized $5.2 million and $4.0 million of interest, respectively.

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Depreciation expense related to property, plant and equipment was $13.1 million and $7.3 million for the three months ended June 30, 2024 and 2023, respectively. Depreciation expense related to property, plant and equipment was $24.7 million and $12.8 million for the six months ended June 30, 2024 and 2023, respectively.